Why does my lathe bore tapers?
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  1. #1
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    Default Why does my lathe bore tapers?

    First of all let me state a few things which should eliminate a lot of conjecture. My lathe does not turn OD tapers. I have however had problems over the years boring tapers. I have tried different bars with more or less the same results. This is not heel drag, tool geometry, or any of the common mistakes that I can see. I almost always need to take a couple of spring cuts to creep on a dimension so I have to assume it is flex in the bar which is all too common. The bars I use (and I have quite a few) are always as large as possible and range from tool steel to solid carbide. I get these results even boring a 1.5" hole with a 1" bar. Now here is my question. If it is flex in the bar, why does it change the farther down the bore hole you go? That seems to me to be the only reason why you would get a tapered bore. The taper can be as much as .002" in a 5" bore of 4140, not that it should make any difference what I am boring and it doesn't. Anybody else experienced this? Thanks.

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    The common way to get around a worn lathe boring tapers,is to lock the saddle ,and bore with the compound (correctly aligned ,of course).....this will produce a straight parallel bore suitable for ball or roller bearings.

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    Getting a taper with boring but not turning... I'll assume you are using the same speeds/feed rates during both operations. I also assume the taper is always in the same direction? Always getting smaller on the head stock end?

    Seems to me like there are only two differences between these operations.

    1- Cutting forces are in two different directions. Make sure you are taking the backlash out of both your cross feed screw and compound screw and that you have both screws with their feed force into the workpiece. Don't know what kind of lathe you have, but also check for anything in the works that could be more secure in one direction than the other i.e. cross feed nut secured properly, compound nice and tight, etc..

    2- Since you have a boring bar in the tool post, the saddle is farther away from the chuck/head stock while boring that it is while turning. Maybe your ways are more worn in one position than the other and you are getting a little taper from bed twist or wear. Or there is twist in your bed that is more pronounced in one area than the other. You could try tweaking your bed leveling legs and see if you can improve things.

    Good luck,
    Ted

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    interested in the why also. none of my machines does it, but i have been hearing about this for ages, as if it were normal. they usually say the hole gets smaller, i believe. i have not heard a logical explanation for it so far. why should the bar deflect as it goes deeper in the hole?

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    There are lots of possibilities. You seem to have eliminated several with your large bars. You don't say which way it tapers, there is the possibility of the gripping force of the chuck distorting thinner workpieces.

    There are some significant possibilities you haven't considered.

    One is the position of the carriage when boring vs. the position of the carriage when turning. Bore a suitable piece and mark the positions of the ways. Now turn the od on a suitable workpiece (held by the chuck only, no tailstock). Inspect and compare the measurements for identical carriage positions.

    Another is the position of the cutting tip relative to the size and length of the carriage. When you are OD turning, the cutting tool tip is near the center of the carriage ways. Yaw error tends to be negligible as the carriage tends to revolve about the center. When you are boring, the tool tip is near the front edge of the ways. Yaw error moves the tool tip much more. There is often the leverage principal at work as well, where error movements are amplified when the tool tip is projected.

    IME, the carriage needs to be removed and have the center relieved so all bearing is on the ends. Since this is a repair situation rather than a rebuild situation, chances are you won't get perfection. Anything more than removing 1/2 the error is a win, removing 90% of the error is a big win.

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    Od tuning the cutting forces are down and toward the operator position. Typical Id (boring)the force is down and away from the operator position. This would twist the carriage one way for turning and the other way for boring. I don't know why the carriage might twist, but perhaps boring with the spindle/headstock in reverse and the boring bar/bit on the far side ID may bore straight.
    This is worth a try.

    Oh, perhaps due to the gib clearance in the compound going/pushing one way at OD turning, and then the other way (pulling away) for boring.

    Running an indicator with no forces would not give evidence of the problem, but with cutting forces might tell what is going on. The real proof is with doing the operation. A two-hand push-pull on the compound might show possible action/movement.

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    As a test try setting up a longer piece of work in a steady rest and bore. That should eliminate problems with the ways near the head stock, chuck issues, and head stock bearing issues. If you still have the same taper, then it has to do with the saddle, compound, or tool set up.

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    Well, one difference between boring and outside turning is the accumulation of chips.

    On a deep bore, chips can accumulate which will push against the bar. With a boring bar extended well past the carriage there is always some flex - it may be in the bar itself, in its toolpost mounting, in the compound, cross slide or carriage gibs, whatever. The deeper you go the more chips, and the more deflection.

    What’s the solution? Most will withdraw the bar when the chips build up, clear the chips, then resume cutting. Problem is, you’re then effectively taking a spring pass at the beginning of the bore before you get into the uncut area, which will open it up a bit.

    Using a smaller bar can help, as you can get greater chip clearance by projecting the cutter more or by angling the bar. Seems counterintuitive, but that way you’ll have the same chip pressure at the end of the cut that you do at the beginning - none.

    A coolant-through bar with a flow rate sufficient to keep chips flushed out is the better solution.

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    everything that has been said is right, but doesnt explain why the bar should deflect more (or less) and the start of the hole than at the end of it.

    edit: actually the bore should have less deviation as it doesnt deflect compared to work sticking out of the chuck.
    Last edited by dian; 09-12-2021 at 12:17 PM.

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    Thanks for the replies. Yes it always is smaller toward the chuck/headstock. Yes I have tried using a steady and getting the saddle away from the headstock. The lathe is bound to be worn some but certainly not badly and there does not seem to be any twist in the ways. I test if from time to time by turning collars on a test bar between centers. Half a thousandths is no problem. The lathe is pretty stout (Monarch) but I notice the same thing on my SB as well. As Dian said, I have heard of others having this same issue. I agree that the forces are totally different when boring vs turning but why do the forces change? That's what has me stumped.

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    Don't compare taper between boring and OD turning between centers. If you want a good comparison you need to compare taper between OD turning unsupported and boring. Using a center alters the dynamics of the situation since the tailstock can pull the end of the piece wherever it needs to be to cut straight.

    Get yourself a piece of ~4" OD tubing with a ½" wall and use that for a test cut on the OD unsupported at the tailstock end. You can then use the same piece to do a test cut on the ID with a boring bar. Compare those tapers and see what you've got.

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    QT:[ why do the forces change? That's what has me stumped.| One thing that changes forced is that OD turning pushes the saddle toward the operator and ID work pulls the saddle away from the operator.

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    With OD turning, the cutting point is relatively close to the center of the tool post.

    When boring, the cutting point is fairly distant, especially close to the headstock, and there is much leverage from the long boring bar. Is the tool post perhaps rotating?

    It would not need much rotation to explain the ID taper, and probably could not be seen by eye.

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    so you figure the cariage is rotating as you cut? taking up the slack from od turning? then its simple, give it a bump before you start. boring on the backside should produce no taper, right? cross, something to try, no?

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    You are looking for a simple answer to a very complex system of force vectors.

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    I feel like its related to the forces and the way the cross slide is constructed.

    In OD turning the tool is right next to the post and the tool point pressure which pushes straight back towards operator is twisting the toolpost in a "loosening" manner when looking from above.

    In ID turning/boring, the tool is much longer and is more aggressively twisting the tool post (think of a tightening Z rotation looking from above) and the twist has the action of reversing and opening the "normal, OD turning" clearances in the dovetail and its gib. Might not happen all in one pass but as it occurs its magnified by what I think is a larger stickout.

    Taking the gib out, flattening it, and scraping it for better contact, I believe can help. Some test indicators mounted here and there could prove the theory from a more convincing numerical position.

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    Quote Originally Posted by dian View Post
    so you figure the cariage is rotating as you cut? taking up the slack from od turning? then its simple, give it a bump before you start. boring on the backside should produce no taper, right? cross, something to try, no?
    QT{You are looking for a simple answer to a very complex system of force vectors.} Yes-> make it do the task.

    Mostly I'm saying to try a boring test. To bore a bore with the tool bit at the far away side of the bore, the cutting edge facing up, boring on the in-going feed, and the headstock turning in reverse.

    Likely whatever is happening would be less likely to happen with the forces at boring, the same way as in a turning operation.

    Much easier than analyzing factors, or rebuilding the machine.

    Yes with being sure the clearance under the cutting edge clears as is good in any boring task. Be sure The rake attitudes are proper for what ever material is being bored as normal.

    Other factors: (likely there are more)
    Normal boring pulls the forces the other way than turning.
    The action is tight on the other side of the bed ways.(yes they should be even but who knows.
    The clearance of the compound gibs is pulled the other way (yes once tight should not matter)..
    The bar holder/tool holder is pulled inward not pushing away.
    Not well seated a lathe can bend and twist with different forces.
    and as someone said the length of the boring bar puts on more stress than would be to a tool bit.

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    If the lathe bed was so worn out the tool bit went lower than center so it would cut a taper OD turning, but the Op said it cuts a straight OD turning...just ruling that out.

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    Thanks guys. I've got some good ideas from you how to nail this down. Next rainy day I'm going to look into all that you have suggested. Thanks again.

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    Quote Originally Posted by crossthread View Post
    ...I test if from time to time by turning collars on a test bar between centers.....
    Take the tailstock off the lathe and turn the collars without a center. They'll show the same taper as the one you see when boring.


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