magic 127 TOOTH ?

[Michael GilliganParticipant @michaelgilligan61133]

Nice clear tabulation of some very useful numbers, Rod.

Thanks for posting it.

MichaelG.

[Gerard O’TooleParticipant @gerardotoole60348]

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?

[not done it yetParticipant @notdoneityet]

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?

[Anonymous]

Posted by Gerard O'Toole on 13/01/2020 11:04:11:

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

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

[Guy LambParticipant @guylamb68056]

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

Guy

[BWParticipant @bw]

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

[GeorgineerParticipant @georgineer]

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

[ZebethyalParticipant @zebethyal]

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.

[IanTParticipant @iant]

Posted by Gordon A on 12/01/2020 18:51:21:

I use a combination of a 127T and 50T gear on a Myford ML7 to cut metric threads as this gives a ratio of 2.54 to 1.

The setup has to be run without the gear guard though!

Gordon.

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

[Clive FosterParticipant @clivefoster55965]

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

[Howard LewisParticipant @howardlewis46836]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Howard Lewis on 15/01/2020 12:30:50:

[…]

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.

[Neil WyattModerator @neilwyatt]

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.

[Michael GilliganParticipant @michaelgilligan61133]

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

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 15/01/2020 17:13:37:

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

MichaelG.

Humour….

Neil

[Author]

Posts