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Would you cut this thread? 3/4" 5tpi ACME

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  • Would you cut this thread? 3/4" 5tpi ACME

    I have to single-point cut a .750" 5tpi LH internal ACME thread 1.800" deep in hard brass/bronze. Home made tool. To allow me the clearance to withdraw the tool that means using a bar about .400" diameter with some kind of home-made insert set in the end (probably ground from a broken carbide end mill).

    Is it a viable prospect? I'm thinking that a mild steel bar might be too limber to reach nearly 2" and even that thin I think I'm just going to pack the gap with chips.

    Would you attempt this? If so, got any advice?
    Peter - novice home machinist, modern motorcycle enthusiast.

    Denford Viceroy 280 Synchro (11 x 24)
    Herbert 0V adapted to R8 by 'Sir John'.
    Monarch 10EE 1942

  • #2
    I would try it in a scrap piece of aluminum first

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    • #3
      I can't say I have done this before, but I would use a neutral rake on the tool (being how brass/bronze likes to "grab"), and take a lot of very light cuts. Do not use scotch on this job.

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      • #4
        Is it a through hole? In these circumstances, I usually make a tap first from silver steel (OK, drill rod to you guys), flute it, harden and temper, then gash cut the internal thread with a square tool, to depth plus 10 thou. Then you can pick up the thread again using an Acme profile tool, having got most of the metal removed with the square tool, and finally finish with the tap. Works for me anyway.
        'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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        • #5
          Mild steel boring bar would not even tempt me to try it.

          A 10 or 11mm HSS drill blank with the carbide silver soldered to the end would be more my choice.
          With light cuts and careful attention to clearing the chips, I think you’ll be ok.

          The aluminum test piece can give you some idea what to expect before the real deal.
          Illigitimi non Carborundum 😎
          9X49 Birmingham Mill, Reid Model 2C Grinder, 13x40 ENCO GH Lathe, 6X18 Craftsman lathe, Sherline CNC mill, Eastwood TIG200 AC/DC and lots of stuff from 30+ years in the trade and 15.5 in refinery unit operations. Now retired. El Paso, TX

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          • #6
            Originally posted by Peter. View Post
            Would you attempt this? If so, got any advice?
            bragging rights aside, I'd probably try and adapt a Mcmaster Carr round nut. $48 US in bronze, although not quite as long as you want. If I had to cut it, there'd be some chin scratching.....and would probably end up pursing roughing it single point followed by a home made tap, possibly staged.....$48 starts sounds better and better
            .

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            • #7
              Originally posted by Tim The Grim View Post
              Mild steel boring bar would not even tempt me to try it.

              A 10 or 11mm HSS drill blank with the carbide silver soldered to the end would be more my choice.
              With light cuts and careful attention to clearing the chips, I think you’ll be ok.
              .
              I'd be thinking the other way around - hss soldered onto the end of a carbide bar. afaik the modulus of elasticity of hss isn't much different than mild steel
              .

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              • #8
                Originally posted by Mcgyver View Post

                I'd be thinking the other way around - hss soldered onto the end of a carbide bar. afaik the modulus of elasticity of hss isn't much different than mild steel
                Yield point is, though....... Soldering should not anneal that HSS
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  Maybe use a narrower form tool and after you have pitch and depth then widen the valley. Depends on how much lathe you have I guess.
                  *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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                  • #10
                    Originally posted by J Tiers View Post

                    Yield point is, though....... Soldering should not anneal that HSS
                    ?? the point was, for that reach, you'd really benefit from a carbide bar for its stiffness. solder a mount for the HSS on the end if you're worried about annealing the HSS
                    .

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                    • #11
                      Was thinking the bar might just bend..... hence yield point.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        Originally posted by J Tiers View Post
                        Was thinking the bar might just bend..... hence yield point.
                        could be.....depends. The technique is to whittle away the profile with a smaller tool, not try to do it as one big profile. Hence the possible need for a tap chaser. Unless one really wants the adventure (which can be a worthy reason), its grueling enought I'm still in favour of buying the nut
                        .

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                        • #13
                          Keith Rucker recently did a job like this on his youtube channel, using a custom made carbide bar. Most of us ain't into that kind of money tho, I sure ain't. I do wonder if a decent HSS bar would work with very light cuts. Or treat it like boring a cylinder, with the nut mounted on the lathe carriage and the bar mounted between centers, advancing the cutter a few thou after each pass to get the thread.

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                          • #14
                            JFDI. Plenty of clearance on the bit. Not a lot of rake. Shallow cuts. As has been noted, the stiffness of mild steel and HSS are pretty much the same. The strength is much less for the mild steel. If you can make the tool shank from drill rod and harden/temper it, it'll be good for heavier cuts, but still limited by the stiffness. As a datum, I made 1"x5tpi ACME nuts in cast iron with a 1/2" bar holding an HSS bit.
                            Location- Rugby, Warwickshire. UK

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                            • #15
                              Don't over think it, I used to weld Hss blank to the end of a bolt or rod, then finish profile with Acme guage.
                              sometimes I would relieve the back side of the rod for clearance giving it more of an oval shape.
                              Sometimes I would take 3/4 square steel, then turn part of it round and weld the tip on. Then its easy to clamp in the toolpost.,

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