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  • Internal acme thread

    I was recently asked to make a replacement part for an actuating mechanism. The part was basically a 1.5" long "nut" with 5/8 - 8tpi (internal) acme thread. The original was plastic and had failed - decided to make a steel replacement.

    For cutting the thread, my first thought was to buy an acme tap, but dismissed that idea when I found that the cost was around $100 (Cdn) for a tandem style - single pass acme tap.

    Switch to plan b - found a local source for acme threaded rod & nuts and proceeded to fabricate the part using 2 nuts and some barstock - a quick & easy job.

    Plan c would have been to single point.

    I'm interested in hearing how others would have proceeded with this threading job if the pre-threaded nuts had NOT been available.

    So...again, you need to cut an internal 5/8-8 acme thread into a nut that's 1.5" long.
    It's a 1 off part so you don't want to spend much on tooling. The material is CRS...well okay I guess I could let you use something like 12L14 if you want to. What's your procedure?



    [This message has been edited by Herb W (edited 03-04-2003).]

  • #2
    First you need an Acme thread tool grinding gauge,then you grind a tool with proper clearance angles to fit a boring bar that will fit the minor diameter of the thread with about .005 clearance. Thread just like would a regular thread, but compound should be set at 14 1/2* to the left. Others on this board may be able to add things I may have forgottenb but you need a tool gauge first

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    • #3
      At that length conventional single pointing is not a good option. Maybe a stack of thin nuts could be single pointed and fastened together for the length?

      Make a tap, although if your time is worth anything you'll have far more than $100 into it by the time you're done.

      Another solution (but not the type of answer you're looking for) would be to rough out the nut blank, then get on the phone to some of the local shops to find one with the tap in stock. Pay them to tap the blank nut.

      Yet another solution is to make a custom bar with a running fit in the nut blank. Use a tiny cross mounted bit which does the cutting. For each pass the bit has to be incremented out from the bar for the depth of cut. Tedious, but it does work.

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      • #4
        Recently had to make a 3/4" x 5tpi L.H leadscrew nut for a surface grinder. Due to the Laws according Murphy it was the one L.H Acme tap I didn't have. A call to Tracy Tools in Dartmouth, Devon, UK had a suitable tap in the post next day ~ cost آ£22 (about $34 USD). Every exhibition I've been too I've always searched out Acme taps - they are worth getting even if just to sit in your tool draw! It's almost like insurance don't have it and you will need it, have it and you won't!

        What worries me though, Herb is why do you want to make the nut out of steel, why not bronze, or even good Aluminium??

        As to the point of the question - I think given that Tracy Tools had not existed I would have bought a length of Acme threaded rod and made a tap out of that; tapering, fluting it, and case hardening for a once in a life time pass!! Other contenders would be a TC boring bar ground to form and then single pointing it - this is what I once did to produce a 19.7mm x 1.66mm pitch internal butress thread.

        RR

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        • #5
          If the original was plastic and that was sufficiently strong for the application, then I would suggest using polyurithane 2 part casting compound (I've used Synair product). It produces smooth and strong finished product. Just remember to apply plent of parting compound on the acme rod so that you can unscrew it out.

          If you want to machine it, then I strongly suggest using brass instead of steel. You'll need a pretty rigid boring tool (read solid carbide) to go that deep, but if you go really slowly with a very sharp HSS I think it's doable.

          Good luck.

          Albert


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          • #6
            I needed to make a 1.5X10tpi X3 acme half nut. I single pointed it using a very sharp and polished T-15 cutter that I ground by hand, using a threading guage. At the end I took light cuts and ran the cutter threw without any feed to get a nice finish. Worked great!

            Bob

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            • #7
              For that job, I probably would've made a tap, assuming a lathe had been handy. Boring a thread of such small diameter to such a depth is tough. Recently had to do this in making up sets of replacement half-nuts for an old South Bend. They were LH, 1.125"x6. I seem to make a lot of one-off taps and takes about an hour with heat-treating. Heat-treat for tapping steel only, generally.

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              • #8
                First, original part was plastic. If shaft was steel I would use a dissimilar material for the nut, probably bronze and probably bearing bronze. Since the bronze is of higher strenth than the plastic I would consider reducing the lenth.

                I think I've already got a tool made up for this one actually. Chunk of tool bit welded to a shaft and ground.

                Single point away, cut till it fits.

                Don't put steel on steel if you can help it, posibility of galling.

                Another way would be to make up a outer casing and pour babbit.

                [This message has been edited by halfnut (edited 03-05-2003).]

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                • #9
                  So here I was, living in a very small town in the middle of nowhere, and I got this lathe and it was great except that the cross-slide nut (LH 7/16 x10 acme )was as loose as a old screen door..060 backlash . where o where do you get a tap...no where is the answer!

                  Took a piece of bronze and bored it and single point threaded as good as I could, then removed the "nut" and put it in the deep freeze for a while.
                  Took the old leadscrew and noted that it was good on the end...most wear was in the middle.
                  Took the screw and fluted the end like a tap , using a dremal tool and making the flutes as sharp as possible with positive rake for about 1/2 inch long....didn't care if the flutes were even or not , as it was a hand made tap.
                  when as sharp as possible, I took the frozen nut out and ran the tap through it . It was a bit tough, but when the nut warmed up, I had a nice fit for the slide and all the play was gone.

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                  • #10
                    very slick solution, Rick! That's what makes this board aces. With enough money we wouldn't need to fix old stuff, right? These are the kind of ideas that are golden, thanks.
                    I'm here hoping to advancify my smartitude.

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                    • #11
                      Herb W:
                      If this is important I would consider getting a threading bar with the proper acme insert. And use bronze for the nut - you will thank me later. Easier to cut, and it should last forever if properly lubed.

                      The problems noted in cutting with a single pointed thread is avoided with a boring bar made for acme threads - it has an angled seat under the insert to give proper clearances. The boring bar may be usable with other regular inserts with a different angle seat under the insert. Worth a look if you plan on doing acme threads in the future.

                      Alternately you could cut your own tap from O-1 - starts with a v thread and then transitions to the acme - the sharp v thread roughs out the groove for the acme tap.

                      [This message has been edited by Thrud (edited 03-06-2003).]

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                      • #12
                        Thanks guys! Good advice...as usual

                        As mentioned in my original post, I used a couple of acme nuts & fabricated the part.

                        Steel on steel is not the best choice, but in this case it's a very light load - low speed application so I think it'll be ok.

                        Had the nuts not been available, I probably would have used the leaded steel (12L14)...no Al of the right size on hand...stock of bearing bronze running low and I'd have wanted to keep it for a more demanding application.

                        I was concerned about single pointing to that depth in a 5/8 bore...appreciate the advice on how to get it done. The boring bar for acme threads sounds interesting - have to check that out. The wish list just keeps getting longer...

                        Send it out for threading...yes, entirely practical, but, unfortunately, I'm entirely stubborn.

                        Making a tap seems like a reasonable option too. Believe I'll have to try that next time the need arises. Seem to remember seeing an article on making a tap in one of the mags.

                        Casting...I'm sure it would work but since I'm not up to speed on the procedure, it would just take way too long. Would like to try it sometime though...



                        [This message has been edited by Herb W (edited 03-06-2003).]

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                        • #13
                          I'd probably single point it in the lathe. The method I use is to set the compound parallel to the spindle and infeed with the cross slide.

                          Start cutting the thread normally, but when the going gets tough, start advancing the compoung a couple thousands or so each pass with no cross slide infeed, ie, cut with the leading edge of the tool. Maybe take a few with the compound retarded as well, cutting on the trailing edge. Then after a while go back to zero on the compound and start infeeding with the cross slide. Finish up with very light passes. It's tedious as hell but gets rid of the chatter. Takes a lot of passes also. If the mating part won't fit once you've reached predetermined depth, advance the compound a bit and widen the groove until the mating part fits. A little dykem will tell you where the mating part is rubbing. I hate cutting acme threads but when it goes okay, it's rather neat.

                          I cut external acmes and very coarse vee threads this way also. (6 tpi or coarser)

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