Shop argument! Must have metric ballscrew on cnc lathe to make metric threads?
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    Default Shop argument! Must have metric ballscrew on cnc lathe to make metric threads?

    I have never heard this before, but I'm being told that you can't make a metric thread on a cnc lathe with an english standard ballscrew, but you can make a english standard and metric thread on a cnc lathe with a metric ball screw. I'm saying this is Bullshit!
    Am I wrong?
    have fun
    i_r_

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    I have no actual knowledge but my friend with multiple CNC lathes cuts both all the time. It's a freaking computer with high res encoders. The ballscrew could be from Mars and have a pitch of 0.348 times Marvin the Martian's left thumb knuckle and still cut any thread you wanted. But maybe there's some limitation I don't know about?

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    Well, if it's true, my CNC lathe doesn't know it. I have cut metric threads with an inch ballscrew.

    It really doesn't make any sense to think you couldn't do it.

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    The guy I was argueing with is a cnc maintenance tech and would not take my word for it, even though I've been programming cncs for 40 years!
    thanks
    have fun
    i_r_machinist

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    Bullshit. Cnc lathes understand Feed, or pitch. Be it in mm or inch. Almost all lathes present today are able to do this.

    If you need, say, 8 tpi thread, you can always do as 3.175 mm pitch OR use "inch mode" if default config is mm.

    Sent from my Redmi Note 7 Pro using Tapatalk

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    It's a load of rubbish, all that I guess really matters is that the pitch of the ball screw is consistant along the length.

    I have two American built CNC lathes (presumably with imperial ball screws) and one European which does have metric ball screws and both happily cut any thread pitch asked of them.

    As above when I need an imperial thread I just specify the feedrate as the pitch converted into metric to keep the programs consistent.

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    Quote Originally Posted by i_r_machinist View Post
    I have never heard this before, but I'm being told that you can't make a metric thread on a cnc lathe with an english standard ballscrew, but you can make a english standard and metric thread on a cnc lathe with a metric ball screw. I'm saying this is Bullshit!
    Am I wrong?
    have fun
    i_r_
    It's extremely difficult to grind a truly precise (cnc type) ball screw, and almost all ball screws* on a cnc machine need to be calibrated using a laser-interferometer to create "Pitch error" compensation tables.

    The accuracy comes from the calibration - not what type of ball screw is used re: "Metric" / "Imperial" or other systems to define a ball-screw.

    Also remember on some machines the ball-screw will be pre-tensioned (stretched) and there will in some cases be special pre-loaded spring in the casting block / seating to take up more massive differences in thermal expansion/ contraction.


    __________________________________________________


    * A lead screw on a Moore jig grinder or jig borer is a different kettle of fish from a very different pond.

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    That is ridiculous beyond belief. Someone is trying to be funny, or just does not know what the hell they are talking about. He calls himself a technician? What a joke.

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    Of course it's bullshit. Instead of being geared to the spindle the leadscrew on a CNC lathe is driven by a servomotor controlled by software. With the right programming they could even cut a thread that starts as an English pitch and then smoothly morphs into a very different metric pitch.

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    For this OP question, the guy is talking BS.

    But I was told by a MTB, that when you're getting into absolute accuracy, metric is ultimately more accurate because of least increment and less rounding errors....

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    As others have said, this person is mistaken. To convince him, you might point out that the inch length standard is defined in terms of meters, so even an inch ballscrew is technically a metric ballscrew (just in terms of 25.4mm increments rather than 1mm increments). Now that I think of it, he might be right in the sense that all ballscrews are metric when you get down to it...

    Quote Originally Posted by barbter View Post
    For this OP question, the guy is talking BS.

    But I was told by a MTB, that when you're getting into absolute accuracy, metric is ultimately more accurate because of least increment and less rounding errors....
    Yes, for example if you have a +/-.0001" tolerance, a lathe programmed in inches will have no more than 3 wear offsets that are in tolerance (more like 2), whereas the same lathe programmed in metric will have 4-5 wear offsets that are in tolerance (of course whether the machine repeats well enough to use this extra resolution is subjective). Also smaller rounding errors in the basic control programming for features that aren't nice round numbers.

    Of course this is strictly a relic of minimum programming increment choices on the part of machine tool controls, and has nothing to do with fundamental physical precision of metric vs inches.

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    Quote Originally Posted by barbter View Post
    For this OP question, the guy is talking BS.

    But I was told by a MTB, that when you're getting into absolute accuracy, metric is ultimately more accurate because of least increment and less rounding errors....
    Precise, slightly, yes, but more accurate, no, not likely. There will likely be more error in screw pitch and pulley mounting runout than there will be in rounding error from inch to metric. (some builders will have better components than others). Also would depend on the control. For a Fanuc and most others yes they are mainly "based" in metric. I also doubt you will see much pitch error compensation in the economy grade lathes, if there is, I postulate it would only be because they have known issues with the quality of their screws, that customers have in turn complained about. Most thread applications can handle a fair amount of pitch error, probably far more than a shitty ball screw would produce. Also, pitch error tends to be cyclical, and typically not cumulative if the machine axis has been checked for displacement accuracy over the length of travel (proper scaling).

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    Depends.
    Consider the fact that a inch screw machine can not make an exact 1 mm move.
    Why, because everything is done inside in counts (integers). 1mm is .0393700787 so one can not hit it perfectly as the control cannot see partial counts and stop on them.

    Real world and you don't track within a count while moving and screws and not made to this sort of tolerance so it is a theoretical argument.
    If one had a perfect machine in a vacuum with no friction and zero tracking error the pitch would cyclically vary back and forth an itty-bitty amount as the revs go by.
    Same/same with making ten .0001 inch size adjustments on a mill with one micron resolution metric screws. All the adjustments will not be the same amount of motion.

    Also related to the infamous problem that a digital computer can not understand 0.1. Floating point vs integers. Real numbers vs counts. How about BCD as a inside solution?
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    Quote Originally Posted by CarbideBob View Post
    Depends.

    Consider the fact that a inch screw machine can not make an exact 1 mm move.

    Why, because everything is done inside in counts (integers). 1mm is .0393700787 so one can not hit it perfectly as the control cannot see partial counts and stop on them.

    Real world and you don't track within a count while moving and screws and not made to this sort of tolerance so it is a theoretical argument.
    If you scrounge up an old G&L manual (pretty sure it was G&L but could have been K&T) they went into this at some length. Their computer did not use either inch or metric, but measured turns of the lead screw in machine units. Don't remember what the screws themselves were, if they made their own could have been anything. Seems like that level of thought/complication is not really necessay ?

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    Can you run a part on an imperial cnc machine if the part was made in metric using solidworks?!?!?!?!

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    Quote Originally Posted by metalmadness View Post
    Can you run a part on an imperial cnc machine if the part was made in metric using solidworks?!?!?!?!
    You could before the Dassault purchase.

    Darn French...

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    Quote Originally Posted by CarbideBob View Post
    Depends.
    Consider the fact that a inch screw machine can not make an exact 1 mm move.
    Why, because everything is done inside in counts (integers). 1mm is .0393700787 so one can not hit it perfectly as the control cannot see partial counts and stop on them.
    But what if your encoder has a multiple of 254 steps?

    it is a theoretical argument.


    Perfectly.. You sound like a carpenter..

    There is nothing perfect or exact in metal working.. At some point you are splitting helium light bands..
    You know that.. Otherwise you would be a carpenter.

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    Do it all the time in Inventor. They're just units that get converted into hexadecimal in the end anyway 😄

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    Quote Originally Posted by CarbideBob View Post
    If one had a perfect machine in a vacuum with no friction and zero tracking error the pitch would cyclically vary back and forth an itty-bitty amount as the revs go by.
    Perhaps OP needs to machine spherical cows?

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    Quote Originally Posted by cameraman View Post
    It's extremely difficult to grind a truly precise (cnc type) ball screw, and almost all ball screws* on a cnc machine need to be calibrated using a laser-interferometer to create "Pitch error" compensation tables.

    The accuracy comes from the calibration - not what type of ball screw is used re: "Metric" / "Imperial" or other systems to define a ball-screw.

    Also remember on some machines the ball-screw will be pre-tensioned (stretched) and there will in some cases be special pre-loaded spring in the casting block / seating to take up more massive differences in thermal expansion/ contraction.


    __________________________________________________


    * A lead screw on a Moore jig grinder or jig borer is a different kettle of fish from a very different pond.
    Is that really how a cnc lathe works? I would have assumed they used closed loop control with a position sensor, like dro uses, to determine carriage position while the ball screw just drives to position and could be anything.


    Sent from my iPhone using Tapatalk


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