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Thread: Chuck backing plate question.

  1. #11
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    There is no way that you are going to get an accurate idea of the thread size from an OD measurement. The proper way of measuring a thread is to measure the pitch diameter with thread wires or a thread micrometer and the appropriate tip.

    That being said, yes, it probably is a 1.25" x 10TPI. But fit is everything.

    I have cut internal threads on backplates in my lathe (SB9) and was successful on the first attempt. Sorry Forrest. What I did was to check the progress of the thread as I cut it. I removed the chuck that was holding the new backplate and just tried screwing it on the lathe spindle. This worked like a charm. What you can do is bring the dividing head to the lathe as you cut and try the fit of the thread while the work is still mounted. Always be sure to deburr the thread before checking, yes every time you check.

    As for the bore size, using my tap drill calculate and a percentage of thread of 75%, I get a bore diameter of 1.153". This will leave a 12.5% flat on the crest of the internal thread. This is probably a sufficient percentage, but it is always possible that you could need more. That crest must fit in the root of the external thread on your indexer. If the flat on the crest is too small, that crest can interfere with the root of the external thread and give you a false indication that the thread's pitch diameter is still too too small (for an internal thread) and that you must continue cutting deeper on the thread. This is why thread wires or thread mikes are used to measure the thread's pitch diameter. The nearest standard drill size is 1 5/32" which is 1.156" or slightly over the calculated size: this would do no real harm.



    Quote Originally Posted by RichR View Post
    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
    Last edited by Paul Alciatore; 07-19-2017 at 03:09 PM.
    Paul A.

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

  2. #12
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    This is Great info Gang.

    I just got my HSM magazine and guess what, an article about measuring threads. In the article it talks about the 3 wires being the correct size. I do not have a set of measuring wires, but I remember reading in past posts about using drill bits. In the table for 10TPI it says .055, I don't have any that small, will .035 MIG wire work?

    I will measure the ID tonight using the 3 wire method and see what I get if the .035 wire will work..

    The tool is Asian made with a California tool Co. label on it. The screws are metric allen heads and threads. This makes it a best guess as mentioned earlier about Asian and imperial threads not being consistent.

    I used a Starrett thread gage to measure the pitch, but I did not look for pitch degrees and clearances to the gauge. I did look at the tops of the treads with my 10x loupe and they do have a small flat top on them consistently all the way. The suggestion of using the mill and a DTI may be the other way to try and get a ID reading.

    Forrest, I do plan on a sample piece first, and even a plug test piece to check everything as I go. I will pull the shaft out of the dividing head as a final check when I cut the back plate for the final time.

    Again I appreciate the help.

    TX
    Mr fixit for the family
    Chris

  3. #13
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    Use 3 wires to get the pitch diameter of the spindle thread. Make a gauge, single point the thread and match the PD of the spindle. Use the gauge to check the threads in the backplate as you machine them.

  4. #14
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    If you are just using the wires to make a COMPARATIVE measurement so you can make a matching thread then it really does not matter what the diameter of the wires are as long as the diameter provides two things. First is that the diameter is small enough that the wires seat against the flank sides of the thread form and not resting on the edges between the crests. Second is that the wires are large enough that the wires stick up higher than the crests of the thread so you can measure the distance between the wires on each side.

    So I suspect that the .035 mig wires will fail on a couple of points. First off is that they will be too small and won't extend up higher than the thread crests. Second is that the consistency of the wire diameter might be suspect.

    On the other hand I suspect that a set of three 1/16 drill bits would work just fine to use the shanks.

  5. #15
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    I wonder if it is 32 X 2.5 metric?
    The thread will very likely be 60 degree, use the male spindle as a thread gauge.
    Last edited by old mart; 07-19-2017 at 04:49 PM.

  6. #16
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    0.057735" is the best wire size for 10 thread per inch.

    That's pretty close to a #53 drill which is 0.0595 on the chart. A 1.5 mm drill is 1.590. A standard twist drill has "back taper" that is the lip size on the chart size but the shank might be several thousandths smaller. This reduces binding with the hole wall. The #53 drill might measure 0.0595 at the lip but the shank size may vary depending on manufacturer. Cheap drills may not have a back taper. The drill size may be stamped in the shank. If so, there will be raised metal surrounding the stamp impression. This should be lightly stoned off (not filed) before using the shank diameter for measuring purposes.

    Small number drills are cheap. Go to your nearest fastener store and buy them. You might find an envelope of ten is about the same price as three separately. That is if you have a use for ten,

    Since your checks will be comparative the actual wires size within a range will work. The wires don't even have to be all the same size provided they are in range and used in the same positions during the measurements.

    Don't pick a wire too small because the top of the wire will fall below the crest of the thread. A wire too large may not contact the flanks. Also wires differing from the best wire size are influenced by minor flank angel error. There is a range of wire sizes between which satisfactory pitch diameter checks can be made.

    The best wire size contacts the thread pitch diameter. Over and under sized wires contact over or under the pitch diameter (duh.)

    The best wire size is 0.5773 (Tan of flank angle times thread pitch.)

    The range for 60 degree thread wires is 0.056" to 0.090" These figures are from Machinery's Handbook(16th Ed) and corss check with ASME B1.1 (2003).

    Before you jump into cutting threads you might review a comprehensive machine shop reference like "Machinery's Handbook" in which there are many pages of charts and narrative on the topic of screw threads. Cutting a thread to fit a thread is no big deal but it's nice to have a background before you start.

    Look before you leap. A monkey see - monkey do is a hard slog to competence.
    Last edited by Forrest Addy; 07-19-2017 at 05:52 PM.

  7. #17
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    When i had to cut the spindle thread for my er collet chuck . I bored the minor diameter then a counter bore .002 larger than thread diameter one full pitch deep. the counter bore was blued then cut threads until i just scratched the blueing. collet chuck fit up great with zero run out. john b.

  8. #18
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    just to parrot what others have said - make a spindle copy (good practice too!) and cut the counterbore and look for scratching at full depth. Remember to set your compound at 29.5deg the other way (pointing at the wall behind your tailstock instead of the wall behind your headstock), set the cross slide to zero when the tool touches the bore and then increase depth of thread cut using the compound. PVC pipe is great stuff to practice threading on, cuts really easily and it's cheap as chips.

    Take your time and you'll be fine - it's surprisingly straightforward.

  9. #19
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    I'll just toss in a reminder for doing internal threads that you allow some space for the cutter to come out the other side and not run into the chuck body.

  10. #20
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    Hello Group,

    Resurrecting an old post as an update and a second question. After a long time I've gotten back to the dividing head and the chuck backing plate. I tossed in the towel and had the threading done by a local machine shop after many attempts with sample pieces, just couldn't get it to mate correctly. Anyway now that I have the backing plate threaded and it fits the dividing head shaft, my next step is to cut the face of the backing plate to register the chuck on center. I've done this and it fits real well and has less than 2 thousandths runout when held in place by a live center in the tail stock and a piece of round stock in the chuck.

    OK the second question. How do I get the mounting holes in the back of the chuck (they do not go all the way through) transferred to the backplate so I can drill, countersink, and bolt it to the backplate?

    Look forward to any help or suggestions. Yes I took the lazy way out on the threading.

    TX
    Mr fixit for the family
    Chris
    Last edited by Mr Fixit; 08-15-2019 at 01:38 AM. Reason: spelling

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