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The Artful Bodger
08-14-2014, 09:15 PM
If I have an old imperial type lathe with change gears can I really 'convert' it to metric operation just by including a 120/127 pair of gears in the drive chain? It seems I could, but with limitations.

According to the change gear table on my metric lathe, which has 120/127 compound gear for cutting imperial threads, there are 16 inch gears I can cut using the same change gears as used for metric threads but including the compound gear.


Metric threads .4, .5, .6, .75, .8, 1, 1.2, 1.5, 1.6, 2, 2.4 3, 3.2, 4, 4.8, and 6mm.

Imperial 60, 48, 40, 32, 30, 24, 20, 16, 15, 12, 10, 8, 7.5, 6, 5 and 4tpi.

(Lets do a reality check on one from the list.. a 1mm thread is 25.4 threads to the inch, 25.4/127 * 120 = 24.)

There are a lot more metric and imperial threads on the lathe thread table but these are the gear box ratios that appear in both metric and inch tables.




I presume that if I added a 120/127 pair to an imperial lathe the same relationships could be expected and I could cut all the metric threads in the above table provided the lathe had the imperial threads in the above table?

Arcane
08-14-2014, 10:31 PM
You should be able to. This is the gear chart for my 9" Model A South Bend to enable it to cut metric threads. My lathe needs a 127/100 tooth gear in the gear train. I seem to recall someone saying once that their lathe used a 120/100 tooth count gear to achieve metric threading ability but it's been awhile and my memory being what it is, I'm not sure on that point.

http://img.photobucket.com/albums/0603/Arcane/Gear%20chart/metricgearchart-1.jpg (http://smg.photobucket.com/user/Arcane/media/Gear%20chart/metricgearchart-1.jpg.html)

The Artful Bodger
08-14-2014, 10:49 PM
Thanks, so it seems it would work as I expected.

BTW, so it appears that should anyone want to cut metric threads on an imperial (i.e. "inch") lathe and has the 127/120 compound gear the easy formula to find what tpi thread to set on the gearbox for a particular metric gear
is tpi= 24/mm.

Don Young
08-14-2014, 10:54 PM
For any particular metric conversion setup, there is what i like to call a 'metric transposing factor'. For the setup you describe, the factor is '24'. This is the number that results if you multiply the English TPI on the gearbox by the resulting metric MM pitch. So if you divide the TPI into this factor, you get the MM thread that TPI on the gearbox gives you. So 24TPI gives you 1MM, 16TPI gives 1.5MM, 40TPI gives 0.6MM, etc.. If you need a thread you can't get with this setup, you can change the first gear in the train to get a new factor.

Some time ago I wrote a simple spreadsheet that gives all the metric and English threads a gearbox gives when you change the gears. It requires you to enter the original gears, the original threads in the coarsest (usually the top) row, and the new gears. Once the original gears and threads are entered you can change any gear and see what the resulting threads are. It does not select the gears for you like some programs do but it is handy for seeing what threads are provided for all the gearbox positions once the gears are selected. After you use it a while you can sorta figure out what gears will give you what threads. The spreadsheet is generic so it works with any gearbox lathe, but I have adapted it for some specific lathes.

PStechPaul
08-14-2014, 11:42 PM
I have a similar Excel spreadsheet that I may have adapted from the one you did. I added a lot of other gear combinations:

http://enginuitysystems.com/pix/tools/9x20ThreadChart.pdf

9x20ThreadChart.xls (http://enginuitysystems.com/pix/tools/9x20ThreadChart.xls)

It seems that above a thread pitch of 1.0 the increments are 0.25mm and above 2.0 the increments are 0.50mm.

http://www.engineeringtoolbox.com/metric-threads-d_777.html



Size - Nominal Diameter
(mm)
Pitch1)
(mm)
Clearance Drill
(mm)
Tap Drill
(mm)


M 1.60
0.35
1.8
1.25


M 2.00
0.40
2.4
1.60


M 2.50
0.45
2.90
2.00


M 3.00
0.50
3.40
2.50


M 3.50
0.60
3.90
2.90


M 4.00
0.70
4.50
3.30


M 5.00
0.80
5.50
4.20


M 6.00
1.00
6.60
5.00


M 8.00
1.25
9.00
6.80


M 10.00
1.50
12.00
8.50


M 12.00
1.75
14.00
10.20


M 14.00
2.00
16.00
12.00


M 16.00
2.00
18.00
14.00


M 20.00
2.50
22.00
17.50


M 22.00
2.50
25.00
19.50


M 24.00
3.00
27.00
21.00


M 27.00
3.00
30.00
24.00


M 30.00
3.50
33.00
26.50


M 36.00
4.00
40.00
32.00


M 42.00
4.50
46.00
37.50


M 48.00
5.00
53.00
43.00


M 56.00
5.50
62.00
50.50


M 64.00
6.00
70.00
58.00


M 68.00
6.00
74.00
62.00

PStechPaul
08-14-2014, 11:43 PM
Size - Nominal Diameter
(mm)
Pitch1)
(mm)
Tap Drill
(mm)


M 1.0x0.2
0.20
0.80


M 1.1x0.2
0.20
0.90


M 1.2x0.2
0.20
1.00


M 1.4z0.2
0.20
1.20


M 1.6x0.2
0.20
1.40


M 1.8x0.2
0.20
1.60


M 2x0.25
0.25
1.75


M 2.2x0.25
0.25
1.95


M 2.5x0.35
0.35
2.10


M 3x0.35
0.35
2.60


M 3.5x0.35
0.35
3.10


M 4x0.5
0.50
3.50


M 4.5x0.5
0.50
4.00


M 5x0.5
0.50
4.50


M 5.5x0.5
0.50
5.00


M 6x0.75
0.75
5.20


M 7x0.75
0.75
6.20


M 8x0.75
0.75
7.20


M 8x1.0
1.00
7.00


M 9x0.75
0.75
8.20


M 9x 1
1.00
8.00


M 10x0.75
0.75
9.20


M 10x1
1.00
9.00


M 10x1.25
1.25
8.80


M 11x0.75
0.75
10.20


M 11x1
1.00
10.00


M 12x1
1.00
11.00


M 12x1.25
1.25
10.80


M 12x1.5
1.50
10.50


M 14x1.0
1.00
13.00


M 14x1.25
1.25
12.80


M 14x1.5
1.50
12.50


M 15x1
1.00
14.00


M 15x1.5
1.50
13.50


M 16x1
1.00
15.00


M 16x1.5
1.50
14.50


M 17x1.0
1.00
16.00


M 17x1.5
1.50
15.50


M 18x1.0
1.00
17.00


M 18x1.5
1.50
16.50


M 18x2.0
2.00
16.00


M 20x1.0
1.00
19.00


M 20x1.5
1.50
18.50


M 20x2.0
2.00
18.00


M 22x1.0
1.00
21.00


M 22x1.5
1.50
20.50


M 22x2.0
2.00
20.00


M 24x1.0
1.00
23.00


M 24x1.5
1.50
22.50


M 24x2.0
2.00
22.00


M 25x1.0
1.00
24.00


M 25x1.5
1.50
23.50


M 25x2.0
2.00
23.00


M 27x1.0
1.00
26.00


M 27x1.5
1.50
25.50


M 27x2.0
2.00
25.00


M 28x1.0
1.00
27.00


M 28x1.5
1.50
26.50


M 28x2.0
2.00
26.00


M 30x1.0
1.00
29.00


M 30x1.5
1.50
28.50


M 30x2.0
2.00
28.00


M 30x3.0
3.00
27.00


M 32x1.5
1.50
30.50


M 32x2.0
2.00
30.00


M 33x1.5
1.50
31.50


M 33x2.0
2.00
31.00


M 33x3.0
3.00
30.00


M 35x1.5
1.50
33.50


M 35x2.0
2.00
33.00


M 36x1.5
1.50
34.50


M 36x2.0
2.00
34.00


M 36x3.0
3.00
33.00


M 39x1.5
1.50
37.50


M 39x2.0
2.00
37.00


M 39x3.0
3.00
36.00


M 40x1.5
1.50
38.50


M 40x2.0
2.00
38.00


M 40x3.0
3.00
37.00


M 42x1.5
1.50
40.50


M 42x2.0
2.00
40.00


M 42x3.0
3.00
39.00


M 42x4.0
4.00
38.00


M 45x1.5
1.50
43.50


M 45x2.0
2.00
43.00


M 45x3.0
3.00
42.00


M 45x4.0
4.00
41.00


M 48x1.5
1.50
46.50


M 48x2.0
2.00
46.00


M 48x3.0
3.00
45.00


M 48x4.0
4.00
44.00


M 50x1.5
1.50
48.50


M 50x2.0
2.00
48.00


M 50x3.0
3.00
47.00

Paul Alciatore
08-15-2014, 02:18 AM
Yes, the 100:127 tooth combination does give a perfect conversion from one system to the other. This is because one inch has been defined as 25.4 mm. That is an exact conversion because it IS the definition of an inch. There are no further decimal places. PERIOD. But how does that convert to the 100:127 ratio. Well, gears can only have tooth counts that are integers. You can not have a half or a third of a tooth, just 1, 2, 3, 4, etc. Integers only. Now 25.4 is not an integer so we must multiply it to get to one and if you start multiplying it, the first multiple that is an integer is 25.4 X 5 = 127. (25.4, 50.8, 76.2, 101.6, 127). The 100 part is somewhat arbitrary and other round numbers would work. 50 has been used and 200 would also work, but 100 seems to be the most popular. 127 is a prime number so we can not reduce it to a smaller factor by dividing both 100 and 127 by some common factor. Anyway, that is why 100:127 is an EXACT conversion.

But why are some common threads not possible when we are using a QC gearbox? This is because the English system threads and the metric system threads were set up in two completely different, complementary ways. Now, both systems were set up with a series of fairly round numbers for the threads that are used. But, in the English we measure threads by how many threads there are in a unit of length (Threads per Inch or Threads / Inch) and in the metric system we measure threads by how long a single thread is (just plain mm). English threads tend to be in multiples of a basic series or integers: 10, 11, 12, 13, 14, 15, 16, 18, 20, 22, 24, etc. TPI while for metric threads the series of numbers looks like 0.1, 0.2, 0.25. 0.3, 0.4. 0.5, etc. mm.

In addition to being based on a different basic length, they ARE RECIPROCALS of each other. When converting from one to the other, we need to divide a constant number by the measure we start with to get the measure in the other system.

MP (Metric Pitch) in mm = 25.4/TPI

and

English Pitch in TPI = 25.4/MP

When we divide some of the above numbers into 25.4 we can get some fairly round numbers. 25.4 / 10 TPI = 2.54 mm with on further decimal places. But 25.4 / 11 TPI = 2.3090909... which is an infinitely repeating decimal number. What this means is that when we convert from one system to the other, there will be different rations between the threads in the two series. For instance, going from 10 to 11 TPI the ratio is 1.1 X. but you will not find that ratio between any two of the metric threads. So a quick change gearbox for an English lathe will have one set of ratios from one setting to the next while a gear box on a metric lathe will have an entirely different set of ratios. It is perhaps typical that only two or perhaps three of the eight settings on the right control of an English lathe QC gear box will actually be useful for cutting metric threads. That will typically give ten threads and some of them may not be really standard metric ones. The situation going the other way is very similar.

With a lathe that uses separate change gears on a "banjo" bracket, you can usually find more gear combinations for metric threads as you can combine the gears from the set in any combination, not just the fixed combinations that a QC gear box allows. Many lathes with QC gear boxes also allow you to mount separate gears and with that you can have the best of both worlds. In either case, you may need to purchase or make some extra gears that are not part of the basic gear set provided by the lathe manufacturer, but with the proper external gears, it should be possible to cut ANY thread of either system. Another thing that can be helpful is additional compound gears. For instance, I have 1:3, 1:4, and 1:6 compounds for my SB-9. These are specially helpful for cutting fine threads.

You can see in the SB chart that Arcane posted above that all the metric threads are produced with one of the QC knobs in either the 1 or the 3 position and a wide range of stud gears are needed on top of that. The other numbered positions do not produce any useful metric threads.

In addition to that, with a good set of external gears, you can cut almost any pitch needed, as in making some worms for worm gears. At least this can be done to a very good degree of approximation as some worms will be expressed as an infinite decimal in both systems because the irrational number Pi is involved.

"Screwcutting In The Lathe" by Martin Cleeve is a good reference for all of this and more, much more.

RichR
08-15-2014, 02:42 AM
Very nice write up Paul. One thing I've noticed on the QC gearbox on my old Atlas is the row with coarsest threads form the basis for the succeeding
rows each one with twice the number of threads of the preceding row. The metric sizes, at least from what I can see, doesn't seem to have that
kind of repetitive consistency.

Paul Alciatore
08-15-2014, 05:18 AM
Well, if you have to use different stud gears like the SB chart above shows, or some other external gear change, then that destroys the order.

This is really one area where the English system worked out better than metric. The metric guys really should have used TPC (Threads per Centimeter). That would have made a much nicer system to work with.



Very nice write up Paul. One thing I've noticed on the QC gearbox on my old Atlas is the row with coarsest threads form the basis for the succeeding
rows each one with twice the number of threads of the preceding row. The metric sizes, at least from what I can see, doesn't seem to have that
kind of repetitive consistency.

Baz
08-15-2014, 07:50 PM
If you look at current 'import' lathes say a 12x24 with imperial gearbox it has the 127/100 compound gear and 3 extra stud gears. Then the adverts and manual proclaim metric capability. Technically true but only when you try it you find a couple of common metric threads .7 and .8 are not in the tables because that would require them to have provided another stud gear. Not the end of the world but annoying.

Don Young
08-15-2014, 11:30 PM
There are metric lathes that use a gearbox where all of the cone gear positions produce common threads. In these metric gearboxes the cone gears are 'driver' gears where larger gears produce coarser threads. In an English gearbox the cone gears are 'driven' gears where larger gears produce finer threads. There are some current imported lathes that actually use a metric gearbox with an English leadscrew. These lathes require gear changes to cover the most common English threads. There are also lathes, seemingly mostly European, that use a gearbox where the direction of power flow thru the cone gears is reversed for metric and English threads. On the web there are some pretty good illustrations of the South Bend metric gearbox and the TOS combination gear arrangements.

Most transposed metric threading gearboxes seem to require about 5 stud gears for a reasonably complete range of metric threads.

The Artful Bodger
08-16-2014, 02:02 AM
Metric threads on inch lathes and inch threads on metric lathes are not surprising but I never thought I would hear of metric lathes being fitted with inch leadscrews!

Don Young
08-16-2014, 08:58 PM
I wasn't too clear. The lathe in question was available in metric or English form. The English form used all English screws and dials but the gearbox design was still the metric type where the cone gears were driver rather than driven gears. It belonged to one of the posters on this forum or Chaski and I have the details somewhere.