Retention Knobs

• Black Dragon :

All the manufacturers that specify material spec 8620. Iscar is the the only one that I saw spec hardness, 58-60 (I assume Rc).

Thanks Kelly.

8620 sounds appropriate as that's what tool holders are typical made from. 58-60 RC sounds a bit hard (to me) for the application so I checked some BT and CAT (40) knobs. A few of the CAT knobs were around 45 RC on the business end and 30ish RC on the very end of the threads. The BT knobs (which we are in need of specials of) I checked were 45-50 RC end to end. 48-52 RC sounds more reasonable to me, but I'm not Iscar or a machine tool designer so WTF do I know? :)

Since 8620 is a case hardening material, know of any reliable ways of measuring case depth?

I think it is Big Kaiser that makes theirs out of H-13....very high strength & east to heat treat.

Pretty sure at the lazy b whenever we used 8620 and similar steels then travelling along the order through to heat treat we also processed a "test coupon" made from the same lot of steel that roughly approximated the part size /shape, and then cross sectioned it via cold saw--hardness readings are taken from the outside and the inside of the part and finally the case depth was measured by visually observing the depth where micro grain structural change is actually observed either by eye or by using a microscope if the depth was relatively shallow.

FWIW, usually a second and perhaps more coupons were also made, and used for tensile, shear, compressive and similar (destructive) tests..

• Black Dragon :

I'd send it to a lab, if I wanted reliability & it's not something I did regularly.

A previous employer made alot of shafting and observed the micro-structure after sectioning, as PrecisionmachinisT described previously. IIRC they also used a method which involved passing an electrical current through the sample. But they had a real metallurgical lab.

We decided to make them from Viscount 44. If other companies are making them from H-13, shouldn't be a problem. It's not a high stress application anyway, we're just going to hold a CAT 40 boring system in a BT 40 machine with manual tool changes vs buy another complete boring set for 1 machine.