Metric Threads

[Bryce.R]

Ok so i have only just started cutting threads on my lathe and I am looking for some advise. I know all about how to use the thread dial indicator to cut inch threads but i have read that you cannot use it for metric threads Can someone explain this for me?

[high country]

It's going to depend on your lathe, but most standard leadscrew lathes can cut metric with a proper change gear (most I see are 127 tooth instead of 120 for standard) and the rough part is once the half nuts are engaged.....they need to stay there. If threading into a blind hole, it is near impossible unless the tool goes to the back side and upside-down which allows cutting from the blind out, and reduces the crash risk.

[The Artful Bodger]

If I understand correctly, you can use the thread dial indicator to cut metric threads on a lathe that has a metric leadscrew but even then you may have to change the pinion on the thread dial indicator for different metric threads.

[oldtiffie]

It's going to depend on your lathe, but most standard leadscrew lathes can cut metric with a proper change gear (most I see are 127 tooth instead of 120 for standard) and the rough part is once the half nuts are engaged.....they need to stay there. If threading into a blind hole, it is near impossible unless the tool goes to the back side and upside-down which allows cutting from the blind out, and reduces the crash risk.

A 127 tooth gear is standard on most inch and metric lathes for inch>metric and metric>inch leadscrew conversions.

There are 25.4mm in 1 inch.

5 x 25.4 = 127 (teeth).

If cutting a thread in a "blind" hole you will need a "relief/clearance" groove say two threads wide and you can use either method.

Some lathes cannot fit the tool on the far side of the cross-slids so in that case its best to have the tool inverted on the near side (front) of the cross-slide and run the lathe to cut in reverse when cutting the thread.

If cutting a metric thread on an inch lathe as well as an inch thread on a metric lathe the clasp nuts must remain engaged the whole time.

In both cases on both types of lathe the threading dial is of no use.

[oldtiffie]

If I understand correctly, you can use the thread dial indicator to cut metric threads on a lathe that has a metric leadscrew but even then you may have to change the pinion on the thread dial indicator for different metric threads.

True. My metric lathe has a 3mm pitch lead-screw and a 72 tooth pinion which does work quite well but is limited - so I, like many who cut inch threads on a metric lathe leave the half nuts engaged as it is nowhere near as straight forward as screwing inch threads on an inch lathe.

http://i200.photobucket.com/albums/a...d-cutting7.jpg

http://i200.photobucket.com/albums/a...d-cutting8.jpg

http://i200.photobucket.com/albums/a...d-cutting9.jpg

http://i200.photobucket.com/albums/a...-cutting10.jpg

[.RC.]

If cutting a thread in a "blind" hole you will need a "relief/clearance" groove say two threads wide and you can use either method.

Some lathes cannot fit the tool on the far side of the cross-slids so in that case its best to have the tool inverted on the near side (front) of the cross-slide and run the lathe to cut in reverse when cutting the thread.

Nah, you just need skill...

135mm X 2mm thread cut to a shoulder @ 110RPM, lathe has imperial lead screw..... Just needs practice to do...

[dian]

is that a carbide tool? do you take it out of the cut before stopping the lathe?

[.RC.]

Yea carbide as for moving the tool out, you move it out of the cut, cut the power and hit the foot brake at the same time... It just takes practice to do.. I actually like cutting threads on the lathe...

[MarkA]

Hi guys,

Like .RC., I also quite like cutting threads on the lathe. Good thing too, as I've had quite a few to do lately.

While most of my thread cutting is imperial, I've also done a few metric threads on my lathe with an imperial lead screw, and I do dis-engage the half nuts.

I can't find my notes atm, so writing this from memory...

Set up the lathe to cut your thread as required. Choose a number on the thread dial, and stick to it. I'll use "1" as an example.

Engage the half nuts when "1" comes around, and make your first pass. When you get to the end of your thread wind out (for external threads)
the cross slide and dis-engage the half nuts. Now stop your lathe. It's important that you stop the lathe before the thread dial makes a complete revolution.

Now, and this is the clever bit, reverse your lathe and watch the thread dial. When your number, "1" in this case, lines up with the pointer, engage
the half nuts and let the lathe drag the tool back the the start position. Don't disengage the half nuts when you get back to the start position.

Return the cross slide to the previous position and put on your next cut depth as per the threading method you are using.

Repeat as before until your thread is done.

Because you are engaging the half nuts on the same number in reverse you are effectively engaging the lead screw at the same point you dis-engaged it.

I've done quite a few metric threads this way, at reasonable speeds, 200-300 rpm, and haven't stuffed one up yet.

Hope that helps.

Best regards
Mark

[John Stevenson]

It's best if you operate the clasp nut lever with the cheeks of your arse. Believe me they are the fastest acting muscles in a human body.