Indexing Headstock Spindle

[ericdwilso]

I just bought a new lathe that I am going to turn into a small turn/mill machine. I have been struggling on how best to index the spindle. I think I need somewhere in the 60+:1 gearing ratio. Here is what I have considered...

1. Using a timing belt, but I expect that the backlash will be too great between load and no-load. It would also take multiple belts to get the ratio or I have to down grade the angular accuracy/resolution. - rejected!
2. I have also been looking at getting a harmonic drive off eBay which I believe is the best option except it blocks the 27" diameter spindle thru-hole. Further, the backend of the spindle has no extra length; might use some kind of expanding "collet" to couple the drive to the spindle.
3. Use some kind of a gear train drive with anti-backlash feature. There is room in the spindle headstock where the drive for threading current resides.

Have not decided between servo or stepper, just depends on the final solution. Either will have a brake included. Also, I think that I will have to mechanically decouple the drive from the spindle in turning mode otherwise the motor will be overdriven; not to mention where to put the generated power. Because of this, the encoder will need to be connected to the spindle separately; I think I will use a timing belt here.

I would really appreciate some feed back on the above either from those that have turn/mill machines on how the "big guys" drive their spindles or from others that have considered this problem.

Thanks in advance, Eric!

[Stepper Monkey]

Harmonic drives are awesome, but be careful about using them in any heavy work, the reason they aren't commonly used instead of planetary gearheads for machining is that harmonics can't take heavy loads or a lot of moment.
How much backlash are you worried about? Planetaries can get down to just a few arc-minutes, and in practice I can't detect any measurable backlash on the NemaTrue precision heads I'm using for one of my systems.

BTW, PM me if you need either a Harmonic or a Planetary, I scored a decent stock of both still in the boxes in various sizes and ratios I from a business liquidation, more than I need.

[RICHARD ZASTROW]

Eric, I'm working on something similar right now. Have you considered using hollow shaft servo's like the Kollmorgen DDR? You might also consider some sort of brake or locking device to hold the spindle in place while machining in a stationary position. As for attaching the servo to the spindle, there are clamping devices from Ringfeder, Spieth and others available. Belts work as well. After break-in and readjusting tension, they really do not stretch any more. The ability of the servo to hold position (servo-lock) is a bigger problem.
One of the problems I've encountered is finding a servomotor with enough speed range and torque to cover the speed range needed at the spindle.

[ericdwilso]

Wow... What a quick response!

I am planning to use a 2Hp motor & VFD (I have the VFD already), this gets away from demanding that the spindle motor handle the indexing functions for milling as well as the speed torque for turning. I will disable the VFD when indexing; the speed will not be very high so I do not think that it will hurt while milling. If someone has a different opinion, let me know. I plan to use the exisiting spindle gear train.

For indexing; I am thinking that when the part has indexed to a stationary location the spindle brake will engage and disengage only when ready to index to a new location.

As for positioning accuracy, I would like to get down to less than an arc-minute with 2 arc-min being the most I would like to see.

I like the hollow shaft Kollmorgen DDR servos! Not sure that they would fit my budget though!

I had a look at the . I have 0.860" protruding from the rear of the spindle: 2pcs. 0.38" wide lock nuts + 0.100" extra thread. I can free up some space by making the nuts thinner or converting one of them to a drive and jamming the other against it with some sort of locking mechanism. The shaft has a 41mm dia thread x 27.5mm bore. I do not see any other means of connection. I did not see anything on the Ringfeder & Spieth sites that would fit right out of the box. (The Speith anjustable gibs look neat!)

More later... gtg... Eric

[RICHARD ZASTROW]

Eric, What increment or angle do you want index? There are devices like Curvic couplings etc. that index at increments like 1 deg., 5 deg., or ? But if you want to index to ANY angular position to 1 arc min.(say 29 1/2 deg.), you will need an accurate angle encoder and a clamping device that will not distort or move the spindle while clamping.

As far as your clamp, consider mechanical lock, power unlock. That way a loss of power will not release your spindle. Good Luck.

[ericdwilso]

I want to rotate to any location 0 - 360 deg. I am heading toward using an oversized gear reducer and preload the backlash out of it.

I am currently planning on using a timing belt with an accurate, 1800 or 2500 cpr encoder. A 6:1 ratio with the timing belt would give 30 arc-sec. resolution between edges (4:1 - 45 arc-sec). US Digital has 2500 cpr encoders but they do not have an index which I believe I will need.... I am assuming that the index indicates a "zero" location. Model: S6S-1800-IB $69.36/Qty1 - Ball bearing with shaft. I could assemble my own for for $50 for 2500 CPR with index + bearings, shaft etc. The ball bearing version has a max speed of 10,000, so theoretically this could stay engaged continually.

[digits]

Hi, I hope this isn't off topic, but how do the spindles in a setup such as the one in this video manage to both run slow enough to be a rotary table, and fast enough to be a serious lathe?

http://www.youtube.com/watch?v=_4vLqfIx8X8

[skmetal7]

im guessing they use a very powerful ac servo motor and a high end, and i mean very high end!, ac servo drive

[digits]

Originally Posted by skmetal7 im guessing they use a very powerful ac servo motor and a high end, and i mean very high end!, ac servo drive

Sounds a little pricey for a home-shop! I suppose though that as long as you can control backlash, a two speed gear box or pulley set would work fine - you don't care about position when going from indexing to turning, and when going back to indexing, as you have an absolute position encoder on the spindle, you just need to do a move to the required index once you've changed gears back again.

[RICHARD ZASTROW]

Eric, Assuming you want to hold in position as opposed to milling a annular groove, you will need a substantial brake to hold position. You may also have a problem with a motor trying to adjust/correct position. (de-energize while brake on?) FYI I have some literature on a pneumatic clamp system called RotoClamp. The internet address they have on the literature is www.hema-gmbh.com . Their Canadian number is 905 688 6151.

Good Luck

[digits]

Did anyone get anywhere on this, it is still something I am very interested in doing?

For a new machine, I did think that it might be possible to mount a lathe head on top of a rotary table, with some method of locking the lathe head in position. The lathe head would be driven via a shaft that runs through the rotary table's central hole, and an encoder would also be fitted to this shaft. For lathe ops, the lathe spindle would be unclamped, and spun up via its shaft. When indexing is required, the lathe spindle is clamped, and the rotary table handles the indexing. As the lathe head is locked to the rotary table, the encoder will still record the exact position of the spindle, even during the change over. This is probably vastly over complicated though!

[RICHARD ZASTROW]

digits, Is the lathe headstock axis horizontal and the rotary table vertical? (you said on top of). Might be a rigidity problem without some heavy duty (expensive) bearings and couplings etc. I did see some interesting roller gear stuff at the AGMA gear show in Detroit. www.nexengroup.com Might work for some kind of indexing reducer.