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Boxford metric lead screw fitted to imperial lathe?

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Cupboard25/10/2019 22:45:02
9 forum posts

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!

SillyOldDuffer26/10/2019 17:26:33
4864 forum posts
1021 photos

Bump really, as I don't own a Boxford!

Didn't someone else recently start a thread about an Imperial Boxford with a Metric lead-screw or maybe it was the other way round?

The lathe will certainly cut threads but as Tom has spotted, it may not be easy to find the gear combinations needed. I won't attempt to explain as I'm dreadful at maths but knowing which gear wheels are available and the lead-screw pitch allows the calculation. There's a helpful forum member who's written a book covering the subject but I'm having a senior moment. It's embarrassing! Can someone with a brain remind me who I'm thinking of?

Dave

Meunier26/10/2019 17:50:51
263 forum posts
1 photos
Posted by SillyOldDuffer on 26/10/2019 17:26:33:

There's a helpful forum member who's written a book covering the subject but I'm having a senior moment. It's embarrassing! Can someone with a brain remind me who I'm thinking of?

Dave

Perhaps because I bought a copy rather than the brain
Brian Wood, available from ARC or even on-line. Great book and deserves a pat on the back
DaveD

Thor26/10/2019 17:52:31
1138 forum posts
32 photos

Hi Dave,

I'm assuming you are thinking of Brian Woods book, Gearing of Lathes for Screwcutting?

Thor

Meunier26/10/2019 18:03:26
263 forum posts
1 photos
Posted by Thor on 26/10/2019 17:52:31:

Hi Dave,

I'm assuming you are thinking of Brian Woods book, Gearing of Lathes for Screwcutting?

Thor

Yes indeed Thor, that's the one, thanks for adding the title !
DaveD

Cupboard26/10/2019 19:29:00
9 forum posts

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.

Ian P26/10/2019 20:01:27
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2254 forum posts
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I'm not sure what bed length the lathe was but whilst I no longer have the lathe I do still have the leadscrew which I am sure is imperial. Whereabouts are you located.

I no longer have a Boxford lathe but the 5" metric AUD with gearbox I am sure was able to cut metric and imperial threads without swapping any changewheels (or maybe my memory is playing tricks)

Ian P

JasonB26/10/2019 20:10:42
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Posted by Cupboard on 26/10/2019 19:29:00:

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

Is that an exact 17 threads or almost as you are very close to 3mm pitch. Which is what I would expect if box is imperial as the geartrain for 8tpi would give 8tpi with imperial screw and 3mm with metric.

Try 24tpi abd see if you get 1mm pitch, 12tpi 2mm pitch, 18tpi 1.5mm etc.

Cupboard26/10/2019 20:13:13
9 forum posts

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.

Clive Brown 126/10/2019 20:59:32
287 forum posts
7 photos

If you change the leadscrew for imperial, then you will have a fully imperial lathe for which the 127/100 gear will then give you a full metric range. The replacement leadscrew should really be matched to imperial half-nuts, but wear just might take care of that if the existing ones are indeed metric.

As the lathe stands, as JasonB points out, you should get a number of metric pitches, with no cost involved, The 135/100 gear is intended for a fully metric lathe and will need some thinking out if used with your imperial 'box.

PS, if buying a leadscrew, some early AUD lathes were 16" centres, with correspondingly shorter leadscrews, but they are mostly 22".

Ian P26/10/2019 21:07:18
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2254 forum posts
91 photos

The leadscrew I have came off a lathe that had no gearbox and I dont know what the between centre distance was, do you know what the overall leadscrew length should be if a gearbox is fitted?

Ian P

Cupboard26/10/2019 21:07:21
9 forum posts

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

Clive Brown 126/10/2019 21:59:30
287 forum posts
7 photos

The overall leadscrew length of a model B/C 22" crs. Boxford is 44" approx. and 6" shorter for a model A.

The longer leadscrew can be shortened in order to fit a gearbox, using the lathe itself.

fishy-steve26/10/2019 22:10:30
121 forum posts
30 photos

It does look like you will need to purchase an imperial leadscrew and clasp nuts. Also 24,26,28,32,36,44,48 and 127/100 change gears if you have an imperial model A but want full metric screwcutting functionality .

+1 for Brians book.

JasonB27/10/2019 06:25:08
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It would have been 16 and 2/3rds of a thread over 50mm. When you are back in the shop try measuring over a length that is a multiple of 3, if you threaded the rod a bit more then 51mm would be ideal (17 x 3)

You can get the imperial threads buy using the same 135/127 approximation as that will mean your leadscrew turns approx 1/8" rather than 3mm and in effect converts it back to an imperial. as 135/127 is just a touch more than 3.175/3.

Need to decide what you do more, leave as is for some true metric pitches with approx imperial, or change back to an imperial screw to get exact imperial and approx metric.

Edited By JasonB on 27/10/2019 06:28:38

DC31k27/10/2019 08:02:33
82 forum posts
Posted by JasonB on 27/10/2019 06:25:08:

You can get the imperial threads buy using the same 135/127 approximation...

I am confused by the above. Is it simply a typo? Surely if you have a 127 gear and use it correctly, it is no longer an approximation but is an exact conversion.

To the OP, your fundamental issue is working out all possible input: output ratios of the gearbox, forgetting at this stage the change gear train (or set or assume it is 1:1).

You need to see what one turn of the gearbox input shaft does to the output shaft. If you do this systematically for all possible lever positions, that will tell you the ones that will be useful for metric screwcutting.

If you look at the picture here: http://www.lathes.co.uk/boxford/page7.html the left lever is a range lever, that goes from 0.5:1 (A) to 8:1 (E). My guess is that B is 1:1, hence the numbers in the B-row are likely to be the actual gears in the gearbox (the tooth counts seem sensible for a Norton gearbox).

Thus use of the 16 setting will be useful in gearing your 3mm leadscrew by factors of 2 (1.5mm, 0.75mm). The 18 setting has a factor of 9 in it which might not be useful (Jason's original post above where he says try 18tpi for 1.5mm is, I think, a typo - it should say 16tpi); the 20 a factor of 5 (0.8mm or 4/5, 1.25mm or 5/4); the 22 a factor of 11 (not too useful for metric, unless you want 5.5mm or 11mm pitches); 23 not useful at all; 24 has 3 in it; 26 has 13 (not useful); 28 has 7 (perhaps for 1.75mm 7/4).

Your overall calculation will go: [change gear ratio between spindle and gearbox input] x [range box ratio] x [Norton gearbox ratio].

Edited By DC31k on 27/10/2019 08:03:15

JasonB27/10/2019 10:09:26
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Posted by DC31k on 27/10/2019 08:02:33:
Posted by JasonB on 27/10/2019 06:25:08:

You can get the imperial threads buy using the same 135/127 approximation...

I am confused by the above. Is it simply a typo? Surely if you have a 127 gear and use it correctly, it is no longer an approximation but is an exact conversion.

Well I can get the 100/127 to work as 100/127 x 3.175 = 2.4999997 so near enough 2.5

but when it try 135/127 x 3 I get 3.1889763 not 3.175

Unless I'm doing it wrong.

DC31k27/10/2019 10:38:03
82 forum posts

OK, assuming it has the same gearbox as that shown at http://www.lathes.co.uk/boxford/page7.html BUT with a 3mm pitch leadscrew, the following should work.

mm is metric pitch required. TPI is gearbox setting. CG is change gear ratio. Only needs 35, 40, 45, 50, 60 and 70 gears with one duplicate. Best to copy and paste somewhere using a monospace font.

mm TPI CG
3.0 8 1/1
2.5 8 5/6
2.0 12 1/1
1.75 12 4/7
1.5 16 1/1
1.25 24 5/4
1.0 24 1/1
0.9 24 9/10
0.8 24 4/5
0.75 32 1/1
0.7 24 7/10
0.6 40 1/1
0.5 48 1/1
0.45 48 9/10
0.4 48 4/5
0.35 48 7/10
0.3 80 1/1
0.25 96 1/1
0.2 96 4/5

DC31k27/10/2019 11:22:52
82 forum posts
Posted by JasonB on 27/10/2019 10:09:26:

Well I can get the 100/127 to work as 100/127 x 3.175 = 2.4999997 so near enough 2.5

but when it try 135/127 x 3 I get 3.1889763 not 3.175

Unless I'm doing it wrong.

We should keep firmly in our heads that we are discussing machines with a (Norton) gearbox. The ratios in the gearbox of a native imperial machine will be different to those in a native metric machine. The imperial Boxford above is a good example - it has, for example, a ratio with an 11 in it. You would not find a native metric machine with an 11 in its gearbox.

[Interestingly, it has a 23, half of which is 11 1/2, used for American pipe threads but not UK ones and does not have a 19 which we do use. Uncle Tony's site says it was a copy of the US South Bend and this is very apparent here].

The difference in gearbox ratios affects our choice of translation gear, so a gear to go from imperial to metric on an imperial machine will differ from that used going from metric to imperial on a metric machine.

If I might say so, there is no place whatsoever for decimals in any discussion of screwcutting. To avoid confusion, fractions are the only acceptable currency. One inch is defined as exactly 25.4mm, or 254/10 or 127/5.

Thus our 8tpi leadscrew has a pitch of 1/8" or 127/(5x8) = 127/40 mm. If we drive this with a 100/127 combination, the 127's cancel and we get 100/40 or exactly 2 1/2mm .

This is where use of decimals and a calculator lets us down. With fractions, you can see the numbers and you know that the 127's must cancel, so you know that one side of the multiplication sign has to have 127 as numerator and the other side of the multiply sign has to have 127 as denominator. If you have a 127 gear, the conversion will always be exact. If you come to a place where it seems approximate, you have taken a wrong turn somewhere.

Now onto our 3mm pitch leadscrew and our 135/127 gear. 1mm is 5/127 inches. 3mm is 15/127 inches. 15/127 x 127/135 gives 1/9 inches exactly. So our 3mm pitch leadscrew is behaving as if it is 9tpi. This number seems strange at first sight, but it probably combines better with the metric-native ratios in the gearbox to give the required imperial pitches.

not done it yet27/10/2019 11:28:32
3583 forum posts
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I somehow think the gearbox is a bit of a red herring. Any lathe with a gearbox and some change wheels seems quite able to cut either imperial or metric threads. The gear box is usually set for most common ratios, not a particular thread size as ‘most’ threads are ‘sensible’ ratios of another thread pitch.

My lathe with a QCGB simply involves changing a couple of change wheel gears to enable it to cut all the threads I am likely to need, some that I will likely never need - and only unable to cut the odd thread which I doubt/hope I would ever need!

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