Thread: Feed Screw Wear

After reading comments regarding making a replacement acme thread nut for a feed screw recently, I started thinking about the feed screw for my lathe as I was investigating reasons for the slow takeup of backlash when using the taper turning attachment. I wish to minimise this.

I have removed the screw and measured it at 25mm intervals using wires. I got a variation of 0.018" (0.457mm) over the wires. Wondering how much wear this represented axially, I drew it in CAD. The result I got was that the thread form was worn 0.059mm. That is, the flanks of the thread form were worn 0.059 further apart at the worst measured point. This is further complicated by the fact that the thread would have suffered from more wear close to the major diameter due to the nut wear increasing its minor diameter. The nut does have quite noticable wear. It is a 2 piece nut which has a close fitting pin joining the 2 pieces and a tapered end grub screw to seperate the 2. One piece slides on the pin. The grub screw does not seem to fully adjust anymore, but this may be due to limited movement in the holes for the clamping bolts. The nut needs replacing fairly soon.

The 0.072" wires sat pretty high on the thread. There was more wire diam outside the thread than inside. The next smallest wires sat too low at the worn section.

What would be considered as acceptable wear for the feed screw?

I have started putting out feelers to buy a new screw and nut, but I suspect this may be a pretty expensive proposition.

I don't think either are particulary difficult to make except for the thread for the nut. The thread is LH 3/4" diam, 3mm pitch. These lathes were available in both metric and imperial. I measured the thread diam as 19.05mm. The nut is silver/grey in colour and pretty light. I don't know what my chances are of finding a 19mm X 3mm LH acme tap.

Dean

How long a piece of threaded rod do you need Dean? I know bearing suppliers like Gardners (in Mt Gambier as well as Adelaide) can supply ACME threaded rod and I presume nuts too. It may be worth seeing whether you can get a length of that and a nut and then adapt them.

Michael

If its truly metric the machine may be using metric trapezoidal threadform so maybe look at a trapezoidal thread table and see if any of those match, you would then be able to find a tap out of europe somewhere.

Dean nice thread mate
What sort of lathe do you have.
I've got a taper turning attachment for my old Colchester that's working part of the lead screw is really worn . Because I have a non tapper set up as well I havnt dealt with that problem yet.


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It doesn't matter if it's ACME, metric trapezoidal or square thread. They all work. You can buy ACME and trapezoidal, square is the easiest - IMO - to cut yourself.

I'd buy rather than make.

There are a number of places in the USA where you can buy ground ACME thread in LH, matching nuts etc, in various diameters & pitches. I've been meaning to buy some to replace my Monarch cross slide screw myself. Yeah, it's not all that cheap but I'd rather pay the money for a known accurate thread than screw (hah) about making one using a leadscrew that might be less than perfect itself.

PDW

The thread form is metric. I checked with a gauge today as I was anticipating the imperial thread suggestion.

According to Wikipedia the metric trapezoidal threadform chart jumps from 16mm to 24mm. The 16mm has a pitch of 4mm so it is not a standard thread according to Wikipedia anyway.


Cross Slide Feed Screw.jpg

This is the 2 piece feed screw. I did not detach the rear end. There are thrust bearings inside the black block. The piece behind it is the TTA mount.

The threaded section is roughly 500mm long looking at the picture. I have put it back together so I can't measure it. The dovetails in the picture are about 460mm long.

Azzrock, I have a Yunnan Machine Works CY1640 lathe. For a Chinese machine it is very solidly built (1500kg), well designed and well finished.

Making a screw would be my last choice. I will look into getting some threaded stock and a matching nut or two. It would be a simple matter to machine the feed screw then. The nut also should be relatively simple. I just hope I can find 3mm pitch at a suitable diameter.

The lathe works fine as it is, but I think it is better to fix it now, befor the nut strips right out. I am still not getting a huge amount of backlash from the nut. I did manage to adjust it a bit better today.

Dean

Accugroup in UK has 14mm x 3mm x 500mm Trapezoidal Lead Screw for $10.00 and bronze cylindrical nuts for $22.70. The lead accuracy is given as 100µm / 300mm. I presume the "Â" is irrelevant. The lead screw is made from C45 medium carbon steel.

Roton in the US, who I have dealt with before, has 16mm x 3mm Trapezoidal Lead Screw for $23.64 per ft or $47.28 for 2ft which I will need. Bronze cylindrical nuts are $44.45. The lead screw is made from special quality low carbon steel and are "cold formed using a unique, highly accurate thread rolling process which guarantees a smooth, high quality thread".

I would imagine either of these would be suitable, but I wonder whether the extra diameter of the Roton screw would be a benefit. I do need to check the ends of my screw to see what diameters are needed. I will have to pull it out again. I hope I can do it without too much trouble as I put the chip guard back on today. It is large, heavy and awkward to handle. I had it off to fit the DRO scale and finally got around to doing that over the last week or so. Now I have the DRO Z axis working at least.

Dean

I do need to check the ends of my screw to see what diameters are needed. I will have to pull it out again. I hope I can do it without too much trouble as I put the chip guard back on today.

Well you can't have everything. I could not get the screw out with the chip gaurd in place, but I decided to just cut a hole big enough to slide it out. Problem solved.

I am having trouble finding much information on Trapezoidal Screw Profiles. If what I have worked out is correct, a 3mm Trap. thread has a depth of 1.75mm. This means that the minor diameter of a 14mm thread will be 10.5mm. I measured the screw today and need 12mm diam for the ends. I guess that rules out the 14mm diam screw.

I have also found a German company, Bornemann Gewindetechnik GmbH & Co. KG that lists many different diameter 3mm Trap. threads from 12mm to 60mm diam.



Dean

Hi there,

So does that mean that the 16mm x 3mm thread from the US would have a 12.5mm minor diameter? and you need 12mm. Have I missed something or would it be a simple process of turning down the ends by 0.5mm and fit this on you machine?

I'm with PDW, definately buy, don't make. Especially for those prices.

Simon

That is correct Simon, however due to the scarcity of information about the trapezoidal thread form I am not 100% sure about my figures. They are based on a drawing and table from the website of Bornemann Gewindetechnik. This is the only mention of thread form dimensions I have found on the net. My feed screw has deeper threads than this information would suggest. Using a small thread wire I have measured a depth of 1.8mm. Thread wire diam + depth caliper reading. The thread wire was not siting right on the bottom. Bornemann Gewindetechnik lists sizes in a table with the diameters set out in 3 columns headed "Array 1, Array 2 and 3. I don't know what this indicates. They list a 24mm under Array 1 and a 22mm under Array 2. The only limitations are the 30mm bore the screw has to fit in and the nut has to fit in a slot about 40mm wide.

I received a reply from Bornemann Gewindetechnik yesterday. Sergii Goman told me that they service Australia themselves (no local distributor) and asked me to send drawings of the screw and nut. I emailed back to say I was only looking at buying the parts and machine them myself. It was good to get a reply as the 3 other emails I have sent have not been answered.

Dean

Dean,
Does this help?
Metric Trapezoidal Threads.xlsx

Andrew.

Yes that helps. It provides more details than I had previously. I think all the samples I have found have been made under the DIN 103 standard. I was going to have a look at the standards.

Dean