metric on imperial

Mike,

Thanks for the response, that is pretty much what I guessed.

However, I would be unlikely to come up with any of my above combinations based on the standard set of 16DP 14.5 PA gears for my 4tpi leadscrew Mellor lathe (20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 85, 90, 95, 100, 105 and 127) - I am missing 127 and all of the gears 50 and below from my set which in itself presents a challenge, I am slowly making the missing ones myself from dural plate.

The 127 gear will be the biggest challenge - at 204mm or 8" Crest Diameter it is bigger than what I can turn on my 3.5" non gap bed lathe, so I will have to make it entirely on my milling machine and rotary table.

Edited By Timothy Moores on 10/06/2016 15:40:22

**LINK**

Hello Timothy,

The approximation 47/37 was mentioned earlier in the thread, it will give perfectly adequate results for all but the most demanding of pitch cutting. The actual value is 1.2703 to four places of decimals and in real terms the error in use is plus 0.27 tenths of a thou per inch of thread.

It is not quite as good as the combination 80/63 which gives a negative error of - 0.16 tenths of a thou per inch of thread, but it does offer the real chance of making gears in those sizes on your lathe.

Yes, the tooth count is awkward, but that is unavoidable.

Regards Brian.

Hi Brian,

I am not disagreeing with anything you are saying, and it was actually one of my posts in this thread that first mentioned 47/37.

That combination along with the other options I mentioned were all found by setting up a quick spreadsheet of 127x127, one value divided by the other, and then looking for values as close to 1.27 as I could find.

My following questions were more around trying to see if any of the various websites, apps, etc could also find these or other alternatives to a 127 gear, using values outside of ones known gear set, that might help others less inclined to run up a spreadsheet and manually search for possible close division values.

Hello Timothy,

Ooops, sorry. I am usually quite critical, privately that is, of others who only read the number plate in front of them so to speak and here I am doing the same!

I am not clever enough to generate spread sheets nor interpret what they try to tell me, so I use other methods like continuous addition. I don't though see how your approach will alter the fact that only certain fractional combinations will come close to the magic value of 1.27. It must surely depend on what error one is prepared to accept, there are other combinations that give larger errors and with gears that are reasonably sized.

Regards

Brian

A possible solution if the 127T gear is too big to machine is to change the DP. If the DP is chosen to be 40 then the gear will be half the diameter of a 20DP gear. It would of course mean you would need a similar transition gear to mate with it which is also 40DP and to compound it with the other gears in the train. Whether such gears are readily available from a supplier is another moot point!

One solution I was considering a while back was to purchase some 56DP gears - 127 is only 58mm, 150 is 69mm, 100 is 46mm and 50 is 23.5mm (all Crest Circle Diameters), these would easily fit on pretty much any lathe.

These gears are readily available from Sherline at $19.50, $38.00, $16.25 and $9.75 respectively, simply bore out the centre hole to your desired size and cut keyways as required, postage to UK should not be excessive as these are pretty small and light.

Sherline actually makes use of different DP gearsets anyway, most of their change gears being 24DP, then using 56DP for their metric translation options.

Edited By Timothy Moores on 13/06/2016 11:13:38

Problems are likely with a Boxford and similar re changing DP as the headstock gear can not be changed as it is part of the main spindle and drives the tumbler gears.

Should still be possible as you will be able to change the leadscrew gear and the gear that drives it, have these as 56DP and whatever others you require at whatever DP is normal on your lathe - 127/100 (or 50 or 150) does not have to be a compound gear, just needs to be in the gearchain.

For example 1.25mm from 8TPI lathe - A=40, B=80, C=100, D=127, compound 80 at your DP with 100 at 56DP

Edited By Timothy Moores on 13/06/2016 11:51:13

I've found in the past that both of these applications work

**LINK**

It's been a long time since I used either of them but vaguely remember that one has a precision setting and that it's worth playing around with that. I only used the with gearbox one and I may be confusing usage with something else.

On Boxfords the 100/127 gear probably has more merit on lathes that do not have the screw cutting gearbox. Where lathes have the gearbox there isn't much point in buying it - eg one of the charts in my album

If some one wants to produce charts like this on lathes with the gearbox it can be worth forgetting 100/127 etc and producing a spread sheet that works through all of the pitches the lathe can do and then trying combinations of different change wheels driving it. Curiously they seem to have inbuilt ratio's that help obtain very close approximations.

John

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Edited By Ajohnw on 15/06/2016 09:31:47

Dave Smith the 16th,

Much earlier in this thread you were posing the conversion problem the other way round; ie to convert metric to imperial on a Boxford lathe. The factory combination wheel is 135/127 teeth for the conversion, that might be about as hard to find as unicorn hair now.

A perfectly reasonable, and accurate, conversion is possible using a ratio of 17/16, or suitable multiples thereof. I hope that helps you.

Regards Brian