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08-19-2009, 07:49 AM #1
Metric threads and transposing gears
I was cutting some fine metric threads for the first time on my Logan, but suspect this applies to SB and other lathes as well. Reading on the 'net about 127/100 and 47/37 gear sets, all seems simple and easy. It probably is for 1mm and easy division pitches. Silly me. What I discovered is that for fine threads, say 0.35mm pitch, there are only a few possible combinations of gears, and of those, there are combinations that simply can't be made to fit and/or engage without interference. I ended up cutting several new change gears I didn't have, and remaking various spacers and fasteners, so I could get the teeth engaged without collisions. This is just a warning to anybody contemplating very fine metric threads to really think about the gear train and clearances.
On a similar topic, I was hoping to someday stumble across a cheap used Starrett gear vernier. Apparently that's not going to happen. I just saw the new price- why in the world is a 456 gear vernier way over a thousand bucks???
08-19-2009, 08:06 AM #2
Not sure how your gear train is set up, but on my SB 9A, it's fairly simple: replace the 80-tooth idler with a 100/127 compound gear, install .46" thick spacer behind 56-tooth gearbox gear, replace 20-tooth stud gear with 28-tooth gear, then set gearbox tumblers to D-3.
This is based on my experience with the gear set from Tools 4 Cheap. It includes the same gears as the original SB metric transposing set, and will cut all standard metric pitches from .20 to 6.00mm (though 4.00 is a more practical limit for this size lathe).
Since you selected the 100/127 gear, I assume your Logan has an 8 TPI leadscrew? Is your lathe a change-gear or quick-change type?
08-19-2009, 08:24 AM #3
Hi Paula, actually I went with the 37/47 combo. So far as I can tell, there's no way to cram a 127 in my fairly small (10") machine and still have room on the banjo to install much of anything else. Even with the 37/47 it's a tight squeeze to get the other reduction pair in there. I have to wonder if maybe Logan supplied an alternate banjo with longer slots to accommodate larger gears. This isn't a QC, just plain change gear- I wonder if having a QC actually improves the whole clearance issue?
When I get a chance I'll post a photo of what I've got because I keep having this nagging feeling that there's some alternative way of setting it up that hasn't occurred to me.
08-19-2009, 08:42 AM #4
28/127 x 100/56 x 16/32 x 16/32 x 16/32 x 32/20 x 28/20 x .125" = .013779"
.013779" x 25.4mm/in. = .35mm
Notice the three 16/32 multipliers -- these are all within the cluster of gears controlled by the left-hand tumbler. I think that this compact arrangement of (4) 16/32 compound gears (and one 16-tooth single gear) greatly aids the clearance problem with high-ratio pitches.
08-19-2009, 09:49 AM #5
Ah ha! Yes, the QC box moves a lot of conversion off the limited space at the end of the lathe. My .35 solution is:
0.125 (leadscrew pitch) * 37/47 * 28/50 * 16/64 = .01377"
The 16 is the stud gear and the 50 is the leadscrew gear, leaving me with the 37/47 stack and the 28/64 stack- you can only put two stacks on the banjo, so whatever is to be accomplished has to be done with that limitation. Because the 28 is small compared to the 64, the 64 wants to hit the shaft of the leadscrew. I had to replace the normal spacer/bushing on the leadscrew with a slotted ring. The slot is for the key, since the key goes around within a few thou of the 64 tooth gear. Probably be clearer in a photo. Anyway, with the plain change gears (call me weird but I do actually like change gears) this isn't just a matter of the math working out, but the parts have to fit- my guess is that finer *metric* pitches might not be possible. Normally it's no problem to go 250 tpi or more because the transposing gears aren't using up the space where another reduction can take place. Hmmmm... visions of a three slot banjo are forming!
08-19-2009, 09:57 AM #6
You can work with the 47/37 compound as well. 94/37 gives you a 2:1 reduction for example. Unfortunately 47 and 37 are both prime so you can''t break them down.
There is another ratio that gives very close to 1.27 or 2.54, but I can't remember it. The tooth counts were 60's or 70's IIR, and my vague recollection was that one of the tooth counts was not prime and could be broken down, but maybe not in a way that gets you closer to .35mm pitch.
'Course, making or sourcing these big oddball gears is either time-consuming or expensive, so there are practical limitations as well.
Custom banjo is definitely a possibility. There's really nothing magical about how many slots or their orientation.
08-19-2009, 10:57 AM #7
I saw that "other" ratio someplace here too, but can't remember it. The 94 tooth solution buries the other gear too deep to engage with anything, as did the nice 100 tooth gear I cut and then hung on the wall as a useless decoration. I like my dividing head, but not enough to want to cut any more 100 tooth gears! The Logan change gears are 0.440" thick, 16DP and 14.5PA- IMHO plenty robust for the application. Overly robust IMO, so I make my own specials out of Delrin. Works great and I haven't seen any durability issues.
08-19-2009, 11:15 AM #8
08-19-2009, 11:42 AM #9
I think that's it! Both are non-prime, so you can do all sorts of potentially useful factoring.
08-19-2009, 01:04 PM #10
08-19-2009, 02:07 PM #11
On a QCGB-equipped lathe, you will probably find, at most, four Imperial threading positions to be useful for metric threading.
Those, say, four positions, are required to be supplemented by additional so-called reverse-shaft (Monarch-speak, I admit it) gears, sometimes called stud gears on other maker's lathes.
Example: a Monarch Series 60 lathe has a 75 teeth gear on its so-called "quadrant".
This 75 teeth gear must always be present, yet it may be compounded with a 127 teeth gear to form, for this particular series of Series 60 lathes, a 75/127 metric transposer.
(Other lathes may have a 100/127 or a 60/127 or a 48/127 metric transposer, but the central point here, is that there is a 127 teeth gear present as a compound gear, and which, effectively, accomplishes a division by 254 or 2.54, depending upon one's point of view. By international definition, there are 25.4 centimeters per Imperial inch, precisely, and while 254 is not a prime, 127 certainly is, hence the 127 factor will always be present ... somewhere ... in the gear train, in the denominator of a fraction. The reverse would be true of a metric lathe with a 3mm lead screw pitch, which had available for it an Imperial transposer, in which case the 127 would appear in the numerator).
Now, for the Series 60 example, only 11, 10, 8 and 5 tpis, and their multiples, are useful for metric threading.
But, these would be supplemented by reverse shaft gears of 35, 40, 45, 50 and 55 teeth, which are all 16 DP 14.5 PA gears, and which engage with the 127 teeth gear (the remaining gears in the gear train are all 10 DP 14.5 PA gears), and which, combined, can produce every conventional and model-maker's metric pitches from 0.25 to 6mm.
Other maker's lathes would be quite similar.
08-19-2009, 02:49 PM #12
Joe, I've perused the Logan info, but it's somewhat generic. That's what led me to believe metric setup would be an easy no-brainer! Goodness, though I'd like an HLV, I think I'd like a Monarch even better. Still, like a Ford man or a Buick man, I'm a Logan man. Having beat this to death, here's a couple photos. The 64 tooth gear at the lower center is only about 4" in diameter, so you can see that a 127 tooth gear would cover the banjo and much of everything else. Fortunately the 37/47 combination works just fine. Also note how close the 64 tooth gear is to the key on the leadscrew! It does just clear, and I don't know if I've mentioned that the M6-.35 parts I had to make turned out just beautifully.
08-19-2009, 03:08 PM #13
Another option is to go to a higher DP. Yes, you'll need to make a different stud gear, but there's no reason the end gears have to be the same DP as the originals, or the same as the gears in other trains. As long as you're running low-speed and light loads, 20 DP or even 24 DP should stand up just fine.
64-tooth in 24 DP is only ~2-1/2" OD.
08-19-2009, 04:28 PM #14
Picking up on Finegrain's comment.
I have a 405 which uses 20 DP gears all the way through.
I have, for years had a 100 tooth gear in my set. About a year ago I got a 127 tooth 20 DP gear and mated it to the 100 tooth for a metric translation set.
Now the standard SB 0.35 metric pitch says that with a change gear lathe to use a 100 tooth gear on the screw and a 56 tooth gear on the stud gear, with a 4:1 compound in between. I don't have a second 100 tooth gear, nor do I have a 56 tooth gear, nore do I have a 4:1 compound. I have a 5:1 compound
I can get around this because however. The 405 has also a 2:1 compound.
With a 72 tooth gear on the screw, which I do have, and the 2:1 compound, and a 20 tooth gear on the stud I come within 0.8% of the 0.35 metric pitch. (close enough).
Now almost every 18 DP gear you need can be obtained here:
At very good prices.
Yes I have used them and they fit fine and the shipping is reasonable.
No I do not have any affiliation.
Almost every 20 DP gear you need can be obtained either from Boston Gear through a distributor (MSC is one) or
About 2/3 of the way down the page.
No I have not used this source although I am going to.
08-19-2009, 04:33 PM #15
Finegrain- excellent and clever thought. That's what Sherline does- two entirely different DPs to solve the diameter problem because their lathe is so small. I actually thought about the fact that they do it, but never made the mental leap that it wouldn't be all that difficult to convert at least one pair of mine. Thanks!
08-19-2009, 07:00 PM #16
Another way you can attack this problem is to use a 21 tooth gear. 21 is a factor of 63 which is in the 63/80 approximation for 100/127. The 63/80 approximation has about half the error of the 37/47 approximation.
The 100/127 ratio effectively converts the 8 TPI lead-screw to a 2.5mm pitch. To cut a 0.35mm pitch we then need the fraction
0.35/2.5 * 63/80
If you play around a bit you can convert this to
28/50 * 21/40 * 18/48
The numbers on the top of the fractions are driving gears and those on the bottom are driven gears. So you could put a 28 on the stud, driving a 40/18 compound gear, which then drives a 50/21 compound gear and a 48 gear on the leadscrew.
So if you arrange that your gears can be paired to make compounds and have a 21 tooth gear you could be set to cut all sorts of metric threads. That's how many English lathes work, with most gears in multiples of 5, where as American lathes typically have more change gears and don't rely on compound gears for most threads.
So this forms a possible space saver. Not saying that this is necessarily the best solution, but it is something that's nice to know.
08-19-2009, 07:27 PM #17
"... but there's no reason the end gears have to be the same DP as the originals, or the same as the gears in other trains ..."
And, there is no reason to retain the same tooth form and PA as the others in the compounded gear.
You could step-up the DP and change to 20 PA for the first step, and then retain the original DP and PA down to the box.
The Monarch 10EE went to 32 DP for the first step, that which involved the 127 teeth gear, and then returned to 16 DP for the remainder, as the 60 teeth quadrant gear (16 DP) is 3.875" dia, whereas the 127 teeth transposer (32 DP) is 4.031" dia, and just about 4.125" is available in the enclosed end gear unit.
For a 20 DP-based lathe, such as many Hardinges, I could see going to 40 DP, provided the loads remained low by taking shallow cuts.
08-20-2009, 03:42 AM #18
Wow, plenty to think about there - I'm going to have to bookmark this thread for when I get round to organising some transpose gears for my 13" SB.
In most of the SB catalogues etc. I've seen it looks like the transpose set was originally supplied with a Y shaped banjo for all the threads that need more gears than just the 127/100. I have already considered that I will probably want to make a custom Banjo for the finer threads, fortunately my main thread is going to be M20x2.5 which should just need the 127/100 - as srwaaa says.
Sorry I can't add anything yet, but thanks for all the info!
08-20-2009, 03:42 AM #19
Using the 100/127 transposer alone, there are six metric pitches selectable directly from the QCGB, including .25, .50, 1.00, 1.25, 2.00, and 2.50mm. A pretty sweet setup, I think.
08-20-2009, 11:09 AM #20
The South-Bend metric transposition set-up for the Imperial QC box is undoubtedly one of the neatest and easiest to use versions as almost all the gearbox and gear settings are in logical ascending order. Some other makes have you hopping up and down the box in a manner guaranteed to cause confusion. If anyone plans to do a variety of metric threading they may well find it useful to make two additional sub-tables listed by drive gear and by gearbox setting rather than by pitch. The different view being very useful for verifying that the set-up actually is right. I've found it unwise to always rely on the error being big enough to be obvious on the witness scrape.
One intriguing feature of metric conversion is the way the 100/127 compound gear method is pretty much restricted to smaller and, mostly, lighter lathes. Usually those with change gear versions in their ancestry or in simultaneous production. Most of the heavy metal does it differently for example the Holbrook D series use the 127 as the gearbox driver with 7 smaller gears to give 36 pitches. Given what a major PIA the 127 is in the middle of the banjo one wonders why the approach isn't used on smaller lathes. Longer legs between bed and bench is hardly a major problem.
When struggling with fitting things in on change-gear lathes it would help to have access to older issues of the UK magazine Model Engineer which has frequently covered the issue. Some of the combinations proposed being seriously arcane and the target machinery distinctly un-prepossessing. As I recall matters several of the readers letters discussions took proper account of the errors involved enabling mathematically incorrect trains producing real world acceptable results to be used. I don't know if the on-line indexes, such as that provided by Colin Usher (http://www.colinusher.info/), go into enough detail to enable the articles and letters to be identified but if they do my collection goes back to 1971 and I can easily look things up if someone PMs me with the issue data. Allegedly the book "Screw-cutting in the Lathe" by Martin Cleeve includes a good selection of compact gear trains for change gear lathes. Never having seen the book I can't vouch for the accuracy of this claim. If you really are stuck for space its perfectly acceptable to hook a secondary slotted arm between the banjo and a suitable point on the lathe. Which doesn't even have to be straight! Brackets may be needed. A strong pillar off the drip tray might work if the free end can't be hooked direct to the lathe.
These days programmable electronic lead-screw drives can be got at affordable prices e.g. http://www.autoartisans.com/ELS/ and are far more convenient. So much so that teaming a Model B South-Bend, preferably got cheap 'cos most of the change gears are lost, with an ELS system probably produces "best of breed" in the 9" South-Bend pantheon.
One, to me, mildly irritating feature of the ELS phenomena is the knowledge that back around 1980 I incorporated all the important gubbins needed to make an inexpensive analogue ELS into the scanner control circuitry on a concept demonstrator inexpensive IR homing head. Modify the sensors, add switch gear, divider chains, a windscreen wiper motor and DC driver circuit. ELS way before affordable microprocessors and stepper motors. Only took about 25 years for the penny to drop! Naturally I no longer have the designs and I'm pretty sure that all the paperwork has been purged from the UK MoD research departments archives.