Trying to figure out tpi for an old lathe head stock for chuck insert

[John McKay]

Very new to turning and ive purchased my first chuck to attempt bowls. Not realizing I needed an insert adapter for the headstock.Trying to post a pic in hopes of someone being able to identify so i can purchase the proper insert and avoid damaging the lathe but i cant seem to link it to the post. The lathe is a vintage Boice Crane from around the 30s.

[Michael Mills]

The diameter should be easy to measure. Assuming it is not metric, for the TPI measure from one peak to the last for a full inch. You really want to know the number of valleys between them not the actual peaks. If in one inch you have 9 peaks then the TPI is 8; if you have 13 peaks then the TPI is twelve, etc.

[Marvin Hasenak]

If you have a face plate, take it to a hardware store and see what size bolt fits it. If not start here http://www.lathes.co.uk/boicecrane/ Then go to the Old Woodworking machinery forum (owwm.org) and trace it down.

[Brian Kent]

Do you measure the diameter on the peaks or the valleys of the threads?

[Dennis Ford]

Measure across the peaks and consider that they have worn down slightly. I would expect brand new threads to be 0.010" +- less than the nominal size. An old machine might have some wear and be slightly smaller across the peaks.

[John K Jordan]

Do you measure the diameter on the peaks or the valleys of the threads?

You measure the diameter at the peaks.

Don't be alarmed if the diameter isn't exactly the Nominal (Basic) dimension. For example, the 1-1/4"x8 threads on the three lathes in my shop are all .015-.020" undersized. There may be a little wear over time, but they are actually machined that way with the tops of the peaks flattened (actually rounded).

Thread charts list the dimensions for the diameters at the peaks and the troughs (major and minor diameters). You won't see 8 tpi for larger spindles in many charts since they appear to be in the coarse thread Constant Pitch series, the 8un. I'm no machinist but this is my understanding, I trust someone will correct any misunderstanding.

This web page (and many others) describe thread measurements and have tables. http://www.efunda.com/designstandards/screws/screws_intro.cfm (This one is good but wants you to sign up if you use it more than a few times.)

JKJ

[John K Jordan]

Measure across the peaks....

Ah, you beat me to the post! It takes me a long time to type a message with running to measure and fact checking my memory. For some reason the memory seems to be less reliable every year. :)

JKJ

[Bill Jobe]

If you have an android phone...

https://play.google.com/store/apps/details?id=kr.sira.thread&hl=en&referrer=utm_source%3Dgoogle%26utm_medium%3Dorgani c%26utm_term%3Dandroid+app+for+determining+thread+ pitch&pcampaignid=APPU_1_Q8EBWKuwN9OB-QGkpJygDw