Four Index Thread Indicator

It's not the same thing the OP is asking. Oxtools is still reversing the lathe and engaging the halfnuts to move the carriage back to the starting point. He just uses the indicator dial to facilitate this. Simples, and been around for years, at least since 1986 in Martin Cleeve's book Screwcutting in the Lathe. But Franco wants to avoid using reverse and simply and quickly wind the carriage back by hand.

Franco, that same book by Martin Cleeve describes the gearing required to make your own geared indicator dial, but it does not sound simple. It also describes a way of using a carriage stop in conjunction with the English indicator dial to cut metric threads, but only shorter ones of 2 inches or less in a Myford lathe with 8tpi leadscrew. So not a great advantage because it is the longer threads that take so much time to reverse back to the start!

Now that's a new one one me, but seems blindingly obvious when you think about it, the 127 tooth conversion gear and all. Although, I can't quite visualise exactly why. I shall have to give it a try.

In the absence of a DRO, could one use the imperial thread indicator dial to measure carriage movement in inches? On the Myford with its 8TPI leadscrew, and its 16 tooth gear on the TDI, one full turn of the thread dial equals 2 inches of carriage movement when cranking the carriage traverse handle and the leadscrew is stationary. So 5 inches would be two and a half turns of the dial.

Personally I liked screwcutting on the old DSG toolroom lathes. They had a combined clutch and brake lever on the carriage so you could screwcut up to a shoulder at 400rpm. Just start slipping the clutch and slow down as the tool approaches the shoulder. Then slam the brake on just as the tool is about to crash into the shoulder. Retract the tool and reverse flat out at 400rpm back to the start and reset tool depth and repeat process. Not for the feint hearted but no apprentice was going to admit to that.

This is the article i referred to in the the posting at 05/02/2022 22:11:28.

Using this method allows the motor to remain running ,the spindle
can be left turning ,the half nut are dis-engaged at the end of the
pass the carriage returned to the start position ready for the next pass.

To get the best use from this a true 127 gear for metric conversion
must be used.
The article is worth reading and covers all of the points the OP
asked for.

I use the same 127/60 tooth gears as in the article,here is
the change wheel list for all the metric pitches.

John

Hello John,
I was blown away when I read your message. This was exactly what I was looking for. With the engine always running, disengage the lead screw at the end of the thread, return the carriage quickly backwards and engage the lead screw again. Two things remain to be clarified:
1 the number of teeth that the thread dial indicator gear has and how many notches it has on the dial; Also what module does it have for milling teeth.
2 I have the Norton box on the lathe; how should i adjust it?

Regards, Franco

Edited By Franco Convertini on 06/02/2022 19:11:31

Hi Franco,

i have sent you a PM .

The article shows all of the information required ,Unfortunately you have the disadvantage in having a screw cutting gearbox ,i think there is lash up arrangement using 33 and 34 tooth gears which is not as good as a true 127 for imperial to metric conversion ,this would work for you if you could fit in a 127 gear some ware in the gear train ,i have a change wheel machine and find that is far better than a boxed machine.

John

Not a lash up but a very practical solution at minimal cost.

Tony

Hi John, I have already replied to you on the MP.
I realize that for this project it is easier to have a lathe where you change the wheels. Is it possible that the box of my lathe does not have a 1: 1 ratio?

Edited By Franco Convertini on 06/02/2022 19:35:06

What I find hard to get my head around is rather than 8tpi, which is a nice round figure of 0.125" pitch, the Smart & Brown has an imperial leadscrew of 6tpi which is not a nice round figure,0.166666----------". The 24 digit threading dial cannot be disengaged from the leadscrew. That has a slight advantage, as before engaging the nut, you can quickly glance at it and be sure which way the saddle is going to move.

In the article it's mentioned that the system works for integer pitch metric threads. What does one do when cuttiing non-integer threads which, let's face it, would be the more common?

Pete ,

You need to read the full article it covers all the pitches.

John

Well I just had to try this method on my imperial Myford ML7 because it sounds too good to be true -- and it does not work, as above pic shows. Lathe change gears were set up to cut a 2mm pitch thread with an 8tpi leadscrew. Carriage movement was measured with dial indicator to be exactly 2mm per one revolution of the chuck. Lathe was stopped at the end of the first pass before disengaging halfnuts and carriage was moved by a carefully measured 5.000 inches and halfnuts re-engaged then lathe started and second pass taken.

First pass is visible at the left. Second pass, after using the above method resulted in the double thread on the right hand portion.

And I can not see mathematically any way that this method could possibly work. You stop the lathe, disengage the half nuts then move the carriage 5.000 inches or 127.00 mm to the right and re-engage the halfnuts. That would work ok on a 1mm pitch threads. You are re-engaging the halfnuts 127 full pitches away from the starting point. All good.

But on a 2mm pitch thread, 127 mm is halfway between two pitches. It needs to either engage at 126mm from the original disengagement point, or 128mm, which would be 63 and 64 pitches respectively. (But of course the halfnuts will not engage at these two points because they are each 1mm out of line with the thread on the leadscrew. )

And it does not work mathematically for any other metric thread pitch either. They all work out to oddball measurements, none of them 127mm. As 127 is a prime number, the only pitch that divides into it without getting into fractions of a pitch, is 1mm. It does not work for any other pitch, as far as I can see.

Edited By Hopper on 10/02/2022 11:02:32

OK, so on further thought: If winding the carriage back 5" works for a 1mm pitch, then winding back 10" would work for a 2mm pitch thread, ie 10" = 254mm = 127 full threads at 2mm pitch. And on the same principle, 15" (= 381mm = 127 x3) would work for a 3mm pitch.

But what about 1.5mm and 1.75mm pitches etc? Hmm further thought required. As Curly from the Three Stooges said, I tried thinking about it but nothing is happening.

Edited By Hopper on 10/02/2022 12:09:32

Edited By Hopper on 10/02/2022 12:13:01

Mathematically the number needed is the least common factor of 127 and a multiple of the metric pitch to make it an integer.

For 1.5mm pitch it is 2x1.5x127 = 381 = 15"

Andrew

Edited By Andrew Johnston on 10/02/2022 12:18:08

Aha. Thank you. I can see that. Basically half a 3mm pitch so the same 15" point.

So I guess .75mm pitch would also be 15". As in 4 x 0.75 x 127 = 381 = 15"?

And for .5mm pitch you could use 127mm/5" because it would equal 254 threads.

But what about 1.25 and 1.75? I'm thinking 1.25 x 4 x 127 = 635mm = 25", which is probably off the end of the Myford bed someplace!

Going the other way, 0.8 x 5 x 127 = 508 = 20". h

Could be handy for long jobs for sure. I think I will have to write them all down on a chart though. i will never remember the calculations by the time I ever need to use it!

And from my own faffing around in the shed this afternoon, I reckon one could use the thread indicator dial to measure off the carriage movement, rather than wrestling with a digital caliper as I did. One full turn of the TDI dial is two inches on a Myford. (16 tooth gear engaged with a 8tpi leadscrew). So to move five inches, it was a matter of use #1 on the dial for the first pass, then wind back two turns plus a half to get 5", so the #3 on the dial is lined up with the mark. 10 inches would be a matter of five full turns of the dial etc. As long as the halfnuts engage with the stationary leadscrew at that number,you are ready to go. Neat!

Edited By Hopper on 10/02/2022 13:48:16

I recall an aged turner many years ago recounting screwcutting with out the use of a thread indicator. His explanation centred on what he called chalk marks. Apparently if my aged memory serves the procedure was to mark the bed where the carriage was located establishing a start point. The lead screw was also marked along its length against a static part of the bed. On a Myford say the tailstock end leadscrew bearing so the point at which the half nuts were engaged was indicated. Thus a start point for the leadscrew and a corresponding start point for the carriage were established. The marks enabled the turner to return to the same start point relationship after each cut.

I've not tried the method but am not able to see why it wouldn't work and on the face of it should work for any pitch.

Or am I way off?

Maybe one of the ancient tomes on lathework might have a description although I'm unable to recall seeing it described in print.

John

Yes i think Martin Cleeve's book Screwcutting in the Lathe mentions the old chalk mark system but not sure it would work for metric on an imperial. I think to do that would require a chalk mark also on the chuck and a very long wait for both the chalk mark on the chuck and the leadscrew to both line up with their reference points at the same time. I used to try to do stuff like that using the leadscrew dog clutch on my Drummond for cutting odd TPI threads etc and it's a test of patience. Will have to have a read up on it in the morning. Getting late here now.

I got hold of that full article from MEW 149 too, so plenty of reading over breakfast tomorrow!

Edited By Hopper on 10/02/2022 13:54:57

Edited By Hopper on 10/02/2022 14:00:40

Hopper, I realised when I came back to this thread that I had forgotten that the pitch needs to go into 127 an integer number of times. So 1mm, 0.5mm, 0.25mm will work. You can also do 2mm with a 254mm (10" ) move.

Martin C

Thanks Martin. With Andrew's help I think we figured it out. Handy if you have a 9" long job, not so much for a 6" long job as you still have a long time to wait before the tool gets to the job, so not that much quicker than the reversing the lathe rotation method. And when you get out to using the 15" and 20" settings, even more so.

Reading the full article in MEW 149, it looks like that involves quite a long wait for the one mark to come around on either the 40T gear driven dial for integer threads, or even longer on the very large non-integer improvised set up for non-integer pitches.

And Martin Cleeve in his book on screwcutting mentions geared indicator dials but says they are no huge advantage because of the time needed for the mark to come around into alignment.

So not sure if any of the methods really offer a truly quick way of cutting metric threads on an imperial lathe, albeit they are in some cases a bit quicker than the reversing method but not by a huge amount. Will have to play about with it some more.

Meanwhile I am off to make some new changegear studs for the Myford, the type with the clamping nut on the easily accessible outer end. Sick and tired of wrestling around with those hidden nuts in behind the quadrant. Just noticed Brian Wood's book on lathe gearing for screwcutting has a nice drawing of the later type with the better nut location. Should be a nice little project for the day. Another day of screwing around in the shed!

Even with the nuts easily accessible changing the gears is still a pita

Martin C

Yes but it should downgrade if from a Right Royal PITA to a British Standard PITA. Every bit helps.