Servo - January 2009

MAN UFACTUR IN G:

Moving On With Your Rob t

● by John Frizell

So, you’ve got yourself some battery powered drills and have started to convert them into gearmotors to drive a robot. (There are lots of good articles about how to do this on the web, as well as one in SERVO November ‘06.) But what do you do next? How do you mount it in your bot, attach a wheel, and power it up?

The tough plastic case of a cordless drill is designed to hold its motor and gearbox rigid as you drill through things so it makes a good mount. And it’s free. Photo 1 shows a 9. 6 volt cordless drill mounted to a piece of plywood with three pieces of 4 mm all-thread rod. (All-thread rod was used because the 4 mm bolts available weren’t long enough.)

This sort of mounting wastes a

directly on to the thread. You can try Team Rolling Thunder www. teamrollingthunder.com/Kitbots/ Parts/ body_parts.html or email jamesncooper@btconnect.com in the UK (Photo 2).

Big Wheels Will Make For A Faster Bot; Small Wheels Will Give It More Pushing Power

PHOTO 1. A 9. 6 volt gearmotor ready to take a wheel.

bit of volume around the motor but doesn’t make the unit any longer. The rods are arranged in a triangle outside the body of the gearmotor. You could put two short bolts under the gearmotor through the bottom of the plastic case, and the third to one side — it works but it won’t be as rigid.

You also could mount the case directly onto the base of your robot but having the unit on a plywood plate makes it easier to get the gearmotor in and out without having to take the case apart every time. If you haven’t already taken off the adjustable torque mechanism, don’t bother. Just turn it to its highest setting and leave it.

If it slips in combat, it will probably be protecting your motors from burning out or overloading your speed controller.

The shaft that once held the chuck has a 3/8” UNF thread. The “bolt people” are gradually standardizing on one thread — metric — but this has a long way to go and hasn’t reached the cordless drill industry yet. The best way to deal with this is to buy a pair of wheels that screw

PHOTO 2. A blue wheel from James Cooper, mounted on a

7.2 volt drill gearmotor.

If you want to make wheels, the easiest way is to buy cheap plastic ones and tap the existing axle hole for the 3/8” thread. Clamp the wheel in a vise and go through it carefully with an 8. 5 mm drill. Try drilling half way from each side — if you don’t have a drill press, you will be amazed at how much the drill can wander. Then, wind your 3/8” UNF tap through (see Photo 3), remembering to make half a turn backwards for each one forward.

Cheap wheels are not designed for being driven and so the rim will often separate from the body of the wheel. Put a screw through to stop that from happening. Put lots of

Loctite on the threads and screw the wheel into place. Make at least one spare one — these wheels don’t tend to last too long at events as Photo 4 illustrates.

For a more durable wheel, buy a 3/8” UNF bolt and some nuts at an auto supply store. Cut some steel strap ( 30 mm by 4 mm is good) to a length just a bit shorter than the diameter of