metric on imperial

hi all i have a boxford model A ive read that to cut metric threads i nead a 127/100 change wheels these seem expensive and hard to come by i saw on a john mills video that he used a different combination my question is

A /can you use a different combination of redilly available change wheels.

B/ where can i get a chart for the the gearbox settings.

You can get close without the 127 gear. I have the opposite at present. I have a metric machine and need to cut an 8TPI thread.

I can get close with with a 20/66 although gearsVB6 says 40/66 ?

or a 24/80 Gearsvvb6 says 33/52? Not done the maths yet to see which ones closest.

Thread on here recently (i think) where a 66 gear was suggested instead.  Or maybe i deviated to another site?

Edited By Dave Smith the 16th on 08/06/2016 20:50:42

If 127:100 works so will 80:63 as near as makes no practical difference.

127/100 = 1.27

80/63 = 1.26984

Error is 0.012% or one tenth of a thou in an inch.

Neil

Edited By Neil Wyatt on 08/06/2016 22:17:52

The 127 tooth gears are enormous and difficult to fit into a train. I have one for the Myford but have always ended up using the 63 tooth approximation anyway. To fit it in the train I would have to cut a slot in the base the lathe is mounted on. I guess if I wanted to screwcut a leadscrew for a lathe I might go to the trouble, but nothing I have ever done has needed any better accuracy than the approximation.

John

This simple gear calculator from lathenovice is easy to use.

http://www.imagesalad.com/lathenovice/lathegears/lathegears.html

Mike

However 63 is not a common gear in the general Boxford compatible second hand market.

Hang on a bit as Brian W was looking at alternatives a while ago like the simple ones for the Myford. he might come on and tell where he got to.

That gear calculator mentioned above comes out differently for my machine.

My metric machine with a 2mm leadscrew.

Picture below on my manuals figures and the figures from the calculator above.

(although i do wonder if my manual is correct, no mention of the high/low gearbox and the motor drives the spindle directly)

Martin Cleeve's inexpensive book "Screwcutting in the Lathe" (Wortkshop Practice Series of books) includes a couple of right handy charts of change gears to cut metric threads on imperial lathes without needing any special gears, just compounding the standard set. Well worth the cost of the book just for the charts (about 7 quid, delivered.)

Hello Kevin,

This table may be of interest to you.

a Hercus model A lathe is identical to your Boxford model A.

Regards John

Raglan used the fact that 44/52*30 almost equals 25.4 for the Raglan gear train. Might that help to sort out an alternative? Probably not, but just a thought....

Hi Dave,

If your leadscrew is 2 mm pitch and you want to cut an 8tpi thread, which has a pitch of 3.175 mm, then you need to turn the leadscrew by 1.5875 turns for every turn of the spindle. Your first combination (your manual) gives a ratio of 72/40 = 1.8 which is hopelessly out. The second combination (gear calc) gives a ratio of (20/24) x (72/60) = 1 which is also hopelessly out .

If you use the lathe novice calculator that I posted above the best combination is A=50, B=72, C=80, D=35. This gives a ration of (50/72) x (80/35) = 1.5873 which is an error of 0.013%. This is an insignificant error.

Mike

A result of '1' suggests the online calculator is set for a lathe with an imperial leadscrew...

Neil

Some other possibilities for imperial/metric translation gears, depending on which gears you may have available (or be willing/able to make) in decreasing levels of accuracy:

47/37 = 1.27027 or 0.021% error (2 tenths of a thou in an inch)

66/52 or 33/26 = 1.269231 or 0.061% error (6 tenths of a thou in an inch)

61/48 = 1.270833 or 0.066% error (6 tenths of a thou in an inch)

The lathe novice program actually 17 different combinations of gears which have a low error.

Mike

The availability of gears outside those produced by Boxford themselves is close to zero and while you can probably do most metric pitches if you have a a full set of gears, and BA pitches too, the need for a conversion gear doesn't ever really go completely away. For what it's worth I can't rever recall seeing an 84/80 compound gear as in the Hercus chart on sale in the UK.

The 100/127 easily fits inside the end cover, from my reckoning it's around £46 new from http://jwlatheparts.blogspot.co.uk/2011/10/new-british-made-lathe-parts.html

Edited By Martin 100 on 09/06/2016 17:35:00

Michael Cox 1, Not an 8TPI there.

The left hand set is what my manual says i need for a 2mm pitch thread. The right hand set is what the link that you gave shows i need to cut a 2mm thread.

Something not quite right there.

The reason 63 is a more useful number than some others is that (1) for most leadscrews and gearboxes it can be mated with 'common' gear sizes, meaning only one special is needed and (2) it factors to 21 and 7, which mean it has other uses.

Neil

Dave,

If you click on the arrow at the side of the leadscrew box it will give you a long list of both imperial and metric leadscrew pitches so just select the pitch you have which is 2 mm. Where it says target you can input a pitch in mm or a tpi that you want make. You must then click to indicate whether it is tpi or mm oi the box to the right of the target. You can put in 8 as the target and click tpi. Then press search combinations and it will come up with the best combination. To reveal other possible combination then click the drop down button at the side of the results.

You can also add in other gears in the boxes below the row headed my gears, if you have others that are not listed.

If you have a 2 mm pitch leadscrew and you want a to make a 2 mm thread then the final gear ratio required is 1 which is the result given by the gear calculator but not by your manual.

The LatheNovice gear calculator is extremely versatile and I have been using it for a couple of years and it works very well.

Mike

How do I persuade it to come up with combinations outside of my set of available gears?

My examples above are possible low error alternatives to a 127 gear, none of which are in my current gear set.

I would contemplate making 63, 37, 47, 66, 52, 33 or 26 tooth gears as they are easily divisible with my standard set of dividing plates. 61 is a prime number, so 61/48 not really feasable.

Timothy,

In the rows labeled "My Gears" you can put in what gears you have and any other gears that you might want to try and then see what answers come up.

Mike