Cutting metric thread on imperial lathe

Can someone remind me how to cut a 1.5mm thread using an 8tpi lead screw. I don't have a metric conversion gear, but I have the normal steps of 5 up to 65 including many duplicates and a couple of 38 gears
(which I have never used so maybe this is what they are for). I think its about 16.9tpi, so maybe 17tpi is good enough - but thats not in my thread chart either.
Cheshire Steve
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Well if it helps at all (maybe you already know) the spindle needs to turn 2.1167 times faster then the lead screw.
0.125 / 0.059055 = 2.1167
So just need to find a ratio close to that.
An 80 tooth on the screw with a 38 tooth on the spindle with direct gearing would give you a ratio of 2.1053 for a lead of 0.0594" or 1.508mm should be close enough for most things.
I hope I got that right. _________ Andre' B.
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1.5.93 and the following gives 16.923 so try the following 20 on mandrel driving 55 keyed to a 65 which drives the 50 on the leadscrew John Quirke
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snipped-for-privacy@gmail.com wrote:

John.. you are a star. That is easily good enough, and considerably closer than anything I came up with.
Thanks .. Steve
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Cheshire Steve wrote:

You don't say what lathe in particular, but if you can get three ratios in there, you would want 38/45 40/65 50/55 which gives 1.4999223mm with 8TPI leadscrew.
If you are restricted to two ratios then you want 65/50 20/55 for 1.500901mm, these are the same numbers posted by John Q.
I've written a piece of software that allows you to play some tunes on the different division rations, you can download it at http://metal.duncanamps.com/software.php
--
Duncan Munro
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And a really excellent job it is too. Thank you Duncan.
--
Charles Lamont

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from Duncan Munro

. . . . and an excellent piece of work it is too! . . . even though I have a 127 tooth wheel it has got me out of 'centre-distance' problems many times . . . if you have an extended set of wheels it will take account of those as well.
JG
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Duncan Munro wrote:

Thanks Duncan,
The lathe is a 1950s Winfield, it has a single banjo slot rather than the forked type you find on the Myfords (I bought a Myford banjo, but it is a completely different fitting, and have put it to one side for the moment). I vaguely remember looking at your excellent software, and I think I concluded that it was a bit Myford specific, or maybe it assumed I had gears that I didn't. That may have been a bit hasty on my part - although it can't give the gear layout, it clearly can give the ratios.
Can it be used for a single banjo like I have ? If the software allows you to insist that all the gears are on the single long slot, then that would work be perfect as the lengths are almost identical - its 4 3/4 inches between driver and leadscrew ! I sometimes have a real struggle getting the gear train right - and I am just off to the workshop to see how I can fit the ratios you have offered. I need to turn up a radiator plug to get my Land Rover on the road.
Steve (Cheshire)
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Cheshire Steve wrote:

Hi Steve, it was build around the forked style of banjo rather than a parallel double or single.
However... I'll see if it's possible to fool the software into using a single, if not I can add an option to take the Myfordesque checks out so it will just concentrate on ratios rather than "will they fit".
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Duncan Munro
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Hi Duncan,
What tools do you use to produce the app + installer package?
Steve
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Steve W wrote:

Hi Steve, the app is produced with Turbo Delphi 2006 Professional (Borland), and the installer package is GP-Install (Quality Software Components).
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Duncan Munro
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Thanks Duncan.
Steve
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Duncan Munro wrote:

Duncan,
It proved impossible to get the large gear combination on the single slot of the Winfield, but I downloaded your excellent program and was able to find 38/65 20/25 which I could manage and was (had to be) good enough for the task in hand (1.485mm instead of 1.5mm). Job done.
I'll think again about modifying the Myford banjo to fit, but I should be OK for now as I normally only have to cut imperial threads, and in a month or two my Cromwell lathe will be up and running so I will have a full thread cutting gearbox - at last.
Steve (Cheshire)
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On the ML7, you get 2 x 20t gears as standard, and also a 38t, which foxes many people.
The 38t is your best friend when it comes to metric pitches.
0.6, 0.7, 0.8, 1.0, 1.25 and 1.5 pitches can all be cut accurately with errors better than 0.02 leadscrew revolutions using the 38t somewhere in the chain.
If anyone wants the chart I have prepared with all the ratios and pitches, just PM or email me.
chris
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On a South Bend 9" a 100/127 gear will do most of the magic.
--
Uffe Brentsen



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"Uffe B�������������������������������������" wrote:

A 100/127 gear will work the magic on any Imperial lathe that can accept it, because 127 is half of 254, and there are 25.4mm in 1". The trouble is that some smaller lathes simply don't have sufficient space for such a large gear. Unfortunately, 127 is a prime number, so anything smaller has to be a compromise between accuracy and usability. A 38t gear does just that.
--
Regards, Gary Wooding
(To reply by email, change gug to goog in my address)
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lemel_man wrote:

B�������������������������������������" wrote:
63t can be pretty close (poor man's 127/2) if you have the room for that.
Bob
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63 is what Atlas use in the geartrain.
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On Sun, 29 Aug 2010 12:00:48 +0100, Bob Minchin

The relevant ratio in this case isn't:-
63*2/100=1.26
its:-
80/63=1.2698
Which is pretty damned close.
Mark Rand RTFM
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