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The Operation - Hold on a minute!

Remember when I said that the spindle (Dremel Tool) would not be under computer control? Well... I have had second thoughts on that subject. I think it would be easy to have the spindle toggle on or off using one of the unconditioned channels available on the motor controller. There are 3 unused channels, so let's pick pin #14. Just using pin 14 as the power source for the dremel tool is just silly, that's not how these things work. Pin 14 has TTL level logic on it (3 to 5 volts at less than 10 milliamps). What's needed here is some sort of relay that can switch 120 Volts AC from a TTL level voltage. Fortunately, several companies already make such things, they are called "Solid State Relays" strangely enough.

I figured that I would not want to cut the Dremel tool cord to make the connections so I worked out a little junction box that the Dremel tool can just plug into.

A quick scan through the Mouser catalog finds all the needed parts.

Korbiconn 173-0603 power cable
Crydom OAC5 120VAC 3A Solid State Relay
Schurter 4300.0705 snap-in polarized 2 connection plug
Bud CU-3003-A Aluminum Enclosure
A rubber grommet, some 4-40 standoffs and a simple hand made circuit board fashioned to solder the SSrelay and wires.

I did a little re-wiring inside of the power supply to provide switched raw unfiltered power for the Dremel tool. Filters are by their very design current limiters and we dont want any power loss going to the Dremel. I fabricated the junction box as shown in the above pictures, hooked it up, programmed the channel for pin 14, powered it up and crossed my fingers.

Shazam!!! it works, toggle the spindle on and the Dremel tool starts-up, toggle it off and the Dremel turns off... that's neat! but wait a minute... it's not that neat, the Dremel tool seems to be running weak and slow, now what!

A little measurning of things with a volt meter shows me that I'm only getting about 65 volts AC, not the expected 120. I decided to give my friendly Crydom tech support person a call. I have never got such detailed and high quality support as I did from Crydom, they knew exactly where I messed up. In a nutshell... I selected a relay which had the right voltages and current ratings but was a "zero crossing" type which behaves badly when switching a highly inductive load (like a Dremel tool). I was supposed to use a "random" crossing type like their CX240D5R relay, these types deal with inductive loads like a duck takes to water. Crydom said that the "R" at the end of the part number signifies random.

One more little order to Mouser and that should do the trick. It's strange though, the correct relay is smaller and has a higher voltage and current carrying capacity as you can see from this picture. Luckily, they both have the same pin spacing and pattern so unsolder the old one and install the new one.

I hook things back up, apply power and now it don't work at all, that figures!

After a bit of poking, prodding and measuring I found the problem. The old relay drew less current than the new one and as a result, when the computer switched pin #14 it went to 2 volts instead of 5. What we need here is to shift the voltage swing up a bit towards 3 volts or so. This can be done using an 1/8watt 1K ohm resistor between the pin 14 connection and a 5 volt source on the controller board... a pull-up resistor. I soldered one of these to the solder side of the controller board, hooked things back up and as Steve Jobs would say: "Bam!!!" it works!

Now that everything works correctly it's time to button things up. I mounted the junction box to the bottom of the cnc-drill frame, dressed the wiring and that's it. Now it's finished, of course... there are still 2 more unused channels on the controller for any notions I may have in the future like fixture clamps, vacuum or fluid control.

One cautionary note here is that if you have the software off and energize the cnc_drill, the Dremel tool will startup. This is the fault of the pull-up resistor and without any computer signals to regulate it, it pulls pin #14 high (+5Volts) causing what you would think is a normally off condition to actually be normally on. Make sure to bring up the control software before powering on the cnc drill and there will be no suprises or... if you want, bring up the cnc drill first and then startup the computer, all the while listening to the dremel tool whine and when the software finally starts-up the dremel tool will switch off... it's up to you.

EPILOG 12/06/06
It's been about two years since I built this machine now and I still use it regurlarly. I have not had to replace or even adjust anything in this time and the thing still looks like new, except that the table is a bit chewed up from work. I have drilled dozens of boards and even milled some plastic with very acceptable and professional results. In retrospect we all have a list of things we would have liked to or should have done in the first place and I am no different than anyone else in these respects, I have some druthers too.

1. I would have made it bigger! While approx. 8" x 8" seems like enough it really never is. I found myself designing boards to fit this limitation making some tightly stuffed boards in the process.

2. I would have made it stronger! The contruction of this machine is fine for light drilling but it is totally inadequate for milling anything harder than balsawood or soft plastic (soft plastic even gives it some problems at times). The deflection of stages when milling and the resulting "chatter" can make things exciting considring that your milling tool is spinning a few 10 thousands of RPM's in your face!

That's it! everything else is just as I dreamed it could be and better. I am thinking of a bigger beefier mill using a router motor, some bigger steppers and a table maybe 4 foot by 8 foot but that an issue for another time and a much different task.

-- William Stephens