Which Bronze to use ?

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Bob.Murphy
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Which Bronze to use ?

Postby Bob.Murphy » Sat Aug 17, 2019 10:34 am

A quick one for you Metallurgists out there . . .

I'm finally getting around to re-furbishing my 1950s Denford Viceroy Lathe. The main problem is slack in the Cross-Slide and Top-Slide nuts.

The 'nuts' are tubular pieces of bronze bored with a 1/2 inch 10 TPI British ACME Thread (29 degree cutting angle - Metric ones are 30 degree).

I have two lengths of 1 inch Bronze round bar - '660 Cast Bronze' and 'PB1 Phosphor-Bronze'.

The '660' is good for bearings and is easy to machine.

The 'PB1' is harder and resists wear better.

I don't want to wear the screw threads that they run on, but want them to last the distance.

My initial thoughts are - "Use the Phosphor Bronze" . . But what do the experts think :?: :?:

Thanks in advance.

Bob.
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Re: Which Bronze to use ?

Postby john.whiting » Sat Aug 17, 2019 12:05 pm

I d use the easy to machine bronze.............IMHO ,the main cause of screwnut wear is lack of oil......topslide ,Id check the play in the thrust bearings......usually there is no wear in the nut there............Cross slide screws are often LH acme ,and sometimes 2 start.....Tracy tools used to sell suitable taps.....you will will find some bronze is very difficult to tap,due to closing on the tap .You can also buy replacement acme rod in the us ,even hardened ,specially for lathe screws........some lathes had a two piece nut,and adjusting a wedge will reduce slack................you cant eliminate it without a new screw.

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Re: Which Bronze to use ?

Postby pappleton » Sat Aug 17, 2019 7:30 pm

I recently had this problem with my '48 colchester master lathe. The crosslide leadscrew had around .025" backlash, making parting off somewhat scarey. It was 5/8"x 5tpi LH Acme. I could not find any in Europe, but found 10tpi in the USA. I ordered that and a CI nut. Chopped the old leadscrew off and spliced on the new. I had to machine the nut to size, but in the end I ended up with .002" backlash and parting off is now a breeze.The crosslide is now also direct read. I can't see any problem with a CI nut on a steel thread - so long as it is lubed. The company I used was Green Bay Mfg, Marsh Hansen the guy I spoke to. Total cost with shipping and customs etc £110
Paul Appleton '53 VH plunger, '53 VHA rigid - in many boxes, '58 H.D. pan/shovel rigid

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Re: Which Bronze to use ?

Postby cmfalco » Sat Aug 17, 2019 9:06 pm

I have six types of bronze on the shelf, including phosphor and 660. For this purpose I wouldn't hesitate to use 660. However, your real problem will be cutting threads in it with less backlash than you already have. Sorry if I'm telling you something you might already know, but taps come in various classes of fit, i.e. slightly different diameters. The taps (and dies) most commonly available in all sizes and thread forms are fairly sloppy and this is what you will get unless you specify otherwise. You might have to inquire of several suppliers before finding one who has them available with a higher class of fit.

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Re: Which Bronze to use ?

Postby Bob.Murphy » Sun Aug 18, 2019 10:21 am

I have 1/2" x 10 TPI ACME 29 taps & dies in both Left & Right hand but I'm reluctant to use them for the reasons you state. They are a bit of a blunt instrument.

I have also got "Indexable" carbide tips for internal 10 & 8 TPI but they are quite substantial and the boring holder they fit is about 18mm diameter so too large for a 1/2 inch female thread.

My current plan is to find (or make) a small diameter tool holder that takes a square section cutting tip - fitted at 90 degrees to the axis. I'll then hand grind a 10 TPI tip (wish me luck). I haven't found such a tool yet and to cut an accurate square hole through a piece of round bar is going to be tricky. I'll need a square broach or something if I'm to avoid Pin Files and bad language. It looks simple enough when Adam Booth does it on You-tube (look up 'abom79', he has a vast number of 'Machine Shop' vids and a huge assortment of enormous and fascinating machine tools).

The screw threads look OK but are a bit worn in the centre as one might expect. The Top Slide should be quite straightforward to machine from round steel bar but the Cross Slide incorporates a gear for the powered cross-feed. This is machined into the shaft - not separate. Also I haven't yet worked out how to dismantle the Cross Slide shaft; the Top Slide has a small pin through the hand crank but the Cross Slide doesn't :!: I don't want to damage anything by trying to unscrew it. I had thought about re-machining the screw to make it consistent (and smaller) across its length. I'll then cut the thread in the 'nut' on the (other) lathe to suit it. What could possibly go wrong :roll: .

Its all interesting stuff which sounds simple enough but comes with a host of problems - just like every other 'project' in the shop :lol: .

Thanks for the feed-back, I'll go with the 660 Bronze ;) .

Bob.
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Re: Which Bronze to use ?

Postby Bob.Murphy » Sun Aug 18, 2019 10:29 am

pappleton wrote:I recently had this problem with my '48 colchester master lathe.


Ooooh nice ! I'm currently looking at Colchester Triumph lathes - 50" between centres - as I'm becoming more & more disillusioned with my Chinese 'Warco 280VF'. Can I justify such a thing after spending all that money on the two lathes (and a Mill) that I've got. Its just for 'Hobby Jobs' after all :roll: .

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Re: Which Bronze to use ?

Postby nevhunter » Sun Aug 18, 2019 1:07 pm

You would be better to split the nuts into two down the middle normal (90 degrees)to the axis and have a means of adjusting them angular wise to each other .You can then adjust the play out of the equation for a long time until the threaded shaft wears unevenly enough for it to matter. Nev

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Re: Which Bronze to use ?

Postby cmfalco » Sun Aug 18, 2019 5:13 pm

Bob.Murphy wrote:taps... are a bit of a blunt instrument.
I mentioned different classes of fit with taps because I didn't know your background. This is a case of the "right" way of doing something being more complex than the much easier "possibly-OK" way and I didn't want to be a person who insists it must be done the right way in case you weren't in a position to do it that way.

I have indexable threading bars (and even Whitworth-form inserts) with shanks down to 1/8"-ID, but even with the right bar and insert the problem in making this bespoke nut is getting the size right. By 'right', I mean having the desired tight, but not too tight, fit on a threaded rod that is buried in the lathe. If the rod were separate you could sneak up on it by testing the fit after every pass that removed another thou. But, you can't do that without disassembling your lathe, and if you disassembled your lathe you couldn't make the nut.

In any case, the only way you'll know if you succeeded is after you've made the nut and disassembled the lathe to test its fit on the shaft. Given this, something to think about is buying a tap that would give the tightest fit (i.e. having the class of fit giving the smallest ID), making the nut, disassembling the lathe, and testing it. If it gives an acceptable fit, great. However, if the nut you just made is too tight it now is a no-go gauge, and if too loose you can use it to judge how much smaller it needed to be. Then reassemble the lathe and make the next nut with an indexable threading bar using the information obtained from the first nut.

Nevhunter's split-nut suggestion would be a good one to pursue, but only if the cavities surrounding the two nuts are large enough to fit the required clamping mechanism.

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Re: Which Bronze to use ?

Postby pappleton » Sun Aug 18, 2019 6:39 pm

This is a bit Heath Robinson (Rube Goldburg to Charles) But Colchester used to almost cut through the crosslide nut and then had a tapped hole in the crosslide above the nut. A pointy grub screw could be wound down into the nut, forcing the two apart. This is OK if the leadscrew is worn evenly (ha de ha), but may it take some of the backlash out. I'd be a bit reluctant to replace either the nut or leadscrew on its own.
BR, Paul
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Re: Which Bronze to use ?

Postby cmfalco » Sun Aug 18, 2019 7:12 pm

pappleton wrote: I'd be a bit reluctant to replace either the nut or leadscrew on its own.
That's an excellent point. Using the DRO I added to my mill a few months after buying it, I measured the backlash of the X and Y lead screws. For the X the backlash is 0.001" at one end and 0.002" at the other end, 24" away. However, it increases toward the center, reaching a high of 0.023".

The near-zero backlash at the ends means that my mill's equivalent of the nut in your lathe has essentially no wear. This means the backlash in the center is entirely due to wear in the center region of the lead screw. But, even if the backlash at both ends were, say, 0.010", a new nut could reduce that to ~0 but it still would leave the backlash in the center ~0.013" because of the additional wear in the center region of the lead screw. If I wanted to reduce the backlash in the center region of travel, only a new lead screw would fix that problem.

Note that backlash doesn't just affect rigidity on your lathe, it also affects accuracy. Always approaching the work from the same direction of the dial, to eliminate the backlash, doesn't solve this accuracy issue. As a hypothetical example, assume you set the dial on your cross slide to '0' when it first touches the OD of a 4.000"-dia. piece, and the nut/lead screw has zero backlash at that point but, say, 0.024" backlash when the tool reaches ~2" dia. If the wear on the nut/lead screw is symmetric it means the position of the tool will be 2.012" when the dial indicates it's at 2.000". My mill has the same issue, but the DRO takes care of it as long as I go by its readout and not the dials on the handles


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