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Thread: Internal Thread Cutting

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  1. #1
    Join Date
    Nov 2008
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    28

    Default Internal Thread Cutting

    When cutting external threads you set the compound to 29.5 degrees, and advance into the cut with the compound. This causes the cutting to occur on one edge of the thread, and the other edge is cleaned up with the 0.5 degree difference between 29.5 and 30 degrees.

    What is the preferred method of cutting internal threads? One way would be to reverse the lathe's direction of rotation and cut the far wall of the tube, leaving leave the compound at 29.5 degrees, keep the same cutting direction as you use on an external thread (from the end of the tube inwards). This would require a right hand threading tool.

    A second way would be to set the compound to 60.5 degrees beyond the 90 degree point, and again advance the compound to deepen the cut.

    I suppose the simplest is to use the cross slide to deepen the cut, however this cuts both side of the tread at the same time, perhaps leading to poorer threads.

    What method is preferred?

  2. #2
    Join Date
    Aug 2006
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    Taylorsville Ky
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    when you cut a right hand external thread the compound is angled to the front right 29/29.5/30 deg as you like and the thread is cut toward the headstock. The compound is used to advance the depth of cut and the crossfeed is used to back the cutter out of the thread and is usually set at "0" on the dial and returned there to start the next pass.

    For an internal thread you can set the angle of the compound to the front left or to the rear right. The compound is still used to advance the cutter into the thread for each pass.

    I prefer to set the compound to the front left as long as I have clearance to do so. Sometimes the compound will have to be set to the right rear to clear the work or the lathe.
    It's only ink and paper

  3. #3
    Join Date
    Mar 2005
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    If I'm cutting a matching bolt and nut I leave the setup as is and reverse the spindle for the nut. Otherwise I don't have a preference.

  4. #4
    Join Date
    Jan 2009
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    Southern WI
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    Default

    And when setting the compound to 29.5 degrees, I was taught to take the last couple of thousandths with the cross slide to clean up the thread profile on both sides of the vee.

    Keith

  5. #5
    Join Date
    Feb 2004
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    1,017

    Default

    If I do any internal threading I just turn the tool upside down and thread the back of the bore instead of the front.

    The lathe turns the normal way,the cross slide or saddle moves in the same direction towards the back of the lathe as doing external threads and the chips fall away from the tool to the bottom of the bore.
    It makes it easier to set the tool at tip at 90 degrees and you can see what's happening easier.

    Allan

  6. #6
    Join Date
    Sep 2006
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    Southwestern Ontario, Canada
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    Default

    Quote Originally Posted by Allan Waterfall
    If I do any internal threading I just turn the tool upside down and thread the back of the bore instead of the front.

    The lathe turns the normal way,the cross slide or saddle moves in the same direction towards the back of the lathe as doing external threads and the chips fall away from the tool to the bottom of the bore.
    It makes it easier to set the tool at tip at 90 degrees and you can see what's happening easier.

    Allan
    Won't this try and "lift" the saddle up

  7. #7
    Join Date
    May 2002
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    Beaumont, TX
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    7,260

    Default

    Quote Originally Posted by Carld
    .......

    For an internal thread you can set the angle of the compound to the front left or to the rear right. The compound is still used to advance the cutter into the thread for each pass.

    I prefer to set the compound to the front left as long as I have clearance to do so. Sometimes the compound will have to be set to the right rear to clear the work or the lathe.
    But remember that the tool would need to be sharpened either left or right hand depending on which angle you choose.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  8. #8
    Join Date
    May 2003
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    52N 122W Western Kanuckistan
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    When doing internal threads I just advance with the cross slide. With a well sharpened tool it works quite well enough. Just remember that the deeper the tool cuts the more it cuts so reduce the advance correspondingly.

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  9. #9
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    Nov 2002
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    Kenosha, not the pass the other one
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    I normally would just advance the cross slide (or is retract????????) as it gives me more posite control as to the end point of the threads if there is a relief groove at the end of the threads. Another reason also for cxutting on the backside with the spindle in reverse. With a properly ground tool bit, good still boring bar and sufficent cutting lubricant this should pose no real problem. Now these are internal threads. Not the high buck 6" scale

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  10. #10
    Join Date
    Mar 2005
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    Quote Originally Posted by Spin Doctor
    Another reason also for cxutting on the backside with the spindle in reverse. With a properly ground tool bit, good still boring bar and sufficent cutting lubricant this should pose no real problem. Now these are internal threads.
    You wouldn't happen to have a picture of the cutter used for that? I'm grinding one (several attempts so far) to do the same.

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