Making threads on a lathe
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  1. #1
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    I am making two suppressors on ATF form 1's next year. My first can had welded on end caps, but I decided I need to use threaded on caps for my next two projects. I will be using aluminum tubing and bar stock. The tubing is 2" wide and has 1/4" walls.

    I have yet to decide the exact dimension of the threads, but I want to cut about half way into the tube wall cutting the threads. I am thinking of ordering a small 60 degree thread cutting tool bit from a mail order catalog.

    Is a slow speed the best way to cut? I can only get my lathe down to about 180 rpm. Maybe a variac can slow it down a bit. I practised a little using a 90 degree tool, and wound up with threads that were rather rough. How do I avoid rough threads? Since the end cap will center itself on the barrel using a shoulder cut into the cap I do not have to worry about slightly uneven threads. I would use a tap/die if I could get them cheap, but any tap/die over an inch will cost 3 times more than all the materials I am using to build the suppressor. I will be using a steady rest to hold the tube in position while cutting the threads.

    Any other suggestions? Thanks.

    Ranb

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    Lets see if I got this right, you want to cut the threads 1/2 the thickness of the walls, .250/2=.125/2=.0625 depth per side or 16 TPI. To find the depth of a sharp V thread devide 1 by the number of threads. To find the drill size for a tap subtract the depth, number above, from the major dia. and use the nearest drill size (next size up is better than a smaller one). If your scared about running into the shoulder, you can turn from the inside out. run the spindle in reverse and feed from left to right. On inside threads I find it better to feed stright in without useing the compound as the infeed. Rough threads? A good sharp tool or a full profile insert ( the best but will cost more than the tap) setup on center, and WD40 is a good cutting oil for AL. If you decide to bite the bullet an buy a tap and die, start the threads on the lathe first and when there just about done chase them with the tap this will asure that there stright and clean. 180 rpm is fine, but I've ben doing this for 36 yeears.

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    I would spin the chuck by hand myself.

    I know, It's gutless, but it's hard to disengage at exactly the right time without practice, and if I were you I wouldn't want to practice on my first finished product.

    and if you leave the saddle engaged you can just back the cutter out and spin it backwards out of the hole and move the cutter back in and wheel through another cut. and you don't have to worry about the thread counter.

    you should feed in with the cross slide set at 30 degrees (29.5 really but who's picky)

    I feed in only with the compound, and back the cross slide in and out to "0" for each cycle. you must back the cutter out ov the thread completely to retract, or the slop in the leadscrew will make the tool destroy the thread.

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    Sounds like good advice guys. Thanks. I will be practising on tubing first. The tube and end caps are the easiest to make, so I will practise, make the can/caps, then bring them in to the engraver before I start the baffles.

    My lathe is a chinese made grizzly, 31"x12". The chuck is belt driven, I have to change gears to adjust for thread size. The only instructions in the owners manual were how to plug it in and turn it on. I am still looking for a good book to read on it, otherwise I am picking it up as I go along.

    How much metal should I take of with each pass? I have looked at other machines, and as far as I can tell, I do not have a thread counter. I cut my first threads by merely disengaging the saddle then backing out for another run. I have lots of practising to do.

    Randy

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    if you don't have a threading dial, and I'm not sure your machine would automatically know to pick up threading in the correct index, then you will have to leave the saddle engaged the whole time, and stop the motor (or clutch?) and reverse the tool back to the start and start it going forward again.

    I imagine you will be trying somewhere around 32 - 24 tpi? I would feed .005 each pass with a single point tool.

    if you always use the compound to feed in, and always use the cross slide to retract and reset to 0 it is very easy to keep track of.

    since you don't care about coming out to any particular dimension since you will not be making interchangable production parts just make a nice complete female thread form on one part, and keep feeding in on the mating part. run a fine file flat over the male threads to nock off the very crest before you try the fit. Go until they fit very very snugly. they will wear in a bit.

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    I tried making threads again tonight. Very much improved now. Not rough at all. The key for me and my machine was not disenaging the feed screw (or whatever it is called) after making each pass. I tried turning the chuck by hand, but it just wore me out, so I turned the lathe on and then off several revolutions before I needed to stop then the rest of the way by hand. Backing the tool bit out then reversing the machine set me up for the next pass. I had to take off only .002" at a time after the first few passes.

    I tried a little bit of math for threads/inch verses depth and bit angle.

    2 x Depth x tan(tool angle/2) gives the distance between threads. Divide this product by 1 to get threads per inch. For .0625" deep threads and a 60 degree tool bit, I get 13.8 threads per inch. This sound right? Thanks.

    Ranb

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    Yep sounds good to me, 14 TPI would be .06186. I screwed up my math above, what I gave is for pitch only not for the depth. Machinery's Handbook has D=P*COS 30 DEG.=P*.866=.866/TPI.

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    Maybe you already have done this- Set your compound at 29.5 deg. & make your cut increments with the compound, this will effectively eliminate tool pressure on the back of the pitch & better your results.
    OOPS- already mentioned above!

    [This message has been edited by Gastrap (edited 01-17-2004).]

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    I finally bought a tool bit to cut inside threads. However, the bit has a rather blunt tip. Making fine threads will result in shallow cuts. Is there a good way to grind the tool bit down a bit to make the tip more pointed? I have a bench grinder.

    Ranb

  10. #10
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    You can buy a 60 deg. gauge(enco p#890-1357 page 246) 14.00$ and use this to check the angle on your tool when you grind, I personaly would just buy a boring bar with an indexable insert of the size thread pitch to cut, that way if you brake or chip your tool indexing the insert takes but a minute of your time and grinding a new one, setting it, and getting back on the pitch will take you a lot longer.
    good luck and have fun.

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    The standard thread profile has a flat on the end of the cutter that is 1/8 of the pitch, so to make a standard thread, you need a different cutter for each different thread pitch. For a low stress use of the thread, you can do a sharp V thread which goes to a sharp point and can be used for fine or coarse threads. But it is much better to use the standard thread for high stress applications as it is stronger.

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    its been my experience that if i need to stop threading at a critical place on a part i will set up a 1" travel indicator with approx 3/4 rev of travel .075" i thread around 100 - 150 rpm depending on the pitch with some practice you should be able to disengage the half nut within .003" or less try to stop at 0 for some self gratification also the 29 1/2 deg compound i find to be best. also if you have the proper flat on the tool point you will find that advancing the compound at 75 % of the pitch works great ex. 20 threads per inch = .050 pitch compound advance .75 x .050 = .035 try out thread at .005 short of total to fit

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    SIR,
    when you have completed the threads, be sure to apply
    antiseize compound on the threads of both parts, before
    you screw them together. otherwise you may have problems.
    wlbrown

  14. #14
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    Ranb,

    what cal are your cans for? If it's for .22LR there are other ways of doing what you want to do.

    Mike

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    toolmaker Jim
    just did couple quick calcs & & ur 75% of pitch for compound feed is w/ in tenths of dead on for a sharp pointed 60 deg. bit....tnx for a very QUICK way for "figgerin "

    best wishes
    docn8as

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    shud have noted that w/ a sharp point bit ,the .75 Pitch calc. is for direct feed in ...for compound use , wud need to trig out the feed ,depending on angle used ,29,29 1/2, 30 ...(THINK i figured it correctly )

    FWIW ...the olde tymers wud catch a thrd by setting a hard stop where carriage was first engaged ,& a chalk mark on chuck ,or the dog noted at 12:00 ..then return carriage to stop & throw it in when chalk or dog is at 12:00 , eliminating the wait in reversing ,when a long thrd was cut ....having had to cut a few thrds w/ crossfeed on an oldie w/out a compound , a gooseneck tool hldr allowed cleaner thrds than with a solid tool hldr.

    best wishes
    docn8as


    best wishes
    docn8as


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