Trugrip conversion

So I got a crawford trugrip chuck bundled with my lathe but it didnt come with any collets. After a bit of searching it seems that some people machine a new inner collar so the more widly available 5C collets can be used.

So this sounds like an ideal opportunity to use my lathe to make something for my lathe!

So my research would suggest the external thread on the 5C collet is a 1.238 x 20 tpi thread. But I'm have no idea would to do with this to get to the dimensions I need to establish (major and minor?) Before I start cutting the thread.

Any help would be appreciated.

Cheers

Colin

The major diameter is 1.238". If you want to verify, buy a 5C collet and measure it.

I am guessing that the thread form is 60 degree included angle.

Most of the major threading insert manufacturers provide tables showing the infeed for a particular pitch and thread form.

Below is one example. Another very good one is the Seco one:

https://www.mitsubishicarbide.com/application/files/5314/7191/5783/tec_threading_depth_external_en-us.pdf

Use the numbers as a guide. The final dimension you require depends a lot on the cutting tool you use - a hand ground HSS one will be different to a multi-pitch insert (A, AG, G, etc.) and will be different to a single (specific) pitch insert.

If you are set on this route, it might be wise to buy your collet set and cut a test piece in the same material as the real version to verify that it fits every collet. If you cut the final version to a set of numbers, what happens after the chuck is assembled and one of the collets does not fit?

If you implement this modification I believe I am correct in thinking that the Trugrip collet has a left-hand thread whilst the 5C collet has a right-hand thread. Therefore, when closing a modified chuck the key will have to be rotated anti-clockwise. Can someone else confirm this is the case?

Edited By Chris Crew on 20/10/2021 23:50:29

That is the case. Trugrip is left hand and 5c is right handed