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Need Help! Lathe Cutting A Taper!!!

Discussion in 'QUESTIONS & ANSWERS (Get Help Fast Here!)' started by blaser.306, Dec 27, 2016.

  1. blaser.306

    blaser.306 Active User Active Member

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    I need some input and or suggestions. I have a 10x22 king lathe, ( I know , sorry for your luck) all kidding aside. It seems that all of a sudden I am cutting a taper .002" over about an inch, two inches out from the headstock. I am using an ER 40 collet chuck that I built. The work is slender so I am expecting some deflection, but even with spring passes the taper continues. I have made a test bar and re adjusted my tailstock to less that .001 run out turning between centers. I checked for chips under the mounting register and face of the chuck, re adjusted the headstock bearing tightness and can see no appreciable movement there I have done the compound hold down mod for added rigidity, any suggestions where I should be looking. The only thing that I haven't looked into is leveling the lathe bed, and do not know if it can even be done as the lathe is in my basement and not anchored to the concrete ( yet ). As I had mentioned this is a new problem to this machine and I have been running it for 3 years with very little in the way of issues. Any help is greatly appreciated and thanks in advance.
     
  2. 12bolts

    12bolts Global Moderator Staff Member Active Member

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    Are you using the compound or the carriage?
    Check your cross slide gibs

    Cheers Phil
     
  3. blaser.306

    blaser.306 Active User Active Member

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    The cuts being made are using the carriage and power feed. Backlash has been taken up and gibs are adjusted, in both the cross and compound slide.
     
  4. petertha

    petertha Canada H-M Supporter - Premium Content H-M Supporter-Premium

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  5. blaser.306

    blaser.306 Active User Active Member

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    My taper is toward the headstock ( smaller end ) large outer dimention. One thing I do know , after doing the compound mod ( 4 bolt plate ) the backlash adjustment screw is near impossible to keep adjusted. I am thinking of countersinking a button head cap screw over the edge of the adjusting screw for it to lock it in place.
     
  6. petertha

    petertha Canada H-M Supporter - Premium Content H-M Supporter-Premium

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    A smaller headstock diameter & larger tailstock diameters can typically be caused by cutting tool deflecting the unsupported end of work, so its not cutting as much so the net diameter is larger. As it gets to the chuck thats the rigid side so it cant deflect that portion as much. The skinnier the material and/or less tensile strength, the more pronounced taper effect. I'm not sure if that's what you are saying but yeah, that's when tailstock support is for sure required. Or a travelling steady.
     
  7. blaser.306

    blaser.306 Active User Active Member

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    The material is o-1 1/2" diameter with 2" protruding from the collet too small for a live or dead center and definitely to short for a follow rest! will have to try and slow down the cut and use HSS see if that helps, like I had mentioned this just seems to have arisen as of late.
     
  8. 12bolts

    12bolts Global Moderator Staff Member Active Member

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    I'm surprised you cant sneak a tool point in next to a centre. Alternatively can you extend your work piece out further, (say another 1"), and then use a centre but start your cut away from the centre. It will mean burrowing into the work and wasting the extra 1" at the TS end. Perhaps its time for one of these to be added to your collection
    183318952_rhm-26571-type-670-carbide-tipped-half-point-dead-center.jpg

    Cheers Phil
     
  9. Ebel440

    Ebel440 United States Active User Active Member

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    First thing I would do is to try the same cut but with a larger diameter piece of material. That would let you know if its deflecting or a lathe issue.
     
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  10. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    You might try a dab of LocTite on the backlash adjustment setscrew.
     
  11. alan morris

    alan morris United States Iron Registered Member

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    Blaser;
    I had nearly the same issue a few weeks back with my Precision Mathews 11X27. It turned out that my headstock had become misaligned with the bed of the lathe. I found a technique called Rollie's Dads Method for checking the alignment. http://www.neme-s.org/Rollie's_Dad's_Method.pdf
    It worked great for me. I used a plunger style indicator with the tip removed so that the reading was being picked up on the horizontal and not affected by being a little above or below center. I had a piece of 1" stock about 20" long. I was cutting .001" per inch and after tweaking my headstock got it to .0015" in 18".
     
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  12. rock_breaker

    rock_breaker United States H-M Supporter - Premium Content H-M Supporter-Premium

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    I must have overlooked any mention of cutting tool sharpness and it's shape. The height of the cutting tool could be a factor as well.
    Have a good day.
    Ray
     
  13. Tozguy

    Tozguy Active User H-M Supporter-Premium

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    How sure are you that your ER40 chuck is holding tight enough?
     
  14. Tozguy

    Tozguy Active User H-M Supporter-Premium

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    Do you mean that you have done a similar turning job in the past without getting a taper?
     
  15. blaser.306

    blaser.306 Active User Active Member

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    Yes, I have turned many progects with no perceivable taper, I plan on spending the day cleaning and inspecting today . I will report my findings. As before ( good cuts ) I was using ccmt 2151 tooling @ 1200 rpm and .010" depth of cut.
     
  16. blaser.306

    blaser.306 Active User Active Member

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    A day of cleaning and stewing over the miss alignment issue! I tried cutting with 1" diameter aluminum shaft 6" long protruding, .005" depth of cut @1200 rpm with a brand new ccgt ( aluminum cutting ) insert on a holder that presents a 75 deg. approach angle at the lowest rate of advance per rotation possible . The cut is the same .001"per 2" run or slightly better towards the head stock. I also ran a test indicator along the side of a ground HSS reamer blank and had the same measurable result. My next test will be to take a facing cut and indicate across the end of the piece to see if there is a " taper " there. I talked to the machinery service dept. @ Quality Cutting Tools here in town today and he suggested indicating the side of the 3 and 4 jaw chuck to find if I get similar readings, thus indicating the headstock was actually angled to the side.
     
  17. petertha

    petertha Canada H-M Supporter - Premium Content H-M Supporter-Premium

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    From what you have described, it may well be that your headstock is pointing slightly toward the rear side of lathe. That was my measurable conclusion (starts about post #17).
    http://www.hobby-machinist.com/threads/tail-stock-alignment.53621/#post-446343

    Traversing across the chuck face should theoretically tell you the same thing, however its a shorter distance so more prone to error measurement. It also presumes chuck face is ground perpendicular & sitting 100% accurate on spindle nose etc. Maybe facing off a face plate & measuring dish or V might be more indicative?

    I just received a precision ground test bar, MT3 on one end, ~8" constant diameter on the other. Its supposedly very accurate ...if I believe all those 0.0000 zero's in the ebay advertisement from India :) My lathe headstock is MT5 spindle & it came with a ground MT5/MT3 adapter sleeve. I dry assembled the parts & they stuck like glue so I'm going to install the assembly & traverse down the ground surface bar sticking out with DTI. No chuck or tailstock involved. If that confirms the same taper amount I last saw from my last cutting test, I think this will be a good & quick way to check validate the lathe every so often. MORE often! Its probably not a replacement for actual turning test, but I also think turning can introduce some degree of ambiguity depending on material, diameter, cut, cutter, unsupported length, diameter measurement etc.

    Personally, at least after my own experience, I think this whole lathe jacking thing should be the second thing not the go-to initial fix. For sure the lathe should start out level (meaning without twist) with best instrument you can lay hands on. But a likely culprit is headstock rotation & it only takes a smidge to achieve taper. That was also how the story ended in the 2 YouTube videos I referenced in my post from pretty experienced guys. Put another way, if lathe bed was actually 100% level (non-twisted) & unbeknownst to you the headstock was the culprit, then shim jacking the lathe bed to impart compensating twist now means you have 2 problems fighting one another, not just one root cause. And its probably not doing the bed casting any favors either being twisted outside of how it was initially ground.

    After adjusting mine back I've also been wondering about the set screws that adjust the head in position prior to locking down with vertical bolts. I kind of viewed them as jacking screws t assist rotation which is probably what they are. But if for whatever reason they backed off or loosened on their own over time, then thousands of repeated cutting pressure traverses might slowly serve to swivel the headstock over time? (viewed from top). Who knows on that one, but I would for sure check after moving the lathe. I'm still not finished my alignment tune up but now I'm glad I have at least an appreciation for what's going on.
     
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  18. blaser.306

    blaser.306 Active User Active Member

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    I just came up from the shop, trying to eliminate possibilities. I removed my collet chuck and installed my 3 jaw and repeated the prior tests and measurements. The results were the same larger diameter away form the headstock and smaller near to it. I guess I know what I will be doing tomorrow. Wish me luck.
     
  19. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    All lathes "cut a taper' regardless of cost or age, the question is how much is acceptable to you.

    If your answer is ZERO then you are in the wrong game, however there are several ways to get happy here, one is to purchase a CNC lathe and program the taper out, this is what I do. I never achieve ZERO taper.

    A cylindrical grinder is an option as well, much better then a lathe for this work..

    Good Luck
     
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  20. blaser.306

    blaser.306 Active User Active Member

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    Realistically I am not expecting 0 but 1 thou over 2 inches an inch out from the headstock when added in a linear way adds up to a lot more than 1 thou over the length of my bed 22" .
     
  21. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    There is a grinder in your future.
    I suspect that the set up and material are causing problems, a machine of that size with a collet chuck can't hold over a 1" bar I imagine. As a rule of thumb do not extend a piece of stock more then 3 diameters from the collet or chuck face.
    If using a tailstock then just move it over until the taper is acceptable, done deal.

    I often run a 24" X 100" lathe that will sometimes taper yet not every time, I can turn a 60" long part within .002" on the diameter other times it will be all over the place depending on the stock and setup.

    Again, Good luck
     
  22. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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  23. blaser.306

    blaser.306 Active User Active Member

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    Yes I do agree, there is a grinder in my hopefully not too distant future!
     
  24. petertha

    petertha Canada H-M Supporter - Premium Content H-M Supporter-Premium

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    Aside from my PITA inaccessibility of 1 hold-down bolt to loosen (which is located behind my gears) the actual headstock re-alignment on my Asian lathe was nothing more than tweak of 2 adjusting screws while watching the dial movement. Then you're back in the game & will wonder what all the fuss was about. Now that I know what its all about, seriously, it takes me as long to dial in stock on a 4-jaw chuck.

    Tonight my level arrived, a Starrett 98-12 good for 0.005" per foot. Not over the top precision but good enough for now & price was right. Turns out my guess & by golly foot setting was within a half division so call it 0.003"/ft twist difference between headstock & tailstock over nominal 40" bed. Then I installed my MT3 test bar + MT5/MT3 socket assembly into the headstock. Chuck is completely removed & no tail stock involved. This parallel stub section is only 5" long, but I measured exactly the same headstock rotation direction & amount as my last recorded coupon cutting test when prorated to this length (0.0010" over 5"). This tells me I was chasing my tail on the lathe levelling as suspected.

    As mentioned, I'm not suggesting this MT bar as a replacement for test bar cutting method, but its convinced me of the ease & reliability of spot checking this way. My preference would have been this longer bar (ebay 49$U, same MT3 section but a 245mm = 9.6" straight section) for same price. It would be the exact same procedure but even more accurate because its over double length. Theoretically you could probably go longer yet but a) haven't seen one longer b) its ~1" steel but might start seeing cantilever droop issues towards tailstock so DTI off-centering? c) does it really matter at this point relative to cutting things even tool post grinding because you way past what should be tailstock domain & now that dominates alignment?
     

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