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Thread: Chuck backing plate question.

  1. #1
    Join Date
    Nov 2010
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    Default Chuck backing plate question.

    Hi Gang,

    Back in January I posted about a dividing head I got. I have measured the nose of the shaft and it reads 10TPI x 1.214" OD. I'm just not finding any parts with the threads that I measure.
    Now, I have looked for a small 4 jaw independent chuck and backing plate for this, and I purchased a 1" x10TPI back plate, but of course it does not fit.

    Is my only choice to make my own backing plate to fit, by boring the 1"out and cut threads for the 1.214 x 10TPI threads? What size of hole do I bore to?
    I've never cut internal threads, and I don't want to waste the cast backing plate. I do have some aluminum 6061 stock that I could make a test piece from, but I'm not sure where to start.

    Looking for some guidance here for a novice HSM that is willing to try. Southbend 9A lathe, HSS tooling, and BP clone mill for machines to work with.

    TX
    Mr fixit for the family
    Chris

  2. #2
    Join Date
    Aug 2004
    Location
    York, PA
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    166

    Default

    Mr Fixit,

    If you have never cut internal threads, you need to try it on a scrap piece of material. When you are ready to move to the backing plate, make sure the face and chuck register journal are running true on your lathe (if they are finished). If not, you can re-cut them later.

    Next thing, you need to measure you dividing head spindle thread. It does not look standard to me. It is neither imperial or metric. You need to get the OD, ID and pitch of the thread.
    The ID may be difficult to measure. I would remove the spindle (you need to do it anyway to try the backing plate fit before you remove it from the lathe) and measure it on my microscope. The alternative method is to set it in your mill vise and use a dial indicator with a sharp point to check the height of the thread. OD - 2*height = ID.

    Next open up the bore in your plate to about .020-.030" bigger than you measured spindle thread ID. Accuracy is not very important here.

    Next start cutting your thread with a tool, which has a little sharper point than your spindle thread. Go approximately as deep as your measured thread height. When you get close, start checking the fit with your spindle (which you removed before hand). After the tread is done don't forget to cut the face, which contacts the spindle.

    If needed, you can set the spindle on the lathe between centers with the backing plate assembled and cut the chuck mounting surfaces to make sure they are true.

  3. #3
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    Long Island, N.Y.
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    Default

    It's probably 1.25X10TPI. This thread should be of some help, it even deals with the size in question:
    http://bbs.homeshopmachinist.net/thr...130#post947130

  4. #4
    Join Date
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    Default

    Standard method to determine hole size for an Imperial thread is to divide 1 by the thread pitch and subtract that from the OD. In your case 1/10=0.1" hole size would be 1.114". I do agree the thread is likely 1-1/4"-10 unless it is an Asian import, then all bets are off. Make sure to confirm that it is a sharp V-60* thread form.
    Jim H.

  5. #5
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    Jan 2002
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    Huntsville Ala
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    5,878

    Default

    Quote Originally Posted by JCHannum View Post
    Standard method to determine hole size for an Imperial thread is to divide 1 by the thread pitch and subtract that from the OD. In your case 1/10=0.1" hole size would be 1.114". I do agree the thread is likely 1-1/4"-10 unless it is an Asian import, then all bets are off. Make sure to confirm that it is a sharp V-60* thread form.
    JCH, don't you mean ..."divide 1 by the TPI and subtract that?" In other words subtract the pitch.

    I thought that rule of subtracting the pitch holds equally for all 60 degree form threads.

  6. #6
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    Quote Originally Posted by JCHannum View Post
    I do agree the thread is likely 1-1/4"-10 unless it is an Asian import, then all bets are off.
    I considered the possibility of a metric spindle. The difference in pitch between 10TPI and 2.5mm is only 0.0015" and might go unnoticed when
    measuring. The diameter works out to be 30.84mm giving you 31 x 2.5mm if there were such a thing.

  7. #7
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    Aug 2004
    Location
    York, PA
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    Quote Originally Posted by RichR View Post
    I considered the possibility of a metric spindle. The difference in pitch between 10TPI and 2.5mm is only 0.0015" and might go unnoticed when
    measuring. The diameter works out to be 30.84mm giving you 31 x 2.5mm if there were such a thing.
    There is no such thing as 31 mm standard metric thread. That is why I think it is most likely imperial 1 1/4-10 thread, but since good fit of the adapter plate to the spindle is required the thread needs to be treated as special. The measurements of the spindle thread OD, ID and pitch are required and the plate can be threaded to fit.

  8. #8
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    Thread pitch/TPI early in the morning yet. The caveat about Asian threads is that they can be an approximation, and be neither Imperial or metric. Checking thread profile and count with a good quality thread gage should clear up any doubt.
    Jim H.

  9. #9
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    It very likely, almost certainly in fact, is a nominal 1 1/4-10 size. The difference being that the crests of the thread have been dressed off by .018". Likely to make the crests less prone to dings during handling. Or perhaps to give any crud in the threads somewhere to squeeze into. And then there should be a section at the base of the thread which has a shoulder and face for the final ultimate register just like on a lathe spindle.

    Faced with the same issues of making my own tooling to fit my lathe headstock the first thing I did was make an identical threaded test stub that also has the same size shoulder and face register surfaces. I then used this test stub during the internal threading on a new face plate and the 5c collet holder I made. It's a rather handy thing to have for this sort of work. In your case the spindle isn't buried inside a chuck but it's pretty hard to hold up the dividing head and offer it up to the thread you are cutting and have any sort of meaningful feel for the fit of the thread.

    The trick to exactly match the thread of the dividing head on your test stub is to use the three wire method for matching the thread diameter.

    A trick you could use for obtaining the ID to bore out the face plates to for starting the internal threading is to make your threaded stub a few threads longer than needed at first. Once you have a good match using the three wire check then cut away the first couple of threads down to where you just have a scratch mark of the thread root left. Measure that and make note of it. Cut off that excess from the stub so you have the proper length to match your dividing head. As a final touch I'd say stamp in a note on the end that says something like "BORE ID 1.xxx" from that last measurement.

    With this test stub you've got a handy way to gauge your thread fit on any face plate you make and can even test the registration shoulder for a nice firm hand turn fit as well.

    I like the thread to spin on easily and have just a slight, barely felt amount of play. That way the registration shoulder fit isn't hampered by the thread and can do its job.

  10. #10
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    Minor diameter is simple: nominal diameter - the thread pitch. In this case 1.214 - 0.010 or 1.114" That's the bore size you start cutting threads in.

    But not so fast. You have to be able to check the thread you're cutting. You can't measure it directly with equipment you're likely to have on hand. You have to try the mating thread (try the dividing head spindle as you get to what you think is the finish size.) Trying to screw the whole dividing head in the thread you cutting is plain impractical. Disassemble the dividing head to remove the spindle can be done but it's a PITA.

    I suggest you make a gage duplicating the spindle thread. Measure the spindle thread over wires, turn a short remnant of bar stock to 1.214" OD and cut a thread to the same size over the same wires. This little gage is easy to handle and when it fits you know the spindle it represents will fit.

    I know it sounds like extra work but I've cut a zillion oddball threads to fit clumsy stuff to handle and never once missed.

    Don't sweat boring a thread. It's about the same as turning a thread but inside out. I suggest you practice a few on remnant material. Make one of your practice pieces the size of the thread you have to cut, then check it with the gage. This is called "proving the process."

    I've been a machinist since the dinosaurs roamed the earth and I still practice welding, threading, etc if it's been a while since I last used the skill. Make the expensive part right the first time: practice on scrap.

    Noob wood butchers are notorious for painting them selves in a corner. They get a set of plans, buy a load of very expensive wood (figured walnut, quilted maple, etc,) then, after they get all the pieces cut to size, they want to know how to cut their first dovetails - starting with the project parts!!

    Practice, practice, practice.
    Last edited by Forrest Addy; 07-19-2017 at 11:47 AM.

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