Is a concentricity tolerance of .0001" on oval diameters even possible?

Other Brother · Feb 15, 2017

A customer is trying to hold .0001" concentric on 6 or 7 oval diameters, the largest is approximately 1.2500" and smallest is .7500". The material is titanium and the length of the part is about 6 inches. The lathe is a newer LB3000EX with y-axis, box way slides and live tooling. The lathe had a bad crash. The spindle run-out is good, the turret was repaired and aligned, the gibs on all axis have been adjusted, ball screws checked for lost motion etc. The customer claims that since the crash they can't hold the .0001" concentric tolerance. Next step is a ball bar test. Trying to figure out what I am missing.

Your thoughts?

Jashley73 · Feb 15, 2017

#1 - How are they inspecting...
#2 - Do they have any parts from before the crash, to verify that it actually was possible - and then actually done...

#3 - What's the head-stock alignment like? Parallel to Z in the X/Z & Y/Z planes? Part between centers? How's the leveling? Y-axis offset's the same?
#4 - How are the actually machining these ovals? Some kind of trick fixturing, live/tools & milling, cam/follow turning?

Just a few thoughts that come to mind...

ewlsey · Feb 15, 2017

The ball bar can tell you if there is an issue with arc motion, but it's not going to tell you why. I agree with the headstock alignment, but you probably already check that. Did you check the live tool holders themselves?

Other Brother · Feb 15, 2017

Jashley73 said:
#1 - How are they inspecting...
#2 - Do they have any parts from before the crash, to verify that it actually was possible - and then actually done...

#3 - What's the head-stock alignment like? Parallel to Z in the X/Z & Y/Z planes? Part between centers? How's the leveling? Y-axis offset's the same?
#4 - How are the actually machining these ovals? Some kind of trick fixturing, live/tools & milling, cam/follow turning?

Just a few thoughts that come to mind...

#1 and #2 are questions that will be asked tomorrow. Those are questions we already have prepared.
#3 Headstock alignment is good as is all other machine geometry. Leveling? This could be an issue. The machine is mounted on a 6' thick concrete pad surrounded by 3 inches of rubber to isolate it from a large press. The machine shakes as it moves. I have never seen anything like it.
#4 I am assuming that they are milling with live tools. Another good question.

Thanks

Other Brother · Feb 15, 2017

ewlsey said:
The ball bar can tell you if there is an issue with arc motion, but it's not going to tell you why. I agree with the headstock alignment, but you probably already check that. Did you check the live tool holders themselves?

I have not even looked at the live tools other than removing them when aligning the turret.

Jashley73 · Feb 15, 2017

Other Brother said:
#1 and #2 are questions that will be asked tomorrow. Those are questions we already have prepared.
#3 Headstock alignment is good as is all other machine geometry. Leveling? This could be an issue. The machine is mounted on a 6' thick concrete pad surrounded by 3 inches of rubber to isolate it from a large press. The machine shakes as it moves. I have never seen anything like it.
#4 I am assuming that they are milling with live tools. Another good question.

Thanks

If the leveling is off, then your alignment means little... And how did you check headstock alignment? Cut a part & measure at both ends, or mount a test-bar to the spindle nose & sweep that? Did you indicate along it's length in two places, or just one? (Parallel with X-axis, then 90*/perpendicular to X as well...?)

The machine shakes as it moves? What does that mean? It shakes when the press hits, or the machine shakes when you rapid-traverse it? If it shakes under rapid, then I would assume the ball-screw, or motor coupling is bent? And/or the ball nut/thust bearings are crushed? If it shakes under rapid, then I would assume that's a big red flag that something is bad-wrong... Is it noisy under rapid?

Or are we talking about the spindle shakes/vibrates? If so, is the chuck-cylinder swept/indicated concentric to the spindle? What's the work-holding like now? Is the chuck damaged after the crash?

Again, just spitting out thoughts...

MCritchley · Feb 15, 2017

Hey brother!
# 2 would be my first question.
Milling within a tenth? Seems like that would be tough.

Other Brother · Feb 15, 2017

As far as the offsets, we have set zero for each tool offset in x and y using dowel pins, then ran a program to check repeatability. We are checking after each tool comes in with a Blake co-ax mounted in the spindle and sweeping each tool. It's hard to check tenths with a .0005" indicator, lol. I need to come up with a better way to check. So far it seems like the positions are not repeating, but a co-axial indicator isn't a precision measuring device. Maybe a tenth indicator and adjust for gravity?

ewlsey · Feb 15, 2017

Here's how I check it. No worries about gravity.

Other Brother · Feb 15, 2017

Jashley73 said:
If the leveling is off, then your alignment means little... And how did you check headstock alignment? Cut a part & measure at both ends, or mount a test-bar to the spindle nose & sweep that? Did you indicate along it's length in two places, or just one? (Parallel with X-axis, then 90*/perpendicular to X as well...?)

The machine shakes as it moves? What does that mean? It shakes when the press hits, or the machine shakes when you rapid-traverse it? If it shakes under rapid, then I would assume the ball-screw, or motor coupling is bent? And/or the ball nut/thust bearings are crushed? If it shakes under rapid, then I would assume that's a big red flag that something is bad-wrong... Is it noisy under rapid?

Or are we talking about the spindle shakes/vibrates? If so, is the chuck-cylinder swept/indicated concentric to the spindle? What's the work-holding like now? Is the chuck damaged after the crash?

Again, just spitting out thoughts...

Headstock was checked to a test bar in 12 inches of travel.

When I say the machine shakes, it's when it makes any rapid move. It seems like the concrete pad moves in the rubber mount. All rapid moves are quiet.

yardbird · Feb 15, 2017

MCritchley said:
Milling within a tenth? Seems like that would be tough.

My first thought also! Even in a perfect world that would seem tuff to do.

Brent

Jashley73 · Feb 15, 2017

Other Brother said:
Headstock was checked to a test bar in 12 inches of travel.

When I say the machine shakes, it's when it makes any rapid move. It seems like the concrete pad moves in the rubber mount. All rapid moves are quiet.

As in, the machine is quiet & tight, but the entire pad shakes? Does it shake when the rapids are turned down? Setup an indicator so that the base is one pad, and the needle is on the other... Traverse the machine - does the indicator reading change? Do this with an indicator at all 4 corners of the pad, and you could see pretty clearly if the pad is moving... You did say a 6-foot thick pad, and 3" of foam between the lathe's pad, and the rest of the floor, right...? That seems like a lot of mass, and not much lateral support it sounds like - although I know very little about soil, concrete, etc...

Milland · Feb 15, 2017

If the thick concrete pad is to act as an "infinity stiff" mounting base due to the nearby press, that has some merit, but to really see if it works you need an accelerator test chart that measures motion/vibration on the key elements while the machine and press are in action. Some vibration will come through any damping medium, you have to see if there's any coupling with the axis of motions to find out if it's really working at whatever the natural frequencies are.

But interesting challenge, I'll be curious what you find out.

ewlsey · Feb 15, 2017

I thought it was implied that they were somehow holding this tolerance before the crash. It seems unlikely that the foundation would have been damaged in the crash. Marginal or not, it was working before...

Milland · Feb 15, 2017

ewlsey said:
I thought it was implied that they were somehow holding this tolerance before the crash. It seems unlikely that the foundation would have been damaged in the crash. Marginal or not, it was working before...

If there was undiscovered damage to ballscrew thrust bearings (or similar), then (as you say) what was marginally working before may no longer be. I'm not a vibe/acoustics expert by any stretch, but I've worked with them and done much testing on machinery and spaceflight apparatus. Lots of stuff happens that you'd only see with recorded accelerometer readings or high speed cameras.

Jashley73 · Feb 16, 2017

ewlsey said:
I thought it was implied that they were somehow holding this tolerance before the crash. It seems unlikely that the foundation would have been damaged in the crash. Marginal or not, it was working before...

I would want to see the proof that it was making parts in-spec before the crash though. That's a tight tolerance on a tricky form. Seems like a stretch to me.

barratt · Feb 16, 2017

Have you been given the job of rebuilding this machine?
Milling to within 0.0001" on a lathe seems like taking the piss.
Maybe they just don't want to pay for the rebuild.
What are the original specs for the machine?

michiganbuck · Feb 16, 2017

fighting such close size might be better served with a part run between centers on a grinder IMHO.

SND · Feb 16, 2017

This doesn't seem to make any sense, at least without a sketch.... Sounds like they're milling the OD of a part to be oval, over 6" length, and wonder why it isn't concentric, concentric to what feature?

B-Mathews · Feb 16, 2017

To me, this goal sounds impractical in most materials let alone Ti. Pissing in the wind?

Is a concentricity tolerance of .0001" on oval diameters even possible?

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