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Thread: Threaded chuck back plate advice needed

  1. #1
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    Default Threaded chuck back plate advice needed

    I'm making a threaded back plate for this chuck. http://bbs.homeshopmachinist.net/thr...-Help-ID-chuck It's a 1-1/2" 8 thread, I'll be making a threaded plug gage first and measuring with triangles. The chart shows 1.710" measured over the triangles, I checked my spindle and it measures 1.700". My gut tells me to make the gage the same as the spindle, any thoughts? What kind of fit should it be? Stu

  2. #2
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    Quote Originally Posted by Stu View Post
    I'm making a threaded back plate for this chuck. http://bbs.homeshopmachinist.net/thr...-Help-ID-chuck It's a 1-1/2" 8 thread, I'll be making a threaded plug gage first and measuring with triangles. The chart shows 1.710" measured over the triangles, I checked my spindle and it measures 1.700". My gut tells me to make the gage the same as the spindle, any thoughts? What kind of fit should it be? Stu
    Fit can be anything from loose to very loose, doesn't make practical difference.

  3. #3
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    The spindle thread is probably slightly softened (smoothed) on the tips of the threads by .005. I didn't bother making a plug gage, as the chuck I held my back plate with was small enough that I could remove chuck and work to test the fit on the spindle itself. This would work if machined on a faceplate as well. Whether you use a gage or not, pay attention to getting the register correct, shoulder contact true, and then cut it to fit the new chuck after it is running true on the spindle.

  4. #4
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    If you make the fit with 0.0005" clearance, it will always be a PITA to fit. I have 10 backplates with clearances from 0.0005" up to at least 0.010" and they always repeat perfectly. I would aim for 0.002" clearance. The thread should not be too tight and when threading and boring the register, remember to face off the end which contacts the spindle before removing from the chuck/faceplate.
    Screw on the new backplate several times before final facing and producing the chuck register. I set the back gear and use a strap wrench to remove anything screwed onto the spindle, it is less of a strain on the gears.
    Last edited by old mart; 05-20-2019 at 03:51 PM.

  5. #5
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    Yes, you do not have to obsess over the fit of the thread on a threaded back plate. The Vee thread, by it's nature, will self center to a very high degree of accuracy. Leave it a few thousandths loose and you will be fine.

    Also, don't worry about what is called the "register" area. It also can be a few thousandths or even more loose. The Vee threads will handle the centering.

    I am with Gary on just unscrewing the chuck you use to hold your in-progress back plate and turning the combination around to check the fit on the actual spindle threads. It will go back on in perfect alignment with the original set-up and you can continue cutting your thread from there. Just don't take the back plate out of the chuck. That is how I did it: I removed and replaced the chuck/back plate combination several times. There is no real need for a replica of the spindle thread to check your progress.

    They sell back plates that are already threaded and ready to just screw on to your spindle. I guarantee you that they do not hold them to a ten thousandths fit.
    Last edited by Paul Alciatore; 05-20-2019 at 04:27 PM.
    Paul A.

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

  6. #6
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    I have taken a nut of the size I need, weld it to a plate, screw it on the lathe spindle and machine the register. Has worked very well for me.

    Barry


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  7. #7
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    Quote Originally Posted by Paul Alciatore View Post
    Yes, you do not have to obsess over the fit of the thread on a threaded back plate. The Vee thread, by it's nature, will self center to a very high degree of accuracy. Leave it a few thousandths loose and you will be fine.

    Also, don't worry about what is called the "register" area. It also can be a few thousandths or even more loose. The Vee threads will handle the centering.
    The ability of a V thread to self center is over rated. It's centering is only accurate to the clearance between the crests and roots of the mated threads. I learned this while making flashlights which were supposed to have hidden joint s where they screwed together. That's why there are registers and tapers. A slip fit register will repeat very well.

    This assertion is validated by the way the nut on a hardware store bolt will move off center as well as tilt from side to side.

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

  8. #8
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    A lot of good info, I think I have a handle on it, thanks guys.

  9. #9
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    The V thread does a fine job of centering..... WHEN IT IS TIGHTENED.

    What it is like when loose is of no particular interest. A Morse taper rattles all over the place before it is seated.... but if the angle matches, it centers. it will even center if the angle does not match, if held straight, which the back seating surface of the backplate takes care of.

    It is a SERIOUS MISTAKE to make the so-called "register" a close fit unless your machining is essentially perfectly accurate.

    If you do that, you must have the threads, and the "register" very accurately centered to each other, or the two will not agree, and will "fight" each other, probably preventing the rear seating surface from touching evenly, and throwing the chuck out of true. Just use the thread to center, and it will be fine. It will be centered far better than the inherent accuracy of the chuck itself, which should not be counted on to be better than 2 to 4 thou, at best.
    1601

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  10. #10
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    What Jerry said is + 1
    Make sure your thread cutter profile matches the Thread angle ( ie.60 degree) as perfectly as possible
    Make it a loose thread and always FACE the adapter during the the thread cutting process- they must be done at same setup

    Forget the so called Register, that is bogus. My 6 jaw Buck Chuck repeats to under .001" between mounting and Buck has the clearance at .025" on the so called register.

    Rich

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