Chapter 8: Tinkercad: Application of Mechanical Blocks of Robotics

The robotics club at a school in Jaipur decided to build a small four-wheeled robot. The students bought two motors, four wheels and a sheet of plastic for the base. On Saturday morning they marked the sheet with a pencil and started cutting.

By noon, nothing fitted. The holes for the motor screws were a few millimetres off, so the motors sat crooked. One wheel rubbed against the edge of the base. The sheet was already cut, so there was no way to move anything. They had used up the whole sheet and had one wheel that would not turn freely.

Their teacher suggested something simple for the next attempt. Before cutting anything, draw the robot on a computer. Place the base, the wheels and the motor mounts on screen. Check that the holes line up and that the wheels turn without touching anything. Change a measurement, look again, change it once more. A mistake on screen costs nothing.

That is what Tinkercad is for.

Diagram

Fig. 8.0-1 – two halves of one picture. Left half: a student cutting a plastic sheet by hand, with a crooked motor and a wheel rubbing the edge, and a bin with wasted offcuts. Right half: the same robot base shown on a computer screen as a neat 3D model, with the wheels and motor mounts in place and a measurement arrow between two holes. Label the left half “Cut first” and the right half “Design first”.

Curiosity Corner

Q1. Why do engineers design a robot on a computer before building it? Q2. Apart from 3D shapes, what else can you build and test inside Tinkercad? Q3. How can one shape be used to cut a hole in another shape? Q4. What is the difference between a revolute joint and a prismatic joint? Q5. Why is a robotic arm with two turning joints called an RR mechanism?

By the end of this chapter, you will be able to answer all five of these questions.

8.1 Designing Before Building

A tailor does not cut cloth the moment it comes off the roll. He measures, marks the cloth with chalk, checks the marks once more, and only then picks up the scissors. Cloth that has been cut wrongly cannot be joined back. A chalk mark can be rubbed out and drawn again in seconds.

Building a robot works the same way. A robot is not one solid object. It is many parts that have to fit one another — a base, motors, wheels, screws, a battery, a circuit board. If a single hole is a few millimetres out of place, the motor sits crooked and the wheel rubs. The part is already cut, so the mistake cannot be undone.

Checking a design on screen

This is why engineers draw the whole robot on a computer first. Every part is drawn to its real size, and the parts are placed exactly where they will sit in the finished robot. On screen you can check three things before you cut anything:

Fit — does the wheel fit the motor shaft, and does the motor fit the hole meant for it?
Clearance — is there enough empty space around each moving part, so nothing rubs?
Size — is the finished robot small enough for the table, the track or the box it must work in?

If any answer is no, you change a measurement and look again. Nothing has been spent. The same check on real material would cost you the material.

A design like this is a plan of the real object. Once you are satisfied with it, you build the object from the plan. The first object you build to test the plan is called a prototype.

Definition

(A prototype is an early, trial version of a product, built to test whether the design actually works before the final version is made.)

Engineers often go around the same loop several times — design, check, change, design again — and build the prototype only at the end (see Fig. 8.1-1).

Diagram

Fig. 8.1-1 – a loop of four boxes joined by arrows: “Design on screen” → “Check fit, clearance and size” → “Change a measurement” → arrow back to “Design on screen”. A fifth arrow leaves the “Check” box, labelled “all correct”, and points to a separate box on the right marked “Build the prototype”.

There is one honest limit to all of this. A drawing on screen shows shape and size, but it does not tell you how heavy the robot will feel, whether the plastic will bend under the weight of the battery, or how much a running motor will shake the base. Those answers come only from building the prototype. Designing first does not remove testing — it removes the wasted material.

Curiosity Corner

Q1. Why do engineers design a robot on a computer before building it?

A. Because a mistake on screen costs nothing, while a mistake in cut material wastes the material and the time spent on it. On the computer the parts can be placed at their true sizes and checked for fit, clearance and overall size, and any measurement can be changed and checked again as often as needed. Only when the design is correct is the first real version, the prototype, built.

8.2 What Is Tinkercad?

Section 8.1 said that a robot should be designed on a computer first. But which computer program should a school student use? Professional design software is expensive, has to be installed, and takes months to learn. That is too much for a first robot.

Tinkercad is made for exactly this situation. It is a free online design tool from Autodesk. You open it in an ordinary web browser at www.tinkercad.com, sign in, and start designing. There is nothing to buy and nothing to install, so it works the same on a school computer, a home laptop or a college machine.

Definition

(Tinkercad is a free, browser-based online platform used to create 3D models, build and test electronic circuits, and write simple code for them.)

Tinkercad is not one single tool. It has three separate workspaces, and you pick the one that suits the job (see Fig. 8.2-1). All three are useful in robotics, and this chapter uses the first one the most.

Diagram

Fig. 8.2-1 – three labelled panels side by side under one heading “Tinkercad”. Panel 1 “3D Design” showing a cube and a cylinder on a grid. Panel 2 “Circuits” showing a small board wired to an LED with a resistor. Panel 3 “Codeblocks” showing three coloured instruction blocks stacked one below the other with an arrow pointing to a simple 3D shape.

8.2.1 3D Design

The 3D Design workspace is where you build the shape of a part. You drag ready-made shapes — a box, a cylinder, a tube, a cone — from a panel on the right onto a flat grid, and then set their length, width and height to the sizes you want. Placing several shapes together gives you a part such as a wheel or a robot base.

What you build here is a 3D model: a description of an object held in the computer, showing its exact shape and size from every side. It is not a photograph and not a flat drawing. You can turn it around and look at it from underneath if you want to.

A finished 3D model can be saved in the file formats .STL and .OBJ. These are the formats a 3D printer understands. 3D printing is a way of making a real object by laying down thin layers of melted plastic, one on top of the other, until the whole shape is built up. So a wheel designed on screen can become a plastic wheel you can hold.

How It Works

the 3D Design workspace What it is: The part of Tinkercad where solid shapes are combined into a 3D model of an object. How it works: You drag basic shapes onto a grid called the workplane, set each shape’s measurements, and move the shapes so they touch, overlap or sit inside one another. Shapes can then be joined into a single object. Used for: Designing the mechanical parts of a robot — a wheel, a base, an arm — before any material is cut. Watch out: The model only carries shape and size. It says nothing about how strong the material is or how much the finished part will weigh.

8.2.2 Circuits

A robot is not only mechanical parts. It also has wires, sensors, an LED or two, and a small computer board that controls them. Getting a circuit wrong can damage a component, and a beginner cannot always tell a wrong connection from a right one just by looking.

The Circuits workspace lets you build the circuit on screen instead. You drag out a breadboard, a resistor, an LED, a sensor or a microcontroller board such as an Arduino Uno — a microcontroller is a small computer on a single chip that reads inputs and controls outputs — and join them by drawing wires with the mouse.

Then you press the button that starts the simulation. A simulation is a run of the design inside the computer, in which the computer works out what the real components would do and shows you the result. If your wiring is correct, the LED on screen lights up. If you have connected it the wrong way round, it stays dark. Nothing is damaged either way.

Compare Table

Table 8.1 – 3D Design vs Circuits

Feature	3D Design	Circuits
What you build	The physical shape of a part	An electrical circuit
What you drag in	Boxes, cylinders, tubes, cones	Breadboards, LEDs, resistors, sensors, boards
What you get out	A 3D model, saved as `.STL` or `.OBJ`	A working circuit that can be simulated
Question it answers	Will the parts fit together?	Will the circuit work when switched on?
Used in this chapter	Yes, throughout	Only mentioned

8.2.3 Codeblocks

The third workspace, Codeblocks, builds a 3D model by following instructions instead of by dragging shapes around. You stack coloured blocks such as “add a box” or “move by 10”, and Tinkercad carries them out in order and draws the result.

The advantage is that the instructions stay visible and can be edited. Change the size written inside one block, run the blocks again, and the whole model is rebuilt at the new size. Dragging shapes by hand does not give you that.

Writing code for a microcontroller is a separate job, and it is done inside the Circuits workspace, not in Codeblocks. There you can write the program either as blocks or as text, and the simulated Arduino board then runs it.

Common Error

thinking Codeblocks is where Arduino programs are written What goes wrong: Codeblocks and Arduino programming both involve coloured blocks, so they are easily mixed up. Students then look for the Arduino board inside Codeblocks and cannot find it. The fix: Remember what each one produces. Codeblocks produces a 3D shape. The code editor inside Circuits produces a program that runs on the simulated board.

Curiosity Corner

Q2. Apart from 3D shapes, what else can you build and test inside Tinkercad?

A. Electronic circuits. In the Circuits workspace you can place components such as LEDs, resistors, sensors and a microcontroller board, wire them together on screen, and run a simulation to see whether the circuit works. You can also write the program for that board there, either as blocks or as text.

8.3 Getting Started with Tinkercad

Knowing what a tool does is not the same as being able to use it. This section takes you from an empty browser window to a finished part with a hole in it. Everything here happens in the 3D Design workspace.

8.3.1 Opening Tinkercad and Starting a Design

Getting in takes two steps.

Open a browser and go to www.tinkercad.com.
Sign up or sign in. You can use an email id, or sign in with a Google or Apple account. Students whose school has set up a class can instead use the option to join their class with a code from their teacher.

Once you are signed in, the dashboard opens (see Fig. 8.3-1). Down the left side are the workspaces you met in Section 8.2 — 3D Designs, Circuits and Codeblocks — along with Lessons. The middle of the page shows the designs you have made so far, most recent first, and a button to create a new design.

Diagram

Fig. 8.3-1 – screenshot of the Tinkercad dashboard after signing in, with the left-hand list of workspaces (3D Designs, Circuits, Codeblocks, Lessons) marked, and the “Create new design” button marked.

Every design is saved by itself as you work, on Tinkercad’s servers rather than on the computer you are sitting at. That is why you can start a design in the school lab and open the same design at home. Tinkercad also gives each new design a random two-word name of its own; rename it to something sensible, or you will not find it again next week.

8.3.2 Working on the Workplane

Click on 3D Designs and then on the button to create a new design. What opens is a large blue-green grid with a panel of shapes on the right (see Fig. 8.3-2). That grid is the workplane — the flat surface you build on, standing for the ground under your object.

Diagram

Fig. 8.3-2 – screenshot of the Tinkercad 3D design editor, with the workplane grid labelled at the centre, the Basic Shapes panel labelled on the right, and the view-control buttons labelled at the top left.

To place a part, drag a shape from the panel on the right and drop it on the workplane. Once it is there, you can change it:

Size. Click the shape. Small white handles appear along its edges. Drag a handle to stretch the shape, or type the exact length, width and height into the boxes that appear.
Height above the ground. The small black cone on top of a selected shape lifts it off the workplane. This is how one part is made to sit above another.
Colour and transparency. Click the word Solid in the shape’s settings panel to pick a colour, so different parts of a robot can be told apart at a glance.
Copies. Hold the Alt key, click the shape and drag. A second, identical shape comes away from the first. This is much safer than drawing the same part twice, because the copy is exactly the same size.

Two more controls are about looking rather than building. The mouse wheel zooms in and out. Holding the right mouse button and moving the mouse turns the view, so you can look at the model from the side or from below. If you lose sight of your object altogether, select it and press the F key to bring the camera back to it.

As you drag a shape across the workplane, Tinkercad shows the distance from the shape to the edges of the grid, and to other shapes. You do not have to guess whether two holes are far enough apart — the measurement appears while you move.

8.3.3 Solid Shapes and Hole Shapes

Real parts are rarely lumps of plastic. A wheel needs a hole in the middle for the motor shaft. A robot base needs holes for screws. So Tinkercad needs a way of taking material away, not only adding it.

Every shape you drag in can be set to one of two states, using the two buttons in its settings panel:

A solid shape is ordinary material. It shows in a colour, and it is part of the finished object.
A hole shape is empty space. It shows as a see-through grey shape, and it will remove any solid material it overlaps.

The important point is that a hole shape by itself does nothing yet. It sits on the workplane looking transparent, but the solid under it is still whole. The removal happens later, when the shapes are grouped (Section 8.3.4). Until then you are free to slide the hole shape around until it is exactly where you want it.

Diagram

Fig. 8.3-3 – the same cylinder drawn twice, side by side. On the left it is coloured and labelled “Solid — adds material”. On the right it is drawn see-through with grey hatching and labelled “Hole — removes material”. Below both, show the settings panel with the two buttons “Solid” and “Hole”, with an arrow from each button to the matching cylinder.

Because a hole shape is a shape like any other, its size decides the size of the hole. A cylinder used as a hole leaves a round hole whose radius is the cylinder’s radius. So the sizing has to be worked out before the shape is placed.

Example 8.1

sizing a hole cylinder for an M3 screw

Given: An M3 screw has a shaft 3 mm across. The screw should pass through the hole freely, so the hole is to be 0.5 mm wider than the shaft.

Find: The radius to set for the cylinder used as the hole shape.

Working:

Diameter of hole = diameter of shaft + clearance = 3 mm + 0.5 mm = 3.5 mm

Radius of hole = diameter ÷ 2 = 3.5 mm ÷ 2 = 1.75 mm

Set the cylinder’s radius to 1.75 mm.

Common Error

typing the diameter into the radius box What goes wrong: The screw measurement you are given is a diameter, but Tinkercad asks a cylinder for its radius. Typing 3.5 instead of 1.75 gives a hole twice as wide as intended, and the screw wobbles in it. The fix: Halve the diameter before typing it in, and read the label on the box you are typing into.

8.3.4 Grouping Shapes to Build Complex Parts

The shapes Tinkercad gives you are simple — boxes, cylinders, tubes, cones. No robot part looks like any one of them. Real parts are made by combining several shapes, and grouping is the command that does the combining.

Select two or more shapes and click the group tool at the top of the screen. What happens next depends on what the shapes are.

Solid grouped with solid: the shapes are joined into one object. The whole thing takes a single colour, which is how you can tell the grouping worked.
Solid grouped with hole: wherever the hole shape overlapped the solid, the material is removed. The hole shape itself disappears.

So making a hole is a matter of putting a hole shape in the right place and then grouping.

Algorithm

making a hole through a part Input: A solid shape, and the size and position of the hole wanted. Output: The same solid with a hole through it, as a single object. Steps:

1. Drag the solid shape onto the workplane and set its length, width and height.
2. Drag a cylinder onto the workplane and set its radius to half the hole diameter.
3. Set the cylinder's height taller than the solid, so it passes right through.
4. Click the Hole button in the cylinder's settings panel.
5. Move the cylinder to the exact position where the hole is wanted.
6. Select both shapes and click the group tool.

Why it works: Grouping treats the hole shape as material to be taken away, so every part of the solid that the cylinder passed through is removed.

Diagram

Fig. 8.3-4 – three stages left to right, joined by arrows. Stage 1: a coloured cuboid and, beside it, a tall see-through cylinder. Stage 2: the cylinder moved so it stands through the middle of the cuboid, still see-through, labelled “positioned, not yet grouped”. Stage 3: the same cuboid in one colour with a clean round hole through it, labelled “after grouping”.

Step 3 is the one students most often get wrong. If the cylinder is shorter than the solid, or sits too low, it removes material from only part of the thickness. The result is a shallow dent rather than a hole that goes right through, and you will only notice when you turn the model over.

Try Yourself

Make a cuboid 40 mm long, 40 mm wide and 10 mm thick, and cut a 3.5 mm hole through it near one corner. Now move the hole cylinder halfway off the edge of the cuboid before grouping, and predict what shape the cut will leave.

Grouping is not permanent. The ungroup command separates a grouped object back into the shapes it was made from, and a hole that was cut comes back as a hole shape you can move. This is what makes the design loop of Section 8.1 practical — you can open last week’s chassis, ungroup it, shift a hole by two millimetres and group it again.

Curiosity Corner

Q3. How can one shape be used to cut a hole in another shape?

A. By setting that shape to Hole instead of Solid, positioning it where the hole is wanted, and then selecting both shapes and grouping them. A hole shape counts as empty space, so grouping removes every part of the solid that the hole shape overlapped. Until the two are grouped, the solid stays whole and the hole shape can be moved freely.

8.4 Where Tinkercad Is Used and What It Offers

Tinkercad is a school tool, but it is not only a school tool. Knowing who else uses it, and why, tells you what kind of work it is good at — and where it stops being the right choice.

8.4.1 Applications of Tinkercad

Tinkercad is used wherever someone needs a simple 3D model or a quick circuit test, without a long wait and without expensive software. Five uses come up most often.

Education. Classrooms use Tinkercad to teach 3D modelling, electronics and coding together. A student who has only read about a circuit can build one and see it work in the same lesson. Because it needs nothing but a browser, a whole class can use it at once.
Prototyping for 3D printing. Designers use it to draw a quick trial version of a part, print it, hold it, and find out what is wrong with it. Only after the shape is settled do they move to heavier professional software.
Electronics and robotics. Circuits can be built and tested before any component is bought. A sensor circuit, a robotic arm’s wiring or a small smart device can be laid out, coded and simulated on screen, and the mistakes found there cost nothing.
Architecture and interior design. Simple models of rooms, buildings and furniture can be put together quickly, so a designer can show a client the idea rather than describe it. A piece of furniture can be modelled at true size and checked against the space it must fit.
Modelling for animation and visualisation. Basic models made in Tinkercad can be exported and taken into animation software as simple characters or objects. Teachers and researchers use the same approach to model structures that are hard to picture, such as a molecule or a cell, for a demonstration.

The thread running through all five is the same. Tinkercad is used at the start of a job, when an idea has to be turned into something you can look at and check.

8.4.2 Benefits of Tinkercad

Why Tinkercad rather than one of the professional packages? Several reasons, and most of them matter especially to a beginner.

Easy to learn. Everything is dragged and dropped. Software such as AutoCAD or SolidWorks can do far more, but a beginner spends weeks learning the controls before making anything. In Tinkercad you can build a simple part in your first lesson.
Web-based. There is nothing to download or install. It runs in a browser on Windows, macOS or Linux, so an old school computer works as well as a new laptop.
Free. No licence has to be bought, which is what makes it usable by a whole class rather than by one machine in the corner.
Work saved online. Your designs sit on Tinkercad’s servers, so you can open the design you started in the lab from any other computer.
Versatile. The same account covers 3D modelling, circuits and coding. Models can be exported as .STL or .OBJ for 3D printing, so the work does not end at the screen.
Supports STEM education. It brings design, electronics and programming together, which is what science, technology, engineering and mathematics (STEM) teaching tries to do.
Simulation before building. Circuits can be run on screen first, so mistakes are found before components are wired up and possibly damaged.
Community and collaboration. Users share their designs publicly. You can open someone else’s design, look at how it was built, and change a copy of it. Teachers and project teams can work on shared designs together.

Ethics Watch

A design you share is visible to strangers, and anyone may copy and change it. Never put a person’s name, address, phone number or photograph into a design, and never upload a classmate’s work as your own. If you build on someone else’s shared design, say so.

Where Tinkercad stops

Being simple is also Tinkercad’s limit. It builds shapes by combining boxes, cylinders and tubes, so smoothly curved or finely shaped parts — a gear tooth profile, a moulded casing — are awkward or impossible to make well. It is not built for assemblies of hundreds of parts, and it cannot tell you whether a part will bend or break under load, which professional software can. It also needs a working internet connection; with the network down, the lab computer will not open your design.

None of this makes it the wrong tool for this chapter. It makes it the right tool for the first step, and the wrong tool for the last one.

8.5 Joints: How the Parts of a Robot Move

So far every part you have designed has been a single rigid piece. A robot arm is not one piece. If it were, it could not move at all.

Hold your own arm out straight and keep it stiff. Now you can only move the whole arm from the shoulder. Bend the elbow and the forearm moves on its own. Your arm is made of stiff bones with a movable connection between them, and that is exactly how a robot arm is built.

The stiff pieces are called links — a link is a rigid part of a robot that does not bend or change shape. The movable connections between them are called joints. A joint holds two links together and allows one to move relative to the other, but only in one fixed way.

That last phrase matters. Your elbow bends, but it does not slide, and it does not twist freely. It allows exactly one kind of motion. Engineers describe this by saying the joint has one degree of freedom — one independent way in which it can move. Because each joint moves in only one way, an arm’s position can be described completely by one number per joint, and a computer can control it.

Definition

(A joint is a connection between two links of a robot that allows one link to move relative to the other in one definite way.)

Robots use several kinds of joint. Two of them are the basic ones, and almost every simple robot is built from them: the revolute joint and the prismatic joint.

8.5.1 The Revolute Joint

Think of a door on its hinges. The door does not go anywhere — it swings. Every point on the door travels along a circle around the line of the hinges, and the door’s position is given by a single angle: how far open it is.

A revolute joint works in the same way. Two links are pinned together, and one turns about the pin while the other stays put (see Fig. 8.5-1). The joint’s position is one angle, measured in degrees.

Diagram

Fig. 8.5-1 – two straight bars labelled “Link 1” and “Link 2” joined at one end by a small circle labelled “revolute joint”. Link 1 is horizontal and fixed, shown hatched at its far end. Link 2 is drawn in two positions, the second swung upwards, with a curved arrow between them and the angle between the two positions marked with the Greek letter theta.

You have already built one without knowing it. In Section 8.3 you made a hole in a shape. Put a cylinder through that hole so that it fits but is not tight, and the two parts can turn about each other. A pin in a hole is a revolute joint.

How It Works

revolute joint What it is: A joint in which one link turns about a fixed line through the joint, called the axis. How it works: A pin passes through a matching hole in the other link. The pin cannot move sideways or lengthwise, so the only motion left is turning. In a real robot a motor at the joint sets the angle. Used for: The shoulder, elbow and wrist of a robotic arm, and the axle of a wheel. Watch out: The turn is usually limited. The links themselves get in the way after a certain angle, and wires running across the joint will twist and eventually break if it keeps turning the same way.

Because the far end of a link travels along a circle, you can work out how far it moves for a given turn.

Example 8.2

how far the tip of a link travels

Given: A link 20 cm long is fixed at one end to a revolute joint. The joint turns through an angle of 90°.

Find: The distance travelled by the tip of the link.

Working:

The tip moves along an arc of a circle of radius 20 cm.

\[\text{arc length} = \frac{\theta}{360°} \times 2 \pi r\]

Here \(\theta\) is the angle turned through in degrees, and \(r\) is the length of the link in centimetres.

arc length = (90 ÷ 360) × 2 × 3.14 × 20 cm

arc length = 0.25 × 125.6 cm

arc length = 31.4 cm

The tip travels 31.4 cm.

8.5.2 The Prismatic Joint

Not every motion is a turn. A drawer in a table does not swing — it slides straight in and out. A lift does not rotate — it goes straight up and down. In both cases the moving part keeps facing the same way throughout; only its position along one straight line changes.

A prismatic joint gives a robot this kind of motion. One link slides along a straight track or inside a sleeve on the other link (see Fig. 8.5-2). Its position is one distance, measured in millimetres or centimetres, not an angle.

Diagram

Fig. 8.5-2 – a fixed hollow rectangular sleeve, hatched, with a narrower bar passing through it. The bar is drawn in two positions, the second pushed further out, with a straight double-headed arrow between them labelled with the letter d for the distance moved.

How It Works

prismatic joint What it is: A joint in which one link slides in a straight line along the other, without turning. How it works: The moving link is held inside a sleeve or on a rail that fits it closely. The fit blocks turning and sideways movement, so straight-line sliding is the only motion left. Used for: The vertical column of a lift, the up-and-down movement of a 3D printer head, and machines that push a part into place. Watch out: The joint can only reach as far as its track is long, so its range is fixed by the size of the part. Dust or grit in the track makes the slide stiff or jams it altogether.

Compare Table

Table 8.2 – Revolute joint vs prismatic joint

Feature	Revolute joint	Prismatic joint
Kind of motion	Turning about an axis	Sliding along a straight line
Position given by	An angle, in degrees	A distance, in millimetres or centimetres
Path of the moving end	A circular arc	A straight line
Everyday example	A door on its hinges	A drawer in a table
Simple way to build it	A pin fitted in a hole	A bar fitted inside a sleeve
Limit on its range	Links clash, and wires twist	The length of the track

Safety Note

The gap at a joint closes as the joint moves, and fingers can be trapped in it. Keep your hands out of the path of a joint on a real robot while power is on, and switch the power off and disconnect the battery before adjusting, tightening or rewiring anything at a joint.

Curiosity Corner

Q4. What is the difference between a revolute joint and a prismatic joint?

A. A revolute joint turns. One link swings about a pin, its position is given by an angle, and the moving end travels along a circular arc — like a door on its hinges. A prismatic joint slides. One link moves in a straight line inside a sleeve or along a track, its position is given by a distance, and the moving end travels in a straight line — like a drawer. Each allows one kind of motion only.

8.6 Designing the Mechanical Components of a Robot

You now have everything you need: shapes, holes, grouping, and an idea of how joints work. This section puts them together to design two real parts of a wheeled robot — a wheel, and the frame that carries everything.

Work in millimetres throughout, because that is what Tinkercad uses and what a 3D printer expects.

8.6.1 Designing a Wheel

A wheel looks like a simple disc, but it is not. A solid disc has nowhere for the motor shaft to go. What a wheel really needs is a ring of material with a hole down the middle, sized to grip the shaft.

Tinkercad has a shape that is already a ring: the tube. A tube is described by two measurements — its radius, taken to the outside of the ring, and its wall thickness, which is how much material the ring is made of. Subtract one from the other and you have the size of the space inside.

Diagram

Fig. 8.6-1 – a tube seen face on, drawn as two circles one inside the other with the ring between them shaded. An arrow from the centre to the outer circle is labelled “radius”. A short arrow across the shaded ring is labelled “wall thickness”. A dashed arrow from the centre to the inner circle is labelled “inner radius = radius − wall thickness”.

The wheel is built from two tubes, one inside the other. The outer tube is the tyre — the part that touches the ground. The inner tube is the hub, the part that grips the motor shaft. For them to fit, the outer radius of the inner tube must equal the inner radius of the outer tube. If it is smaller, a gap is left; if it is larger, the two overlap and the model is wrong.

The sizes therefore have to be worked out before either tube is dragged onto the workplane.

Example 8.3

sizing the two tubes of a wheel

Given: The tyre tube is to have a radius of 10 mm and a wall thickness of 3 mm. The motor shaft it must fit on measures 4 mm across.

Find: The inner radius of the tyre tube, and the radius and wall thickness of the hub tube.

Working:

Inner radius of tyre tube = radius − wall thickness = 10 mm − 3 mm = 7 mm

The hub must fill this space exactly, so radius of hub tube = 7 mm.

The hole down the middle of the hub must match the shaft, so its inner radius = 4 mm ÷ 2 = 2 mm.

Wall thickness of hub tube = radius − inner radius = 7 mm − 2 mm = 5 mm

The tyre tube has an inner radius of 7 mm. The hub tube has a radius of 7 mm and a wall thickness of 5 mm, leaving a 4 mm hole for the shaft.

With the numbers settled, building it is quick.

Algorithm

building a wheel from two tubes Input: The tyre radius, the tyre wall thickness, and the width of the motor shaft. Output: A single wheel object with a central hole for the shaft. Steps:

1. Work out the inner radius of the tyre tube, and the radius
   and wall thickness of the hub tube.
2. Drag a tube onto the workplane. Set its radius and wall
   thickness to the tyre values. Set its height to the width
   of wheel you want. Colour it dark.
3. Drag a second tube onto the workplane. Set its radius and
   wall thickness to the hub values, and its height to the
   same value as the tyre.
4. Move the hub tube inside the tyre tube so that the two
   share the same centre and the same base.
5. Select both tubes and click the group tool.

Why it works: The hub’s outer surface and the tyre’s inner surface are at the same radius, so the two rings meet with no gap and no overlap, and grouping makes them one part.

Diagram

Fig. 8.6-2 – screenshot of the finished wheel in the Tinkercad 3D design workspace, seen face on: a dark outer ring with a lighter ring fitted inside it and a small round hole at the centre.

Common Error

making the two tubes different heights What goes wrong: If the hub is shorter than the tyre, or sits at a different height above the workplane, the two rings do not line up. The finished wheel has a step in it, and a 3D printed version may not sit flat. The fix: Give both tubes the same height, and set both to sit on the workplane before grouping.

Try Yourself

Build the wheel above, then make a copy of it and change the tyre radius from 10 mm to 15 mm without touching the wall thickness. Look at the new inner radius, and say whether the hub tube you designed will still fit.

8.6.2 Designing the Chassis

Everything else in the robot has to be carried by something. That something is the chassis — the frame that all the other parts of a robot are mounted on.

A chassis has three jobs. It has to be stiff, so the robot does not sag or twist. It has to hold the motors in a fixed position, or the wheels will not stay parallel. And it has to give the battery, the board and the sensors somewhere to be bolted down.

The base plate

Start with the flat piece that everything sits on. Drag a box onto the workplane and set its length, width and height so it becomes a thin, wide cuboid — long and broad enough for the parts you plan to carry, and thin enough not to be heavy.

Now add the mounting holes. Each electronic part is held down by screws, and the screws need holes. Use the method from Section 8.3.4: drag in a cylinder, set its radius from the screw size, set its height taller than the plate so it goes right through, switch it to Hole, and position it. Copy the hole cylinder with the Alt key for the others, so every hole comes out the same size. When all of them are in place, select everything and group.

Diagram

Fig. 8.6-3 – a thin cuboid base plate seen from above, with four small round holes arranged in a rectangle near the middle and two more towards one end. Label the plate “base plate” and one hole “mounting hole for a screw”.

The side mounting walls

The motors cannot be screwed flat onto the base plate, because their shafts have to point sideways, out towards the wheels. They are held instead by two upright walls, one on each side.

Make a wall the same way as the plate: a box, made thin and given the height you want, then rotated to stand upright. Each wall carries two holes, one towards each end, for the two motors on that side. Place these holes far enough apart that the wheels, when fitted, will turn clear of the corners of the base plate. Then copy the whole wall with the Alt key, so the second side is identical to the first.

Finally, bring the base plate and the two walls together so that the edges of the plate meet the walls, and group the three parts into one chassis.

Diagram

Fig. 8.6-4 – the assembled chassis seen from one corner: a flat base plate with mounting holes, held between two upright side walls, each wall carrying two holes near its ends. An arrow points to one wall hole with the label “motor shaft passes through here”.

The chassis you have made is one grouped object, but it is not final. If the battery turns out to be wider than expected, ungroup it, move a hole, and group it again. That is the loop from Section 8.1, and this is where it earns its keep.

8.7 Designing an RR Mechanism

A wheeled robot moves itself about. A robotic arm does something different — it stays where it is and reaches for things. To design one, you have to decide how many joints it needs and what kind.

Start with one joint. A single revolute joint with a link on it can swing, but the tip of that link is stuck on one circle. It can reach a point on that circle and nowhere else. It cannot come closer in, and it cannot go further out.

Now add a second revolute joint at the far end of the first link, with a second link on it. The first joint decides which way the arm points; the second decides how far the arm is folded or stretched. Between them, the tip can be brought to any point over a whole area rather than a single circle. This is exactly what your shoulder and elbow do together.

An arm built this way is called an RR mechanism. The letter R stands for revolute, and there are two of them, one after the other.

Definition

(An RR mechanism is a robotic arm made of two links connected by two revolute joints, one at the base and one between the links.)

The parts of the arm

Fig. 8.7-1 shows the arrangement. At the bottom is a fixed base that holds the whole arm steady. The first revolute joint sits on the base and carries the first link. The second revolute joint sits at the far end of that link and carries the second link. At the very end is the end effector — the tool at the tip of a robot arm that actually does the work, such as a gripper, a claw or a suction cup.

Diagram

Fig. 8.7-1 – a side view of a two-link arm. A wide flat disc at the bottom labelled “base”. A small circle on it labelled “joint 1 (revolute)” with the angle from the vertical marked as theta one. A bar rising from it labelled “link 1”. At its top end, a second small circle labelled “joint 2 (revolute)” with the angle between the two links marked as theta two. A second bar labelled “link 2” going out from it, ending in a two-fingered claw labelled “end effector”.

Give the two joints angles and the whole arm’s position is fixed. Nothing else is free to change. That is why two numbers are enough to tell this arm where to go.

How far the arm can reach

The two link lengths decide the area the tip can cover. When both links stretch out in one straight line, the tip is as far away as it will ever get. When the second link folds back along the first, the tip is as close in as it will ever get.

Example 8.4

the reach of an RR arm

Given: The first link is 60 mm long and the second link is 40 mm long.

Find: The greatest and the least distance from the base at which the tip can be placed.

Working:

Greatest reach = length of link 1 + length of link 2 = 60 mm + 40 mm = 100 mm

Least reach = length of link 1 − length of link 2 = 60 mm − 40 mm = 20 mm

The tip can be placed anywhere between 20 mm and 100 mm from the base.

The set of points the tip can actually be brought to is called the arm’s workspace. For the arm above, it is a flat ring: everything closer than 20 mm or further than 100 mm is out of reach (see Fig. 8.7-2).

Diagram

Fig. 8.7-2 – a top view showing two circles about the same centre, marked “base”. The outer circle is labelled “100 mm — arm fully stretched” and the inner circle “20 mm — arm fully folded”. The ring between the two circles is shaded and labelled “workspace: points the tip can reach”. The small disc inside the inner circle is left unshaded and labelled “cannot be reached”.

Building the arm in Tinkercad

The whole arm is made from shapes you have already used — tubes, cylinders and boxes. A cylinder standing in a tube’s hole is the pin-in-hole revolute joint from Section 8.5.1, and a cylinder laid on its side, rotated by 90°, makes a link.

Algorithm

building an RR mechanism Input: The two link lengths, and the size of the base. Output: A single grouped model of a two-link robotic arm with a claw. Steps:

1. Drag in a tube with thick walls to act as the base.
   Stand a short cylinder in its central hole as the first
   joint, and group the two.
2. Drag in a cylinder for link 1. Rotate it and set its
   length, then place its lower end on the first joint.
3. Place a second short cylinder across the top end of
   link 1 to act as the second joint.
4. Drag in a cylinder for link 2, rotate it to the angle you
   want, and place its end on the second joint.
5. For the claw, make a thin box, copy it with the Alt key,
   angle the two copies towards each other, and join them
   with a flat cylinder between their ends.
6. Group the claw parts into one end effector, place it at
   the free end of link 2, and group the whole arm.

Why it works: Each joint is a cylinder shared between two links, so the parts meet exactly where the arm is meant to turn, and the model shows the arm in one real position of its workspace.

Diagram

Fig. 8.7-3 – screenshot of the finished RR arm in the Tinkercad 3D design workspace, seen from the side: a round base, an upright first link, a bent second link, and a two-fingered claw at the end.

Common Error

grouping the arm before it is finished What goes wrong: Grouping early turns the separate links into one solid object. You can no longer pick up a single link to move or resize it, and matching the next joint to the last one becomes guesswork. The fix: Group each small part as you make it — the base, the claw — but leave the arm as separate pieces until every link is placed. If you have already grouped it, use ungroup to get the pieces back.

What the model does and does not show

The model you have built is one snapshot. It shows the arm frozen at one pair of angles, with the parts in the correct places and the correct sizes. That is enough to check that the links do not clash and that the claw is the right size for the object it must pick up.

It does not move. Nothing in the model makes the joint turn, and Tinkercad’s 3D Design workspace will not swing the link for you. Seeing the arm move is a separate job, and it is the subject of Section 8.8.

Curiosity Corner

Q5. Why is a robotic arm with two turning joints called an RR mechanism?

A. Because R stands for revolute, the kind of joint that turns, and the arm has two of them one after the other — one at the base and one between the two links. Engineers name a mechanism by listing its joints in order, so two revolute joints give RR. The two joints together let the tip reach any point in a whole area, which one joint alone cannot do.

8.8 Visualising the Motion of the Designed Components

The arm in Section 8.7 sits at one pair of angles and stays there. That is a fair test of the parts, but it is not a fair test of the robot, because a robot is judged by what it does while it is moving.

Think of a cupboard door in a small kitchen. Closed, it fits perfectly. Open, it fits too. Halfway open it strikes the handle of the drawer beside it. Checking only the two end positions would have missed the problem completely.

Robot arms fail in the same way. Two links can be clear of each other when the arm is folded and clear again when it is stretched, and still clash somewhere in between. So the design has to be looked at all the way through its movement, not only at the start and the end.

Definition

(Visualising motion means viewing a design at successive positions of its movement, so that the whole path of every moving part can be checked.)

Stepping through the movement

Tinkercad’s 3D Design workspace does not play an animation. What it does let you do is set a joint to any angle you like, and copy the whole design. Put those two together and you can build the motion yourself, one position at a time, rather like the pages of a flip book.

Algorithm

checking a joint through its range of movement Input: A grouped model of the arm, the range the joint must cover, and the size of step to check at. Output: A row of copies showing the arm at each position, and a list of any positions where parts clash. Steps:

1. Decide the range of the joint, from its lowest angle to
   its highest.
2. Choose a step size that divides that range evenly.
3. Copy the model with the Alt key and place the copy beside
   the original.
4. Ungroup the copy, rotate the link at the joint by one
   step, and group it again.
5. Repeat steps 3 and 4 until the whole range is covered.
6. Look along the row from the side and from above, and mark
   every position where two parts touch or overlap.

Why it works: Each copy is the same arm at a different angle, so laying them out in order shows the path the arm takes and reveals a clash wherever one occurs.

Diagram

Fig. 8.8-1 – the same two-link arm drawn three times in a row, sharing one base position. In the first the arm is folded close in, in the second it is halfway out, in the third it is fully stretched. Faint curved arrows join the tip positions to show the path travelled. Label the row “one joint, stepped through its range”.

The step size is a judgement. Too large and you can step straight over a clash. Too small and you spend the afternoon making copies.

Example 8.5

how many positions to check

Given: A revolute joint must turn from 0° to 180°, and the design is to be checked every 15°.

Find: The number of steps, and the number of positions to be looked at.

Working:

Number of steps = total range ÷ step size = 180° ÷ 15° = 12

Every step gives a new position, and the starting position must be counted as well.

Number of positions = 12 + 1 = 13

12 steps, giving 13 positions to check.

Common Error

checking only the folded and stretched positions What goes wrong: The two end positions are the easiest to picture, so they are the ones students check. A link that swings through the base, or a claw that catches on the second joint halfway round, is then missed entirely. The fix: Check the middle of the range as well, in steps small enough that no part moves more than its own thickness between one position and the next.

Seeing the swept area

Once the positions are laid out, a second thing becomes visible. Each moving part does not merely end up somewhere — it sweeps out a region of space on the way, and nothing else may be placed inside that region.

Diagram

Fig. 8.8-2 – top view of the arm’s base with the shaded ring workspace of Fig. 8.7-2 shown faintly. Over it, a shaded wedge from the base outwards, labelled “area swept as joint 1 turns from 0° to 90°”. A small box drawn inside the wedge is crossed out and labelled “nothing may be placed here”.

This is what tells you where the battery can sit on the chassis, or how close to a wall the arm may be mounted. A design that is correct in every single position can still be wrong as a whole, if something has been parked in the space the arm needs.

Two other ways to see it move

Stepping through copies is not the only route, and the other two are worth knowing.

The Codeblocks workspace builds a model by following instructions, as Section 8.2.3 described. Because the angle a part is rotated by is written inside a block, you can change that one number and run the blocks again, and the whole model is rebuilt at the new angle. Watching the blocks run also shows the model being assembled step by step.

The Circuits workspace goes further, because there the motion is real rather than drawn. Place a servo motor and a microcontroller board on the workspace, wire them, write the program that tells the servo which angle to hold, and start the simulation. The arm on the servo turns on screen as the program runs, and it stops at the angles your program asked for. This is the closest Tinkercad comes to showing a robot actually working, and it is also a test of the program rather than only of the shape.

None of the three tells you everything. All of them work on a model, and a model has no weight, no friction and no loose screws. When the arm is finally built, it must still be moved slowly by hand, with the power off, through its whole range before it is switched on for the first time.

8.9 Summary

A robot is made of many parts that must fit one another, so engineers design it on a computer before cutting any material. A mistake on screen costs nothing; a mistake in cut material wastes the material.
On screen you can check three things before building: fit, clearance and overall size. The first real version built from a finished design is called a prototype.
Tinkercad is a free, browser-based platform for making 3D models, building and testing circuits, and writing simple code for them. Nothing has to be installed, and designs are saved online.
Tinkercad has three workspaces. 3D Design builds the shape of a part. Circuits builds and simulates an electrical circuit. Codeblocks builds a 3D model by following instructions.
A 3D model can be exported as an .STL or .OBJ file, which is what a 3D printer needs to make the real part.
Shapes are dragged onto the workplane and given exact measurements. The mouse wheel zooms, the right mouse button turns the view, the F key brings the camera back to the object, and the Alt key makes an identical copy.
Every shape can be set to Solid or Hole. A solid shape adds material; a hole shape removes it. A hole shape does nothing until it is grouped with the solid it overlaps.
Grouping joins shapes into a single object. Solid with solid gives one joined part; solid with hole cuts the hole. Ungrouping separates the object back into its shapes.
Tinkercad is used for teaching, for prototyping before 3D printing, for electronics and robotics, for simple architectural models, and for basic models used in animation. Its strengths are that it is free, easy to learn and web-based; its limit is that it handles only simple shapes and cannot test whether a part will bend or break.
A robot arm is made of rigid links joined by joints. Each joint allows one kind of motion only, so one number describes its position.
A revolute joint turns about an axis. Its position is an angle, its moving end travels along a circular arc, and it is built as a pin fitted in a hole.
A prismatic joint slides along a straight line. Its position is a distance, its moving end travels in a straight line, and it is built as a bar fitted inside a sleeve.
A wheel is built from two tubes, one inside the other. The outer radius of the inner tube must equal the inner radius of the outer tube, where inner radius equals radius minus wall thickness.
The chassis is the frame that carries every other part. It is built from a base plate with mounting holes and two upright side walls that hold the motors.
An RR mechanism is an arm with two revolute joints, one at the base and one between the links. Two joints let the tip reach any point in an area, which one joint alone cannot do.
The greatest reach of an RR arm is the sum of the two link lengths, and the least reach is their difference. The set of points the tip can reach is called the workspace.
The tool at the tip of an arm is the end effector — a claw, a gripper or a suction cup.
A model shows the arm at one position only. Motion is visualised by copying the design at successive joint angles and looking along the row, so that a clash occurring partway through the movement is not missed.
A moving part sweeps out a region of space, and nothing else may be placed inside it. A design can be correct at every single position and still be wrong as a whole.
Motion can also be seen by changing an angle in Codeblocks and rebuilding, or by simulating a servo motor in Circuits, where the part actually turns on screen. None of these replaces moving the finished robot slowly by hand, with the power off, before switching it on.

8.10 Key Terms

3D model — A description of an object held in the computer, giving its exact shape and size as seen from every side.
3D printing — A way of making a real object by laying down thin layers of melted plastic, one on top of the other, until the shape is complete.
Chassis — The frame of a robot, on which all the other parts are mounted.
End effector — The tool at the tip of a robot arm that does the work, such as a claw, a gripper or a suction cup.
Grouping — The Tinkercad command that joins selected shapes into a single object, and that cuts a hole wherever a hole shape overlaps a solid.
Hole shape — A shape set to remove material rather than add it, shown see-through, which cuts away the solid it overlaps when the two are grouped.
Joint — A connection between two links of a robot that allows one link to move relative to the other in one definite way.
Link — A rigid part of a robot that does not bend or change shape.
Microcontroller — A small computer on a single chip that reads inputs and controls outputs.
Prismatic joint — A joint in which one link slides in a straight line along the other, its position given by a distance.
Prototype — An early, trial version of a product, built to test whether the design works before the final version is made.
Revolute joint — A joint in which one link turns about a fixed axis, its position given by an angle.
RR mechanism — A robotic arm made of two links connected by two revolute joints, one at the base and one between the links.
Science, technology, engineering and mathematics (STEM) — The group of subjects taught together so that design, electronics and programming are learnt as one skill rather than separately.
Simulation — A run of a design inside the computer, in which the computer works out what the real components would do and shows the result.
Solid shape — A shape set to add material, which forms part of the finished object.
Tinkercad — A free, browser-based online platform used to create 3D models, build and test electronic circuits, and write simple code for them.
Workplane — The flat grid in Tinkercad’s 3D Design workspace that shapes are built on, standing for the ground under the object.
Workspace — The set of all points that the tip of a robot arm can be brought to.

8.11 Objective Questions

Which file format can a Tinkercad 3D model be saved in for 3D printing?
1. .docx
2. .mp3
3. .STL
4. .png
Fill in the blank: A shape set to ____________ removes material from the solid it overlaps when the two are grouped.
State whether true or false: A hole shape begins to cut the solid as soon as it is placed over it.
A tube is given a radius of 12 mm and a wall thickness of 4 mm. What is its inner radius?
1. 3 mm
2. 8 mm
3. 16 mm
4. 48 mm
Match each item in Column A with its correct description in Column B.

Column A	Column B
a. Revolute joint	i. Carries all the other parts of the robot
b. Prismatic joint	ii. Turns about a fixed axis
c. Chassis	iii. The tool at the tip of an arm
d. End effector	iv. Slides along a straight line

Name the key that is held down while dragging a shape in Tinkercad to make an identical copy of it.
An RR arm has a first link of 50 mm and a second link of 30 mm. How far from the base can its tip be placed at most?
1. 20 mm
2. 30 mm
3. 80 mm
4. 150 mm

8.12 Concept Questions

Define a prototype, and state why one is built before the final version of a product is made.
Give two differences between a solid shape and a hole shape in Tinkercad.
Explain why a robot is designed on a computer before any material is cut. State one thing about the finished robot that the design on screen cannot tell you.
The two side mounting walls of a chassis are identical. Give a reason why the second wall should be made by copying the first rather than by building it again from a fresh box.
An RR arm has a first link 70 mm long and a second link 45 mm long. Work out the greatest and the least distance from the base at which its tip can be placed, showing your working, and describe the shape of the region the tip can reach.
What would happen if the cylinder used as a hole shape were made shorter than the thickness of the plate it is placed on, and the two were then grouped?
A student checks a robot arm only in its fully folded position and its fully stretched position, finds no problem, and declares the design correct. Explain why this check is not enough, and describe what should be done instead.

8.13 Case-Based Questions

The robotics club of a school is designing the base plate of a line-following robot in Tinkercad. Their measurements are given below.

Item	Measurement
Base plate	60 mm long, 40 mm wide, 5 mm thick
Screws holding the circuit board	M4, shaft 4 mm across
Clearance wanted around each screw	0.6 mm on the diameter

Work out the radius the club should set for the cylinder used as the hole shape for one screw, showing your working.
The club sets the height of this cylinder to 5 mm. Explain what is wrong with that choice.
Name the command that turns the base plate and the positioned hole cylinders into a single part with holes through it.

A workshop has a small robotic arm bolted to a bench. It is an RR mechanism whose two links are each 60 mm long. A toolbox is kept on the bench, 100 mm from the base of the arm, directly in the path the arm sweeps as its first joint turns.
1. Work out the greatest and the least distance from the base at which the tip of this arm can be placed.
2. Describe the shape of this arm’s workspace, and explain how it differs from the workspace of an arm whose two links are of unequal length.
3. State whether the toolbox is in a safe position, giving the reason, and say what the workshop should do about it.