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  1. #1
    Chuck W. is offline Aluminum
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    Default Cutting Square threads

    Well, I have the lathe in pretty good shape and have been learing how to turn and face pieces. Now I am trying to reproduce my first M274A2 Mule part, and am having trouble. This is a bolt with square threads, 12 TPI, about 3/16" in diameter. Can anyone share their tricks for grinding a tool to cut these threads? My efforts to date have been pretty bad!

  2. #2
    AlfaGTA is offline Diamond
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    The trick is to get enough clearance on the tool bit on the lead face and to grind it straight withe the tool bit. I make my squair thread tool like a short parting tool. For a squair thread the height and width of the thread fourm are the same. The width of the root and crest are the same as well. I grind the tool narower than the caculated thread width. (P=1/number of threads, so the pitch on 12 TPI is .0833" , therefore the thread width = .0833/2 or .0416" ) grind the tool narrower than this width...say .035" and give the lead side enough clearance so it won't drag in the cut. Believe the caculated helix angle for that lead at the major diameter is about 12.5*but at teh root it is about 16* due to the smaller daimeter so you are going to need about 20* at least for the front clearance. The total length of the tool tip only needs to be the same as the width so the tool only needs to be about .045" long..(note this is a pretty small thread to be learning on)

    I run the compound at 90* like a european lathe hand would for this and take the slack out moving the compound forward some and setting the collar to zero.

    Now begin making your depth cuts using just teh cross slide. Easy does it on something this small. If possible i turn the beginning end of the thread to the minor diameter (major dia. less the pitch) Use this diameter to gage when you are to depth.

    Turn to depth using lots of oil and light cuts. Once at depth now correct the width by advancing the compound a slight amount and chasing the thread...this will scrape the lead face of the thread. Mic. the width of the groove using a small hole gage or feeler stock and continue scraping the lead face until the groove and crest are the same and the depth has been reached.

    Cheers Ross

  3. #3
    Finegrain is online now Titanium
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    Is it a square thread, or an Acme? Looks like Acme from the picture. Either way, you gotta account for helix in the metal under the cutting edges. I can explain more if you like.

    Where is it not cutting well? What are you using to gage the cutting tool angles (assuming you're grinding your own HSS cutter)? What RPM? What material? Lube? How are you advancing the tool between cuts -- straight in, or angled (assuming Acme)?

    Regards.

    Finegrain

  4. #4
    Finegrain is online now Titanium
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    I just reread your post and you say 3/16" OD? Ouch. That's gonna be a bugger to cut on the lathe. Can you run a die instead?

    Finegrain.

  5. #5
    Chuck W. is offline Aluminum
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    CRS.....should have been 3/8"......I can't even remember stuff for as long as it takes to walk from the basement to the computer! Getting old $ucks!
    These threads have a square sholder at the bottom, if I understand it right, the Acme has a taper?
    I'm trying to grind my tool to match the width, depth, and angle of the sample screw I have. The material is 1/2" cold-rolled round stock (cheap) and I'm feeding straight in. I've got the speed at 410 rpm, and just using some MMO for lube. I think I need to be using the tailstock, as the piece seems to be deflecting when I cut the threads.
    Thanks for the help, this is all new to me.

  6. #6
    jim rozen is offline Diamond
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    MMO = marvel mystery oil, right?

    I do love that pepperminty aroma, but it is a terrible cutting fluid.

    Get some sulfurized cutting oil from Home Depot (available in small containers) and
    that will help a bit.

    Square threads are possibly the toughest threading job you could possibly imagine.
    You sir have picked the hardest novice project ever imagined!

    You need to have the tool set up with side clearance on each side, so it will be
    largest at the very tip, tapering back slightly when viewed from above.

    It also has to have more relief on one side, than on the other, to match the helix
    angle of the thread. Basically it will try to be scuffing more on one side than the
    other so that side needs more relief. I think the machinery handbook has a
    photo of what that tool needs to look like for square threads.

    Also you want to be cutting this at the slowest back gear speed your lathe has,
    and be taking a thousanth or a half thousanth of infeed at a time.

    Good luck.

    Jim

  7. #7
    jim rozen is offline Diamond
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    Err, looking again at the photo, this isn't a double start thread by any chance, is it?

    Jim

  8. #8
    AeroncaChamp is offline Cast Iron
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    the clearance angle needed for the leading (cutting) side of the tool is:

    arctan{pitch/(pi*Dmin)} +1 deg, where Dmin is the thread minor diameter.

    The angle for the following side clearance is opposite the leading angle, and is:

    arctan{pitch/(pi*Dmaj)} -1 deg, where Dmaj is the major diameter of the thread.


    Looking cutting end on, the shape of the cutter is something like (oops - ascii art doesn't work here) like a tall trapazoid that's squashed sideways. (if this doesn't make sense I can scan a sketch for you)

    Found this a while back while looking into cutting a B&S 29deg worm threads for an old drive restoration - Apparently square and Acme threads (as well as the B&S worms) can be made with the same clearance angles.)

    Best reference I found for this was: "Machine Tool Operations", Vol 1, McGraw-Hill, Inc, 1959, pg 457ff (had the picture that made it all make sense for me.)

    I see what Jim's seeing about double start thread but to my eye it's not clear... double start thread = at least double the fun to cut! (and pitch in my equations above becomes lead - how far the screw advances for one turn, which would be twice the pitch for a 2-start)

    have fun!
    Last edited by AeroncaChamp; 01-07-2008 at 11:32 PM. Reason: removed attempt at ascii art

  9. #9
    Jim B.'s Avatar
    Jim B. is offline Hot Rolled
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    In restoring my Springfield shaper I have had to cut several square threads a 5/6-10 LH, a 1-4 LH and a 3/4-6 RH. In general the advice you got form AlfaGTA is quite good.
    I hand ground all my tool bits. The 5/8-10 I ground a bit oversize and just remade the screw. I have been using Air hardening drill rod unhardened.

    One "trick" I picked up is to first start with a 60 degree V thread and go only as deep as the width (less a few thou) of the square thread. I start off plunging in increments of 0.005 then 0.0025 and finally 0.001 as the thread gets deeper.

    I turn in back gear on the lowest speed. You doo need to support the free end and I also use a follower rest. Of course my screws are longer. Here is the setup for the 1-4 thread:

    Here is the first 5/8-10

    and here is the 3/4-6.

    Another point, If you are going to support the free end with a center be sure to leave enough stock so the carrage does not hit the tailstock. I did not and had to make an extension center. (Having tow lathes is good)
    I find a steady rest is better than the tailstock.

  10. #10
    Finegrain is online now Titanium
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    Quote Originally Posted by Chuck W. View Post
    CRS.....should have been 3/8"......I can't even remember stuff for as long as it takes to walk from the basement to the computer! Getting old $ucks!
    These threads have a square sholder at the bottom, if I understand it right, the Acme has a taper?
    I'm trying to grind my tool to match the width, depth, and angle of the sample screw I have. The material is 1/2" cold-rolled round stock (cheap) and I'm feeding straight in. I've got the speed at 410 rpm, and just using some MMO for lube. I think I need to be using the tailstock, as the piece seems to be deflecting when I cut the threads.
    Thanks for the help, this is all new to me.
    Couple thoughts -- if it's cold-rolled 1045, or drill rod, you might should just pitch it and get ahold of something free-machining like 12L14 or 11L44. Even 4140 (annealed) would be better.

    Acme has a straight (parallel to the axis) thread root, with 14.5 degree sloped sides. Square thread is just that, straight thread root, 0 degree sides. True square threads are pretty uncommon. If there's any taper to the sidewalls at all, it's probably Acme.

    Ditto on the sulpherized oil. Just ask for "pipe thread cutting oil" at the hardware store.

    400 RPM is "really dang fast" for trying to cut threads manually. I bet your leadscrew is humming right along :p. You aren't trying to cut the whole thread in one pass, are you? At 400 RPM, you're gonna have a helluva time backing out at the same spot at the end of each pass. Try 5-10 thousandths feed between passes.

    Use the tailstock. You might need a long-nose center to get the proper clearance.

    Regards.

    Finegrain

  11. #11
    dullvinci is offline Aluminum
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    Suggestion: it can be tricky for a novice to tell a square thread from an Acme. If you have a set of number size drill bits, find the one that's the closest fit in the bottom of the thread, (use the shank end) and look straight across the screw. If the sides of the thread angle away from the sides of the drill bit, you have an Acme. If they're parallel then you have a square thread. For cutting you have lots of good advice in the previous posts.
    chas

  12. #12
    Chuck W. is offline Aluminum
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    Thanks to everybody for all the tips, I see a lot that I was doing wrong, and now I have a pretty good idea what to try.
    Chuck

  13. #13
    Chuck W. is offline Aluminum
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    Well, I finally managed to cut some square threads that seem to work O.K. for this project.

    I must admit to a bit of "stupid", the first one I made just would not thread into the nut, I kept looking at it and finally realized I had cut the threads left-handed! Duh!

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