Pinch turning length to diameter limitations

Group,

Has anyone ever done any pinch turning, and what was the longest length and smallest diameter? By pinch turning I mean two turrets simultaneously turning at once. The challenge I have before me is .875 diameter x 44 inches long. No steady rest, tailstock only. I think the best approach will be roughing with the top turret and finishing with the bottom turret. The key to equalizing pressure will be to split the difference between the stock size 1 1/8 hex and the finish size .875. (1.3-.875)/2 = .2125. So I would be roughing and finishing with a .2125 depth of cut. I think the finisher should follow the rougher as close as possible say .01. The machine has a Y axis so I can adjust tool center line if needed to control deflection. The material is 316 stainless. Does anyone that has done it foresee any potential problems with the above set up?

Thanks in advance

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If it was possible to prep the end as a seperate operation,I would put a two roll steady in the top turret and the turning tool in the bottom. regards,Mark.

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snipped-for-privacy@ems-fife.co.uk wrote:

That is what I suggested but the customer is adament about the method.

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Bill,

I have machined using balanced turning (pinch) on twin turret machines often but not the method you describe where the rough and finish are in the same balanced pass.

Normally I would use two rougher's balance turning the roughing pass/s followed up by two finish tools balance turning the finish pass. Works well on but not limited to your type of job.

Possible problems:

1) Chip wrap, long stringy chips getting wrapped around the part and tools. Not much space for chips to clear when you have two turrets, two tools and the part all in close proximity. Chip break may be the most critical part of an operation and material like this. You need to PUSH the tools fast enough to break the chips. If you can't get the chip to break you will have problems with coolant flow possibly holding your finish call out.

2) On a part like yours with it's diameter to length ratio and the DOC you will have problems as the inserts wear unevenly.

The most interesting job to me was also the smallest I have run using balanced turning. 1/4" round Titanium A6L-4V, turning and threading an interesting profile around 3" long with the leading end at .215" diameter and finishing up with two tapers 2-5/8" long total ending towards the collet at .015" diameter, no tail stock.

Tom

Bill Roberto wrote:

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brewertr wrote:

I used to run a shop that made dental drill motor spindles. When you tighten a collet into a spindle and grip a .063 deltronic pin it has to have zero runout. The way I did that was to finish turn and bore simultaneously removing the same amount of stock from the i.d. and o.d. on a Citizen. Thats the only experience I have with pinch turning. If the deltronic pin hiccupped then one of the tools were duller than the other and changed the cutting dynamics a zillionth of a degree. When you work to those tolerances you learn everthing has a cause and effect. I figure this project will be the same way.

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Here's a vid from Mori's website showing pinch turning. I don't know if there's any useful knowledge to be gained from it, but it's neat to watch. Dry cutting 316ss as well. http://www.moriseiki.com/english/mail/user/0601ue/0601uec01.html

We've done some pinch turning on our Star SV. We've had the most success by splitting the doc between the two cutting tools just as you've described. On our Y axis turning center (Mori SL150SY) we can't run the main or subspindle unless the Y-axis is zero returned. I've seen several machines that will run without Y-axis zero return but it's worth clarifying if you don't know for sure. The other thing that I'd be concerned about would be harmonic vibration that might take place due to the interupted cutting of hex stock coupled with the reduced .875 diameter of the finished bar. Especially as you get 20 plus inches down the bar.

Sounds like a good one you got there. I'll definitely be following this post to see what you (we <g>).

DanL

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DanL wrote:

Liked the video, love the sound of chips hitting the guards the sound of money being made.

Tom

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wrote:

We have a few machines with Y axis that will all run without the Y axis on zero. Not in the conversational programming though, on one with a Siemens 840D unless we set a Y axis offset in a parameter page, separate from the offset page.

If that Mori has a Fanuc control, in the offset page, if you press the + key to scroll through the softkeys, does one labeled "Ofset.2" show up? This brings up a page with Y axis offsets on one of our machines with Fanuc control. Don't know if this will work for you or not.

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We've tried that unsuccessfully. If the turret isn't in the zero return condition neither spindle will run. If we command G28V0 with an offset in Offset2, the spindle still won't run. We need to cancel the offset, then zero return the turret. Although there have been occasions where it would be helpful to be able to tweak centerline with the Y-axis, it does force things to be in order before moving on to turning ops. Maybe a parameter override would be nice.

DanL

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Bill,

I don't know how much help this will be but, a long time ago I looked at a pinch turning application that one of our sister companies was having a problem with. The part was a rifle barrel. On previous turnkeys they used a programmable follow rest in the lower turret an turned from the upper.

For this application they did a few test cuts in the show room using pinch turning and the results looked good enough for the customer to buy a couple of machines. When the machines arrived that's when things fell apart.

Here is what was learned;

The machines have to be leveled dead on. There can be no twist in the bed, and the tailstock needs to be lined up perfectly. This helped but didn't fix the problem. The machines started out level but had settled after a week or so.

The turning tools need to be ABOVE center. This one thing alone made all of the difference as far as chatter/deflection.

The finisher needs to take a slightly heavier DOC to balance out the load. It would be best to calculate it out.

Both tools have to be programmed at the same feed. The rougher can't be too far in front of the finisher. I don't remember how far exactly but it was less than .05"

The rpm that you run at can create a harmonic. I don't know that that can be avoided ahead of time but it's good to be aware that the situation can exist.

If all else fails, this is a good Swiss application. Farm it out.

--

Dan

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D Murphy wrote:

Put it on the type machine it belongs on? Now your taking all the fun out of it, lol.

Tom

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D Murphy wrote:

I'm an application guy. You know doing the impossible with whatever the customer wants.

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I hear ya.

If all else fails show them this video.

<http://video.google.com/videoplay?docid 30195085869242029>

Or convince them to buy a programmable following rest for the lower turret.

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Dan

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D Murphy wrote:

Yeah, nice. I've done and seen some pretty cool applications, but if you want my attention it is rigid tapping. No matter what machine I see it being done I think its the coolest thing. This project will be a matter of balance of the combined variables of tool pressure, deflection, and centerline manipulation. Once done it will be cool to watch, but nothing like watching 1/4-20 taps at 2500 rpm rigid tapping.

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No doubt. I'm always fascinated by the machines that are set up so that the feed rate overide works during rigid tapping. Slow the feed rate down and the spindle slows down, speed it up and the spindle speeds up. You can crank it down to zero when the tap is engaged then turn it right back up and the tap doesn't break.

Too cool.

On our lathes that have "B" axis you can hit feed hold while drilling an angled hole, flip it into handle mode and hand wheel out of the angled hole.

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Dan

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