14 months after Ian Skeldon 2 tested my threading tool for his Myford VMC, I shall be making the 1" X 5 ACME nut for the knee of the museum's Tom Senior light vertical. Probably just in time, the original nut has only 1/4 of its thread width left. This job also entails raising the main body of the mill 25mm to add to the Z axis height. The price of a slab of steel or aluminium 25mm thick was more than I wanted to pay, so I got a length of channel section mild steel cheaply and Alan milled the top and bottom down to the required 25mm. The channel will sit flat side upwards in two sections under the main pillar and 3" BSF bolts will hold everything down. The Z screw is plenty long enough to accomodate the extra height. I spent several hours getting the knee to move the full distance without binding or becoming loose, and now it is right, the gib screws will not be monkeyed with. I will post some pictures soon when the threading gets done. The original nut has an integral flange which would make it very expensive to make, I will bore it and loctite the new one inside.

One thing I had forgotten was that the knee raising jack goes straight into the coolant tank, I thought it had a sleeve. I have had to plug the hole while the mill is in bits, all the spanners and screwdrivers want to drop inside.

We have completed the new nut, the parts screwed together with a 28tpi thread, LH for ease of manufacture and Loctited, no pins, I'm hoping they will not be needed. The 25mm high Zaxis extension is under the main body and the bore at the top still just misses the building cross brace supports. The longer screws were late coming,1 3/4" long 3/8 BSF standard to 2 3/4" long, so I have made some today as it is not safe to keep adding weight until the body is secured to the base. The screws were through the letterbox when I got home today, so they will be spares for something.

While the mill is in bits, I photographed the casting defect in the Yaxis to show you, good old Tom Senior.

The Y axis nut is nearly finished, I have to think up a way to lock the settings. The original nut has been shortened to 1 1/8" and the new 1 1/2" long nut screws into it with a 40 tpi thread. The nut assembly screws onto the leadscrew after backing it off 1/2 a turn from fully screwed together.

Edited By old mart on 28/07/2022 17:26:50

You would be surprised just how easy it is to pick up an external thread,even that 40 tpi. The backlash must be taken out first by clamping the leadscrew nuts and then turning the spindle in the direction you are cutting in.Then you advance the tool tip close to the original thread and shine a light through the gap with some white paper behind and as the tip of the cutter fits into the vee of the thread, it is easy to make the final adjustment dead central.

I had to pick up the internal 5 ACME thread when I had made an improved stiffer threading tool, that was done by feel with the tool loose in the toolpost to not only set the axial, but also to keep the tool parallel with the spindle.8

With the nut idea, I would have to set up the old nut and face the end true, so a locknut would sit nicely. Shame I didn't think of that when I was threading it.

 Now I'm thinking of it, I could face off 1/8" without loosing anything and there would be about 1/4" of thread for a nut anyway.

Edited By old mart on 29/07/2022 17:33:38

Edited By old mart on 29/07/2022 17:37:13

While the Tom Senior is in bits, I decided to make the Yaxis nut backlash adjustable without didmantling the machine. The pictures show my way of doing it and Mike and myself did some turning, hacksawing and milling to make a tool that can reach under the Y axis.

Nearly, but not quite right, the knee being extended by 25mm to increase the Z height slightly has not exactly gone to plan. The knee stops at plus 25mm from the bottom. Investigation found that the top of the nut was running out of leadscrew thread. Not slowly binding, but stopping dead. It is easy to check that it is indeed the nut running out of thread because if the knee is raised to near the top of its travel,and locked, the nut can be screwed up until it stops and the nut height recorded. Then the nut is returned to the bottom of the leadscrew and the distance that the bed can be lowered is the same.

The nut flange has been thinned, and the height of the cast iron base that the nut sits on shortened. The flanges on the nut base are wide enough to foul the narrow underside of the knee, so they have been turned smaller and the bolt seats have been chamfered to use countersunk head 3/8 W bolts (in the post ).

It is possible, just, to remove the nut and nut base without taking everything off of the knee. I rotated the head 90 degrees clockwise and wound the bed right up to it. The nut and base can be blocked up when lifting the last bit and then the knee locked and chocked. The nut on its own can be screwed up the leadscrew and there is room for the nut base to be removed followed by unscrewing the nut. I have added a picture of the modified nut assembly to compare with the old unmodified one earlier in this thread. The screws remain to be sorted at present.