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Singlepointing a 3/4 x 5 ACME nut.

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  • Singlepointing a 3/4 x 5 ACME nut.

    When we restored the Tom Senior light vertical mill, I made modifications to the X axis leadscrew to accomodate two backlash adjustable nuts. They are 3/4 x 5 ACME RH and as I thought it was impossible to singlepoint them, I bought a tap. To take some of the strain off the tap, I partially singlepointed the thread using an 8 ACME insert. This is a size 16, whereas the 5 ACME is size 22. So before tapping, the thread was about 70% cut.
    Of course, the Y axis is left hand and I didn't want to buy another expensive tap, so I thought I would try another approach.
    There are some Densimet bars at the museum and I have grabbed the lot for the workshop to make tooling. The bars are 1/2" diameter with one end reduced to 12mm over about 3 1/2". I measured the thickness of one of the size 22 5 ACME inserts and it was 3/16". The only 3/16 mills we have are threaded shank HSS and I found a new 2 flute slot drill. The idea was to silver braze a cut down insert into a slot in the end of the Densimet. Even with small cuts and coolant, the end of the mill failed before getting very far. So I decided to use a 5mm solid carbide which is about 0.009" larger. To stop the insert moving when being brazed, I jammed a few strands of copper wire from an electric cable in the gap. In my hurry to get going, I made a silly mistake, putting the insert at the end of the 12mm part, when my boring bar holder is made for 1/2". So the overhang was 3 1/2". My nut could have been 1 1/2" long, so the tool length unsurported could have been 1 3/4", not nearly 3 1/2". I needed to relieve a little of the back of the tool opposite the cutting edge and when making the thread, I could see the tool spring back straight as it emerged from the hole. Even after ten spring cuts, there was still metal being cut. The thread is about 90% done, but I will shorten the bronze to the 1 1/2" and shorten the tool to 1 3/4" before finishing off the nut. The original nut wear not too bad, so I intend to bore 1/3 of the thread length out from one end and screw the new nut part into it. There is room for the modified nut to be longer at the column end of the Y axis without restricting movement.
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  • #2
    Sounds like your describing converting a tungsten armour piercing impactor to a boring bar, that’s got to be an original reuse if ever there is one , thinking densimet was 90% tungsten from the last time we crossed paths ( recycling munitions into steel additives) safer than depleted uranium imho!
    well done
    mark

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    • #3
      Densimet comes in several types, not particularly used for weapons. Being machinable and dense, it excells at sheilding for Xrays and gamma rays. Also the density makes it good for balance weights. The stuff I have got my hands on was probably intended as some type of balance weights in helicopters. It can be machined and I have drilled and tapped it to make boring and threading bars. As you can see, there is very little room to fit in the bore even when the insert is cut right down, so that is why I silver brazed it (we call it silver soldering in the UK).

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      • #4
        Today, I made the shorter version of the threading bar out of another Densimet bar. The 12mm section is now 1 3/4", and I took the time to turn it eccentrically to take off about 0.060" off one side. This relief is opposite the cutting edge of the insert, they have to pass down a hole of about 0.560". That bore could have started bigger, as when I measured the core of the Y axis leadscrew, it turned out to be 0.572". I still needed 4 springcuts to finish cutting that 5 tpi ACME.
        The parts will have to be screwed together using 40 tpi as the wall thickness of the new nut is only about 1/8" after the ACME thread has been finished.
        Mike made a special bolt for the original nut to fit on while he milled the sides true to the axis of the thread, using the rotary table. This works with an ACME thread which still has some of its flat thread tops left. This is so I can set it up true in a four jaw chuck to bore it for the 40 tpi thread.
        Last edited by old mart; 07-27-2022, 03:22 PM.

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        • #5
          Some pictures of the nut and how it fits into the old nut with a 40 tpi LH thread. There is 1 1/8" old thread and 1 1/2" of new. Click image for larger version

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