Thread: 127x100 compound gear

Yes if you have the other gears to cut your desired TPI.
No if you don't have the other gears to cut your desired TPI.

I'm not sure a 127/100 compound gear is the correct one for doing an imperial thread on a metric-leadscrew lathe. That's the one for doing it the other way around - cutting metric on an imperial lathe. The South Bend book, "How to Run a Lathe" has a chart in it showing a 127/135 gear for metric to imperial.
9 inch Hercus is a close copy of the SB.

Jordan

Hi,

The gear required (from what I can read from a label for a 9 Inch lathe) is 127/ 120 which gives you 8 TPI from a 3mm leadscrew.

To cut a range of gears you will need one off each 33, 36, 39, 42, 54 tooth gears to get other pitches.

The 63/64 compound gear also requires a number of additional gears and some fiddling with the norton gearbox.

--
Regards,
David

Any chance of a better image of the 9 inch chart, David?
It's not quite readable as is.

Jordan

Hi,

I may have an alternative source of the plate, I may look tomorrow. (9 Inch spares book/ early text turning?)

The 9" plate previously attached was copied from eBay (fw444400/ jr444400) in approximately 2009. Unfortunately I appear to have copied the thumbnail rather than a larger picture.

Was working out whether to go the 120/127 or 63/64 route and eventually decided on the 63/64.

The labels occasionally turn up when the Durdens decide to sell a few.

I have tried to increase the contrast of the label previously sent and also have attached a typed copy of it. E&OE excepted.

--
Regards,
David

That's better, thanks David. The only bits I can't see now are the second line, and the part number, but the important info is clear enough.
I have some versions of the Hercus books, but I can't find that label in them.
It's for the Hercus A model, evidently 9 inch, so you've got the best answer as to what was the standard arrangement for this lathe.

Did you use 63/64 for a 260 or 9 inch? If 9 inch, what's the advantage over 120/127?

Jordan

There is one on ebay right now:

Hercus 9" Lathe 127/100 tooth gear to suit. | eBay

Hi,

Sorry I haven't replied before now. I have done some searching myself but haven't found any better information than I have posted before.

The advantage of the 127 tooth gear is that it is an exact conversion (half 25.4) and any other combination is a compromise.

The disadvantage is cutting the 127 tooth large prime number gear is costly and smaller gears cheaper.

As you are fiddling with gear combinations and settings on the norton box how accurate either approach is eventually I cannot say but I assume that the settings shown are accurate enough for say at least 100mm. (at least within Imperial tolerances)

What I have never seen has been a list of the errors in the various gear combinations.

Reiterating the 63/64 combination is more of a compromise than the 127 tooth combination.

The gear I went for was the 63/64 tooth gear pair as appropriate for the 260 lathe which I have.

An advantage I found after purchase is that this gear combination will fit within the enclosure over the change gears. The 63/64 tooth gear is say 100 mm dia and the 127 tooth gear would be nearer 200 mm dia. How it would fit I cannot say but I feel that you couldn't fit the 127 tooth gear without significant compromises. For the old 9 Inch lathes without guards over the change gears not a problem and you can occasionally see these gears on these lathes offered for sale eon eBay. For the more modern 9 Inch lathe with change gear covers this might also be a problem.

The fact that the 63/64 was cheaper was also an issue.

Note- what isn't apparent in this discussion is that you will need an extra pivot bolt to mount the conversion gear combination on the banjo plate.

The main reason for getting involved with this was the idea of cutting an 8TPI thread to suit the spindle to make either a dummy for test purposes, to transfer a chuck to other machine setups, thread a backplate or a spindle protector. Once you have the gears for this purpose rest of the set is not expensive.

The inability of the lathe to cut the spindle thread is a major disadvantage of these lathes in metric form. A better idea would have been to go the camlock route for mounting the chuck. I believe this option was offered (D1-3) but not often chosen. It was not an option to me when I bought this lathe.

Anyway I have attached an A6 label which shows the eventual kit.

HercusMet_Imp_A6.pdf

--
Regards,
David

Many thanks gherringh for lucid description. I never thought of the 127 tooth wheel as half 25.4 which it is. My rationalisation of 127/120 compound was that these two numbers are the first whole numbers where metric and imperial exactly align using the 1" standard of 24.5mm exactly. My metric Dainichi lathe (6mm leadscrew pitch) uses a 127 tooth wheel (plus a bunch of others) to cut imperial. When cutting metric, has either 60 tooth or 120 tooth in the place where the 127 tooth wheel was. The maths of this stuff is a bit beyond me...I get little glimpses of logic through the fog and then lose it again with all the different gear teeth numbers.

It is also fascinating that the inch standard is now expressed in precise metric units noting the irritation some have (in Model Engineers' Workshop magazine for example) with metric. Ditto the insistance by many in US to use imperial measures when all their top line machinery that is exported (Caterpillar, John Deere etc) are in metric dimensions.

But, back to topic, is troublesome to cut metric on my 9" Hercus ARM. Is also irritating as you say, that the spindle is nevertheless imperial at 8TPIx1 1/2" meaning that I can never cut a thread to match my spindle on the lathe itself. I did do 10 TPI once by just cutting 2.5mm pitch using 55 degrees rather than 60 - runout over 30mm length of thread was minimal.

Question here guys, maybe I've missed something:


Perfect conversion requires:

127/120 = 1.058333


The hercus compromise conversion:

64/63 = 1.015873


Why didn't they use:

60/57 = 1.052632


According to what Mal has available on on ebay at the moment, both 60 and 57 are readily available gears.

If you used 60/57 to try and cut an 8tpi thread, it would give you an error of 0.057mm for every 10mm of length machined

Is that close enough?

Yep I was wrong, the correct conversion ratio is 127/125, not 127/120 !!


I put this into a spreadsheet, see below, note that 57/56 is your next best compromise after 64/63 if your in a pinch. All other possible combinations with the standard gears are miles off:

Inch	Metric	Ratio
25.4	25.0	1.0160
Driver	Driven	Ratio	Error	Error Per 10mm
127	125	1.0160	0.0000	0.0000
64	63	1.0159	-0.0001	-0.0004
57	56	1.0179	0.0019	0.0062
60	57	1.0526	0.0366	0.1221

My calculator confirms these numbers. So why not 125 tooth rather than 120

Where does the 127 come from?
1 inch is exactly 25.4mm. 127 is the lowest exact multiple of 25.4 (5x 25.4 = 127). Or in other words, 127 is the smallest whole number to contain 25.4. Or else, one may think like this: you can make a 127T gear, but you cannot make a 63.5T gear


Where comes the tooth count for the meshing gear from?

Let's compare two commonly used leadscrews. One 8TPI, the other 3mm.

The pitch of an 8TPI leadscrew is 25.4mm / 8 = 3.175mm

3.175 / 3 = 127 /120

or easier to see:

40 * 3.175 = 127
40 * 3 = 120


Another common example, one leadscrew 8TPI and the other 2mm

3.175 / 2 = 127 / 80

Another common example, one leadscrew 12 TPI and the other 1.5mm

25.4 / 12 = 2.1166667

2.1166667 / 1.5 = 127 / 90


So the number of teeth in the gear meshing with the 127T gear depends on the pitch ratio between the two leadscrews that we are converting from and to. However this ratio additionally varies with the ratio of any other gears present between spindle and leadscrew. And in practice, every lathe design is slightly different here, that is why there are many different transposing gear combinations out there. The only thing universally in common is the 127T gear, as this is the one that makes the correct translation between the metric and imperial world.


There are also many "shortcut" combinations out there, based on 63T or 64T transposing gears (since nobody ever managed to cut the correct 63.5T gear ). But all these "shortcut" combinations contain an error. And this error is incremental, meaning the longer the thread that is cut, the greater the error. One can cut perfectly usable fastener threads with such a shortcut gear combination, as the error is small. But one cannot cut a leadscrew with such shortcut gears, as the error quickly increments into very unacceptable dimensions. It is all a matter of what you want to do with your lathe. Chris

Thanks Chris. Best explanation I have read to date. The derivation of 127 is fascinating...the first whole number (of teeth) you can get based on 25.4mm=1" standard. The really confusing bit about different lead screws leading to different teeth numbers to accompany the 127 tooth wheel is now clear...I am pleased that I have got that out of my system.

David