Metric thread cutting in a lathe

[Chris TriceParticipant @christrice43267]

Something I've recently started doing is if I'm cutting an "odd" thread like this, I'll make a separate short length of it at the same time as the actual item at the same settings to act as a sort of home made thread gauge next time I have make a similar thread. As you get close, it's then a case of observing carefully and very finely finessing the finished thread by frequent offering up.

[EmgeeParticipant @emgee]

Posted by Andrew Johnston on 25/01/2019 12:50:14:

Posted by Barrie Lever on 25/01/2019 10:34:50:

That info is all on the back of the Tracy tools price list.

Some suppliers call it extra fine; Machinery's Handbook says constant pitch so that's what I'll use. I stopped purchasing from TT a good few years ago, for several reasons.

Andrew

Andrew, are not all machine screws constant pitch ?

Emgee

[Anonymous]

I should have said constant pitch series.

For standard thread series the individual threads are designated by their OD, as in 1/4" BSW or M10, the pitch (or tpi) of the thread is implied. Before some smartypants points it out BA is an outlier.

For a constant pitch series the series is defined by the pitch of the thread and is independent of the OD. The OD is determined by the individual circumstances and can be whatever the designer specifies.

Andrew

[JasonBModerator @jasonb]

The metric constant pitches such as 0.5, 0.75 and 1.0mm are much the same as ME 40tpi, 32tpi and 26tpi brass where the pitch is constant over a range of diameters.

Just so happened to be wanting a M18x1 thread this weekend so brought it forward to today. Started with 18mm dia, went into the tapped hole snugly with 0.051" off dia so took another 0.001" for comfort so that is 0.052 cut or 1.3208mm which equates to 0.6606P that was with a partial form insert

 

 

Edited By JasonB on 25/01/2019 16:36:51

[BufferParticipant @buffer]

One big mistake i made was to set my cross slide over to 30 degrees and the thread was very odd. It took a while to work out that i needed to set it to 60 degrees on my lathe. Seems obvious now but it didn’t at the time.

[Howard LewisParticipant @howardlewis46836]

A basic rule of thumb with Metric threads is for Tapping drill to be (Nominal – Pitch) i e 6 x 1, drill 5mm, or 12 x1 drill 11mm.

And for screwcutting, a 12 x 1mm thread, use, again as a rule of thumb, having turned the OD to say 11.95mm, depth of cut will be 1mm (NOMINAL). Allowing for the 0.05 truncation, the depth of cut will actually be 0.975 mm (0.05/2) from contact with the O D.

Note the words, RULE OF THUMB.

For greater precision, use the maths to calculate the more exact figure for truncation and depth of cut to produce the correct root diameter. The reduced O D is to truncate the thread, to prevent the crest of the external thread interfering with the root of the Internal.

Similarly, if you so wish, the internal thread can be truncated by boring the hole slightly oversize, in similar manner.

Howard

[egaParticipant @ega]

Posted by Richard brown 1 on 25/01/2019 17:45:14:
One big mistake i made was to set my cross slide over to 30 degrees and the thread was very odd. It took a while to work out that i needed to set it to 60 degrees on my lathe. Seems obvious now but it didn't at the time.

As you realized, the offset is to the axis of the cross slide. The authorities recommend setting a degree or so less with a single point tool so that the trailing flank takes a fine shaving.

[JasonBModerator @jasonb]

Posted by Howard Lewis on 25/01/2019 18:05:23:

 

And for screwcutting, a 12 x 1mm thread, use, again as a rule of thumb, having turned the OD to say 11.95mm, depth of cut will be 1mm (NOMINAL). Allowing for the 0.05 truncation, the depth of cut will actually be 0.975 mm (0.05/2) from contact with the O D.

Are you sure about that or is your thumb a very loose fit in the nut.

Even if the threads went to a complete point you would only need to feed in by the height of the triangle which is 0.866mm for a 1mm pitch.

Edited By JasonB on 25/01/2019 19:56:45

[Howard LewisParticipant @howardlewis46836]

Ah, a Sine of the times, or are you seeking to Tan me?

The figures are as Jason says, (Whichever way you calculate them), which may produce a tight fitting thread combination which will probably need to be truncated to function.

One way to find out, suck it and see!

BUT

The Rule of Thumb produces the same figures as in the Zeus Charts for Metric Coarse and Fine threads, or is that vice versa? Anyway, the figures are the same. Zeus quotes the nearest stock drill for the (N+0.25) pitches

Perhaps the figures are intended to produce truncation?

I am not the guru, merely repeating what others tell me, and what works in everyday life..

Howard

[JasonBModerator @jasonb]

A drill takes 1mm total off the diameter when tapping. a 1mm DOC will take 2mm total off diameter

Going further Zeuz gives tapping sizes for approx 80% engagement so teh tapping drill for 1.0mm pitch is 1mm less than nominal in this case tapping will be 11mm dia, giving 0.5mm of thread all round and if that 0.5mm is approx 80% then full engagement will be approx 0.6 ( 1.2 x 0.5) if you look at the above posts it is being suggested that DOC is just over 0.6mm and my own figures from todays thread are just a whisker over that at 0.66mm which is a long way from 1mm.

Edited By JasonB on 25/01/2019 20:39:18

Edited By JasonB on 25/01/2019 20:39:47

[peak4Participant @peak4]

On the other hand, if one offsets the topslide by the nominal 30/60° and measure the depth of cut on that graduated dial, then for a 1mm pitch, the depth of cut on the topslide won't be a mile off @ 1mm.

60° equilateral triangles and all that.

Bill

[Author]

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