Need Some Help - Lathe Diameters Not Consistent...
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    Default Need Some Help - Lathe Diameters Not Consistent...

    Ok guys, I have a problem that is driving me crazy. One of our machines is having trouble holding size on a part we run every day in production. This part runs on another machine without issues, so the problem is isolated to this one machine.

    Part is a forging, gets two bearing diameters turned. One at 1.75", one at 2.625" +/- .0004" tolerance. Part is held between centers.

    Here's the tricky part.

    The 2.625" diameter (closer to the headstock) is no problem. +/- .0002" or less, ALL-DAY-LONG!

    The 1.75" diameter (closer to the tailstock) is a wild card. Operators have been writing down sizes of every part, including any adjustments made, and results from re-running the parts... It will hold .0001-.0002" for 5-6 parts, and then suddenly drop -.0004". Occasionally it will go the other way, and jump oversize as well. Likewise, it will jump oversize on occasion.

    But, ONLY on this ONE diameter...

    It will also jump high/low, and be back at nominal on the next part.


    The parts are measured with snap-gages with electronic indicators with 1-micron resolution. Everything indicates that the gaging is accurate, repeatable, and in good working order. (We use these everywhere, and are aware of common issues to look out for...)




    Anyone have any ideas? I am grasping at straws trying to figure out why only ONE diameter would behave erratically like this. Maintenance just aligned the turret for center-height via Radial turret alignment. (Ex CNC field-service BTW - very sharp, capable & knowledgeable.) If you setup an indicator in the machine & test positioning & repeatability, it will repeat, DEAD-NUTS, every time!

    I even went so far as to install a freshly-cut part, setup an indicator in an empty tool pocket & "scan" these diameters with my indicator, with code yanked right from the main program. Repeats with less than .0001" T.I.R. on a test indicator, on BOTH diameters...



    Has anyone ever seen anything bizarre like this before?

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    Just a few ideas floating around my head, at random.

    1-The center height of the finishing tool itself is off. May need to shim the finish tool? Check it's height with a height-gage?
    2-The machine has an NC-servo tailstock, mounted on linear-rails. These rails/trucks are loose, letting the tailstock wonder part-to-part? (FYI, I don't see any taper in the parts, so not sure if this holds water...)
    3- ????

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    Are you saying a turned diameter next to the bearing diameter on the tail stock end does not vary but the bearing diameter does?

    If so, any questions about live center or other tail stock problem are eliminated.

    All that leaves the axis bearings, or something in the inside of the machine (like a hydraulic or coolant line) pushing or pulling on the turret housing.

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    Is this a box way machine? Have you tried checking turret movement at the two locations? Just a thought but an ill-fitting z-axis gib would give you erratic diameters at different locations along the z-axis.

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    How is the pressure on the tailstock, is it steady? Is it aligned well?

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    The tailstock is where I would start looking. Maybe swap out the live center for the one on the other lathe and see if the problem follows?

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    Is this a hard turn operation? Does the turret index or is the same tool cutting both diameters? If hard turn I would monitor the incoming stock size fluctuations. If the turret indexes between the two tools I would look at the turret for repeat-ability. If those check good I would swap the live center from the good running machine to eliminate the live center bearings.

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    What type of tailstock does the lathe have? Is it one piece servo driven or the standard tailbody hydraulic quill type?

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    Quote Originally Posted by Red James View Post
    Are you saying a turned diameter next to the bearing diameter on the tail stock end does not vary but the bearing diameter does?

    If so, any questions about live center or other tail stock problem are eliminated.

    All that leaves the axis bearings, or something in the inside of the machine (like a hydraulic or coolant line) pushing or pulling on the turret housing.
    The part has (2) bearing diameters. The smaller diameter is near the tailstock, and is the problem diameter. The larger diameter closer to the headstock is consistent.

    Machine is a Mazak Quick-Turn "Smart" 350, has linear guides, and a servo-driven tailstock, on linear guide rails. Not sure if the machine has ball or cylindrical roller guide ways/rails. Tailstock thrust is around 1,500lbs.

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    Quote Originally Posted by Jashley73 View Post
    The part has (2) bearing diameters. The smaller diameter is near the tailstock, and is the problem diameter. The larger diameter closer to the headstock is consistent.
    Jashley, are these forgings or castings? If so ANY out of balance at the large end will boss the whole operation around. A fairly quick way to sort it out would be to see if the tailstock end is really-really round.

    If it's not really-really round you have to slow the RPMs down until the operation quits flexing the machine tool. Anchored to a foundation cannot fix this problem, it's a strength of materials thing. BTDT...

    Good Luck,
    Matt

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    I was going to vote for tailstock causing the issue. Sounds like an issue we had with a booster valve in a steady rest once upon a time. It wasn't building consistent pressure because of contamination in the valve. Depending on the length of your part you would still see 'some' variation in the diameter close to your headstock.

    But then i saw the tolerances you were working with and i realised i should keep my opinions to myself.

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    I have seen this before on castings as the center drilled hole is drilled in at an angle or there is a small burr in it.
    check the center hole closely. if the part isnt seating into the chuck perfectly and the center is not perfectly lined up will also cause a problem.

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    Quote Originally Posted by Jashley73 View Post
    The part has (2) bearing diameters. The smaller diameter is near the tailstock, and is the problem diameter. The larger diameter closer to the headstock is consistent.

    Machine is a Mazak Quick-Turn "Smart" 350, has linear guides, and a servo-driven tailstock, on linear guide rails. Not sure if the machine has ball or cylindrical roller guide ways/rails. Tailstock thrust is around 1,500lbs.
    I got to admit that's pretty weird,

    If something was loose on the tail stock I would expect things to be biased towards things being oversized (diameters) for the bearing at the tailstock end.

    But maybe that 1500 lbs thrust is moving things around / pivots the tail stock a smidge and then it gets wedged / something could be loose in a counter intuitive way that then locks down under that thrust and weight ?

    How heavy is the forging ?

    How long is the work piece ?

    AND are you confident that the machine is set onto it's feet and foundation really well ?

    Like it could be rocking across one diagonal pair of feet and maybe you have a floating / partially unloaded foot towards the tail stock end ?

    [Random brain fart ~ but might explain the seemingly random oversize and undersize and on ~ size 5 parts thing ?].

    Wondering if the 'Frame" of the machine heats up … With MAZAK there's thermal compensation on the control that can make 8 micron shifts ++ and + unloaded foot / twist across frame ? [Not sure how much info the control gives you about thermal corrections it makes given that coolant is not circulated through the machine's castings ?].

    stab in the dark ?

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    Ball screw worn
    Ball Screw Thrust Bearing.
    Is the part out of balance sense it is a forging

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    Check your spindle bearing preload. Inadequate preload would give you good tolerancing near the chuck and more random stuff as you move away from the chuck as the potential angular error increases the further out you go.

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    Quote Originally Posted by Matt_Maguire View Post
    Jashley, are these forgings or castings? If so ANY out of balance at the large end will boss the whole operation around. A fairly quick way to sort it out would be to see if the tailstock end is really-really round.

    If it's not really-really round you have to slow the RPMs down until the operation quits flexing the machine tool. Anchored to a foundation cannot fix this problem, it's a strength of materials thing. BTDT...

    Good Luck,
    Matt
    Matt, you obviously have some pretty thorough experience in this realm, as most would never even give this stuff a thought. The part is a forging, it is very A-symmetrical, and out of balance radially. Our face-plates have counterweights to balance the whole assembly dynamically. So it spins in balance. But as you are aware, centrifugal force pulls the part "off-center" at speed. Thankfully, the diameters are still round - way less than .0001" ovality at least as far as we can measure...





    Quote Originally Posted by cameraman View Post
    I got to admit that's pretty weird,

    If something was loose on the tail stock I would expect things to be biased towards things being oversized (diameters) for the bearing at the tailstock end.

    But maybe that 1500 lbs thrust is moving things around / pivots the tail stock a smidge and then it gets wedged / something could be loose in a counter intuitive way that then locks down under that thrust and weight ?

    How heavy is the forging ?

    How long is the work piece ?

    AND are you confident that the machine is set onto it's feet and foundation really well ?

    Like it could be rocking across one diagonal pair of feet and maybe you have a floating / partially unloaded foot towards the tail stock end ?

    [Random brain fart ~ but might explain the seemingly random oversize and undersize and on ~ size 5 parts thing ?].

    Wondering if the 'Frame" of the machine heats up … With MAZAK there's thermal compensation on the control that can make 8 micron shifts ++ and + unloaded foot / twist across frame ? [Not sure how much info the control gives you about thermal corrections it makes given that coolant is not circulated through the machine's castings ?].

    stab in the dark ?
    Eric, you could be onto something regarding the leveling feet letting the machine rock slightly. It's worth a shot, as the machine's been there a while, and could probably use a touch-up.

    RE: Tailstock thrust/movement - Perhaps it could be moving under load. I can always dial-up the thrust & look for a change...

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    have you checked your live center for bad bearings?

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    Quote Originally Posted by Jashley73 View Post
    Matt, you obviously have some pretty thorough experience in this realm, as most would never even give this stuff a thought. The part is a forging, it is very A-symmetrical, and out of balance radially. Our face-plates have counterweights to balance the whole assembly dynamically. So it spins in balance. But as you are aware, centrifugal force pulls the part "off-center" at speed. Thankfully, the diameters are still round - way less than .0001" ovality at least as far as we can measure...







    Eric, you could be onto something regarding the leveling feet letting the machine rock slightly. It's worth a shot, as the machine's been there a while, and could probably use a touch-up.

    RE: Tailstock thrust/movement - Perhaps it could be moving under load. I can always dial-up the thrust & look for a change...
    I took at as a given you absolutely know what you are doing and NICE that you like / can recommend (thumbs up) your service techs etc.

    Balancing along an asymmetric shaft can be counter intuitive, I had a German uncle that used to balance (huge diameter) prop shafts on large passenger liners (civilian cruise ships)… It's kinda funny with the harmonics and dynamic gear changes that on the third level deck that all the glasses and bottles of booze at the bar would jump around and shake until uncle 'Willi" would place these screw in weights at various parts and lengths along the shaft until everything smoothed out (after analyzing the harmonics). But staring at the shaft you don't see anything; the vibrations 'Play out" in weird ways across the whole ship at different RPM.

    So it could be due to various harmonics and an asymmetric part your machine has walked around a little bit (imperceptibly).

    1500 lbs thrust is not trivial, turned through a right angle (tailstock) to flex the frame / casting if not fully supported. It's equivalent to a large European horse perching all four hooves onto one small area/ point. If there is not a counter thrust from the floor/ foundation to support that tailstock a 0.0004"+ movement would certainly be possible (if you have a partially floating leg).

    I think some people put DTI's at the edge of the machine to a plate on the floor to try and pick up any movements, not sure whether the guarding (sheet metal) on you QTS 350 would allow you to do that ?

    I know you like to take big manly ultra efficient DOC's so I wonder if it's easy to push the tail stock around if things are not rock solid whilst gauging and comping.

    Have you got your eye on a new Mori ? ;-)

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    Quote Originally Posted by Jashley73 View Post
    Ok guys, I have a problem that is driving me crazy. One of our machines is having trouble holding size on a part we run every day in production. This part runs on another machine without issues, so the problem is isolated to this one machine.
    Move it back??? Once every 4,5??? To your response to my other post.

    You may have the center in the headstock flexing into an orbit because the load wants to boss things towards the heavy side (this gets big pretty fast).

    Actually had a contractor that couldn’t make a taper and 1 of 2 bearing diameters right (far end - ovality)… Mazak conversational machine, and,, well,,, they run per dia & finish requirement. Spinning a 55lb “T” shaped forging with the small end about 2” with truncated #6 morse taper, 3¾” seal, 3” & 2¼” bearings between, on centers. Ended up having them rough them & we finish ground them.

    I figured when he called me the headstock center was orbiting so I took some cool IRD mechanalysis crap over & took a peek with some transducers & accelerometers. The setup behaved until the RPMs went over 400. Then the dog started waving the tail, most of the displacement in the machine was in the horizontal plane (180° end to end BTW with accelerometers near chuck and the foot end, close to ways). From near base studs to bedway about ¼ amplitude (no phase referenced here). Anyway the horizontal displacement difference end to end was a bit over .008” with the particular part we had in the machine. This was in the early 90’s so getting inside to put a fishtail against a rotating part was do-able without the world blowing up and the piece part displacement (orbit) was a bit more than .001”. I didn’t bother noting the G force’s but they’d be considerable to bend a 30” mazak around like that... I'm out'a bullets...

    Good luck,
    Matt

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    Quote Originally Posted by Matt_Maguire View Post
    Move it back??? Once every 4,5??? To your response to my other post.

    You may have the center in the headstock flexing into an orbit because the load wants to boss things towards the heavy side (this gets big pretty fast).

    Actually had a contractor that couldn’t make a taper and 1 of 2 bearing diameters right (far end - ovality)… Mazak conversational machine, and,, well,,, they run per dia & finish requirement. Spinning a 55lb “T” shaped forging with the small end about 2” with truncated #6 morse taper, 3¾” seal, 3” & 2¼” bearings between, on centers. Ended up having them rough them & we finish ground them.

    I figured when he called me the headstock center was orbiting so I took some cool IRD mechanalysis crap over & took a peek with some transducers & accelerometers. The setup behaved until the RPMs went over 400. Then the dog started waving the tail, most of the displacement in the machine was in the horizontal plane (180° end to end BTW with accelerometers near chuck and the foot end, close to ways). From near base studs to bedway about ¼ amplitude (no phase referenced here). Anyway the horizontal displacement difference end to end was a bit over .008” with the particular part we had in the machine. This was in the early 90’s so getting inside to put a fishtail against a rotating part was do-able without the world blowing up and the piece part displacement (orbit) was a bit more than .001”. I didn’t bother noting the G force’s but they’d be considerable to bend a 30” mazak around like that... I'm out'a bullets...

    Good luck,
    Matt


    Uhhhhh .......

    ???















    ---------------

    It was asked early on page 1, but no answer that I seen.


    There is no turned D right next to the one that you are having trouble with?

    Just one D by the headstock, and one by the tailstock?


    If so, I'd say to change the live center first, and then start looking at the tailstock.
    Maybe starting with less pressure?
    I doo VERY little between center work, but that seems like quite a bit of pressure to seat a 60* incl angle tip?



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