Single point threading problems

  1. daedalusminos Member

    Messages:
    861
    Location:
    Norwich
    I'm trying to cut a 10 TPI ACME LH thread in EN8 with carbide insert on my Harrison M300 and I'm getting variable results. The photo attached shows the problem with the thread profile - in this photo the chuck is on the left and I'm cutting from left to right with 14deg on the compound.
    Cutting depth on each pass is approx 0.1mm to a depth of 1.3mm then much smaller increments to approx 1.4mm.
    The insert has been squared to the chuck, there is some wear in the bed and cross-slide though for the cut the cross-slide was locked and feed was solely by compound (which had been tightened up). The main leadscrew, thrust bearings and half nuts are in good condition.

    It's almost as if the 14deg is too much but I've double checked that.

    Any suggestions greatly appreciated!

    IMG_20200721_205726.jpg
     
  2. ronan

    ronan Forum Supporter

    Messages:
    9,787
    dublin
    What steel are you cutting ? Some steel will not cut cleanly, tends to tear rather than cut. Are you using cutting oil or suds ? Is the tool squealing when you cut ?
     
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  3. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    You need a better quality of steel I would say.

    Try doing your last cut feeding the cross slide in to take an even cut on all faces?

    Is it a brand new insert?
     
  4. Pete.

    Pete. Member

    Messages:
    9,340
    Location:
    Kent, UK
    Looks like you're not feeding at 14 degrees. The compound should be pointing to about half past 6.

    Either that or there's something screwy going on with the leadscrew.
     
  5. rory1

    rory1 Member

    Messages:
    1,159
    Location:
    Macclesfield
    14 degrees for screwcutting will be 76 degrees on the compound .

    you need to be 14 degrees off the x axis (cross slide axis) the compound is oreintated in the z axis (long travel)
     
  6. pressbrake1

    pressbrake1 Forum Supporter

    Messages:
    2,418
    essex england
    Looks like shim is catching
     
  7. rory1

    rory1 Member

    Messages:
    1,159
    Location:
    Macclesfield
    Show us a picture of your compound angle - because now I’m thinking about it , if you had 14 instead of 76 or vide versa the thread would be butchered
     
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  8. pressbrake1

    pressbrake1 Forum Supporter

    Messages:
    2,418
    essex england
    I’d really check that shim clearance. On small diameter acme threads carbide inserts are a pita.
     
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  9. daedalusminos Member

    Messages:
    861
    Location:
    Norwich
    Steel is EN8, the new insert cuts with a satisfying rasp though surface finish is poor. Compound photos attached.

    IMG_20200721_215816.jpg IMG_20200721_215941.jpg
     
  10. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    Threads in EN8 with carbide inserts always cut poor for me. Sharp HSS would be much better.
     
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  11. pressbrake1

    pressbrake1 Forum Supporter

    Messages:
    2,418
    essex england
    It’s funny I machine acme threads most weeks and even though I say so myself I reckon Ive got it sussed and I’ve been ignored.
     
  12. daedalusminos Member

    Messages:
    861
    Location:
    Norwich
    Sorry PB1...if you're referring to shims under the holder, there are none, the 16mm sq insert holder is mounted straight in the tool holder with the insert height set by the Dickson adjuster checking for brass sheet vertical against small (8mm) diam bar in chuck.
     
  13. pressbrake1

    pressbrake1 Forum Supporter

    Messages:
    2,418
    essex england
    Shim under the carbide
     
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  14. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    :flame:
     
  15. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    What Pressbrake1 is saying is there isn’t enough relief under the carbide insert for the depth of thread. If you unscrew the insert there should (if a good quality toolholder) be a shim of softer steel, a similar shape to your insert.

    check that is not rubbing the part. If it is, dress the shim back a bit on a grinder.
     
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  16. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    See the silver/grey piece of soft steel under the carbide Ti Nitrided insert (gold)
     
    • 7A8A3264-7839-44EE-B2CF-3D1FFA7FB7FF.jpeg
    • 7FD6DD2E-4909-4F87-BE47-FA12E15A40EB.jpeg
    • 76564BA0-5E25-4FA7-B052-DEDD71C72402.jpeg
  17. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    Who was it on here that tried turning some material with the shim :scared:
     
  18. ukracer Forum Supporter

    Have you got am image??
     
  19. Brad93

    Brad93 M J B Engineering

    Messages:
    8,519
    Location:
    Essex
    See my post above
     
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  20. Paul in Wiltshire Member

    Messages:
    56
    Location:
    UK Wiltshire
    Other possibility is that your Belleville washers are loose and slipping. But Pressbrake is the expert. ;-)
     
    Brad93 likes this.
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