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  1. #1
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    Default Lathe chuck forcing work piece out???

    This is a new weirdness I encountered yesterday.

    Was machining 30mm dia. 4140 down to a 16mm rod, in my Hercus 9C. Fairly heavy cuts – between 25 and 40 thou. Got a few nice long blue spirals peeling off, until the diameter got too small, tip lost its edge, and started abrading.

    Then I stopped lathe, and looked closely. The job had managed to wiggle about 4mm out !

    IMG_20200521_103924_490.jpgIMG_20200521_104342_513.jpg

    Jaws were still tight on. I just continued roughing down with smaller cuts.


    Now, if I was cutting "the wrong way" (i.e. away from the chuck), I could understand it, but most of these were feed in, disengage half nuts, hand feed close to chuck, then either stop to return carriage, or pull tool post toward me while winding carriage back for another pass.


    Am I going crazy? Or is my job somehow orbiting around between the jaws and gyrating against the cutting pressure?





    P.S. Job is to make a new brake pivot pin for Hafco 336. Here I am doing dodgy milling of flats on the pin:
    IMG_20200521_155331_844.jpgIMG_20200521_155357_587.jpg
    Taking that photo, I noticed how small the shavings were, so put a plastic bag over the bed.

  2. #2
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    Default

    Not sure what may be happening there , unless the 3 jaws are not gripping your material fully along there length.

    If you need to do something similar again try using a 4 jaw if you have one.

    The finish on your material does not look very good for 4140.

    Is that a negative rake tool you are using?

    In your comments you mention pulling the tool post away from the work, can you explain what this means?

  3. #3
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    Like pipeclay wrote the finish is not very good for 4140, looks like tearing. Maybe the insert was cutting on the side rather than the tip? When I turn away from the chuck I use a left hand tool, or opposite to the normal tool. What RPM were you running and can you tell us the feed rate?
    4 Jaws grip way better. If grip is a problem try some fine brown lunch bag paper between the jaws and work piece.

  4. #4
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    Hi Peter.

    1. Don't have a 4 jaw for the Hercus, and this 3 jaw isn't great.
    It could be the jaws need regrinding,
    or it could be what I was gripping wasn't parallel enough,
    but either way; I did clock it, and tightened up well.



    2. Finish, yes its bad. Still working that out. Is sometimes OK at larger diameters.


    3. Rake? IMG_0016.jpg
    I guess that depends on how new the insert is?
    Given that, a few days ago when facing a disc of HARD 4140, this tool was making fireworks*, it wasn't new

    * Sparks, red-hot shavings burning the smaller steel wool shavings


    4. Pulling toolpost away. This is a new trick I learnt from a toolmaker.

    Make a cut. and then grab the top of the toolpost and pull towards you. The tool will move away from the job by at least a few microns, or a few thou on my sad old Hercus. This gives you enough clearance to wind the carriage back for another cut.

    All lathes I have tried seem to do this to some extent – combination of some flex in the tool post, and rocking the carriage on the bed – but I have stayed below 2m long. A larger lathe might need some leverage

  5. #5
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    Flip the tool upside down, set the new centre height, reverse the operation and try again.
    Or just turn use conventional approach towards the chuck.

  6. #6
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    Quote Originally Posted by topari View Post
    Like pipeclay wrote the finish is not very good for 4140, looks like tearing. Maybe the insert was cutting on the side rather than the tip?
    I tried a few different orientations. The natural:

    Screen Shot 2020-05-22 at 9.22.56 am.jpg

    and turning it 45° to the right (which should clear equally well facing left or right?)


    At 40thou, though, definitely cutting a bit on the side.
    The radius on these things is meant to be 0.4mm (seems to be 0.5), so most of that chip is from the left edge?



    Some of the earlier chips (earlier on) seem fairly nice. e.g.IMG_0017.jpg:



    What RPM were you running and can you tell us the feed rate?
    4 Jaws grip way better. If grip is a problem try some fine brown lunch bag paper between the jaws and work piece.


    This Hercus tops out at 628RPM, which below 25mm diameter isn't much?

    Feed rate? I tried varying it manually, but got no improvement.
    Auto, stud gear is 18teeth, so that is about 2.5thou per revolution.

  7. #7
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    Quote Originally Posted by topari View Post
    Flip the tool upside down, set the new centre height, reverse the operation and try again.

    Or just turn use conventional approach towards the chuck.

    I was actually machining the correct/conventional way (towards the chuck).


    But, thanks for the interesting suggestion.

    a) On this lathe (Hercus, screw chuck), not possible.

    b) On the one I am machining a part for (Hafco 336, D4 cam lock chuck), I might try that?

    c) What does that achieve, apart from letting small chips fall away easier?

  8. #8
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    I think you are using the wrong tool holder if you want to turn away from the chuck. The nose of the insert needs to lead.

    When I use carbide I have matching left and right hand tool holders for the same inserts.

  9. #9
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    That tool is negative rake, it appears to be WMNG type insert, it puts a lot of load on your Hercus motor.

    If you can get hold of some tooling that accepts CCMT, TCMT type cutting tips this will provide positive rake tooling for you reducing the load on your lathe.

    Depending on your motor size at the appropriate speed and feed or there about you should be able to take cuts upto 2mm/.080" deep with small blue chips. For the Hercus I would suggest if you change your tooling to go with a tip Nose Radius of .2 .

    Did your tool maker friend explain that pulling the tool post away by hand is really only allowing any back lash in your Compound/ Crosslide to allow it to clear. This in my opinion introduces a lot of problems if you are trying to achieve a finished dimension, I believe you would be better off just retracting the tool with the Compound/Cross Slide and then setting another cut.

  10. #10
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    If you were using HSS the cutting edge will be on left when you turn towards the chuck. If you want to turn away from the chuck you need another HSS bit with the cutting edge on the right. You cannot use the same cutting geometry to go both ways. I have bored out motorcycle hubs where all turning is away from the chuck. This way one cannot over shoot and cause damage on rare parts.

  11. #11
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by nigelpearson View Post
    Hi Peter.
    It could be the jaws need regrinding,
    or it could be what I was gripping wasn't parallel enough,
    but either way; I did clock it, and tightened up well.
    I'd say the movement of the workpiece out is the result of all of the above.

  12. #12
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    Quote Originally Posted by pipeclay View Post
    That tool is negative rake, it appears to be WMNG type insert, it puts a lot of load on your Hercus motor.
    Flat belt, so I have an inbuilt clutch


    I did actually monitor RPM for the deeper cuts. Dropped from 628 to 619. Cadet 1/2HP 3Ø motor. Is that too much load on the motor? I was more worried about the pressure in the cross-slide screw.



    If you can get hold of some tooling that accepts CCMT, TCMT type cutting tips this will provide positive rake tooling for you reducing the load on your lathe.

    Depending on your motor size at the appropriate speed and feed or there about you should be able to take cuts upto 2mm/.080" deep with small blue chips. For the Hercus I would suggest if you change your tooling to go with a tip Nose Radius of .2 .

    Some of my inserts are .2 radius. Most of my cheap holders are negative rake on the insert – only a parting tool, and a Sandvik T-MAX S CTDPL 1212F 11 have a few degrees positive.

    I have always assumed that the moulded edge & chip breaker on the inserts provides enough rake to cut in cleanly for small cuts?



    Did your tool maker friend explain that pulling the tool post away by hand is really only allowing any back lash in your Compound/ Crosslide to allow it to clear. This in my opinion introduces a lot of problems if you are trying to achieve a finished dimension, I believe you would be better off just retracting the tool with the Compound/Cross Slide and then setting another cut.
    If you are cutting, the tool is still loaded against the cross-slide screw! (i.e the back lash is being taken up by the cutting pressure)

    Try it next time you are on a lathe. Auto-feed a very gentle cut, and pull the tool post away from the job. Depending on how rigid your post is, you should be able to zero-depth the cut !

    Assuming the cross-slide wheel is tight, its screw will not rotate. When you take your hand off and release the tension on the post, it usually springs back to the original cut depth.

  13. #13
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    Ever removed a round stake from the ground by pushing it offcenter and then stirring it in a circular motion?
    I think the same is happening here.
    Jaws aren't good, likely bell-mouthed a bit, force on the workpiece pushing it off center and the rotation all combined.

    I'd think supporting the end with a center would stop it walking, mainly from the support perspective, but also as a axial stop.
    My guess is a fixed steady would also prevent it, even without the axial stop component.

    Steve

  14. #14
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    Quote Originally Posted by topari View Post
    If you were using HSS the cutting edge will be on left when you turn towards the chuck. If you want to turn away from the chuck you need another HSS bit with the cutting edge on the right. You cannot use the same cutting geometry to go both ways. I have bored out motorcycle hubs where all turning is away from the chuck. This way one cannot over shoot and cause damage on rare parts.
    Indeed. My favourite way of external threading is to use an internal tool on the outside rear, cutting away from the chuck on a reversible lathe. The only way I trust myself to use automatic feed!

    In terms of left+right cutters, what about these:

    Screen Shot 2020-05-22 at 10.16.16 am.jpg

  15. #15
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    Quote Originally Posted by OxxAndBert View Post
    Ever removed a round stake from the ground by pushing it offcenter and then stirring it in a circular motion?

    Excellent analogy!


    Thanks Steve (and all the contributors to my weird little challenge)

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