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
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.
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.
Bill Roberto 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.
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.
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>).
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.
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
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
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.
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.
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.
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
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