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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.
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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