New knee nut for Tom Senior

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.

Yes Old Mart please do post pics.

I Remember when i stripped mine down during covid to give you some dimensions. Will be interesting to follow.

Steve.

The nut now has threads, and the backlash is better than 0.001", the old nut has 0.018". I will turn down the new nut to fit in the cast iron base and thread the end so that the bronze flange from the old nut can be threaded and screwed on and loctited. The two bits of channel section will fit under the main body and allow an extra 25mm of Z travel and height.

Looking good.

Will there be enough meat on the nut to get descent size screws in to hold that flange.

Steve.

This is what I have in mind.

Ah yes I see what you mean. Fingers crossed. I guess you could even silver solder if needs be. Although thread soak might be an issue. Great idea so far.

Steve.

A 40tpi thread would take up a very small part of the diameter, we don't have enough heat available to silver solder that size. When I renewed the leadscrew nut on the Smart & Brown, I made a thinwall inlay and soft soldered it onto the bored out block, but that had a much greater surface area, about 5 square inches.

I wonder if you could pin it.. just in case the locktite came unstuck.

Steve

Several 1/16" pins fitted axially through the thread would be very secure. I would check the dimensions carefully before machining the parts.

Did you notice the Tom Senior bodge to make things fit? A clue, look at the hole in the green cast iron nut support.

Edited By old mart on 16/07/2022 19:17:34

Edited By old mart on 16/07/2022 19:20:44

That would do it. No chance of the screw binding & undoing the finest threads .

Yes the green casting well of line hole. Going to check mine now.

Steve.

Mine seems OK.

Steve.

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.

It is something I never used. It looks like a large tank on there.

Steve.

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.

Today, I decided to look into making a new Y axis nut. I had already made a pair of backlash adjustable X axis nuts, but the X has a right hand therad and the Y has a left hand thread. The 3/4" X 5 ACME taps are £40 from Tracey Tools and I only had the RH one. The start size is 0.55" for this thread, and the 5tpi ACME laydown inserts are er22 size, so its not going to happen without some modification. I have some 1/2" Densimet bars with one end turned down to 12mm. The er22 inserts are 3/16" thick, so I decided to cut a slot in the end of one of the Densimet bars and silver solder a modified insert in. The new Clarkson 3/16" hss slot mill was destroyed before it had cut 1/8th of the slot, so I had to resort to a 5mm solid carbide which is 0.0093" bigger. Densimet is tungsten, sintered with iron and is not anything as hard as tungsten carbide, is denser and has a higher Youngs modulus than steel. It makes good holders for carbide inserts. In my hurry to get going, I made the tool much too long, even to thread the 50mm hole in the gunmetal I was using. Because of the length of 12mm and the tool support being made for 1/2", the whole of the 12 mm was overhanging by at least 3". The workpiece should have been cut down to 38mm long and the length of 12mm should have been about 43mm long, much better than the 76mm that I used. The thread is about 95% done, but I will remake the tool to the shorter dimension to finish off. With a coarse thread like this, even with a female one, it is easy to realign the tooling if needs be.

Ahh yes and a very fine tool it is OM, although I didn't realise at that time that I was the test pilot. My home made VMC nuts are still going strong and it was only possible for me to make them due to your willingness to let me borrow your tool to which I am extremely grateful.

One of the ACME inserts in the photo is the one that you kindly sent me when you returned the threading tool. Having two was the reason I cut one down to get into the smaller 0.55" start diameter for the Y axis nut it really is pushing the limits for the pitch, a 5/8 x 5 would never singlepoint.

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

Very nice job

As to locking the thread could a simple lock nut be employed. I suppose the fine external thread would have to be extended which might be a bit tricky. I don’t know if it would work in that situation, but it’s an idea.

William

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