Myford spindle information

Discussion in 'General Engine Discussion' started by RonW, Jun 20, 2019.

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  1. Jun 20, 2019 #1

    RonW

    RonW

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    Hi all, I have a vintage Myford 7 circa 1949 and I am trying to make a back plate for a 5c collet chuck. I know the spindle is 1 1/8" x 12 tpi Whitworth form but I do not know what the correct major and minor diameters should be. Has anyone attempted this, I can't be the first, and if so where did you find the information or how do I go about extracting it from the machine itself. Any help would be appreciated.
     
  2. Jun 20, 2019 #2

    fcheslop

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  3. Jun 21, 2019 #3

    Hopper

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    Basic screwcutting. For Whitworth, thread depth = .6403 x pitch.
    So that comes to .053" thread depth.

    Don't get too carried away with trying to cut the perfect thread to exact BS standard threadforms etc. That is the realm, as Martin Cleeve calls it, of "overzealous amateurs".

    Standard procedure is turn the outside diameter slightly undersize. Probably about five to maybe even ten thou undersize at the diameter in question. Then grind up a screwcutting tool bit at 55 degrees and put a small radius on the end by rubbing it on the bench oilstone. The radius on the peak of the threads can be put on by running a file down the thread at the end of the job.

    Aim at cutting to .053" thread depth but when you get close, run the corner of a file down the thread to remove the burrs and try the thread on the lathe spindle by unscrewing the chuck with job complete and turning it around to try the thread. Repeat this fit and try and taking fine cuts until you get it to size.

    Make the thread a slightly loose fit rather than tight. Location is by the plain register collar area on the Myford. This needs to be machined to a nice neat fit.

    LH Sparey's book "The Amateur's Lathe" gives a good run through on all this stuff, specific to old Myford lathes too. Martin Cleeve's book "Screwcutting in the Lathe" is also a standard reference, but delves into more detail than you probably need.
     
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  4. Jun 21, 2019 #4

    Hopper

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    PS, system won't let me edit previous post. If you are cutting a female thread, make the inside diameter of the hole five to ten thou larger than the calculated minor diamter from the formula I gave. This gives tip clearance and ensures threads engage on the flanks, not the tips.
     
  5. Jun 22, 2019 #5

    RonW

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    Thanks Hopper. I guess my biggest question is trying to decide what the major and root diameters will be. This is an oddball size in that its 1 1/8" diameter but 12 TPI so charts of threads don't really help. If I use the outer diameter of the spindle and the internal diameter of the chuck or face-plate is that going to be "close enough" for a good fit? Having already turned part of the blank I'm unsure as to how to proceed without screwing the part. Excuse the pun. I'll consult the library some more and see what the books mentioned have to say.
     
  6. Jun 22, 2019 #6

    Hopper

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    Well, nominally the minor diameter would be 1.125" minus your thread depth of .053" x 2. (As derived from the formula Depth = .6403 x pitch)
    So 1.125" - .106" = 1.019" minor diameter.
    To allow a bit of tip clearance, I would bore the hole to 1.025" diameter.
    Then proceed screwcutting from there, aiming at a depth of .053 but checking carefully for fit from about .043" onwards.

    But don't take my word for it. I majored in English Lit. at college. So do your own calculations and double check before cutting metal. Checking the inside diameter of the thread in your chuck would be a good idea too.
     
  7. Jun 22, 2019 #7

    goldstar31

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    Risking not only censure but being burned at the stake as a heretic, change from 55 degrees coarse Whitworth to 60 degrees United, you have a standard thread.
    How I found out will take too long to explain.

    However firms like RDG supply taps and dies in Whitworth form to supply the needs of many Myford users.

    Myford also designed a 'bastard' tee nut set of dimensions. That is easily explained because of the weak saddles etc. How I found out?????? but there I go again - heading towards the stake!

    Regards

    Norman
     
  8. Jun 22, 2019 #8

    Hopper

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    A second way to double check my calculations above is to use the chart linked to above by fcheslop. It lists a 12tpi thread for nominal 1/2" BSW. So its major diameter is .500". To make it relevant to 1-1/8" diameter, add 5/8" or .625" to the diameters quoted.

    So the thread depth the chart quotes for 12tpi by 1/2" is .053". As per my calculation. Thread depth for a given pitch/tpi remains the same regardless of diameter.

    Then the minor diameter in the chart is .393. Add .625 to that and you get 1.018" minor diameter. Close enough to my 1.019". Then add your tip clearance and you get the 1.025" I calculated.

    So I would call the 1.025" diameter for the hole you bore all good. And thread depth from there of .053", with manual checking at final stages all good too.
     
  9. Jun 22, 2019 #9

    Hopper

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    LOL@ Norman. No stakes involved. There is no doubt about the level of "bastardry" involved in the manufacturing industry. But if you are making your own screwcutting tool, might as well make it 55 degrees to match the spindle. Taps are handy for cleaning up the final thread but I don't fancy the chances of tapping a nice clean 1-1/8" thread dead true from scratch. That's a big tap.
     
  10. Jun 22, 2019 #10

    RonW

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    Thanks guys, I already have a commercially made 55 degree boring bar so hopefully that will produce what I want. I am going to proceed on the 1.025" bore as suggested by Hopper and we'll see what evolves. I have an old super 7 spindle and a RDG produced mount for a rotary table to use as test pieces so here's to careful turning. I'll let you know how it "turns" out.
     
  11. Jun 23, 2019 #11

    bluejets

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    Whether the thread is tight or slightly loose(latter preferred) the above comment is what is important.
    Surely you must be able to get some idea from the headstock spindle thread with verniers...??
    I'd mic the register to be sure but the thread....verniers.
     
  12. Jun 24, 2019 #12

    bazmak

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    For all the fancy geometry and maths just measure the bore on any chuck etc
    and cut the thread to fit the spindle.Its the 1.25 dia and back face that does the accurate locating
     
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  13. Jun 24, 2019 #13

    goldstar31

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    I agree with Barry but a 5" Back plate threaded Myford is only about £23 and I assume that a pair of 'Myford' taps and dies is well in excess of this. Of course, one has to add time, materials and , in this case bought out tooling and a method of fastening the brute onto the faceplate with clamps which probably have to be bought out and to swing a faceplate for screwcutting probably means making a mandrel handle.

    As Hopper rightly mentions I also did 'litt' but I also majored in economics of which cost and works accountancy featured.

    Writing a précis, Not me!

    Norm
     
  14. Jun 26, 2019 #14

    BobsModels

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  15. Jul 1, 2019 #15

    RonW

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    Thanks to all I now have a back-plate blank, mounted to the machine and faced ready to cut the "tenon", for want of the right word, to locate in the rear of the chuck. Its 3.75" x 0.1875", give or take, but the next question is, does the body of the chuck meet with the inner of the back plate (face of the "tenon") or is there a space between the face of the "tenon" and the recess (0.010")so that the outer ring of the chuck body, where the bolts are located, mates with the adapter's outer face. I understand that the "tenon" needs to be turned to accurately fit in the chuck body's recess but which faces meet?
    Ron W The bolt ring or the "tenon"/ recess faces?
    RonW
     
  16. Jul 1, 2019 #16

    RonW

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    I should add that the RDG adapter was a bad choice for checking the threads I cut as the locating shoulder is much shallower than the spindle. I only found this out when I tried the old S7 spindle I had and the #$%^&*( thing would only screw on to the first 1/8" of the locating shoulder. I had to re cut the locator at least another 1/4" deeper to get it to mate with the face. I also had to cut the threads about 0.005" deeper to fit the spindle. The adapter has a smaller o/d by that amount measured across the threads. It was fun trying to set the boring bar back to the correct depth and location to re cut the threads after I had removed it.All in all, it's been interesting test and learning experience.
     
  17. Jul 2, 2019 #17

    bazmak

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    The spigot on the adaptor plate should be shorter than the depth of the recess on the chuck
     
  18. Jul 2, 2019 #18

    goldstar31

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    So that's another 'Round Tuit' to check on my RDG spindle nose to see if it fits Original Myford chucks and faceplates rather than the rest of the tackle made by 'AN Other'

    One bright thing comes to mind, however, and that is the rounded crests of Whitworth threads- which can, of course be truncated. Oh yes!

    My Micrometer thread on my Quorn is truncated as per Prof Dennis Chaddock's words and music both in the original series of articles in Model Engineer and his later book.

    Another item for discussion?

    Norman
     
  19. Jul 2, 2019 #19

    RonW

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    Hi Norm, Don't get me wrong. The RDG adapter definitely fits my two 4 jaw chucks, my driving plate, my three jaw and large 9" face plate with no problem. It just isn't a substitute for the nose of the lathe when it comes to making a new back-plate. Its as deep overall but the register is short in comparison meaning that the spindle runs out of thread before it butts up to the back register.
    RonW
     
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