Four Index Thread Indicator

On my super 7 when I do imperial threads the Four Index Threads Indicator works well, I can unhook the carriage from the lead srew and start re-reading again.
This system does not work for metric threads. For each metric thread I waste a lot of time because for each pass I have to stop the lathe motor, make it turn backwards until the thread begins and start a new pass.
I wanted to know if there is a dial indicator for metric threads?

Edited By Franco Convertini on 05/02/2022 17:49:11

On a lathe with Imperial leadscrew the Thread Dial Indicator only works for Imperial threads. When cutting metric threads on a lathe with Imperial leadscrew, keep the half-nut closed all the time. You can of course, make a screwcutting clutch, to make it easier.

Thor

I wanted to know if there is a dial indicator for metric threads?

Yes but only for lathes with a metric lead screw.

regards Martin

I think my lathe is metric because it has a cross slide and a top slide with a 2mm pitch but the lead screw has a pitch of 3.15 mm

Edited By Franco Convertini on 05/02/2022 19:30:16

That leadscrew pitch of 3.15mm looks a lot like 8tpi which is 3.175mm.

I have seen dial indicators with changable gearing between the leadscrew and the indicator, but have never known what their function is.

During the last couple of weeks, I have been making internal threads of 2.5mm pitch and have to leave the leadscrew engaged. I find that it is just as easy that way, stopping and reversing the spindle as it gives me time to think about the next setting, backing off and making the next depth of cut. The Smart & Brown model A has proved very easy to change spindle direction, and quicker than when I was cutting 40tpi today and waiting for the threading indicator to turn through 24, not 4 divisions as on Myfords. If I was doing lots of threading, I would bother to learn the proper way to use the threading indicator.

Top and cross slide are irrelevant. The leadscrew needs to be metric for dial indicators to work for metric threads. You cannot drop the half nuts in anywhere else on an imperial lead screw when cutting metric threads. Even for imperial threads if you are doing strange TPI's it is sometime neccessary to keep the half nuts engaged.

regards Martin

What you say is true but it is cool to be able to unhook the screw and resume the thread. Here in Italy someone has managed to make a metric dial on a Ceriani lathe https://youtu.be/kk5oWnAkVoI.
I thought some Myfordists had created something like this.
Thank you all for the time you have dedicated to me.
Greetings, Franco

That Ceriani has a metric leadscrew. You can tell because in the comments he mentions two different gears for the dial according to what pitch he is cutting. You cannot do it with a non-metric screw it just isn't possible without either reversing or using a single tooth dog clutch. Or using ELS.

Yes, the metric Super 7 has metric cross and topslide feedscrews but the main leadscrew is 8 tpi. British engineering at its best.

When cutting any multiple of 8TPI on the Myford you can engage the leadscrew at any point - no need to wait.

Edited By ega on 05/02/2022 21:59:43

Posted by Franco Convertini 05/02/2022 17:48:34

On my super 7 when I do imperial threads the Four Index Threads Indicator works well, I can
unhook the carriage from the lead srew and start re-reading again.
This system does not work for metric threads. For each metric thread I
waste a lot of time because for each pass I have to stop the lathe motor, make it
turn backwards until the thread begins and start a new pass.
I wanted to know if there is a dial indicator for metric threads?

Edited By Franco Convertini on 05/02/2022 17:49:11

Yes it can be done,

In MEW 149 an article by Richard Bartlett on
page 36 through to 40 describes a method
for metric screwcutting on a Myford with an 8 Tpi
leadscrew and by dis-engaging the leadscrew.

John

Very interesting John, where can I see? is there a link to this?

Edited By Franco Convertini on 05/02/2022 22:27:42

It CAN be done but you still have to stop the motor and reverse it. The advantage is that you can stop at a shoulder by disengaging the half-nut. Then you reverse the motor, traverse the table back to your start, switch the motor back to forwards and pick up the same index on the threading dial on the same turn. It's no quicker than simply reversing the motor with the half-nuts closed.

When I make fine threads and go back and forth with the engine it goes away so long that I grow a beard. 😉

For short lengths of thread keeping the half nuts closed is not too bad. For a long length of thread you can release the half nuts and move the carriage back in multiples of 5" (127mm) to pick up the correct position on the leadscrew. Your thread dial indicator engagement point will be different for each successive pass. There is a risk of getting the distance wrong if you do not have a good method of ensuring you move the correct distance. Best if the leadscrew is stopped before the half nuts are disengaged otherwise you have to reverse just enough to go back to the thread dial mark that was being used then make the 5" move. A DRO will help as you can set the zero point before moving back. These issues are why, for people who regularly do both metric and TPI threads, the electronic lead screw (ELS) projects are so popular or even going the small extra step of making the lathe CNC at which point thread dial indicators and gear changing for different threads no longer takes place.

Martin C

Hi Martin,
is a good suggestion.
I have to decide as soon as possible to buy a good DRO. I did a search but the ones I've seen cost a lot.

Franco

Have a look at this video on Youtube. Oxtools I have used this method a lot. I get a bit dismayed when people post that you CANNOT disconect the half nuts during metric threading on an imperial lathe. They should really take time to research a subject before commenting. Things change over time & new discoveries are made that make things easier to do. As long as you have an imperial threading dial the method shown in the video works perfectly. If you have a screw on chuck only do this at slow speed as I do on my Boxford.

IHTH regards

That was interesting and obviously works; but seems more of a faff than just withdrawing the tool and throwing the lathe into reverse? It wouldn't work for me anyway, as I removed my imperial thread dial indicator to fit an Ainjest unit.

Andrew

Thread Dial Indicators (TDI) not working cleanly is one of the few disadvantages of the Metric System.

Imperial threads are specified in terms of Threads Per Inch, which relates directly to the gear ratios on a lathe. As the relative positions of the spindle and lead-screw have a simple relationship determined by the change gears, a simple gear driven by the lead-screw turns a dial showing when they are correctly aligned for screw-cutting. It's simple.

Metric threads are different. They're specified in terms of Pitch, which is the reciprocal of Threads per. The pitch convention breaks the simple relationship between spindle, lead-screw and the TDI. Not completely, but if a metric TDI is provided, it will only cover a certain number of pitches, or will come with a selection of drive wheels that have to be changed to suit the metric pitch in hand. It's messy!

As Imperial TDIs won't indicate pitches correctly, and Metric TDIs are more complicated to use, a different technique is mostly used to cut metric threads. Basically, the lathe is worked forward and reversed without ever disengaging the lead-screw so the spindle/lead-screw position is never lost. The method is slow, but has some advantages:

• Removes the possibility of the operator misreading the TDI, or mistiming half-nut engagements.
• Works on metric, imperial and non-standard threads
• Better suited to automatics, and reduces the need for skilled operators. (Manual machines depend on expensive people skills, which businesses hate paying for!)

I don't know why Metric threads standardised on Pitch rather than Turns per. On the face of it, pitch makes life more complicated. My guess is pitch suits Engineers thinking in terms of strength, friction and turning forces, whereas TPI met practical needs at a time before thread theory was understood. Might also be because metric threads were defined just as industry was switching to rolling threads rather than lathe cutting them. Thread rolling machines don't need TDIs or lead-screws and they make better threads faster!

Dave

Interesting an Ainjest unit. Are there any photos?
Any video showing how it works on Myford?

Edited By Franco Convertini on 06/02/2022 11:21:21