Why does my lathe bore tapers? - Page 2
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  1. #21
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    Temperature change can contribute to this phenomenon. Inside the bore the long bar is soaking up heat from the workpiece and swarf flying around in there. Have you ever held onto a micrometer too long and got different readings on the same part? As the bar cuts along and heats up it can move too. If it heats unevenly it will warp. Through coolant has the advantage of being able to more efficiently move the hot chips out and reduce the effects. This problem isn’t as pronounced on OD turning as the tooling is not surrounded by the workpiece and swarf does not build up as easily

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  3. #22
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    Try a left handed boring bar cutting with the spindle in reverse. Might make an improvement,

    The best I’ve ever figured is to use super sharp high positive inserts and small DOC to minimize cutting forces.

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    Quote Originally Posted by crossthread View Post
    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.
    Simplest test, first. Because it was what worked "oftenest" on big, heavy and BADLY worn seriously OLD lathes.

    Change nothing else.

    Rotate the bar 180 degrees on its long-axis.

    Tool-tip is now pointed to the rear of bore centerline, sharp edge DOWNWARD.
    Do NOT reverse the spindle rotation.
    Let it cut on the upsweep of the bore's rotation, chips deflected downward..

    Gain?

    No need for the lathe to even HAVE reversibility.

    You can SEE chips forming better, change your cutting plan , keep them clear more easily at the right times.

    Cutting force is applied into the wear and backlash in the cross feed, not away-from.

    "Perfect solution?"

    Not even close-to!!!

    Improvement? "Only sometimes". But sometimes happens. A lot.

    And it is cheap, fast, and easy ALL the time.

    So I dooo's it. 60 years, and counting...

    Boring solution?

    Assuredly, it is!

    Not a lot of mystery to a lathe.

    You want excitement with your boring bar? Find a good woman with a sense of humour and an appetite for a bit of adventure.

    Not a lathe. Lathes are just tedious.


  5. #24
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    .002" over 5"? Not sure what you're worried about... I've done a lot of lathe work in many shops and can't remember many parts that wouldn't fit in these parameters. You have to work with what you have. So go have way down and take a couple more thou on the last half. Work with what you have...

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    Quote Originally Posted by davehud View Post
    .002" over 5"? Not sure what you're worried about... I've done a lot of lathe work in many shops and can't remember many parts that wouldn't fit in these parameters. You have to work with what you have. So go have way down and take a couple more thou on the last half. Work with what you have...
    Aye!

    Two thou BEFORE he cranks in experience and compensation 'coz he has "learned the ways" of a(ny) given machine-tool?

    Wouldn't that be f*****g LOVELY ....for what most 'ere have actually had put between our "employee" hands by indifferent employers? And/or our own sparse rations..

  7. #26
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    I will give that a shot Thermite. Sounds easy enough and could be revealing. As others have said, it's time to look at the force vectors.

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    Quote Originally Posted by crossthread View Post
    I will give that a shot Thermite. Sounds easy enough and could be revealing. As others have said, it's time to look at the force vectors.
    As said, badly worn lathes. And even a six-inch diameter boring bar will at least TRY to chatter on a large lathe. Biggest I keep handy for the 10EE is only a two--inch, nowadays.

    Vibration is all the more impetus for the cross to move "somewhere" other than where you left it.

    Touchiest boring work wasn't for roller bearings. Most had short seats, long-axis, so easily finessed. Mining machine used a lot of grease-grooved and easily purged Bronze, too. Turning those.. and cutting back corn-cob stick weld in wotever they had to be installed to - had the depth to 'em.

    Unless we had a steep taper to do, mebbe once a week.. we didn't run with a compound ordinarily. If you do, there's yet-another source of movement.

    Big-ass 4-Way, directly on the topslide of the cross, ELSE a bespoke boring-bar holder also useful for mounting clearance drills.

    Also a MASSIVE Cee-Clamp, topslide to underside of the saddle. I did say "OLD lathes"?

    Most of our large lathes were too old to have powered-feed tailstocks, too..but even the really OLD ones did have carriage rapids. One Niles I found meself on said "City of Allegheny" in the TS casting lock-bolt cavity.

    I was born there, too. But it had changed name to "Northside, Pittsburgh" long before even me MUM was born there, 1922.

    Official name change was 1911, actually.

    Niles was an early example of what was to become called a "conglomerate". Bought up Pratt & Whitney, 1900, changed from Niles Tool Works to Niles-Bement-Pond, went on to even more of a buying binge, then went into receivership right after the crash of 1929.

    Wuudna' missed those years for their learnin'
    Surely ain't missed them SINCE, neither!


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    I like to look at the easy things first. .001 to .002 on the diameter of the work is a hairs breath. So what might cause this to happen if everything on the machine is right? Is the tool fresh, honed and sharp. Everything else has been covered.

    Roger

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    Yes the tool is sharp and honed. Finish is excellent. Just a taper in the bore.

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    Part extension from the chuck might be it. Deflection goes as the cube of the length. As the boring bar goes in towards the chuck the length of the part is less at the cutting tool. So should take off more material as it bores inward (I was too lazy to look over all the comments, is it taking off more or less material as it bores towards the chuck?)

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    Quote Originally Posted by BobM3 View Post
    (I was too lazy
    Got THAT much right.

    You did say "Minneapolis?"

    Why are we not surprised?


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    Since I find it much more of a PITA to try to straighten a bore than an OD surface, I typically 'level' my lathes with a boring cut about 4" deep, so that it bores straight enough. If you've only got a wee bit of taper, you might choose to split the difference between 'straight' on the OD and ID.

    So far as OD turning tapered or not, I find it is kind of a crapshoot from day to day. Some days, in some materials, the damn lathe will turn straight for 12" no sweat. On another day, there will be a wee bit of taper one way or the other. With a good DRO reading to .00005", you can tweak the tool position as it progresses along the part.

    You can always figure out how much to tweak one of the levelling screws to gain a thou here and there, to avoid the irritation of long tapered surfaces. If you do this in the middle of a job, just be aware that tweaking a levelling screw affects the diameter considerably, so you need to be cautious to find out where the tool tip is when resuming the job.

    On my 28" lathe which I use most often to bore large motor endbells, the carriage does slew about .0005" when boring coming out of the hole. This is not taper, and I use it reliably if I want to skim cut a half a thou without moving the tool at all, then I just bore coming out of the hole instead of into it.

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    Quote Originally Posted by HuFlungDung View Post
    Since I find it much more of a PITA to try to straighten a bore than an OD surface, I typically 'level' my lathes with a boring cut about 4" deep, so that it bores straight enough. If you've only got a wee bit of taper, you might choose to split the difference between 'straight' on the OD and ID.

    So far as OD turning tapered or not, I find it is kind of a crapshoot from day to day. Some days, in some materials, the damn lathe will turn straight for 12" no sweat. On another day, there will be a wee bit of taper one way or the other. With a good DRO reading to .00005", you can tweak the tool position as it progresses along the part.

    You can always figure out how much to tweak one of the levelling screws to gain a thou here and there, to avoid the irritation of long tapered surfaces. If you do this in the middle of a job, just be aware that tweaking a levelling screw affects the diameter considerably, so you need to be cautious to find out where the tool tip is when resuming the job.
    The very thot of "tweaking" the kinematic three-points of either 10EE.. or the railway-bridge-rigid cast-base Cazeneuve that could take a whole MONTH to reflect the change into the ways?

    I think you just presumed a spaghetti-bed hobby lathe.. or flung some dung?


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    Quote Originally Posted by thermite View Post
    The very thot of "tweaking" the kinematic three-points of either 10EE.. or the railway-bridge-rigid cast-base Cazeneuve that could take a whole MONTH to reflect the change into the ways?

    I think you just presumed a spaghetti-bed hobby lathe.. or flung some dung?

    Or you mistook that 28" as a bed length rather than the swing... Large lathes are very easy to tweak around when they are bolted to the floor.

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    Quote Originally Posted by eKretz View Post
    Or you mistook that 28" as a bed length rather than the swing... Large lathes are very easy to tweak around when they are bolted to the floor.
    28" is a medium, not large.

    Faster and easier to selectively hang a substantial mass of ballast on the arse-end of a four to six-inch boring bar - alter that during the pass(es).

    Even a "company" lathe, you gets to know wot it does and does not do.

    Besides, a Machinist-classification wasn't permitted to mess with the fixin's. Same Union, but that was a Millwright's classification.

    "Seldom seen" folk, our shop, too. Rig-in, rig-out - ELSE emergency repairs. No Fine Way they'd be diverted to "tweaking" any more than they'd be caught "twerking"!


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    Quote Originally Posted by HuFlungDung View Post
    ...
    You can always figure out how much to tweak one of the levelling screws to gain a thou here and there, to avoid the irritation of long tapered surfaces. If you do this in the middle of a job, just be aware that tweaking a levelling screw affects the diameter considerably, so you need to be cautious to find out where the tool tip is when resuming the job.
    ....
    Problem here is he has no idea what you are talking about. His idea of aligning the machine is turning collars on a bar supported by the tailstock.
    Zero chance he will resolve his problem.

  19. #37
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    Quote Originally Posted by thermite View Post
    28" is a medium, not large.

    Faster and easier to selectively hang a substantial mass of ballast on the arse-end of a four to six-inch boring bar - alter that during the pass(es).

    Even a "company" lathe, you gets to know wot it does and does not do.

    Besides, a Machinist-classification wasn't permitted to mess with the fixin's. Same Union, but that was a Millwright's classification.

    "Seldom seen" folk, our shop, too. Rig-in, rig-out - ELSE emergency repairs. No Fine Way they'd be diverted to "tweaking" any more than they'd be caught "twerking"!

    That is true, and those are easy to tweak also... A lot of the larger shops I worked in wouldn't bother tweaking the machines either. They'd just tell you to bump the tool if there was a taper.

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    Quote Originally Posted by eKretz View Post
    That is true, and those are easy to tweak also... A lot of the larger shops I worked in wouldn't bother tweaking the machines either. They'd just tell you to bump the tool if there was a taper.
    Uhh.. If a foreman had to TELL you?

    You'd not have made it out of "Machinist, Probationer" classification? You were meant to already know, and have more than just the ONE way to hit spec, too. Worn-out machine-tools had to be countered by sweat and ingenuity or nobody ate, reg'lar-like.

    Strong Union, once a hand was "in", the United Steelworkers were.
    Company got that one run of free shots to decide whom they had to live with.

    Bet your sweet a** they TOOK ever' one of those shots, too!

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    The best way to find out about the issue is to clamp the work axially for a change. I know there are too few manually driven axial clamping chucks around for conventional equipment. If you have a face plate, treat the part like on a mill’s table.

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    Quote Originally Posted by Mechanola View Post
    The best way to find out about the issue is to clamp the work axially for a change. I know there are too few manually driven axial clamping chucks around for conventional equipment. If you have a face plate, treat the part like on a mill’s table.
    Better-yet, DO it on a mill? Chandler-Duplex. Wallbanger, Narex, Gamet, (Criterion, Flynn..)etc.



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