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  1. #1
    Doug is offline Diamond
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    First off, my customer wants to know how to correctly spec a thread, 1/2-14 NPT internal, for instance. When he gives the drawing to me I run samples until my thread plug gage labeled "1/2-14 L-1" fits to the depth of the flat ground on it. (as shown in Machinery's Handbook)

    This is all fine as long as I do his parts, what will he specify for shops other than mine who may not have the gage? Maybe tell them to buy the gage?

    With these parts the thread depth is fairly important, there's a sensor in the shallow hole under the threads. We don't want a fitting to screw in too far and jam against the sensor.

    Second question, how do you guys program tapered internal pipe threads? The way I do it is to draw in CAD a line at 1.7833 degrees at the approximate diameter representing the taper with the center line. Establish a start point off the end of the part and an end point in the hole. Use the "X" diameters with their respective start and end "Z" values in the threading cycle. With soft materials no taper is cut in the hole prior to threading, let the threading tool handle that. I creep up on size by increasing the "X" depth value in the thread cycle until the gage fits.

    Funny, in 20+ years of CNC'ing I can only remember a couple times I've had to single point pipe threads. And every time I have to think about it and re-learn my method.

  2. #2
    ARB's Avatar
    ARB
    ARB is offline Titanium
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    Sounds like you have it handled Doug. I single point lots of pipe threads. I bought the gages inside and out for all the threads that I cut. That is the only way that I know to make sure that you are cutting them correctly. I would think that anyone cutting pipe threads for a customer would have the gages if the work is critical.
    As far as start points that is pretty much the way I do it. I just went to lathe training at Hardinge and that is how they do it. They actally had a nice spread sheet that filled in the 2 line G76 for pipe threads. Pretty cool.

    ARB

  3. #3
    mrainey's Avatar
    mrainey is offline Stainless
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    And every time I have to think about it and re-learn my method.
    I know the feeling.


    With soft materials no taper is cut in the hole prior to threading
    Does that work? I know a tap will form the taper, didn't realize that a single-point tool would.

  4. #4
    ARB's Avatar
    ARB
    ARB is offline Titanium
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    If you use full form laydown inserts you can get the thread without pre-turning the taper. Other wise you need to turn the taper first. I use full form inserts with good success.

  5. #5
    mikey B is offline Aluminum
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    Are you cutting NPT threads or NPTF threads? Both pipe threads but big difference.

  6. #6
    mrainey's Avatar
    mrainey is offline Stainless
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    I can't visualize it, but I believe you.

  7. #7
    mrainey's Avatar
    mrainey is offline Stainless
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    Found some Vardex full-profile NPT inserts with the angle built in.

    Now I get it (I think).

  8. #8
    ARB's Avatar
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    The angle needs to be in the threading cycle.

    Hey, wait a minute. Are we talking about a lathe or a mill here?

    :rolleyes:

  9. #9
    Doug is offline Diamond
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    I was talking about on the lathe.

  10. #10
    mrainey's Avatar
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    I drew it out to better understand. I had been concerned about the full-form insert cutting into the taper ahead of it. Looks okay though.

    Don't you have to add a number of passes at the start to get rid of the extra material (with a relatively weak, expensive insert)?



  11. #11
    Ox's Avatar
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    This is all fine as long as I do his parts, what will he specify for shops other than mine who may not have the gage? Maybe tell them to buy the gage?

    Everyone should have their own 1/4 pipe guage if they want to make those parts. It's just part of the job. MUCH more necessary than a guage for a reg tapped hole!


    Are you cutting NPT threads or NPTF threads? Both pipe threads but big difference.

    Only diff between NPT and NPTF is tollerancing. NPTF being the tighter of the two. Any part ran to NPTF guages will easilly pass NPT inspection.


    And every time I have to think about it and re-learn my method.

    Figger it out once and cheat off old prog later!


    Think Snow Eh!
    Ox

  12. #12
    John Michael is offline Senior Member
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    I have been in a lot of job shops around Houston and the custom here seems to be that the customer supplies the gages. Gages need to be regularly inspected and certified and the customer doesn't know if the job shop's gages are up to date or not.

  13. #13
    alphonso is offline Stainless
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    I have been in a lot of job shops around Houston and the custom here seems to be that the customer supplies the gages. Gages need to be regularly inspected and certified and the customer doesn't know if the job shop's gages are up to date or not.
    Particularly true for various API gages. I have always insisted that the customer supply the gages.

  14. #14
    mrainey's Avatar
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    the customer doesn't know if the job shop's gages are up to date or not.
    And the customer doesn't know if the job shop dropped the borrowed gage, "adjusted" it (ring), or tried to use it as a cutting tool to get those last few tenths.

    I'd be leery of a job shop that didn't have NPT gages, at least for the smaller sizes.

  15. #15
    Doug is offline Diamond
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    "I drew it out to better understand. I had been concerned about the full-form insert cutting into the taper ahead of it. Looks okay though.

    Don't you have to add a number of passes at the start to get rid of the extra material (with a relatively weak, expensive insert)?"

    Good questions....

    I used a 60 degree laydown style partial profile insert rated for 8-14 tpi. They don't have the "topping" section on the front edge so no worry about cutting into the taper ahead of it.

    This run I only had a few parts to do in 6061, the insert is plenty strong to "hog" in that material. Besides, if I would have bored a taper first that would have involved more calculating on how much material to remove. As it was I crept up on the size by trial running with progressively deeper vee until the gage fit. Lots of threading passes, with only a few parts it didn't matter.

  16. #16
    Doug is offline Diamond
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    Speaking of thread plug gages for internal tapered pipe threads, it just occured to me my gage doesn't tell me much. It only verifies pitch and depth.

    I've always been gunshy in single pointing these threads that I might calculate the taper angle incorrectly. If the angle is off slightly the gage won't always indicate that.

    There must be other gages needed to verify a critical thread for the correct taper.

  17. #17
    mrainey's Avatar
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    I took a second look - it does appear that the full-form insert is improperly removing material from the next thread.

    Pictured is a 3"-8 NPT, with .100 thread height. The leading edge of the insert is cutting an extra .004 off the top.

    This may or may not be a problem with NPT - makes me wonder, though, about the effect on an NPTF thread, which relies on metal-to-metal contact for sealing.




  18. #18
    elskorcho is offline Junior Member
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    "There must be other gages needed to verify a critical thread for the correct taper."

    We use a taper gage for checking API threads. It's an apparatus with a fixed point and an opposite adjustable point attached to an indicator. You zero the indicator at one point, then move back, say 1", then check again. Kind of hard to explain, but it works pretty well.

  19. #19
    elskorcho is offline Junior Member
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    What brand of insert is that? I've never seen one with a shoulder on the left side like in the illustration.

  20. #20
    mrainey's Avatar
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