confused with sandvik c8 collet sizes

toadboy65 · Sep 17, 2020

I picked up a set of Sandvik Coromant STC8 collets, and the sizes confuse me. My understanding is that the part number includes a four digit number, which is supposed to be the size in inches. For example, I have in my hand one of the collets which reads-
"Sandvik Coromant STC8.1..1000.1...TH". This should be a 1" collet, , but the opening when it is "relaxed" is .79". A one inch tool is not going to go in there.
Clearly I am doing something wrong, but I don't know what.
Thanks- T

TeachMePlease · Sep 18, 2020

There's metric fookery about.

.787 is 20mm...

Most Sandvik tooling I've ever seen is expressed in either mm or 1/8" increments. Like their stick tooling, an SDJCR08xxxx is 08/16 or 1/2" shank.

toadboy65 · Sep 18, 2020

Well, it still seems nonsensical. The 0500 is half the size of the 1000. If you are going to the trouble of numbering them by size, and stamping those numbers on the collets, it would make sense to have those numbers relate to something in the real world. I have plenty of metric tools, and even some Japanese ones that use shaku. I am comfortable with that.
In this case, it is going to be difficult to fill out the set unless either I figure out their system, or have the opportunity to measure each collet.

toadboy65 · Sep 18, 2020

Pipe tap shank sizes. That has to be it.

Mud · Sep 18, 2020

toadboy65 said:
Pipe tap shank sizes. That has to be it.

That's it. There should be a reduced diameter at the end of the ID so that the 4 slots catch the square of the tap to drive it Normal C8 collets just say C8.

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