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Originally posted by GordonL View PostI think that we are talking about two different situations. If the feed is only through change gears then lead screw enters into the calculations. If the feed is through a quick change gearbox then the prior gearing was based on the lead screw pitch.
For example if your leadscrew has 6TPI and you want to cut 18TPI, your leadscrew should turn exactly 3 times slower than the spindle. Then you multiply 6 by 3 to get 18TPI. I may have to do the calculation from a scratch if I decide to use a 127 tooth gear for metric threads.

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There are other conversion gearsets which may already exist in the set you have (if you have change gears), depending on the accuracy you require. 37t and 47t apparently give an accuracy of .02%, which would be pretty good for any short thread.
CNC machines only go through the motions.
Ideas expressed may be mine, or from anyone else in the universe.
Not responsible for clerical errors. Or those made by lay people either.
Number formats and units may be chosen at random depending on what day it is.
I reserve the right to use a number system with any integer base without prior notice.
Generalizations are understood to be "often" true, but not true in every case.

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Originally posted by MrWhoopee View PostTry asking your questions here
This is a group to talk about the southbend lathe. It would be neat to talk about the repurposing or upcycling of the older southbend lathe while still maintaining some originality.
here
and maybe here
Here's a readable copy of the chart from
Note that it uses 127/120 instead of 127/100.
I guess there is no easy answer to my question. I will have to make my own calculations to see what is best for my lathe. It looks like my lathe has room for the 127/120 gear block to replace the original 91/86 block. My 8" rotab can divide by 120 and 100, but not 127. So this may force me to finally finish my gear hobbing project. I should be able to cut any spur gears on my vertical mill with electronic syncronization between tool and work spindles. My lathe uses M1.5 gears, so one hob can produce any gear with this module.
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Originally posted by J Tiers View PostThere are other conversion gearsets which may already exist in the set you have (if you have change gears), depending on the accuracy you require. 37t and 47t apparently give an accuracy of .02%, which would be pretty good for any short thread.
https://www.lathe.com/metric_threading.htm
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Originally posted by mikey553 View Post
As I said before I can cut most of the metric threads with pretty good pitch accuracy using just standard gears. But long threads may require an exact conversion.
It is not the overall length that is important but rather the length of engagement.

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Originally posted by The Artful Bodger View Post
The thread can be a long as you like. Suppose you have a length (let's say a couple of yards) or threaded rod and you want to run a nut along it. That will probably work with close but not exact thread pitches but only with a short nut, if you want to use a longer nut closer thread match will be required.
It is not the overall length that is important but rather the length of engagement.
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What you say about the length of engagement being the actual factor to consider is not really true. You are thinking about a screw being used as a mechanical fastener only.
But screws have other uses. An all too obvious one is the lead screw on a lathe or the feed screws on other machine tools, like milling machines.
The real consideration is not the length of thread engagement, but rather the actual use that the thread is being made for. If it is just going to be used as a fastener and, of course, the length of thread engagement follows the normal practice for such use, then the thread can have a fairly large error and still be useful. But even in such use there are considerations. For instance if the screw/bolt will be used near it's yield strength, then you will probably want full engagement along all of the threads in that length of thread engagement. If the pitch is off then only one or two threads may be bearing the entire load while the remainder in that "length of thread engagement" are just gripping air. Then those one or two threads may fail (strip) due to overload conditions. And the next thread(s) in line will then bear the entire load. And those can then fail. And so forth until all the engaged threads fail and the fastener as a whole has failed. What you "see" as a proper length of thread engagement may not be so.
Of course, the uses as accurate sources of distances in machine tools are obviously not tolerant of even moderate errors. And yes, I know that some machine tool OEMs, primarily on the western side of the Pacific Ocean, do get away with some inaccurate feeds.
My point is, it is not the apparent length of thread engagement that should be considered. The actual use of the thread is the better determining factor.
Oh, and the length of the thread is also not a good determining factor. While machine tools often employ rather long threads to produce their precision movements, a micrometer screw is usually around 1.25" long but needs to be as accurate as possible.
Again, it is the intended USE of the screw that must be considered.
Originally posted by The Artful Bodger View Post
The thread can be a long as you like. Suppose you have a length (let's say a couple of yards) or threaded rod and you want to run a nut along it. That will probably work with close but not exact thread pitches but only with a short nut, if you want to use a longer nut closer thread match will be required.
It is not the overall length that is important but rather the length of engagement.Last edited by Paul Alciatore; 03132023, 04:39 PM.Paul A.
Golden Triangle, SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
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I doubt anyone would want to make leadscrew using an approximation.
Although, the 0.02% is not bad if true (I did not calculate it). That would be a couple tenths error per inch, 0.0024 error per foot. A good leadscrew is better than that, but it still is not horrible.CNC machines only go through the motions.
Ideas expressed may be mine, or from anyone else in the universe.
Not responsible for clerical errors. Or those made by lay people either.
Number formats and units may be chosen at random depending on what day it is.
I reserve the right to use a number system with any integer base without prior notice.
Generalizations are understood to be "often" true, but not true in every case.
Comment


Answering this and an earlier post asking about the use of the 127 tooth gear:
This discussion will benefit from a preamble on why a gear with 127 teeth used. The present and totally accurate to any number of decimal places, DEFINITION of the inch is 25.4 mm. That is EXACT. A million more decimal places would be all zeros. That is the very definition of the inch. So the ratio of mm to inches is 25.4 to 1 or 25.4 / 1. We can get rid of the decimal in that ratio if we multiply both numerator and denominator by 10 giving us 254 / 10: same value, just all whole numbers (remember gears can only have whole numbers of teeth). But that fraction is not reduced to it's lowest whole number form and a 254 tooth gear would be rather inconvenient. We can divide by 2 and still have the same overall value. That gives us 127 / 5. Since both 127 and 5 are prime numbers, this fraction is the metric to English ratio of lengths reduced to it's lowest terms. It can not be reduced to any smaller numbers. This is where the "magic" number of 127 comes from. By simple math it is THE smallest gear that will produce an exact conversion from one system to the other. But also notice the 5 as I will use it below as well.
Converting from English measure to metric requires a RATIO in the gear train. And a ratio means that there are at least two gears involved. That is how gears work: how they change speeds or rotation rates. Two gears are used, not just one. This is obvious but also a deep concept when thinking about a gear train.
So yes a second gear is used with the 127 tooth gear and that second gear should have some kind of round number of teeth if the 1.27 ratio is to be had. But not just any round number; the number 5 which I pointed out above is a clue to how many teeth that second gear should have. It must be a multiple of 5 if the pair of gears is going to contain the exact conversion ratio. I have seen lathes where the second gear has 50, 100, and 120 teeth. All of these numbers can work, along with other multiples of 5.
I am sure that the various OEMs of lathes have sat down and tried to calculate the best second gear for their particular lead screw pitch and set of change gears or settings of a QC gear box that their lathe may be equipped with. When I designed an Englishmetric compound gear for the SB9 I did just that. I sat down with an Excel spreadsheet and experimented with different tooth counts on the second gear, looking for the one that produced the best group of usable metric threads. I am not aware of any better way to do this. Oh, and for OEMs I am sure that the cost of making the gears was a factor. A 50 tooth gear would be less costly than a 100 tooth or 120 tooth one.
For my lead screw and set of change gears I found that a 100 tooth gear seemed to be the best. It would also allow me to cut 50, 75 and 100 TPI English threads if it was used without the 127 tooth gear. Yes, those are not standard English threads, but they can have their uses. Another thing I found was that the 127 tooth gear could be used without the 100 tooth to produce some metric threads that could not be produced with the 127/100 combined as a compound gear. Metric pitches of 0.6, 0.65, 0.7, 0.8 and 0.9mm are examples of this. So having two separate gears that could used individually or be assembled into a compound was an advantage, at least with my SB9 lathe.
Therefore my compound gear design was two gears (127 and 100 teeth) bolted together with a spacer between them.
I am attaching the six gear placement drawings that I made for my conversion. They show that the 127 tooth gear can be used in a number of different ways. But when using an English lead screw to cut metric threads, it is always in the position of a driven gear, not a driver gear. This will be true on any lathe.
Figure C shows only two change gears in use: a 60 tooth and the 127 tooth. This is a lucky fit on my SB as the stud and the lead screw seem to be spaced just for it. It illustrates the use of a completely different "second" gear; the 60 tooth one. 60 is a multiple of 5 so it follows the rule I mentioned above. On my SB9 this setup produces a 1.5mm thread pitch.
Due to the BB's limitation on attachments I was forced to delete two. I will add them in a second post below.
If you would like to see the instructions and the Excel file of all threads I found that could be cut with the change gears, compounds, and metric conversion compound here is a link to my Drop Box folder with that Excel workbook.
At one point I tried to work out how to use this 127::100 compound with a standard, SB QC gear box but that effort was never finished. But I am sure a number of additional change gears would be needed for a complete set of metric threads.
PS: I am no longer selling the metric threading compound gear set. There was too little interest at the price that I was forced to ask to make even a small profit.
Originally posted by mikey553 View Post
Thank you MrWoopee, now I have an idea how to use a 127 tooth gear. So there are 2 possible gear blocks  127/120 and 127/100 for cutting metric pitches on the lathe with imperial leadscrew. Are there any other combinations? Which one is better?
I guess there is no easy answer to my question. I will have to make my own calculations to see what is best for my lathe. It looks like my lathe has room for the 127/120 gear block to replace the original 91/86 block. My 8" rotab can divide by 120 and 100, but not 127. So this may force me to finally finish my gear hobbing project. I should be able to cut any spur gears on my vertical mill with electronic syncronization between tool and work spindles. My lathe uses M1.5 gears, so one hob can produce any gear with this module.Paul A.
Golden Triangle, SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
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FWIW, 80/63 is a better approximation than 47/37.
47:37 => 1.270270 (0.021281%)
80:63 => 1.269841 (0.012498%)Regards, Marv
Home Shop Freeware  Tools for People Who Build Things
http://www.myvirtualnetwork.com/mklotz
Location: LA, CA, USA

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Originally posted by mklotz View PostFWIW, 80/63 is a better approximation than 47/37.
47:37 => 1.270270 (0.021281%)
80:63 => 1.269841 (0.012498%)
That 80/63 combination would give an error of 1.5 thou in 12", as opposed to 2.5 thou in 12" for the 47/37. Both can be very significant errors for longer screws intended for "measurement", such as a lead or feed screw, and both are relatively insignificant when cutting a short thread.
For ordinary purposes, there likely is not much to choose between them. I would not put forth much effort to get the two required gears for a slight improvement.
If better accuracy is needed, then one would think that it is obviously better to obtain the 127 tooth gear, with either of the common companion gears. That is an exact conversion, and will give the maximum accuracy obtainable with your machine and lead screw.
Now, the question is whether you can actually improve the overall accuracy at all by using the precise 127/100 combination. That will depend on the lead screw of your machine.
As a point of information, the "ultraprecision" lead screw material obtainable from McMasterCarr has a stated error of 0.0006" per inch. In 12 inches, that is an overall error of up to 7 thou. Either the 47/37 or the 80/63 combinations have an error much less than that.
Roton did not appear to provide ANY precision specifications. I would assume they are about the same as other rolled screws, and in the area of 0.005 inch per foot error
Roton Products is a manufacturer of lead screws, ball screws, & more. View our acme screw engineering data & shop online for your power transmission needs.
Nook Industries precision screws are +0.0005 inch per FOOT error, for the ground screws. That is quite good for nearly any purpose. But you will pay for that.
For their normal rolled screws, the error allowance is +4 thou per foot, again considerably more than the error of either gear combination above.
For their milled screws, the error is + 2 thou per foot, which is approximately the same as the error of either combination of gears above.
Nook screws of these types are easily available for diameters of 0.75 inch and above. Many screws used in machines are of smaller diameter.
Contact Nook Industries if you’re looking for precision acme screws and lead screws. We provide precision inch acme screw and metric acme screw products.
What the precision of your machine and its lead screw may be is unknown to me, and perhaps to you also. My Logan uses what appears to be a milled lead screw, which is likely in the 2 thou per foot category, comparable to the approximate conversions. If you have an NOS 10EE, your machine may be better than 5 tenths per foot.
The wear etc on your machine, combined with its original asbuilt specifications (if any are given), will determine what your final best accuracy spec is. I very much suspect that virtually all of us are working with machines that cannot benefit much, if at all, from a conversion better than either of the two approximations listed above will give.CNC machines only go through the motions.
Ideas expressed may be mine, or from anyone else in the universe.
Not responsible for clerical errors. Or those made by lay people either.
Number formats and units may be chosen at random depending on what day it is.
I reserve the right to use a number system with any integer base without prior notice.
Generalizations are understood to be "often" true, but not true in every case.
Comment


Originally posted by mklotz View PostFWIW, 80/63 is a better approximation than 47/37.
47:37 => 1.270270 (0.021281%)
80:63 => 1.269841 (0.012498%)
It's all mind over matter.
If you don't mind, it don't matter.
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Thanks to everyone who shared their knowledge and experience on metric thread conversion. As I mentioned before my Grizzly lathe uses 91/86 compound gear for such conversion. Its accuracy is in between 47/37 and 80/63 combinations. Not sure why the manufacturer chose to use this gear set instead of 47/37 or 80/63 or the exact conversion 127/120. The end result  all standard metric thread pitches up to 6 mm are available with the standard set of gears. The useful range of metric pitches has a theoretical mistake of .0001 to .0007 mm per pitch, which is almost nothing for short threads.
As J Tiers correctly stated our lead screws may have bigger errors in themselves and trying to make the exact metric thread conversion is a loosing game.
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