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  1. #21
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    As mentioned, check the actual center in the part. We hold those type of tolerances and if the center drill is dull, the center hole it drills will have ovality or a star type pattern. This ovality will cause diameter variation. If you check the roundness of the "bad" part with a roundscan, you are likely to see a difference from a "good" part. Make sure your live center is good. It's very easy for a previous operation to cause issues with a downstream finishing operation.

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    I'd check for tailstock lift/play, and probably try a new/different live center also. Maybe also check that its blueing good with the tailstock taper, maybe its rocking a tiny bit on a piece of dirt or dent in there.

    If the parts that run good in your other later are center-drilled by the same machine/process doing are the ones giving issues in this machine, seems that should rule that out.

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    The parts get forged centers. Instead of a traditional machined, combined drill & 60* counter-sink that we're used to seeing, you have a forged cavity with 3 raised-boss' that locate on the centerpoint in the machine.

    To be perfectly honest guys, I'm having a hard time seeing how a bad live-center/bearings can still leave a diameter to be straight & round within .0002". Usually when our tailstock/center bearings start to fail, we see issues with surface finish before any other failure modes on the parts.

    The tailstock pressure is set [high?] like it is, to control runout on the tailstock end. More center-thrust helps control "whipping"/runout from radial forces from an a-symmetric part. This is something that I can demonstrate & repeat at the machine very easily. (BTW, a Ø3" quill diameter, with 400psi pressure, creates over 2,800 lbs thrust, if you wanted to compare my 1,500 lbs thrust from my servo tailstock, to a hydraulic tailstock...)

    Ovality is also much more directly affected by clamping pressure on the dogs at the drive end. This is also easily demonstrated & repeatable at the machine.



    But back to my parts, the diameters are straight & round. Only the size on the (1) diameter seems to be changing. I can sort of see Cameraman's theory of the machine base moving slightly. Or perhaps the tailstock lifting somehow.

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    I would check the tail center under pressure if it is a live center and expect double the live centers error on the part.
    Next I would get a center lap and lap the part centers to insure a true-round center was on the part, doing that only takes seconds.

    I think a solid high quality live center is best, multi tip versions often don't run dead true.
    One of my jobs was regrinding live and dead machine centers...to very near zero run out. I had to grind lives with loaded pressure. Taking them apart, grinding and then reassembly would not work..If new bearings were needed then they had to be ground after assembly. Brand new bargain brands often would not grind true perhaps because the bearing bores were not aligned, the bearings not high precision , or the bearing line-up marks not in line (?).
    Deads had to be ground spinning on double bearings off their own body..grinding in any holder or work head would not get them dead true. Likely a DeadTru fixture would/might work but that shop did have one so I had to make my own set-up...UNISON DedTru(R) Centerless Grinding Fixture | FlexMech Engineering

    Center Lap Mounted Points | Norton Abrasives
    Center laps should be diamond dressed or ground back to angle and true but will still make a round center when a little out of angle.

    Here is 50 millionths or better and perhaps one of the best/better brands..I was asked to grind to the or better. (50 millionths is a lot of error for a live center IMHO) They should actually run better than the bearings.
    Royal Live Centers

    Note: how can anyone make a precision life center for 20 or 60 bucks when a pair of decent bearing cost more that that..they can't.

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    Is each diameter being approached in the same direction?

    I had an old Mazak that would turn slightly different sizes if there was a lead-in chamfer, ie approaching in X+ (up and over) vs a typical approach in X- (down and across.) If so then backlash may be your issue as cutting forces move your tool around, your tolerances are pretty tight and would represent .0002" of backlash.

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    Are you finishing both diameters in the same operation / same turret index?

    What's your finishing stock allowance? Have you measured the diameters after roughing, and how do they compare?

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    If the part is losing money with having scrap a simple grinder could finish them for one end in perhaps 5 minuets a part. Even a Brown and Sharpe 13 grinder could do that and be used for many other tasks.

    Even a grinder would need a true round center in the part...and could run on a dead machine center.

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    Quote Originally Posted by Nmbmxer View Post
    Is each diameter being approached in the same direction?

    I had an old Mazak that would turn slightly different sizes if there was a lead-in chamfer, ie approaching in X+ (up and over) vs a typical approach in X- (down and across.) If so then backlash may be your issue as cutting forces move your tool around, your tolerances are pretty tight and would represent .0002" of backlash.

    There is that....
    If your last X move was in +, tool pressure/wear variances could move the cut size around with the same servo location.

    I have a big lathe with several thou lash, but I can hold very tight tol by making sure that I attack a feature from "this last X move".


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

    Think Snow Eh!
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    "Centers" are forged into the part, when the part is forged into shape. We can ignore any discussion on machined centers, with maybe the exception of the alignment of the punches used to form/forge the centers into shape - I.E. the alignment of the top & bottom halves of the forging dies...

    And even if the alignment of the dies float in & out of tolerance, we still do not have this same issue present on other machines in the shop that run this same part...


    And FWIW - we used to finish-grind these diameters after being turned. There were many more issues the crept up when you tried to match lengths to a shoulder, runout etc. that comes from moving a part across different machines. Not to mention, the grinder itself wasn't without hiccups & it's own set of problems too.

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    Quote Originally Posted by Orange Vise View Post
    Are you finishing both diameters in the same operation / same turret index?

    What's your finishing stock allowance? Have you measured the diameters after roughing, and how do they compare?
    Both diameters are turned on the same turret index. Finishing stock allowance is .010" per side, for both diameters. That's a little light, but it's also consistent for both diameters on the part, and across other machines as well.

    I have not measured the diameters after roughing, but perhaps it may reveal something. I will look into it.

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    Couple of dumb questions really...

    The two lathes you mention, (one running your forgings fine) the other not; Are they the same make model and year / brand ?

    Do you have climate control in your shop ?

    Is one turning center in a more thermally stable part of the shop than the other ?

    Does the shop change in temperature over the day where the "troublesome" machine is ?

    Reason I ask is that the thermal compensation curves for the QTS 200 match perfectly what you are describing here... For an 8 degree (Celsius) (nearly 15 degrees Fahrenheit) ~ change through a day.

    The curves show first 3 hours continuous machining between 0.00 and - (minus) 5 micron (two tenths) keeping it all below "Zero" for the first 3 hours… Then an hour break (lunch or whatever) while the machine cools a bit but temperature in the space keeps rising and then it's the last two hours of the day where stuff really starts to jump around diametrical tolerances are +++ .

    qts-200-thermal-drift-14-degrees-f.jpg

    ^^^ click and it will go full size.

    Sorry it's a bit fuzzy 'cuz they print it really small in the brochures lol.

    So for QTS 350 and larger temperature changes (in a shop) could be larger variance beyond 0.0004" (also looking at the basic test sheets and tolerances an older machine might be creeping towards it's positional capabilities too. ).

    Another shot in the dark …

    Core cooled ball screws and thermal comp on the control versus machines that have active continuous cooling of the castings + scales ? (shrugging shoulders)… The thermal management is a bit on and off and on and off and lurches about a bit but MAZAK seem to fit to "guessed curves " of most likely "Use" scenarios.

    I like how Hardinge "does it" just places some fans (artfully) in the right places ;-) none of this complex data analysis and faffing around to attempt correct everything lol

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    I wanted to come back and give an update. Yesterday we pulled some way covers & other misc. covers off the machine, so that we could inspect the rails/trucks etc. for the tailstock body. Just a review, the machine has an NC-servo driven tailstock, mounted on linear-ball rails/trucks.

    Long story short, the bolts holding the tailstock body to the bearing trucks were loose.

    I was getting taper in my bearing journals on the order of a couple tenths (ten-thousandths of an inch) upon closer inspection. So while the larger diameter would taper 1-3 tenths yet still be in tolerance, the error magnified would put the smaller diameter out of tolerance.

    We tightened the bolts for the bearing trucks, and the machine has been making parts since yesterday afternoon.



    I'm not terribly surprised. 5 years of running the same part - a heavy, a-symmetric forging which will induce some vibrations into the machine - and heavy interrupted cuts in pre-hardened steel, I'm certainly not surprised that something worked loose in that time.

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    Talking

    Thanks for getting back to us on the problem. I ran a Mazak Smart, and lets say it had its quirks as well compared to other machines with same speeds/feeds/programs.

    I want to also mention that I have ran parts that were casting and interrupted cuts and the tailstock pressure with the vibrations would cause the part to move due to limited gripping surface with a three jaw. This caused the servo driven tailstock to push the part with each "hit" of the interrupted cut and move .001 or more until I either saw it moving or it would alarm out.

    You are one of the smarter guys around so I figured you were aware of this, but letting others know this is an issue with the servo turrets. As well as Mazak quality control as well on other issues.

    Glad you solved it and found a challenge to wrap your brain around

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  17. #34
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    I worked at a shop that was running crankshafts. All of a sudden the crank shafts were going hey wire..we found half the center drills holes were poor and center drills were sharpened poorly..so we went to the forge shop to gather all those poor center drills. threw away most and resharpened the OK ones ..problem went away..
    I would blame the center drill holes and the tail live center as that starting place to solve the problem..

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    Like I mentioned before relating to a problem I had. I don't know why every damn screw and bolt on CNC machines isn't loctited, just blue or purple removable loctite would do it and save a lot of problems.

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    Quote Originally Posted by SND View Post
    Like I mentioned before relating to a problem I had. I don't know why every damn screw and bolt on CNC machines isn't loctited, just blue or purple removable loctite would do it and save a lot of problems.
    Hurco are nothing to write home about on build quality, but one nice thing that I've seen on our Hurcos and not on our other machines - every single structural fastener has a paint marked line between the fastener head, washer, and machine frame, such that if something had backed off it would be apparent.

    It's a small thing, that I've never really given much thought to before, but definitely a nice touch.

    Loctiting fasteners on a machine tool is probably a good idea from the end user perspective, but not so much from the assembly/design/service perspective. Understandable why MTBs in general choose not to depend on it.

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    Pretty much every screw on my First, and Doosan have a white or yellow marker line to say it was torqued, now that I'd really expect them all to do, fairly common of any assembly work, but that really does eff all to keep screws from vibrating loose.
    Its really easy to have some loctite on all the screws, you can even buy screws that already have it on there. It also has zero effect on service if you use the right grade of loctite, some customers use a lot of with for their industrial components, its part of the specifications...
    I still have a few parts I plan to take apart and loctite to make sure they don't come loose, already had one that nearly wiped out the machine and been worried since about having another.

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    Thanks for getting back with the answer. Teaches us to look for the simple answer. IT looked like the tailstock was moving because the tailstock was moving!

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    Quote Originally Posted by SND View Post
    Pretty much every screw on my First, and Doosan have a white or yellow marker line to say it was torqued, now that I'd really expect them all to do, fairly common of any assembly work, but that really does eff all to keep screws from vibrating loose.
    Its really easy to have some loctite on all the screws, you can even buy screws that already have it on there. It also has zero effect on service if you use the right grade of loctite, some customers use a lot of with for their industrial components, its part of the specifications...
    I still have a few parts I plan to take apart and loctite to make sure they don't come loose, already had one that nearly wiped out the machine and been worried since about having another.
    My Doosan has no such marks...

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    Good catch on the bolts. Some time back there was a thread about lockwashers. Turns out they don't work and Loctite is probably the best bet.


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