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Member postings for Cupboard

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Thread: Boxford metric lead screw fitted to imperial lathe?
11/11/2019 12:34:25

Just to close this because the problem is now completely solved. Skip to the last paragraph if you don't want to read the process.

I made a mistake in my initial assessment. The lathe was cutting incorrectly but not for the reasons I thought. I purchased a thread pitch gauge and made some more careful measurements and the leadscrew is genuinely imperial which is, I guess, good.

Using some information from this thread I made a giant spreadsheet with all the various gear ratios, and what pitches each setting would create. I then cut a full range of A, B, C, D, E and 1, 2, 3, .... 8. From that I determined that the ratios between each was correct - so A-B halved the pitch, C-D halved, etc. 1-2 made the equivalent ratio of 8-9 but actually did more like 8.5 to 9.5. Based with that information, I was fairly confident there wasn't anything horrifically wrong with the gearbox.

I checked all the gears in the train under the left hand cover and they all matched what they said they should be, despite the lathe behaving as if the 56 tooth gear was actually a 60 tooth gear (if you put 60 teeth in the spreadsheet, the numbers matched the measurements).

I then took the gearbox off, checked all the gears that transferred drive across the gearbox matched, etc. which they did.

Whilst doing that, I noticed that there was a non-original looking spring washer and spacer washer holding the leadscrew drive gear on. For some reason, that gear had been put on backwards and with a very small tweak from the spacing washer it will mesh with the incorrect gear. It, as it turns out, is supposed to mesh with the final gear (gear 8 under the right hand lever) and it had been put together meshing with the penultimate one, gear 7. That threw the overall ratio out slightly, leading to my issues. I've now flipped the gear round (there's a spacer on one side of it), removed the extraneous hardware and it meshes perfectly with the rest of the gearbox and give the correct ratios.

Once again thank you for all the helpful advice, numbers and support. It gave me enough of an understanding to work out the problem and fix it. Now I have a properly working imperial lathe, I just have to metrify it which is much less daunting with everything as standard!

30/10/2019 17:30:02
Posted by Brian Wood on 30/10/2019 13:59:03:

Interesting as all this has become, it is perhaps easy to overlook the fact that it only came about because of the rogue fitting of a metric leadscrew to the imperial screwcutting gearbox on an imperial lathe, giving rise to the work-arounds that have been proposed to save the owner the expense and hassle of re-equipping the lathe

What might be instructive now would be to see if similar solutions can be found for a case where a metric gearbox has been mated to an imperial leadscrew.

I feel a full bottle of gin might become an important aid in bringing those answers to an eager public!!

Brian

I'm glad this thread had drifted in to such interesting realms!

Although if anyone wanted to flog me a metric gearbox...

28/10/2019 19:26:59
Posted by Brian Wood on 28/10/2019 18:18:23:

Hello again Cupboard,

I have been "playing" again with your dilemma and I think there is even a way round the 3 mm pitch leadscrew [without having to change it] to be able to get imperial pitches as direct reading values as if the lathe was equipped with the standard 8 TPI leadscrew.

What is needed is another set of adjustment gears interposed between the 20 and 56 gears to fool the system into thinking it has the right leadscrew

These are a compound pair, running in place of the 80 T spacer of 55 teeth mated to 52 teeth.

Thanks, that actually makes some sense. One rotation should move the carriage 3.175mm, with a 3mm pitch screw it actually moves it 3mm, so I need to fool it in to moving a little bit faster than it "thinks" it should.

(3/3.175)*(55/52)=0.9994

Which is near enough!

 

I need to find/buy/make a device to hold a pair of gears together so I can make whatever compound I need.

Edited By Cupboard on 28/10/2019 19:30:32

27/10/2019 19:21:59

OK, so instead of the big 80 tooth gear, I'd put a pair of gears in (at the relevent ratio) on the K shaped arms instead.

I've got a friend printing me a 24 and a 32 tooth gear (one in PLA and one in PETG as a comparison) to see what they perform like, and I'll do some experimenting from there.

If I'm taking the big gear out, I think that means I'm going to need another stud to put the additional gear on.

Sorry I'm being a bit slow on the uptake here.

27/10/2019 16:43:52

Thank you so much, and DC31k that table is fantastic.

Just to check I'm understanding things correctly, when you say "change gear ratio", I think 1:1 would mean something like a 40 on the top gear, 80 on the middle/idler and 40 on the bottom gear? Then 7:10 would need a 35 and a 50?

I think my best bet from here, assuming I've understood things correctly, is to get a pile of gears printed for testing purposes, try a few combinations then depending on how I get on and what I decide possibly look at the leadscrew from Ian P

It's quite frustrating being thousands of miles away from my shed, but got to earn the money to buy random chunks of rust somehow!

26/10/2019 21:07:21

It's a model A, not UD and the standard bed length.

The 17 threads was certainly pretty close, nearer that than anything else but admittedly measured fairly quickly with calipers.

Sadly I didn't think of trying some of the other pitches whilst I was at the lathe earlier and I'm now away for a couple of weeks. I am, however, going to want to cut more threads than I think I can currently do and I've no idea how to work back to imperial!

Thank you all for the advice

26/10/2019 20:13:13

I've sent you a PM!

I *believe* that cutting metric on an imperial (or vice versa) requires transposing gears, then certain changewheels and gearbox settings. My plan is to get them 3D printed as the 127/100 tooth gear is about £150 otherwise.

I think going the other way needs a 135/127 gear.

26/10/2019 19:29:00

Just to confirm, it cuts threads very nicely although you'd have a job fitting them to anything.

Set it to 8tpi this morning and got 17 threads in 50mm!

I think my options are

1) buy an imperial leadscrew

2) buy a metric gearbox

3) work out a gear combination with the changegears to get something usable, though I guess that's going to involve a 127 tooth gear

I want to be able to cut both metric and imperial threads on it, though will spend more time in metric land. For that reason a metric gearbox would seem like the attractive option on the assumption that it's changeable, though I guess that will cost a bit more.

Opinions would be gratefully received

There's a leadscrew on ebay at the moment I'm waiting to hear which pitch it is.

25/10/2019 22:45:02

I've got a new to me Boxford Model A that I'm rather mystified about.

It's an imperial lathe, with an imperial gearbox. There is some brazing on the gearbox so it's obviously had a repair at some point.

In the geartrain behind the left hand door it goes out of the tumbler reverse in to a 24 tooth gear that's attached to a 20 tooth gear that drives the 80 tooth idler. That then feeds a 56 tooth gear and in to the quick change gearbox. I also have a 40 tooth gear that's on the same keyed shaft as the 56 tooth gear but isn't currently in the train. So far this matches with the information on this page http://www.lathes.co.uk/boxford/page7.html

I then went to measure the leadscrew which should be 8tpi, but when I set my calipers to a round number of inches they don't really line up with the thread. It's vaguely near to 25 threads in 3 inches, but it's actually a bit less than three inches. If you change over to metric, everything lines up nicely and it's a round 20 threads in 60mm.

I believe I've got an imperial lathe with a metric leadscrew fitted, whether that's a bodge by a previous owner or it's supposed to be like that I've no idea.

Can I actually use this to cut any useful threads, and if so how to I work out what threads it's going to cut?

Thanks in advance!

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