View Full Version : Thread cutting help.

06-28-2010, 04:27 AM
Hi all,

I am going to be having a go at some thread cutting on a lathe and looking for some help.

Anyone point me a link to a chart that will give me the start sizes for internal and external threading,

For example if I am making a cap to go onto the end of something that is say 20mm, I assume to cut the thread on the outside the diameter of the bar is 20mm, but then what is the size of the internal bore to start to cut the matching thread.

Does this make sense...?


John Stevenson
06-28-2010, 05:05 AM
External is the size of the thread so in your case 20mm.

Internal depends on the pitch of the thread, courser pitches are deeper so require a smaller hole.

Working in metric is easy, no tables required.

You take the pitch away from the thread size and that's the size of hole so a 20mm x 1.5 pitch requires an 18.5mm hole.

Works on all metric threads, no exception.

06-28-2010, 07:05 AM

That is really handy to remember, thanks....

06-28-2010, 07:06 AM
Look for a copy of "Zeus" Dave C, gives tables for most thread sizes, also, give yourself a headache with these :-


Regards Ian.

06-28-2010, 07:23 AM
Sir John, wow, are you gunna get it, you just buggered the W-H-O-L-E system, that posting of yours should have been about #50 or 60.

Now what in the hell are all the others going to do, to absolutely confuse the poor bloke, what with that other M-O-N-G-R-E-L thread they use. :-)

06-28-2010, 11:54 AM
I think you really need a chart.. While you can cheat with the -pitch rules and such, I don't think that takes into accout clearance beween ID and OD threads.

And then theres also the diffrence beween threads with rounded crests/roots and not..

John Stevenson
06-28-2010, 12:24 PM
If you get a chart for metric threads it will tell you what you can do in your head.


06-28-2010, 12:42 PM
If you get a chart for metric threads it will tell you what you can do in your head.

But be aware! Things get really complicated, if the dimensions are given in inch and TPI. :D


John Garner
06-28-2010, 01:09 PM
DaveC --

Don't let The Earl fool you . . . the Rule of Thumb he cites, which I'll slightly paraphrase as Hole to be Threaded Diameter = Nominal Outside Diameter of Male Thread - Pitch applies equally to both the ISO Metric threadform and the inch-dimensioned Unified threadform since both of these threadforms are identically proportioned.

To apply this Rule of Thumb for an ISO metric threadform, you need to know that ISO Metric screwthreads are cited "Nominal Outside Diameter of Male Thread x Pitch", which gives you both values directly.

Applying the Rule of Thumb to a Unified threadform is slightly more complex, for two reasons: 1) The Nominal Outside Diameter of Male Thread of a smaller Unified screwthreads is cited as a "Numbered Machine Screw Size" while the Nominal Outside Diameter of Male Thread of a larger Unified screwthreads is cited as a "Nominal Outside Diameter of Male Thread", and 2) the Pitch is cited indirectly as the Number of Threads Per Inch.

To deal with these differences, you need to know a couple of other bits of information:

1. The Nominal Outside Diameter of Male Thread for a Numbered Machine Screw "N" = 0.060 inch + N x 0.013 inch. A worked example? Ok . . . for a Number 8 machine screw, the Nominal Outside Diameter of Male Thread = 0.060 inch + 8 x 0.013 inch, which is 0.060 inch + 0.104 inch = 0.164 inch.

2. The Pitch, in inches, of a Unified screwthread is equal to 1 / Number of Threads Per Inch. So, for a 20 Thread Per Inch screw, Pitch = 1 / 20 inch = 0.050 inch.

Finally, there are a couple of screwthread jargon terms you should learn right now:

1. The jargon term for the Nominal Outside Diameter of Male Thread is "Major Diameter", in millimeters for ISO Metric screwthreads, in inches for Unified screwthreads.

2. The Hole to be Threaded Diameter is most commonly termed the "Tap Drill Size" in screwthread jargon.


06-28-2010, 04:05 PM
Hi John, that is some very good information there, and as a bit of a shock I actually understood it, which is good for me..:D

I don't think I will be trying anything to complicated and will be keeping it simple....

Thanks again for the info, I am finding this place a mine of information.


06-28-2010, 05:29 PM
If you can't find the charts, and can't do the math, you can use online calculators. There is one at http://www.tanj.com/cgi-bin/tpi.cgi that will calulate the major or minor diamter, depending on what you have been able to measure.

It also will give gearing possibilities for the 7x10 and simliar lathes.


06-28-2010, 06:34 PM
tap or bore size is (diameter- 1/threads per inch.) for american threads.

Metric is easy as stated, I love doing metric threads, just make sure its not witworth threads with a 55 degree profile. I guess they call it british standard and its not tough you just need the right tool geometry.

the male thread is the nominal size like 1.25" you may as well turn it to 1.245 or so to make it easy and fast.

Never cut threads too tight, one spec of swarf and it will destroy itself.

good luck.

06-29-2010, 05:11 PM
Hi David
If you are new to threading on a lathe here is a link to a how to video that could be very helpful How to thread on a lathe video (http://www.machinist-guide.com/lathe-threading.html)