A different approach to metric conversion

dullvinci · Sep 25, 2017

While reading the latest posts about Metric conversions, a thought popped in my head. The most common SB leadscrew of 8 tpi. (.125 pitch) is pretty close to 3 mm. Suppose I could find a compound gear pair which with the QCGB set at 8tpi would make it act like a 3mm pitch lead screw. That would also allow a gearbox change to 16 tpi to cut 1.5 tpi, 24 tpi to cut a 1mm pitch, etc.
I would be looking to make or buy those gears already except for a couple of problems.
1) I recently moved to the mountains of NC and am surrounded by unopened boxes and more.
2) My lathe is in pieces, literally. The pipe frame cabinet separated from the underdrive, from the bed, from the headstock, from the carriage, etc. You get the idea.
3) I have no shop to work in. Can't get someone to build it for me with all the storm damage around. A while back I would have said I'll build it myself, but those days are gone. I am sure that I'm one of the oldest members of this forum if not the oldest.
Another thing that bothers me is this: if this is a viable approach, then why hasn't someone already done it in the last hundred years or so. I would much appreciate it if some of the gear gurus out there would weigh in on this. I am not the dullest knife in the drawer, but I also know there are sharper ones in there.

MattiJ · Sep 25, 2017

Google or search this site for 127 tooth gear

Christian James · Sep 25, 2017

Have a look at this video...

Metric Threading on an Imperial Lathe - YouTube

CJ

Monarchist · Sep 26, 2017

dullvinci said:
there are sharper ones in there.

You can't be TOO dumb, as you've started to re-invent what is already done, and apparently on little or no outside awareness of that. That's not a head-slap moment. It's good thinking.

That said.. about "sharper knives".

- The "electronic leadscrew".

- thread "chasing" attachments. Small Hardinge. Seriously large turret lathes.

- Mssr's Bruet, Henri, elder and younger. Both of them highly functional geniuses, and educated as such among others of France's elite.

One of 40,000 or so built of Henri the younger's contribution sits out in my own shop.

The "special apron" on a Cazeneuve HBX-360-BC inch/Metric lathe that doesn't use half-nuts nor even a conventional leadscrew.

All done with gearing, not "directly" with the pitch of a leadscrew at all. Some parts of it bear a closer resemblance to the combination lock on a high-grade vault door than to any other lathe ever built. I did say "functional genius", and it was actually "evolutionary". Henri the elder's lathe designs were uncommon, and uncommonly clever in their day as well.

kitno455 · Sep 26, 2017

dullvinci said:
then why hasn't someone already done it in the last hundred years or so

As said by others, that's exactly how it was done by the factory, and pretty much ever other manufacturer, as well as most home-brew setups. Unfortunately, many common metric threads are not an even divisor of 3mm, so you also need some other ratio in the mix for some threads.

allan

hsracer201 · Sep 26, 2017

I'm just curious what part of NC? All my immediate heritage is from Jackson County.

dullvinci · Sep 27, 2017

Macon County

dullvinci · Sep 27, 2017

There is obviously some misunderstanding of what I was proposing. At the settings I suggested, the ratios are nowhere near the usual 100/127. At 8TPI the ratio of 3mm/.125 is .94488. A gearset with 34/36 or its multiples has a ratio of .94444, close enough for most work. Even if the number of pitches is limited, it MAY be possible to use the thread dial. Wouldn't that be nice? That is where I was looking for informed opinions.

Monarchist · Sep 27, 2017

dullvinci said:
There is obviously some misunderstanding of what I was proposing. At the settings I suggested, the ratios are nowhere near the usual 100/127. At 8TPI the ratio of 3mm/.125 is .94488. A gearset with 34/36 or its multiples has a ratio of .94444, close enough for most work. Even if the number of pitches is limited, it MAY be possible to use the thread dial. Wouldn't that be nice? That is where I was looking for informed opinions.

The capability has been "been around" for over a hundred years. Lathes with built-in inch/metric are not a lot younger.

End of the day, it makes more sense in INDUSTRY to do inch work on ALL inch machines, metric work on ALL metric machines.

I class the HBX-360-BC as "metric" because... clever bisexual threading capability quite aside, the feed dials are all in METRIC.

So... I'm likely to do "inch' threading on the 10EE, metric on the Cazeneuve.

Or what I've done far more often..

Just use the lathe to "power" an ignorant tap or die instead of single-point threading AT ALL. Works a treat. Is always the STANDARD SIZE, and with near-zero effort, too.

kitno455 · Sep 28, 2017

dullvinci said:
informed opinions.

Alright, dull, I'll bite. Do a search on this site or the yahoo SB groups, and you find stuff from me like the following, for the SB Fourteen lathe:
--------------------------

34 tooth stud gear and existing 64 tooth screw gear turns your gearbox 1.417 times faster than normal, which makes the leadscrew act like it was 5.997 mm pitch. This then lets you cut 11 common metric pitches with only 0.046% error, as follows:

QCGB MMPT
144 0.25
80 0.45
72 0.50
48 0.75
40 0.90
36 1.00
24 1.50
18 2.00
12 3.00
9 4.00
8 4.50

Keeping that 34 tooth stud gear, but using your existing 48 tooth gear as the screw gear (in place of the 64) will make you leadscrew act like 7.996 mm pitch, giving you a slightly different list of 11 pitches, with a similar 0.047% error.

QCGB MMPT
192 0.25
160 0.30
96 0.50
80 0.60
64 0.75
48 1.00
32 1.50
24 2.00
16 3.00
12 4.00
8 6.00

-----------------------
So, to again answer your original question, "why hasn't someone already done it in the last hundred years or so?": It has been done, for longer than a hundred, I would guess.

allan

mc_n_g · Oct 11, 2017

I made a 47 and 37 pinned gear combo for the South Bend.
The ratio is within 0.0212% of 2.54 or 25.4 magic ratio for inch to metric conversion.
This is along the same lines of the 127/100 ratio set but is a smaller option.
It is easier and can save some time making 47 and 37 tooth gears.
The amount of slop in my lathe will make up for the error in the cutting.
Just an option.

Ken-Bergen · Oct 11, 2017

mc_n_g said:
The amount of slop in my lathe will make up for the error in the cutting.

Or exaggerate it? :eek:

dullvinci · Oct 11, 2017

allan
Thanks for taking the time to respond. In all of these conversations I was referring strictly to a 34/36 tooth pinned compound gear. In keeping with your tabulation, this is what I get with no other gear changes and sticking only to mm. standard pitches.

QCGB mm pitch
60 .40
48 .50
40 .60
32 .75
24 1.00
16 1.50
12 2.00
8 3.00
6 4.00

Granted, this is a relatively short list, but I was looking at a combination that I personally had never seen in use.
My apologies for the delay in responding, but I don't check the site often. In addition, I still have no place to put my SB, but that is a long and off topic story.

Monarchist · Oct 11, 2017

dullvinci said:
...I was looking at a combination that I personally had never seen in use.

There are things published in the Kama Sutra of Vātsyāyana - written perhaps as far back as 400 BC, and even then, crediting OLDER works - that I've never personally seen in use, or attempted, either.

As with Metrifucating screw-CUTTING lathes, that doesn't mean they haven't been in long use by others .... while we were not paying attention.

billzweig · Oct 11, 2017

I do not see what is wrong with the 127/100 ratio. Or even the 47/37.

SeaMoss · Oct 12, 2017

Monarchist said:
There are things published in the Kama Sutra that I've never personally seen in use ...

Are you tellin' us they don't have porn channels back there ta ol' Virginny ? Wa :bawling:

Monarchist · Oct 12, 2017

SeaMoss said:
Are you tellin' us they don't have porn channels back there ta ol' Virginny ? Wa

Asking the wrong guy. Haven't a klew. Never was into "spectator" sports.

Big world.

Too many women. Too little time.

Eventually you forget why it mattered....

kitno455 · Oct 12, 2017

billzweig said:
I do not see what is wrong with the 127/100 ratio. Or even the 47/37.

No one said anything was wrong. But, if you want a compound in that ratio, 80/63 is closer than 47/37, and you already have an 80 tooth idler in that spot on most SB lathes, just have to add the 63.

allan

A different approach to metric conversion

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