Simple Bots: Inchworm

If you can count on only one thing, it would be a ruler. Now, don't get me wrong. I am not talking about supreme despots for life, or anything of that sort. The rulers that I am referring to are the measuring kind. After all, how can you not count on something with so many sequential numbers? That is why, when it came to deciding what the armature for Inchworm Bot should be, the only thing I could think of was my good, dependable friend, the ruler. After all, in the grand scheme of things, there is merely inches of similarity between the two.

Check out my book Homemade Robots for more projects!

You will need:

(x1) Continuous rotation servo modified for direct drive**
(x1) 4 x AA battery holder
(x1) DPDT lever switch
(x4) AA batteries
(x1) red and black wire
(x1) electrical tape
(x1) 18" aluminum ruler (or longer)
(x2) small plastic container
(x2) 2" fixed casters
(x1) Bic round pen
(x1) comb
(x10) small nuts and bolts
(x4) small washers
(x1) 2" - 3" metal standoff
(x4) zip ties

**Learn how to modify a servo motor for direct drive at this page.

(Note that some of the links on this page are affiliate links. This does not change the price of any of the items for sale. However, I earn a small commission if you click on any of those links and buy anything. I reinvest this money into materials and tools for future projects. However, you are obviously free to source the items as you please.)

Use a hacksaw and cut the ruler into two 9" sections.

(The convenient thing about cutting a ruler is that it comes pre-measured.)

On one of your 9" sections, make two cuts 3/4 of the way through the ruler at 1/2" from the edge and 2-1/8" from the edge.

Place the ruler in a bench vice so that the bottom of the cuts are level with the top of the vice.

Hammer the section between the two cuts flat such that it is made perpendicular.

This will be your motor mount.

Place your servo motor into the slot you just cut and use a pencil to mark where its mounting holes are.

Remove the motor and drill through your marks with an 1/8" drill bit.

Drill a 1/4" hole that is 1/2" from the edge of the the cut ruler section. Repeat on both.

(Obviously, on the piece that you have already worked on, the hole has to be drilled on the opposite side of the motor mount)

Detach your servo horn from your motor (the gear-looking object).

Use your servo horn as a guide to both mark and drill 1/8" holes through the section of ruler that does not have the motor mount.

These holes should be drilled roughly 1/2" in from the edge of the ruler.

It is preferential if you simultaneously drill the servo horn, as to ensure identical sized holes.

Drill an 1/8" hole, centered upon the section of the ruler with the motor mount, that is 5" from the edge with the mount.

Drill a 3/16" hole upon the section with the servo horn mount that is roughly 5" from the edge with the servo horn mount and close to the edge of the ruler. Drill a second larger 1/4" hole that is 4" from the servo horn mount and centered upon the ruler.

Congratulations! You are finally done modding the ruler.

Mount your motor to the ruler, using nuts and bolts. Make sure that the motor mounting holes are on the same side of the ruler that the mounting bracket protrudes towards. The only mounting hardware that should be on the other side of the ruler are the screw heads. This ensures a low profile when the two sections of ruler move back and forth.

Attach that servo motor's horn, if you have not done so already. Use nuts and bolts to attach the other section of the ruler to the servo horn. Again, the screw heads should be facing inward towards the other set to ensure a low profile.

Use diagonal cutters to remove two inches worth of teeth from one side of your comb. However, I found it is beneficial to leave a few teeth on right at the edge. Later on, these few extra teeth will help act as a safe guard to keep the comb on track.

When you are done removing teeth, sand that section of the comb smooth.

Lastly, drill an 1/8" hole on the opposite edge.

Pass a screw through the hole you drilled in the step prior.

Once it is passed through, place 2 - 4 washers on it. The point of these washers is to push the comb towards the other section of ruler (once mounted).

Next, pass the screw through the middle hole, in the section of ruler with the motor, from the inside out. Use your bolt to securely fasten it in place.

Remove your switches' mount nut and insert it into the 1/4" hole on the servo horn section of ruler, Make certain to pass it through, such that the switch lever is facing inwards. Lock it in place with its mounting nut.

Solder a red wire from the bottom left corner pin to the top right. Solder a black wire from the bottom right, to the top left.

While you're at it solder the red wire from your motor to the pin on the bottom of the switch with the red wire and the black wire from the motor to the pin on the bottom with the black wire.

Lastly, solder a long red wire to the middle pin on the side with the red motor wire and a long black wire to the middle pin on the side with the black motor wire.

If you are confused, just look at the wiring diagram below.

Mount the standoff to the servo horn section of the ruler such that it is facing inwards in the same direction as the lever switch.

The comb will rest atop the standoff.

Take your ballpoint pen and remove the ink cartridge such that you are left with a hollow tube.

With a razor blade, cut down on the tube creating two 1" sections.

Take the two sections and cut them in half, creating four 1/2" sections.

Take your two casters and remove the wheels.

Set the wheels aside for some later project.

Center the caster frame atop your plastic container.

Use the holes in the caster as guides for drilling 1/8" holes in the lid of the container.

Zip tie the caster frame in place to create pivots.

On one of the plastic container lids, drill an 1/8" hole in the corner.

Place the battery holder inside and pass the power wires through the hole.

(I know that it shows batteries in the holder, but I recommend not inserting batteries before you solder.)

Using the caster mounting hardware and the pen spacers, attach the rulers to the plastic containers.

Basically, the ruler should have a pen spacer on each side to hold it in place. If it seems loose or tipsy, insert metal washers between the spacer and the ruler until it is not.

Before you start, make sure at least one battery is not in the battery holder or the motor will turn on, making your job here very difficult.

Solder the red wire from the switch to the red wire from the battery holder and cover it with electrical tape.

Solder the black wire from the switch to the black wire from the battery holder and cover it with tape as well.

Basically, it works by reversing the direction of the motor each time the comb hits the switch, which, in turn, forces its "feet" to slide back and forth. Since one side is weighted down with the batteries, it has more forward momentum in one particular direction.

So, as the motor turns, the comb slides to one end and hits the switch. This then causes the comb to slide back to the other end, to hit the switch once more. Rinse. Repeat.

If you find that the motor does not reverse when the switch is pressed and the bot folds itself up into an ugly mess, first REMOVE THE BATTERY! Once you have done that, rotate the switch 180 degrees. Now put the battery back in. When the switch is pressed, it should reverse direction.

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