Need information on building a rotary drive.

[jlwright]

I have a need for a rotary drive to go in my Wire EDM machine. I have parts that need to be indexed between each burn. I envisioned a stepper or servo motor connected to a timing belt driving the 6 inch lathe chuck submerged in the tank. The timing belt is just to keep the motor out of the water. The mechanics are not a problem just the electronics. I know nothing about drives and motors. My Wire EDM machine has terminals that can signal the rotary controller to tell it to rotate so that is no problem. I don't know if I will need a brake or some kind of lock to keep the chuck from moving or hunting for position while Edm'ing. There is very little force on anything during the EDM process. I also need to be able to change the number of indexes in 360 degrees without too much hassle. The torque to turn the chuck would be about the same as turning a small lathe spindle with your hand. I can put a dwell in the Wire EDM program to give the rotary enough time to do it's job so I wouldn't have to signal back to the machine if that's a problem. Thanks in advance for your help. I just need a direction to go to.

[vger]

Interesting problem... One issue you may not have consiedered is the return path for the discharge current. Mounting your part in the lathe chuck, your current path will be through whatever bearings you have it rotating in and on to the table in the tank. You certainly don't want to be "machining" pits in your bearing surfaces. I get around this issue when welding on bearing mounted rotors by always putting the welder ground on the rotor itself rather than the machine frame. I suppose you could come up with some sort of slip ring arrangement with multiple wipers to ensure good low resistance conduction back to ground.

As far as control... There are probably stepper or servo drive systems that will increment a specified amount on command. Other than that, you could use an old PC and a single axis stepper driver to run it. If you are not going to be indexing in either X or Y while running this part, you may be able to build up a drive that you could switch out with one of those to control it. Just an identical drive motor and encoder for one of the axis, and switch to it after that axis is positioned (assuming they are CNC axies). Then it would just be a matter of calculation to for the right G-Code get the desired rotation.

Steve

[jlwright]

Valid question about power return path. There are no bearings. This will be a plain bearing bore and shaft with dielectric grease as a lubricant. I am already doing this with a cheap manual 5-C collet rotary indexer. I have a rotary welding ground that I could adapt as well but it's not necessary. The machine is a Fanuc so buying another Fanuc drive motor would be expensive and I am using both X and Y for postioning. Right now I am indexing the fixture 8 times per 360 degrees every 8 minutes between cuts. I pretty much have to stand at the machine all the time to keep it going. If I had an automated rotary I would only have to change parts every hour or so. That would let me get other things done in the shop besides baby-sitting the wire machine. Here is a picture of the two rotary devices I have now. I will only have one rotary fixture if converted to CNC.

[vger]

Those indexers look exactly like the one I bought for the task of making a worm drive turntable.

For control of the drive motor.... Using a stepper may be the easiest implementation. I would probably use a timing belt system as you suggested to keep the motor out of the tank. Or, if you want to get a bit more involved do a worm drive with a long enough shaft to get the motor out of the tank. A reduction of 18:1 will allow you to use a smaller stepper, have 0.1 deg steps, and still be able to turn the chuck. A Turntable I built (from scratch) has a 180:1 reduction so I have 0.01 deg steps in full step mode and can use fractional steps to get 0.001 degrees.

I have used a program called TurboCNC that is free to download and use. It has a couple of I/O signals called PLC Handshake that can be mapped to pins on the parallel port (printer port) to control program flow. The Function numbers are 5 for output and 22 for input. The M70 - M73 codes are used with these signals. TurboCNC is a DOS program so you take an older PC with windows98 or 95 on it and set it up to boot directly into DOS and run TurboCNC from there.

Wire the EDM machine output that you will use to the PLC Handshake input on the TurboCNC machine. Have code running to do the following.

TurboCNC waits for signal from Faunc with M72 or M73.
Faunc putputs the signal to do the index.
TurboCNC senses the signal and resumes execution of code.
TurboCNC executes the G-Code to do the index movement.
TurboCNC tells the Faunc it is done via M70 or M72.
TurboCNC waits for next signal with M72 or M73
Faunc procedes to next cut.

Of course you will need a stepper driver connected to the parallel port on th computer, and the stepper motor and power supply. To change the index, you just edit the Code in TurboCNC to the new value. You can set up the axis you use as a rotary axis in TurboCNC configuration and just edit the G-Code as needed.

This would be about the cheapest solution I can think of that will still allow you the flexability to change values.

Steve