Grippy Robot Wheels

While trying to build a mini-sumo-robot, we found it difficult to get nice grippy wheels. So if we can't buy it, we will build it! In my research on Instructables I stumbled on the oogoo "movement"; https://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/
I had some experience with silicon but that was not all positive; https://www.instructables.com/id/Arduino-Watch-with-Nokia-3110-screen/
Especially the time it took the silicon to set was a real challenge for my lack of patience. I tried making a wheel with just silicon sealant but the oogoo version came out so much nicer.

You will need

For the rims

• some board material (I used a hard plastic foam)
• glue

Tools

• holesaw
• big hole drill (a Forstner bit)
• drill
• drillpress (a hand drill won't do this time)
• saw
• sandpaper

For the tires (Oogoo)

• silicon sealant
• corn starch
• oil paint (if you want to color your tire)
• release wax
• mold (I used a PVC pipe-cap)
• plastic cup
• nut, ring and bolt
• some water

Tools

• spatula
• caulking gun
• air compressor (helps to get the wheel out of the mold)
• scissors

Optional

• spray-paint
• primer
• degreaser

Before you can start with the rims you need to decide on some things:

• How big do you want your wheels
• What is the diameter of the axle
• What is the size of the ring that you will use to mount your wheels
• How wide do you want your wheels
• What kind of board do you have

My choices:

• Big: 75 mm
• 15 mm wide
• 25 mm rings
• 5 mm axle
• 7,5 mm board

The rims need to be smaller than the wheels (for obvious reasons). The size of the rims will probably be dictated by the size of your hole saw. Mine ended up about 45 mm. The bigger your rim, the smaller your tire will be. You can't make your tires to small or you won't be able to get the oogoo in-between the rim and the mold.
The width of the rim will be dictated by the thickness of your board. It can be 1 x the thickness, or more times. For my wheels it will be 2 times the board thickness.

I started by cutting a strip of board material just a little wider than the hole saw that I am going to use.

Cutting the backplate

• Put your board sturdy in the drill press. (Use a clamp or something)
  Put something underneath the board to protect your hole saw
• Drill the centre hole in the size of the axle (5 mm in my case)
• Do not (re)move the board
• Remove the centre drill from your hole saw
• Change the (5 mm) drill for the hole saw
• Cut out the backplate of the rim
• Nock the backplate out of the hole saw by unscrewing the centre piece and nocking it on the table (that worked for me)

Cutting the front plate(s)
How many front plates you will need, will depend on how wide you want your wheels to be. My wheels will be 2 times the thickness of the board so I will need just one front plate.

• Put your board again sturdy in the drill press with something underneath the board to protect your hole saw
• Drill a hole the size of the ring that you will use to mount the wheel on the motor.
• Do not (re)move the board
• Change the drill for the hole saw
• Cut out the front plate of the rim
• Take the front plate out of the hole saw by putting your finger in the hole and pulling

I smooth out the plates using a belt sander but you can also use simple sandpaper.
By putting a screwdriver trough the hole and holding the plate 45 degrees to the belt, the plate will spin and I won't produce flat spots. If you use this method, be careful not to burn your fingers.

Glue the plates together

• Sand the plates smooth
• You can sand the rim a little bit in an angle to hold the tire in place
• Glue the plates on top of each other. (use the glue that is suitable for the material you used)

Paint the rims
Painting the rims is an option

• degrease the front of the rim
• find a good location to use spray-paint
• spray the front with a suitable primer
• when the primer is dry spay with the color you like
• If you want, you can also paint the back (I didn't)

Find a mold in the size you need. I've found an end cap that was a nice size. It needs to be a little bit flexible. Not a lot, but more than glass or ceramic.

The hardest part is finding the centre of the mold.

• drill a hole in the middle of the mold in the size of the axle
• rub wax (or an other loosing agent) inside the mold
• put a bold trough the hole (use a bolt in the size of the hole)
• put the rim on the bolt with the backside up (the paint must be dry)
• put a ring and a nut on the bolt to secure the rim in place

If you want to know everything about the mixture I used for the tires, you should read the Ible from mikey77 who came up with this recipe; https://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/

But it is so easy to make that I will give the recipe I used.

• put the silicone in a plastic cup
• add a little bit of oil paint (only if you want to color the tires)
• mix the silicone and the paint (do this in a well ventilated workspace)
• add one part corn starch for one part silicone in volume
• stir thoroughly (start slow or you will be covered in corn starch)

It is no problem if you realize that you didn't make enough, you can easily add more when you used this batch.

There is no easy way or trick for this.

• Just get the oogoo in the mold.

I used a small strip of aluminum because it is small enough to fit in-between the rim and the mold.
Try to get al little air bubbles in as possible. This will be hard.

When you managed to get enough oogoo in the mold,

• use a spatula (or putty knife) to smooth out the top and remove excess oogoo. The more attention you will put into this, the better your final product will look.
• Let it dry for a couple of hours (or overnight)

The drying time varies with the mixture of oogoo, the temperature in the room and the moisture in the starch and the air. For me four hours was enough to dry the oogoo.

It can be quite a trick to get the wheel out of the mold, but with some tips it is not so hard.

• take the bolt out of the rim and the mold
• remove some excess rubber (this should be very easy now)
• press the sides of the mold until some space occurs between the tire and the mold
• put some water on top of the mold and work it in-between the wheel and the mold by pressing the sides of the mold
• when you worked the water all the way around in-between the wheel and the mold, push on the bottom of the mold
• to get the wheel out, put your finger on the hole of the rim and blow a little of air in the hole in the mold (with the compressor)
• the wheel should easily come out now

Sometimes the air blows out the rim and not the whole wheel. Don't worry about this, you can easily take out the tire by now. You can glue the tire back on the rim with some silicone sealant.

To finnish this wheel-project I put them on the robot in the making.

They fit the axles nice and snug.

• put the wheels on the axles
• put the ring in the rim
• screw the ring on the axle

For different axles you might need an other method of assembly.

Hopefully I can show an Ible with the finished mini sumo robot soon!