View Full Version : thread cutting question

10-11-2015, 01:53 PM
When cutting a thread with an odd number (5/8 - 11) when do you engage the feed. Seems like I remember seeing that you engage when the thread dial is also at an odd number. Maybe i'm nuts

10-11-2015, 02:59 PM

Even thread any line

Odd thread any numbered line

Good luck


10-11-2015, 03:50 PM
It does depend on your lathe and the lead screw TPI. There should be a guide on your lathe somewhere (or in the manual) that tells you which to use for each thread. The one for my 16 TPI leadscrew looks like this.


The chart uses dots to separate numbers for a list. It uses dashes for a range. The slash means use any. In the case of the 11 TPI, you would use EITHER the mark at 4 or the one at 8.

Other examples, a thread that is 4,8 or 16 tpi can use any mark. (Top chart)

Last example, a thread that is 12 TPI can use any NUMBERED mark. (Lower chart)

Really last example, a thread that is 10 TPI can use marks numbered 2, 4, 6 or 8. (lower chart)

When in doubt, double check by taping a felt tip marker to the toolpost and make a test run to ensure that you get the thread where you want it.


Paul Alciatore
10-11-2015, 08:56 PM
Most thread dials on lathes that have an English thread lead screw are designed to indicate a four inch distance for one complete revolution of the dial. So, the dial is divided into four numbered divisions and eight unnumbered ones. Each numbered division indicates travel of one full inch, so any whole number of TPI (threads per inch) will work at any numbered division.

The eight divisions, four numbered and four unnumbered, each division indicates travel of one half of an inch. Now even numbered threads (example: 10 TPI) are always evenly divisible by two (10 / 2 = 5) so you can start on any of the eight half inch divisions for any even numbered thread.

If a thread has a 1/2 fraction (11.5 TPI) then it does not synchronize in a single inch and you must use a two inch distance. So, you can use the even numbered marks OR the odd numbered marks. Either one will work.

And if a thread has a 1/4 or 3/4 fraction then you must use a four inch distance to synchronize it, so you must start on the exact same mark each time.

And the rules can be relaxed in the other direction. For instance, if you have an 8 TPI lead screw and you are cutting any multiple of 8 TPI (8, 16, 24, 32, etc.) then you can always start on any mark or even any place between the marks where the half nuts will engage. In short, with the above combinations, you can not cut out of sync, even if you try.

All of the above only works on English threads on an English measure lead screw with a thread dial that measures over a four inch distance. Fortunately this is a very common combination. Even then, some lead screw pitches will not work well: like a 3, 5, 7, 10, or 11 TPI lead screw, to name a few. If you have such an oddball lead screw, then the rules will change.

Metric is a whole different ball park. And in a different country.

Rich Carlstedt
10-11-2015, 09:12 PM
Great response, and explanation of the reasons for dial markings !

10-11-2015, 09:37 PM
Paul, excellent explanation!!


10-11-2015, 09:51 PM
That was such a clear explanation, even I could understand it! Thanks, I may have to print that out.


Paul Alciatore
10-11-2015, 11:07 PM
Glad you guys liked it. When you understand how something works, you can use it better.

But don't get me started on metric threads. For once, the English system did it right.