Single point threading help

  1. BelgiqueB Member

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    Novice lathe users, first time threading.

    I keep getting bad to very bad results, the thread is very very rough, looks like it's tearing more than cutting.

    I'm attempting to cut M15 x 1 thread, using a hss bit ground to 60°, 300 rpm, 0,25mm depth of cut. I've tried both the straight in method feeding in with the cross slide as feeding in with the compound at 29°. Both had same result.

    Usually it all goes well till I'm getting near to the final pass, then it starts tearing and often a bit of metal is stuck to the bit (see picture).

    Any advice, thanks Christof?

    20201101_170027.jpg 20201101_170006.jpg 20201101_145455.jpg
     
  2. Brad93

    Brad93 M J B Engineering

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    If it’s just mild steel it never cuts cleanly without upping your SFM.

    you can try grinding a bit more clearance and a bit more positive rake into your tool, this will reduce the shear pressure required. Remember your tool needs to be bang in centre height.
     
  3. Mark Davison Member

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    What grade of steel, en8? If so it does tend to tear
     
  4. waddycall

    waddycall Member

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    Are you cutting with the top of the tool? It looks a bit high relative to the part.
     
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  5. Brad93

    Brad93 M J B Engineering

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    Did you do a few scratch passes to check your gearing was right and you aren’t doing a double thread/cross threading it.
     
  6. Mark Davison Member

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    0.25mm sounds quite deep for a single cut, certainly for a 1mm pitch thread. Have you tried taking shallower cuts?
     
  7. BelgiqueB Member

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    156
    Location:
    Belgium
    First time I ever ground a tool, I'll have to look into it more and 'fine tune'. Tool was on centre height, I double checked that when I had the issues.

    Just mild steel scrap I had laying around.

    The tool is upside down in the holder and I'm threading backwards away from the chuck.

    That I'm sure of, it's not cross threading it's just making very very rough cuts

    I'll give shallower cuts a go, what's the rule of thumb for depth of cut when threading?
     
  8. Brad93

    Brad93 M J B Engineering

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    You’ve actually got heat tint on the tip of the tool, something is definitely off with the geometry.
     
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  9. RichardM Member

    Unknown MS is not something to practice on, but the finish on the diameter doesn't look bad.

    Can you slow it down more, 110 rpm? Take lighter cuts, and allow a few cuts near the end at the same diameter to take any spring out of the material.

    Is the first pic your tool? Are you quenching it when you are grinding? Keep a pot of water by the grinder and dip it frequently to keep it cool. Check you have the correct clearances and you could do with a better finish on the tool, finish it with an oil stone / diamond lap.
     
  10. Kram

    Kram Member

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    Last edited: Nov 1, 2020
  11. Brad93

    Brad93 M J B Engineering

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    Finishing the tool on a stone or diamond isn’t a bad shout. Definitely take a lighter cut.
     
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  12. rory1

    rory1 Member

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    You are at 29degrees to the x axis and not the Z axis?
     
  13. Pete.

    Pete. Member

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    Get a dressing tool for your grinder. Using it will be a revelation once it's dressed. Your tool is burnt along the top edge and ton the tip - an indication of a blunt grinding wheel.

    Also reduce the cut depth as you get deeper into the thread, though unless you have a very light lathe you should be able to cut a 1mm pitch thread straight in without too much trouble.

    I think that most of your trouble lies in the tool grind. Get an eye loupe and really inspect the tip. Don't put a sharp point on it, a tiny flat or a small radius is much better, if you have a sharp tip it'll overheat, break off and start tearing things up.

    Finally, your tool bit looks like carbon steel rather than HSS. If it is carbon steel, getting it blue will ruin the temper so that could be your problem. Carbon steel gives off bright sparks with loads of starbursts whereas HSS gives duller sparks that don't sparkle much at all.
     
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  14. Arc Tourist

    Arc Tourist Forum Supporter

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    A tip I learned from a machinist at Lockheed Aircraft Co. . When threading aluminium, use "Lacquer Thinner" as a cutting fluid; If the tool, tap, or die is in good order, it will give you razor sharp clean threads. It evaporates quickly and leaves no residue.
     
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  15. BelgiqueB Member

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    156
    Location:
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    New day new try :-), more successful this time. Dressed the grinder wheel, reground the bit (definitely HSS, dull sparks), on the lathe I dropped the speed from 300 to 150 rpm and took lighter cuts. Quite happy with the result, M15 x 1 thread. Thanks for all the advice

    :ashamed::ashamed:, compound was the wrong way round.

    20201102_191541.jpg 20201102_191528.jpg
     
  16. Pete.

    Pete. Member

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    Much better well done :thumbup:
     
  17. RichardM Member

    :thumbup:
     
  18. Brad93

    Brad93 M J B Engineering

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    Nice threads
     
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