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[Uploaded Sep08] Drill Speed Controller

By: Frank Cosgrove

For many years I have used a 12 volt drill, stand and transformer. They were all Titan make, but the drill wore out and was replaced with a different make which means great care now to ensure that the drill sits in a precisely vertical position. This worked okay but the terrific speed of the drill was a real problem.

I always start the holes by hand with a sharp drill in a pin-chuck, two or three turns helps locate the hole centre when drilling. At the high speeds involved, GSS drills burn their way through rather than drill. Holes have charred edges and some smaller copper circles become detached from the board by the heat. This has always been a problem to be endured rather than solved because my priorities have always been with the project in hand rather than going off at a tangent and doing something about it.

However, I was recently looking back in 1966 issues of Maplin magazine and saw a drill controller project. It looked neat and simple. I had most of the of the parts so I thought I would have a go. I got an acetate copy of the pcb pattern and made it up. The whole project worked well - on a bulb. The control is via a variable pulse width modulation which shows up well on a scope When I replaced the bulb with the drill, it did not perform so well. At zero setting the drill was stationery but when moving off zero it suddenly jumped into action at a fair speed. I checked everything but I could not solve the problem.

Drill Speed Controller

Having got so far, I felt committed and my thoughts turned to an LM317T variable voltage regulator sitting there. I made a temporary rig with this and it works well; of course, the 317 does not go down to zero but I offset this with a couple of diodes and now it does. I have now made it up permanently and it works well. Holes take a little longer but no more burning up the drills and holes etc. I wish I had made it years ago. Control ip smooth from zero to maximum.

I used a 15-0-15 volt transformer (12vA because I had it but 6vA would be ample.) The drill takes 250mA steadily rising if loaded of course. The 317 (Y0220) needs a good heat-sink. For 0-12 volts I found that a 2K linear pot was needed. I found a dual 4.7k pot in my spares box and used both sections in parallel. I used a metal box 4" x 3" panel, 6" deep. The panel was made with words thrown up on the PC, printed out, cut up and laid out on a plain sheet of white paper. It was then photo-copied onto blue card, covered with self adhesive clear film and stuck onto the box. It looks pretty good. Don't worry if you haven't got a PC to give you the varied fonts. Look through various catalogues and magazine articles and you will usually find the words you want and can get them expanded/reduced on the photocopier.

One thing about the mains transformer, if you are buying (or have) one of the regular clamp mounting types, do invest in a tag cover, at least on the mains side - look in the Maplin Cat. Part numbers DM29, 30 and 31. They are cheap and easy to use and keep stray fingers safe. They don't do 12Va and I can't find them at all in Electromail, Farnell, Rapid or Electrovalue catalogues. I think they should be sold with the transformer. Perhaps they are a dying breed!