Homemade 2 spindle lathe preliminary results

consider reading up on Bode plots and Nyquest Stability Criteria - then take some calibrated data via a frequency sweep from .01 hz to 100 hz and see what your mechanism is really doing

What is the ultimate purpose of your effort? I seemed to have missed the original thread.

Wes

I'd make two flywheels, the master thin and the slave thick, mark reference lines on them and look for the conditions where the slave starts to slip.

There *must* be some relative torsional displacement because this is a feedback control system depending upon an error signal. It could be zero error at times of constant speed if the control system is PI (proportional-integral) but there must be some error during speed changes.

Incorporation of torque sensing could minimize this: sense torque supplied by both drive motor and slave motor and have the control system operate to minimize the difference.

You sorta have that now: torque difference is proportional to the difference in encoder signals and the "twist" in the workpiece.

If the slave spindle need not provide any significant torque, then it may just follow the load like a live center. In that case, the only loads would be dynamic during accelerations and decelerations and the load would drive the slave motor rather than vice versa. In that case, if the torsional distortion in the workpiece is less than one encoder count from drive spindle to slave motor there'd be no excitation provided (or necessary) to the slave motor.

If the glass tube forces the spindles to keep in step, would not whatever you are using this for also force the spindles to keep in step.

Dan

original thread.

So that with a part spinning in one spindle the second spindle can grab it without scratching it. Then the lathe can turnthe back side of the part. ERS

Greetings Don, The two spindles will be at the same speed. What I need is to make sure that whatever speed the master spindle is turning at the slave will match. If the part swap from the master spindle to the slave spindle happens fast enough the part won't be scratched. So I really don't care if the slave lags one encoder count as long as it lags consistantly. And it appears that is happening right now. I will need to take some pictures with a fast shutter to see if the marks always line up. Eric

Eric - what you need to consider is dynamics, not just statics - it is not clear from your limited requiements statement what kind of dynamic tracking is required. I will absolutely guarantee however that under dynamic conditions the two will not track. The amount of error depends on the specific servo loop and its response time (and power) - whether the error is a problem depends on your needs. I had suggested collecting the standard frequency response data so you could evaluate this mathamatically. If you'd rather not do that, then try this - mount an L shaped item (such as a heavy allen wrench) in the main chuck. set up a a steel block so the L will hit the block in about 1/2 revolution from "zero". Set the thing to zero, fire off your high speed camera and hit the L as hard as you can with a hammer (to make the main chuck rotate) - reveiw the high speed camera data to see how good tracking is.

Is this high production? Going slow or stopping would seem to solve the problem. I'm fairly our Mazak Multiplex's and Mori ZT2500Y's are using C axis indexing and are stopped during the transfer.

Wes

A strobe light would probably be more useful to you.

Make the two flywheels like I suggested. They can be thick and thin squares of steel plate balanced on a lawnmower blade balancer, nothing complicated or difficult. Press them onto shafts and put the thin one in the master chuck as a reference and the thick one in the slave to make it have to work hard to keep up.

Line up the corners, then accelerate the master rapidly and slow it gradually. At or above some acceleration the slave will start to slip and the corners will separate.

You could also watch the slave motor current as you twist it out of line with the master. Possibly the slave will hold lock until the current reaches a limit. If so, you could add a meter to see if the current nears that limit in operation.

Then operate it below the maximum acceleration or current limit.

Greetings William, The goal is to drive two spindle mounted chucks at the same speed so after a part is machined on one end in the master chuck the slave chuck can grab it. After the part is clamped in the slave chuck the master chuck releases it. Then the rest of the work can be done on the other end of the part. So the two chucks must be spinning at pretty close to the same speed when the swap occurs. So if the master is spinning at a certain fairly steady rate and if the slave can then spin at the same rate I'll be OK. I realize that the slave will always lag a little as far as angular position goes. But as long as the lag is unchanging then the two spindles can match speeds. Eric

Greetings Wes, If I go to the trouble of putting an extra spindle on the lathe I want to change parts while the chuck is spinning. It's amazing how only a few seconds per part on a long run can add up. So If I can get the spindles to match speed without too much trouble I wanna do it. And not only that either. I just plain wanna figure it out. Cheers, Eric

Greetings Jim, I already am using index marks to watch for slipping. And for sure a strobe is the way to go. Now I'm going to have to make one. Time for a google search for a simple circuit for one. One advantage I have is that the two spindles will not be loaded any more than friction and windage loads. They only need to match speeds during part changes. Cheers, Eric

You could watch both encoders on an oscilloscope. You might not catch a single slip but you would clearly see multiple continuous ones.

I already have digital readouts connected to each encoder. So I know how many counts each one is producing. The counts match exactly after everything stops. The numbers change way too fast to see how close they are during rotation. Eric

On Wed, 10 Sep 2008 00:02:07 -0400, JoanD'arcRoast

Me Too!

Sounds like you are ready to try it out on some parts. I think it will work a treat.

Dan

are stopped

I notice our spindles stop very quickly but if the joy in this is figuring it out, then I wish you well. Get things working really good, then play with spin welding just for fun.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller