Why does my lathe bore tapers? - Page 3
Close
Login to Your Account
Page 3 of 5 FirstFirst 12345 LastLast
Results 41 to 60 of 89
  1. #41
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    2,931
    Post Thanks / Like
    Likes (Given)
    241
    Likes (Received)
    1375

    Default

    Last night I had a bit of time and put a 1" chunk of 12L14 in a collet to eliminate any wonky chuck goings on. Quick test was to drill a 3/4" hole through the stock (stock was sticking out of the collet about 1"). Bored it out about .030" in four passes with the last pass a spring cut. Measured with inside mics and sure enough it was .003" smaller toward the headstock. I flipped the stock around and bored from the other end (.005" cut). Ended up with essentially no taper (.0003"). It seems like it has to be something going on with the direction of forces as others have said although I can't figure out why the forces would change as the boring bar approaches the headstock. Tonight I will try to run everything in reverse from the backside of the bore and see what happens. Just for grins before I ran this test I put a section of ground stock in the collet and a Starrett DTI in the tool post and ran it from about 4" out all the way to the collet (lathe off of course). I got .0004" of taper. I can live with that. Later today I will go out and sacrifice one of my chickens and see if that helps.

  2. #42
    Join Date
    Feb 2006
    Location
    Minneapolis
    Posts
    240
    Post Thanks / Like
    Likes (Given)
    12
    Likes (Received)
    35

    Default

    I don' think flipping the stock around would change the direction of the forces. You're still boring towards the headstock, correct? Boring away from the headstock would change the direction of the axial force.

    Might not be a fair test only taking off .005 when there was .003 taper to start with.

  3. #43
    Join Date
    Mar 2005
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    6,444
    Post Thanks / Like
    Likes (Given)
    2283
    Likes (Received)
    2410

    Default

    Quote Originally Posted by thermite View Post
    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!
    In the places I've worked, foremen don't make decisions like that. They might advise one way or another, but that's a big league decision. Akin to Supe or VP level. And yes, learning to get around those problems does help you start to understand geometry of the machines better and make it easier to combat such problems when they arise. Do they make work in the shop go better or faster? No way. Dealing with them and learning what is going on and how to get around it just makes it so that you can run any POS machine and still put out a good part. But it often costs a lot of time to do that.

    IMO, it is foolish not to set a machine straight if you can, sort of analagous to "penny-wise, pound-foolish." A well set up machine pays dividends in saved time and even sometimes scrapped parts, for a long time. Not a big fan of most of the USW union guys I've worked with. Some are good, many are anything but. I have several close personal friends that work at USS in management/supervisory positions. They call it "babysitting" almost to the man.

  4. Likes tylersteez liked this post
  5. #44
    Join Date
    Oct 2005
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,706
    Post Thanks / Like
    Likes (Given)
    562
    Likes (Received)
    665

    Default

    Late to the party and too lazy to read all the replies but has the lathe ever been properly levelled and headstock aligned with a test bar?

  6. Likes Tyrone Shoelaces, Tom-AMS liked this post
  7. #45
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    27,435
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    6406

    Default

    [QUOTE=crossthread;3812229]Last night I had a bit of time and put a 1" chunk of 12L14 in a collet to eliminate any wonky chuck goings on. ...

    Turn the two collars on that stock, no tailstock used. Bet it shows the same taper you see when boring.

  8. #46
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    21,091
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    12321

    Default

    Quote Originally Posted by eKretz View Post
    In the places I've worked, foremen don't make decisions like that. They might advise one way or another, but that's a big league decision. Akin to Supe or VP level. And yes, learning to get around those problems does help you start to understand geometry of the machines better and make it easier to combat such problems when they arise. Do they make work in the shop go better or faster? No way. Dealing with them and learning what is going on and how to get around it just makes it so that you can run any POS machine and still put out a good part. But it often costs a lot of time to do that.

    IMO, it is foolish not to set a machine straight if you can, sort of analagous to "penny-wise, pound-foolish." A well set up machine pays dividends in saved time and even sometimes scrapped parts, for a long time. Not a big fan of most of the USW union guys I've worked with. Some are good, many are anything but. I have several close personal friends that work at USS in management/supervisory positions. They call it "babysitting" almost to the man.
    "Penny foolish AND pound foolish" some of us rated our employers! Lots of us knew HOW to improve a machine's situation. Damned few were PERMITTED to do.

    To be fair.. as to Union competence, in general? It wasn't that way 50-60 years ago. Folks still had 'standards' to uphold. And pride.

    Rather a lot more pride ....and a great deal less greed and sloth, actually.



    And the lathes & mills?

    Newest one Galis had was Korean War surplus (one lone Cincinnati piss-ant). "Best" were War Two surplus (L&S, several).

    The ones that separated the men from the boys? War ONE and prior through early 1920's vintage (Niles..).

    Irony? Nearly all of those machine-tools were newer THEN than half the "all manual" lathes and mills we are jawing about NOW are .. as to years since left the factory, new!

    As-of 1960, a built-1935 L&S was only 25 years old, and a 1916 Niles only 44 years old.

    WTH? Even some of the "CNC" rigs PM members are earning a crust with NOW are more than old enough to vote!

    Not unusual some are older than their operators?


  9. Likes eKretz liked this post
  10. #47
    Join Date
    Jun 2012
    Country
    UNITED STATES
    State/Province
    Michigan
    Posts
    14,248
    Post Thanks / Like
    Likes (Given)
    4855
    Likes (Received)
    5137

    Default

    Quote Originally Posted by BobM3 View Post
    I don' think flipping the stock around would change the direction of the forces. You're still boring towards the headstock, correct? Boring away from the headstock would change the direction of the axial force.

    Might not be a fair test only taking off .005 when there was .003 taper to start with.
    Normal boring one has the bit/insert facing the operator, so forces go that way.
    Normal turning one has the bit/insert facing away from the operator, so forces go that way.

    OP says the machine cut straight when turning and at taper when boring, .002 in a 5" bore.

    if there was .0004 per inch sloop in something it might get pulled/pushed the other way.
    If a plane bearing spindle some error might come from bearing clearance with push/pulling the other way.

  11. #48
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    2,931
    Post Thanks / Like
    Likes (Given)
    241
    Likes (Received)
    1375

    Default

    Yes I agree Buck. I am going to turn everything around (bore from the backside by turning the bar 180 degrees as someone suggested.) I check the lathe every couple of years to make sure nothing has settled although it is on concrete piers. My precision level indicates no twist nor do I suspect any because It turns OD fine and test bar indicates no taper. It must be something in the geometry of my lathe however. I am going to do the above test and if that reveals anything then I am going to dig into the saddle jibs. The question continues to loom why things would change during the course of the bore. If something is wrong in the saddle then why doesn't it start out wrong?

  12. #49
    Join Date
    Jun 2012
    Country
    UNITED STATES
    State/Province
    Michigan
    Posts
    14,248
    Post Thanks / Like
    Likes (Given)
    4855
    Likes (Received)
    5137

    Default

    Rather than wasting a bore just OD turn a 5" slug with the normal way taking .03 and then .002, then face the tool bit from the backside and spindle reverse and do the same 5" .030 and then.002. You might even have a part/job that needs to take off .070 or so.

    Should be a quick easy task and let us know what happens.

    Yes, the boring bar's long length is another factor that may play some part of the problem. That may try to twist the tool holder or the saddle. Still if so, twisting from the font/pushing bar toward the operator position like in a turning operation would be better than from the back and pushing bit/insert away.

    A 2x2 pry on your spindle nose might be a good check..let us know what you get in the quick move.
    If a plan bearing do this after a 5-minute warm-up.

  13. #50
    Join Date
    Nov 2008
    Country
    UNITED STATES
    State/Province
    California
    Posts
    2,772
    Post Thanks / Like
    Likes (Given)
    21
    Likes (Received)
    736

    Default

    Get a chunk of round same length as your problem then with lathe in reverse use suspect tools and cut on the back side of object.

    Same forces involved in slightly different spot.

    Sent from my SM-G781V using Tapatalk

  14. #51
    Join Date
    Mar 2005
    Country
    UNITED STATES
    State/Province
    Indiana
    Posts
    6,444
    Post Thanks / Like
    Likes (Given)
    2283
    Likes (Received)
    2410

    Default

    Did you ever do the OD test cut without the tailstock? I am with Jim on this one. Before delving in to all the almost limitless possibilities, begin at the beginning. A light finish cut should have very little tool pressure toward or against the tool if you have a properly sharp tool. Most of the pressure will be pushing down.

  15. #52
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    2,931
    Post Thanks / Like
    Likes (Given)
    241
    Likes (Received)
    1375

    Default

    Yes EKretz in this post I described some tests I made and no tailstock was used. Thanks.


    Quote Originally Posted by crossthread View Post
    Last night I had a bit of time and put a 1" chunk of 12L14 in a collet to eliminate any wonky chuck goings on. Quick test was to drill a 3/4" hole through the stock (stock was sticking out of the collet about 1"). Bored it out about .030" in four passes with the last pass a spring cut. Measured with inside mics and sure enough it was .003" smaller toward the headstock. I flipped the stock around and bored from the other end (.005" cut). Ended up with essentially no taper (.0003"). It seems like it has to be something going on with the direction of forces as others have said although I can't figure out why the forces would change as the boring bar approaches the headstock. Tonight I will try to run everything in reverse from the backside of the bore and see what happens. Just for grins before I ran this test I put a section of ground stock in the collet and a Starrett DTI in the tool post and ran it from about 4" out all the way to the collet (lathe off of course). I got .0004" of taper. I can live with that. Later today I will go out and sacrifice one of my chickens and see if that helps.

  16. #53
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    27,435
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    6406

    Default

    You need to do an OUTSIDE DIAMETER turning test with no tailstock. Until you do that and can prove the lathe turns OUTSIDE DIAMETERS with no taper you are just flailing around.

  17. #54
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    2,931
    Post Thanks / Like
    Likes (Given)
    241
    Likes (Received)
    1375

    Default

    Yes Jim. I understand. As stated earlier, my lathe does not turn OD tapers. I have turned some collars on a test bar but also turned the OD using a collet. No taper.

  18. #55
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    27,435
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    6406

    Default

    Quote Originally Posted by crossthread View Post
    Yes Jim. I understand. As stated earlier, my lathe does not turn OD tapers. I have turned some collars on a test bar but also turned the OD using a collet. No taper.
    You did not say that. You said you had a far end supported by a center on the test bar.

    "I test if from time to time by turning collars on a test bar between centers. Half a thousandths is no problem."

  19. #56
    Join Date
    Aug 2004
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    2,931
    Post Thanks / Like
    Likes (Given)
    241
    Likes (Received)
    1375

    Default

    I stand corrected. I did not mention that. I do however turn the OD of a lot of stock with chucks and collets and never get a taper. In my original post I stated that my lathe does not turn OD tapers I just did not specify that I do that unsupported with the tail stock but I certainly do. Thanks.

  20. #57
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    27,435
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    6406

    Default

    So you've done the two collar test with you lathe. Two one inch collars separated by six inches, OD turned, no tailstock support.

    What was the difference in diameters?

  21. #58
    Join Date
    Nov 2003
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    6,070
    Post Thanks / Like
    Likes (Given)
    219
    Likes (Received)
    2096

    Default

    The unanswered questions here are: Why do the internal bores taper when external turning does not? And What is changing to make it so?

    Just thinking and I am assuming this is a used lathe where there is more wear on the ways at the position the carriage will be in when it is as close as it can get to a 3 or 4 jaw chuck. At least, that is how the wear on my used lathe is. The scraping marks are clearly visible directly under a mounted chuck and at the tail stock end, but I can clearly see what must be several thousandths of wear in that position immediately to the right of the chuck and this indicates a low spot.

    So, as someone has mentioned above, when you turn an external diameter the tool normally is above the carriage or very close to it.* That means that when making an external cut toward the head stock, it will ride downward a bit as you get closer to the head stock. But this motion will be roughly the same amount as the wear on the ways and it will not be increased by any lever arm. If the lathe was "leveled" and checked for turning a taper, it was very likely set (the ways are slightly twisted) to make this type of cut as straight as possible. The tool may move downward while making this external cut, but the bed twist is keeping it on the same level as judged by the line between centers. Or, at least as close as possible to that. So external diameters do not taper.

    But when you use a boring bar, the cutting edge on that bar is one or more inches to the left (toward the head stock) And the carriage will be tilting toward the left as it rides DOWN the slope in the ways, heading into that worn area. The extended end of the boring bar will have a longer lever arm so it will dip even lower than the carriage. But the bed still has that twist from it's "leveling" that tries to move the center of the carriage toward the rear to compensate for an EXTERNAL cut. So, if the tip of the cutting edge on the boring bar was set on center, then the tilt of the carriage will be moving it lower as the carriage moves and this will tend to increase the diameter of the bore at the head stock end. But the twist in the bed will be trying to move the center of the carriage to the rear and that movement can also be accentuated by the length (lever arm) of the boring bar so it can easily outdistance the movement due to the downward slope. And if the tool's tip is set a bit above the center line of the lathe as sometimes happens, then both of these motions could contribute to the bore having a smaller diameter at the head stock end.

    One quick experiment you could try would be to set that cutting tip of the boring bar a bit below the lathe's centerline. That could counteract the "leveling" twist in the bed and allow the tilt of the carriage as it rides down that slope in the ways to bring the tool's tip deeper into the metal as it approaches the head stock end. This may require a bit more clearance below the tool's tip to prevent rubbing.

    * I am referring here to the shank of the tool, not the actual tip which will normally be extended off the rear edge of the carriage, toward the rear of the lathe. But it is usually not extended very much off to either side of the carriage and is normally situated above it, near the left edge.

  22. #59
    Join Date
    Jun 2012
    Country
    UNITED STATES
    State/Province
    Michigan
    Posts
    14,248
    Post Thanks / Like
    Likes (Given)
    4855
    Likes (Received)
    5137

    Default

    QT: I will go out and sacrifice one of my chickens and see if that helps.

    Just for a little more confusion about forces...,

    If you chop off a chicken head with first your right, and the next chicken with your left hand what way does each chicken first run?

    Only one answer will be correct.

    Don't blurt it out just say "I Know" if you know.

    Re: there is only one correct answer.

  23. #60
    Join Date
    Jan 2007
    Location
    Flushing/Flint, Michigan
    Posts
    10,916
    Post Thanks / Like
    Likes (Given)
    645
    Likes (Received)
    8760

    Default

    Boring bars are not the same as OD holders.
    They bend, they are hanging out so any twist at the saddle is amplified a lot.
    With many light passes and a new sharp tool for finish is there still a taper?
    Up or downhill run as noted by EPA will mess you up big time but this is rare.
    Bob


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •