magic 127 TOOTH ?

For cutting metric threads on an Imperial lathe with 8 or 10 tpi leadscrew, you can do it with the standard set of change gears, no 127 or 63T needed. And accuracy is in the range of 1 in 3000 to 1 in 8000. IE one thou in eight inches.

Martin Cleeve's book Screwecutting in the Lathe has the gear charts to do it. He uses compound gearing with tandem gears on both studs on the quadrant, but all with the standard set of gears. Luckily for us, he sat down and did all the math and made up the charts ready to use.

I think it was Professor Chaddock who worked out the standard change-gear set gives some phenomenal hundreds of thousands of possible compound combinations and resultant ratios. Almost inevitable that some of them would be almost dead-on metric pitches.

I've used Cleeve's charts and method a number of times to cut metric threads with perfect results by home workshop standards.

Even more magic than a 127 toothed gear is of course the Electronic Lead Screw. An ELS can cut threads of any pitch or TPI, standard or not. And it doesn't require the owner to do hard sums, understand gear trains, or get oily!

Want one!

Dave

Or even more magic, CNC, cut all those threads, tapers, balls, other odd shapes, etc etc....

In the "good old days" an 8tpi imperial lathe was commonly supplied with a set of change wheels - typically 20-65 in 5 teeth steps which would allow the cutting of any desired imperial screw thread. There are 151,200 permutations using any 6 of these 10 gears for drivers or drivens*. Finding a combination of these gears to give the correct pitch for a metric thread needed a systematic approach, hence the use of a 127 gear or the 63/100 ratio. However, in these days of computers we can can look at all those permutations very quickly and pull out a result that is close to the one we require - as in the table below:

The maximum pitch error is 1 micron.

*not really true since a driver combination of 20,30,40 is the same as 30,20,40 but a bigger number always looks impressive

Rod

Nice clear tabulation of some very useful numbers, Rod.

Thanks for posting it.

MichaelG.

All very interesting.

My lathe was made over one hundred years ago ,(Pittler B2) and can only cut imperial threads.

Are the TPI on it based on a longer inch? i.e. the pitch is slightly longer?

TPI is simply the inverse of pitch (and vice versa, of course).

Examples: 8tpi has a pitch of 1/8”; 50tpi would have a pitch of 1/50”; 1.5mm pitch will have a tpmm of 2/3; 2mm pitch would b 1/2 tpmm.

The latter metric examples, such as the 2mm pitch, might be more easily expressed as 5tpcm of course, or in SI units 500tpm. Usable systems just pick sensible values, of course.

Not sure what you mean by different inch lengths? All threads could be quoted in imperial or metric - but most would clearly be pretty useless figures and may not comply with any standards. It’s just a case of maths to convert and common sense to have a sensible working system. Imperial and metric systems both work, if sensible threads are used, but are different systems altogether.

Make sense?

Possibly, in theory. But in practise the standards used in the factories would have been orders of magnitude less accurate than the variations in the definition of the inch.

Andrew

My head hurts now. My blacksmiths brass rule only as 1/8" marked on it !

Guy

Another method is to cut or purchase or print 37 and 47 and introduce them i to the gear train 47/37 is very close to 1.27 ie 1.270270

I use the ML7 gear calculator by Duncan Munro, available here: http://metal.duncanamps.com/software.php

It's a very versatile program which can be set up to calculate which gear trains to use from the standard set, or your own set, or from any imagined set. It can also be altered to calculate gear trains for other lathes.

I used it to work out the gear trains for my own collection of Myford gears, transferred the results into a spreadsheet, tidied it up and use that in the workshop. Out of interest I put in a column calculating errors (the original program doesn't do that) and was surprised at how accurate many of the results are. I needn't have bought the 37 and 47 gears after all.

George

Another good calculator is the one at LittleMachineShop, it allows for any set of gears and any lead screw pitch and will show all possible exact combinations or closest alternatives with the gear set you listed including the error % and error in 10 pitches for each combination. It also allows you to specify min and max values for pairs of gears you can fit on your banjo, so you can restrict it to only show combinations that would actually fit for your lathe.

Me too Gordon. I don't cut that many metric threads on the S7 but when I have, a 127 & 50 gear set-up has worked just fine. You have to keep the cover open (so a bungee cord to keep it out the way is useful) but that's about it. Of course you need the 127 gear - but Chronos used to sell them and I think mine was about £13! May have been a few years back though....

Regards,

IanT

Out in the real world where folk simply need to cut a metric pitch on an imperial lathe the great advantage of the 127 gear is that it allows things to be done in an orderly fashion that can be simply tabulated. Mostly requiring only one gear to be changed for a useful range of threads.

The other effective tabulations using standard gears tend to hop around with the changes needed. Generally something of a pain. Rodericks table is about the neatest I've seen in that respect. Some versions are horrible.

Its all pretty moot if you have a screw-cutting gearbox. 127 is usually the only practical way of going about things. Especially as the folk who made the lathe usually provide you with a spiffy table of what to use when.

Generally the conversion set-up is the only compound gear train needed for the normal range of threads with most screwcutting gearboxes so its frequently practical to leave the 127 gear permanently mounted. My metric Smart & Brown 1024 has a nice roller bearing stud for the intermediate, 120 tooth, gear in the gearbox driving gear train. The metric to imperial conversion uses the 120 driven - 127 driver pair so it was asimple matter to bore out the 127 gear and bolt it on the outside of the 120 gear to align with the gearbox input gear in its conversion position. Changing from metric to imperial threading is now a simple matter of pulling off the spacer and standard 120 tooth gearbox input gear behind it then refitting the spacer followed by the appropriate gearbox input gear. A considerable improvement on the standard process which requires the banjo to be removed and the roller bearing stud replaced with a plain one before fitting the standard conversion gears.

This set-up has cost me 4 module pitches and, I think, 3 small BA pitches which worries me not at all.

I imagine something similar could usefull be devised for other machines.

Clive

It may be possible to copy what some of the far eastern lathes do.

The Warco BH 600, Chester Craftsman, Engineers Tool Room BL12-24 , and probably many of their successors, have Norton gearboxes driven via a 120/127 Idler. To change from Imperial to Metric, or vice versa it only necessary to reverse the position of the idler on the stud, and reset the mesh of the gears.

For some machines, to save space, the 127T gear has been made with a smaller DP or Module, and a matching gear to mesh with it.

Just a thought!

Howard

.

Dragging this out from the archives again:

... Martin Cleeve showed us how.

MichaelG.

If you wake up in the middle of the night worried that if your home-cut threads are going to fail when tested by the NPL wth a laser interferometer, you can always buy Brian Wood's book and use the most accurate ratios possible*.

Neil

*and still make threads that fail the test.

NPL would, like any Accredited Test-House, require a specification against which to certify Pass or Fail.

MichaelG.

Humour....

Neil