Wednesday, 30 May 2007


I have done quite a bit of milling with my Minicraft drill attached to HydraRaptor. I always considered it a bit underpowered for the job but it mills plastic and copper clad board reasonably well. Recently I started using it for drilling plastic. The first thing I drilled was 6mm Perspex. I programmed a pecking action to avoid the drill clogging as Perspex tends to melt when it is drilled at high speed. Again this worked reasonably well. The next thing I tried was drilling 25mm metal loaded resin. No matter how slowly I went I could not stop the drill from stalling once it got to a certain depth.

I decided the only way to solve the problem was to monitor the drill speed, detect when it was about to stall and automatically back off until it speeded up again. To this end I started investigating ways to monitor the speed. Off the top of my head I came up with four alternatives :-

  1. Monitor the current, because it increases as the speed reduces.
  2. Turn it off occasionally and monitor the generated voltage.
  3. Measure the frequency of the commutation noise.
  4. Put a black spot on the chuck and use a reflective opto detector to count shaft revolutions.
I attached the drill to a 12V switch mode PSU in order to take some voltage and current measurements and look at the current waveform on a scope. To my surprise the drill went much faster on the 12V PSU. The PSU that came with it is labelled 11.5V 400ma 4.6W but when loaded by the drill it was only giving about 8V. Then I looked at the drill and saw it was labelled 12V 40W. A bit of a mismatch! When driven from 12V it takes about one amp with no load. No wonder it was stalling so easily. It also explained why when it does stall it does not seem to care. Most drills start smoking pretty quickly if you stall them.

So it looks like the drill is ten times more powerful than I thought. The only problem is that it could do with some speed control as it goes a bit fast for plastic when running from 12V.

So now my next task is to tame it with PWM and monitor the speed somehow. I also want to control the vacuum cleaner with software as I am too lazy to switch it on and off.

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