Tom Senior Advice needed

Belville washers, hmmmm, now there's a thought!

I cant see anything dramatically against the idea, can anyone else?

I would be able to use the old nut if I can think of a way of making a suitable housing. I like the idea in principle all I have to do is figure out the detail. Thats the tricky bit.

I have seen an idea on another site which involves making a thin sawcut in the nut and drilling and tapping a couple of holes so that the sawcut can be closed to reduce of eliminate backlash.

Brian

Not convinced about the Belleville washers idea. It's a simple elegant solution, but introduces elasticity into that direction of that axis. It's got to be elastic or you'd lock the lead screw up solid.

In the direction where the Belleville washer is taking the reaction force from the cutting at the tool, it's a competition between the force to overcome the compression of the spring washer, and the reaction from the cutting forces. It would work OK if you could guarantee never to cut against the Belleville washer, but that's just not going to happen

Rigidity of a machine as all about how steep the stress/strain curve is for the forces resisting that movement at that point in the lead screw. Adding springiness - even if only in one direction on one axis - into the machine is a step in the wrong direction.

We all know that having the cutting forces reacted by positive location is the key to clamping the part successfully, By the same token the leadscrew's function is to provide a controllable and moveable positive location of the work table. The reason climb milling is so dodgy on a machine with back lash is that you don't control the location of the work piece with positive location.

So the squashed up nut is a better option, it has a much stiffer stress/strain characteristic though the point about longevity is well made. Old Mart's description of fettling the leadscrew to have a constant (and therefore correctable) amount of backlash over its usable working length is the corollary.

Rgds Simon

My reasoning for using Belleville washers (springs) is to have a compact spring which can be pretty high rating. The main reason for this is because the uneven wear on the leadscrew threads makes any fixed form of backlash adjustment impossible. If you have 0.002" backlash in the centre of the leadscrew which has 0.010" wear, you will not be able to use more than 1/3 of the X axis movement.

You've got a CVA and you're sending out lathework

How about a version like Old mart's diagram but the two nuts screwed together by a finer thread. Then you would want a way of rotating the second nut but it would only ever be by a small amount once initially set up. You could devise a way of making this happen using a Bowden cable which be infiltrated into the middle of the bed if necessary.

Screwing two nuts together would be an excellent idea if the wear on the leadscrew was non existent. One of the nuts I made screws into a steel bush using a 40tpi thread, and is locked by the washer and screws. I could not figure any way of adjusting the backlash without removing the bed first, but it is easy to remove in five minutes if you know how. With 40tpi it doesn't take much adjusting to change the backlash by 0.001", just look at the barrel of an imperial micrometer.

Edited By old mart on 11/05/2020 21:04:14

Bazyle, you have called me out, if I had a Ferrari it would not make me a formula one driver.

The CVA is in my son,s business premises so in lockdown out of bounds.

You have made me think though.

Old Mart, I get the principle of what you say and it's good, I'm not sure I understand your photos in detail.

a) I'm not sure how the steel component fits the machine.

b) Is the 40 tpi thread on the outside of the bronze nut illustrated and on the inside of the steel component.?

c) Is the steel component reduced in diameter so that it fits in the housing on the machine.?

d) Is there an acme thread in the reduced diameter of the the steel component?

Have I got it completely wrong? Sorry to be so gormless

How do I delete the extra post?

You can’t. But you can edit it (time constrained) and delete the content (replacing with “double posted”, maybe?).

Edited By not done it yet on 12/05/2020 10:11:31

Well! I've ordered an Acme tap, what could possibly go wrong..

I already have an ACME tap which I would have lent to you except for one small problem. It is in my cupboard at the museum, and I have no way to get access to it until the museum reopens. The tap came from Tracey Tools and is a single taper of about 7" long. When I tried it out on a bit of aluminium with the standard bore, I failed to get it to work, it would have needed a tap wrench about 2 feet long, so I partially cut the thread on the lathe (about 80%) before using the tap on the gunmetal which I used for the nuts. Because the bore for a 3/4" X 5 ACME tap is 0.56" maximum, I used a modified 8 tpi ACME insert in a cut down size 16 internal threading bar. This made the tap much more easier to use. The tapped thread has some backlash even on the unworn parts of the leadscrew, which is another reason for my using two nuts.

Andy, all your assumptions are correct except for d) the steel part has a 0.76" hole for leadscrew clearance. It is Locktited in place in the casting.

This pic shows both of the nut housings, one for the fixed nut and the other for the adjustable one. The non standard spacing meant having to add an extra 4" of thread to the leadscrew to maintain the original ammount of travel.

Edited By old mart on 12/05/2020 17:18:12

Good old Tracey Tools, that’s where I ordered mine from. I recognised that tapping would be tricky straight from scratch and having worked out that with the change wheels for my metric lathe at home I an cut a 5 tip thread I thought I would give it a go🤞

The fact that I have only one nut housing is pointing me towards my original idea of a flanged nut with a slit and “solid hinge”, we shall see.

Out of curiosity which museum is referred to in your post.

Andy, when your tap arrives, do produce some test threads before making the nuts. It will give a good idea how much backlash you are going to get. The nut on the museums T S looked like a normal 60 degree thread, there was nothing left of the flats on the crests. It would be worth doing some careful measuring to find out what exactly limits the X axis travel. In one direction, it may be the casting contact, and the other, the end of the leadscrew thread. The other factor in the backlash is the end float of the leadscrew relative to the table. I bought some new thrust bearings for this. They are metric, the nearest size to the old worn out imperial ones. One of the bores had to be opened out a bit on the lathe to get the slightly bigger bearing od to fit.

I'm one of the volunteers at The Helicopter Museum at Weston Super Mare.

Old Mart, Thanks for your advice. Fear not there will be much practicing before the thread is cut in anger. I've got to learn hoe to cut a 5 tpi thread on my lathe.

The thrust bearings on both lead screws are in fine fettle so there is no problem in that direction.

I wasn't sure if I could actually cut a 5tpi thread on my Leinen lathe at home with its 3mm pitch lead screw. I discovered, on the lathes.co.uk web site, a calculator that did the job for me. You input your lead screw, the thread you want to achieve, your whole collection of change gears, the pitch error you are prepared to accept, press the button and bingo there you are.

Not bad eh?

I'm off to do some experimentation now.

Weston Super Mare is quite a distance from Doncaster especially during lockdown!!!!!

I have produced several ACME threads with the help of this useful calculator which is saved in my favourites. When I did the T S nuts and leadscrew, I had to double the lathe leadscrew speed and set 10 tpi, the Smart & Brown model A only goes to 8 tpi normally. While the lathe leadscrew was overdriven, I was worried about the safety link in the mechanism shearing, so I temporarily disabled it. 

*LINK**

Edited By old mart on 13/05/2020 21:41:11

Edited By old mart on 13/05/2020 21:44:21

When cutting a thread courser than the leadscrew it may be better to turn the leadscrew by hand and have that turn the chuck to reduce the strain.

Hopefully my three phase Leinen LZ4S lathe should cope with the job in hand, it's quite sturdy. I will take care and walk up to the job rather than rushing in, thanks for the heads up though.

All this bold talk of cutting an internal Acme thread myself, I gave in and found Richard Stewart of shotmaster.com who is going to do the job for me. My excuse is that I'm comparatively cash rich compared to my time poverty.!

Thanks to all who offered really useful advice, I'll let you know when I'm up and running.

That is a very sensible solution to get a job done rather than risk making a hash of it yourself. There will still be the uneven wear on the leadscrew itself to contend with, do you feel rich enough to ask for a quote from him?