Metric threads on South Bend

[GordonL]

I had occasion where I needed a metric thread of 2mm pitch. I got by using 13TPI which worked for this application. Metric equivalent would be 12.7 TPI.

I have a 13" SB with dual lever quick change gear box. The ideal setup is 120/100 combo gear but 47/37 is very close and many folks seem to be using that and I could probably make those gears. I can find charts and the setup for 9" and 10" but not for the 13" other than a new brass plate on ebay for $80. I just want the information that I can print out. I am not sure if the 9" or 10" information is the same. The lead screw apparently come in 8TPI and 6TPI. My lathe has 6TPI. Does anyone have the information on the 13"?

[mikey553]

I do not have the information for your 13" SB, but I have very nice table of change gears for my 12" G4003 Grizzly. It was developed many years ago by Richard Kinch and includes all possible combinations of standard change gears to produce both imperial and metric pitches. The lathe has 8TPI lead screw. It starts with .0929 mm pitch and ends with 7.9375 mm pitch. This table has 16 pages of printed information and many combinations for metric threads are more accurate that factory recommended ones.

Today I am going to setup my lathe to cut a 1 mm pitch thread. The table gives me the choice of 5 different gear combinations, each one making a 1.0002 mm pitch. Since my thread is short (only 10 mm long) I don't care about this little deviation.

If you know your lathe construction, you can easily develop a similar table for your lathe. Ideally you may include a 127 teeth gear in your gear train to produce exact metric thread pitches. I don't have one and never had a need for it.

[Baz]

What you need to know is firstly whether they use the same ratios inside the gearbox for the 13 as for the 9 and they compensate for the 6tpi leadscrew in the gears between the mandrel and the gearbox input. If this is the case, which is most likely, you can take the table you have and add a 6/8 ratio (leadscrew turns more slowly) in front of that. So you write down the gears the table says and add a 30/40 pair in front. Then simplify the equation.

[Paul Alciatore]

I developed a table of all the possible thread pitches I can cut with my SB-9 which is the manual change gear model. I do have a 127 tooth gear so I can have exact metric conversions. But the funny thing is, even with the full, standard set of SB change gears, some extra change gears not normally included in that set, three different compound gears, and a 100 tooth gear that I use with the 127 tooth, I still can not get every standard metric pitch.

The reason for this is the different ways the English pitches and metric pitches are set up. English threads are measured in threads PER inch while metric threads are simply measured in mm. In addition to using different basic units of length, these two thread specifying systems use numbers that bear a reciprocal relationship to each other. And when they were set up, people naturally choose round numbers for the different threads. 10, 11, 12, 13 TPI and 0.5, 0.8, 1, 1.5, 2 mm etc.

The problem is, due to the reciprocal relationship, choosing round numbers in both of these thread pitch measurement methods produces different mathematical sequences. And that, in turn, means that different sequences of gear ratios are needed for these different systems. So the sequencing of the gears in a QC gear box on an English lathe will be different than one for a metric lathe. Likewise, the individual change gears for manual change gear lathes will also be in different sequences. This is not to say that either one is better or worse, just that they are different.

And that fundamental, mathematical difference in how the two systems specify thread pitch means that if you are going to cut threads of the opposite system as the lead screw, then you are going to need both the 127 tooth gear and also, for some threads, additional change gears. You simply can not count on just the 127 tooth gear to give you all possible pitches in the other system.

And since QC gear boxes have only a few, fixed gear ratios available, they will probably produce the worst problems. This is why almost all tables for metric threads on English lathes with QC gear boxes will also have a number of metric threads which require that a manual change gear be changed. I am sure it is the same the other way around. Lathes with manual change gears have a lot more versatility because, with the set of change gears provided, many more gear ratios can be set up.

[JRouche]

I had run a SB 13" for as long as it took. I was lucky, the machinist that owned the shop let me hang out.. / o am not a machinist.

[mikey553]

Paul, since you developed your own table for your lathe would you mind explaining how the 127 tooth gear should be added to the gear train?

In my lathe I have a QC gearbox for feeds and threads and in addition to that - external change gears. The external gears is where all the magic happens. In my case it can be set as a simple 2 gear pair with an idler in between or a more complicated 4 gear train, which includes a combination 86/91 tooth gear instead of an idler. Where would you put a 127 tooth gear for metric threads? My guess is a combination gear would need to be replaced with a 127/? gear. Am I correct?

[davidwdyer]

I too have a SB 13 and would like more information. In my case, pictures would help about where to put a 127 tooth gear and how it would work.

Also, knowing which metric pitches could be cut and which could not would be hands.

[GordonL]

After thinking about it I do not think that the TPI in the lead screw make any difference. If the gearbox is set for 10 TPI it is going to move 1/10" per revolution regardless of the lead screw. That is already fixed into the gearing which is feeding the lead screw. You need the 127 tooth gear to compensate for the 1" = 2.54mm. 127 x 2 = 254 and the 100 tooth makes 254/100 = 25.4. Therefore metric pitch/25.4 = X, and 1/X = TPI. Then you need a gear ratio to compensate for metric not equalling an even number of TPI. Determining that ratio is the problem.

Still trying to get my thinking straight.

[mikey553]

After thinking about it I do not think that the TPI in the lead screw make any difference.

Not true. Lead screw pitch definitely affects the end result.

[J Tiers]

Not true. Lead screw pitch definitely affects the end result.

?????????

If two machines with different leadscrew pitches are set to the same TPI for threading, there is no inherent difference in accuracy between them. Why would there be?

The gearing to get that pitch is different, but that is obvious and of no consequence to the user because the correct gears and chart are supplied.

[GordonL]

The carriage is going to move 1/TPI no matter what the TPI in the lead screw may be. That is all determined by the gearing prior to the gear box.

[mikey553]

By "the end result" I was meaning the thread pitch on your part, not the accuracy.

[J Tiers]

By "the end result" I was meaning the thread pitch on your part, not the accuracy.

So?

if the pitch is not what is expected, it is not accurate. The lead screw makes no difference at all. ANY pitch lead screw can be used, and there is gearing (which will be supplied) which will make that lead screw create the correct pitch on the part.

Yes, there are stupid lead screw pitches that in the real world won't actually function due to friction, but in a frictionless world they would create the right pitch on the work. Something like 1 turn per 100 yards is like that. Even a 10 inch pitch on a 1" diameter lead screw. There is nothing about the numbers that makes it impossible. There is just the real world friction, and realities of the way gears work that make such things totally impractical to actually build and use.

[GordonL]

I 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.

[J Tiers]

I 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.

No difference at all. The gearbox simply provides the same gears in a different and easier to use form.

For any lead screw pitch there are gears, whether in a box with levers to engage them, or laying out on the table to put on by hand, which will allow that lead screw to make any given TPI of finished screw thread.

Yes, being nit-picky about the statement made earlier, because the statement left it open.