Imperial thread on metric lathe!

Hi Folks I have recently acquired one of the little 7x12 minilathes. Its the metric model and came with a small box marked "gear wheels for imperial
threads". I want to cut a 32tpi on some 1" brass bar. I note on the `Little Machine Shop minilathe manual` that to cut 32tpi I need gear wheels 20,any,any,40 in the order A,B,C,D on the gear mounting. Will this give me 32tpi without a different leadscrew?. I see that to cut metric threads with an imperial lathe a 127 tooth gear is reqd, is there a similar requirement for cutting imperial threads on a metric lathe?. Cheers Steve
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Steve wrote:

One inch is 127/5 mm, so to be exact you need that 127 tooth wheel.
But there are other change gear combinations that come close. If you can get the ratio close enough that the error due to the change gear ratio is significantly less that the error due to your lathe's inaccuracies then you're doing as well as you ever can. If you can get the ratio close enough that the error due to the change gear ratio, plus the error due to your lathe's inaccuracies is less that the tolerance you have to build to, you're doing well enough.
It would be interesting to do the math on your change gears, to see just how close it comes.
--

Tim Wescott
Wescott Design Services
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On 10 Nov 2006 11:42:23 -0800, "Steve"

==================To cut exact metric threads with an inch lead screw or inch threads with a metric leadscrew you must have a 127 tooth gear. ==>The operational word here being exact.<==
Because of size limitations it may not be possible to install a 127 tooth gear, or one may not be available for your machine. Some combinations of [compound] gears will result in a very close approximation to the required 1.27:1, for example 70/55 which produces 1.2727. In many cases the this be so close the lead screw will have more error than the conversion gearing. [ hint -- an Excel or similar spread sheet can be very helpful to determine just what threads you can cut with the gears you have.]
In other cases a "strange" inch or metric thread will be a "close enough" fit. For example, 32 TPI is 25.4/32 or 0.79375 m/m pitch. If you are engaging only a few threads a 0.80 mm pitch may be "close enough.
Be reminded that the thread dial will most likely not work correctly when you you use a 127 tooth gear or one of the approximation compounding sets, so you will have to leave the half-nut engaged.
IIRC -- The Little Machine Shop has lead screw / half nut kits to convert between inch /metric threading. This may be your better option.
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
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F. George McDuffee wrote:

70:55 is a 14:11 ratio. Maybe a 28 : 22 gear set, Another combinaion to consider is the 80:63 combination. This is even more accurate. And can be made from a 20:21 and 4:3 ratio. Common gears that you might have. Pete
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wrote:

There is at least one free software out there that will find the closest matches for you. You choose whether you have a metric or imperial leadscrew and what thread pitch you want, and program spits out one or more gear combinations that will usually get you within a percent or two.
You may find it in the files section of one of the Yahoo groups for the mini lathes. There's one group named 7x10, which is 90% politics and religion, and another group named 7x12, which is much less noisy. The 7x10 and the 7x12 are essentially the same machines, so both groups may be of use to you.
--
RoRo


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Thanks for the replies guys. I don`t know if a 127 tooth gear is available for the minilathe but the suggestions made will help a lot. Have to `suck it and see` how accurate the actual thread will need to be. I need to make an adaptor for a firewire camera I`ve ordered o connect it to the eyepiece of my telescope 1" 32tpi to 1.25" OD .The brass bar is actually 1.5" diam. The threaded part might only need to go into the camera body half inch or so. Thanks again Steve
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wrote:

It may be that the threads are a 55 degree form, rather than 60 degrees. Microscope threads are all 55 degrees. At least all the ones I've done. Not only that, the camera lenses I've had to adapt to were also 55 degree threads. ERS
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Steve wrote:

If it's a C mount the male threads on those usually span about 1/8" or so.
--

Tim Wescott
Wescott Design Services
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On Fri, 10 Nov 2006 17:08:49 -0800, Tim Wescott
<snip>

<snip> From a functional standpoint do you actually need threads or would a plastic adapter made from nylon or delrin or even pvc pipe turned for about a 50% depth-of-thread be adequate?
Many times we get fixated on the end product rather than the function.
This sounds like there should be an adapter commercially available. Have you checked with the camara and teleoscope manufacturer, and the bigger camara supply places [Porter's?] How about some of the shops in the small classified ads in the photo magazines?
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
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Steve wrote:

So, doesn't that mean that you will be removing the 127 tooth gear when you install the 20,x,x,40 gears for imperial threads? I don't see the problem; my old MiniMat lathe cut fully compliant threads both inch and metric (after the gear fiddling).
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On 10 Nov 2006 16:58:31 -0800, snipped-for-privacy@gmail.com wrote:

===========Actually the 127 tooth gear will be compounded with a 100 tooth gear [or some compound train to give the same effect].
What is the pitch or TPI of your lead screw?
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
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On Fri, 10 Nov 2006 22:28:05 -0600, F. George McDuffee

unka george the ratio is 100:254 or halving both to get lowest denominator 50:127
he needs a 50 tooth gear as well not a 100.
it is based on 1"%.4mm
Stealth Pilot
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Stealth Pilot wrote:

As often as not, the gears come together, the 100 pinned to the 127 gear. This was the standard metric trnsposing gear from several tool companies.
Whether you use the 100:127 ratio or 50:127 ratio only makes a little difference. It will all get factored into the gear train.
Most of the transposing gear screwcutting charts I have seen used the 100:127 set, or skipped that alltogether and utilised 21 and 63 tooth gears for somewhat less precise conversions.
The charts I can recall seeing all related to an 8 TPI leadscrew. I am not fluent enough in the math to tell off the top of my head if halving the 100 tooth gear to a fifty would allow direct use of the same geartrains on a 16 TPI leadscrew or not by that simple substitution.
If you have the room to fit the 127 tooth gear on the leadscrew, without it hitting the table, then you also have room to fit the 100 with it. Further to all that, the use of the 100 toth gear provides the ability to make a possible geartrain as opposed to an impossible one, wich works correctly on paper, but cannot be physically produced due to the gear pairs interfering on each other. I would expect some difficulty in that respect out of a 50/127 pair.
Cheers Trevor Jones
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From this discussion I suspect that your lathe does not have a metric feedscrew. I suspect it is 16TPI. SouthBend made both imperial and metric lathes. They recommended that a lathe to cut both metric and imperial threads should have an imperial leadscrew.
Don Young
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On Fri, 10 Nov 2006 22:12:48 -0600, "Don Young"

==============If he does indeed have the inch leadscrew (which the 20,X,X40 suggested change gears suggests), he should be good to go.
If he wants to cut metric, rather than fool around with strange change gearing, get the metric leadscrew kit.
see: http://littlemachineshop.com/products/product_view.php?ProductID91
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
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F. George McDuffee wrote:

Now that's a darn fine idea, and you can hardly complain about the price, either.
That would be a good buy even if you had to doctor up an outboard end extension to fit it to one of the longer versions.
Cheers Trevor Jones
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On Sat, 11 Nov 2006 16:20:56 GMT, Trevor Jones

================Check the website. They have kits to fit the longer lathes.
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
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Hi again chaps Not quite sure whats going on with my little lathe. I`ve just checked the leadscrew and it is 16tpi. All the badges and labels show thread pitches in mm, the guy I bought it off said its a metric lathe so what makes it a `metric` lathe exactly as opposed to the imperial version of this lathe?. It came with gear wheels 2x20,2x80,2x60,1x50,1x45,1x35,1x30 so with the `imperial` leadscrew I appear to have then I should be fine with the 32tpi. I notice in some of the ads for the minilathe a `metric` add on kit to cut metric threads with an imperial lathe?????. Confusing or what!!. regards Steve
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Steve wrote:

well, 20 x x 40 sounds like it just slows the feed down by 2 x. Oh, maybe the 20 x x 40 is the feed setup for a machine with an Imperial leadscrew.
If you just set the lathe for .8 mm pitch, that works out to 31.75 TPI, which is pretty close, 0.7% error.
What is the leadscrew pitch on the lathe? How many teeth are on the spindle drive gear? From that info, you can figure out the gears needed.
The 127 : 100 gear set is ideal as 2 X 127 is the familar 254 (as in 25.4 mm / inch) But, it could be hard to fit a 127 tooth gear into a small lathe unless the gear pitch was really fine.
Jon
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