View Full Version : Insert size - 22_ vs. 21.5_

01-16-2014, 11:20 AM
An old question for me, raised by the boring bar insert thread... Take a boring bar designated as STFPR2. Next to the catalog listing is given "insert type TPMT 21.5_" Will a TPMT 22_ also fit correctly?

Here's my reasoning. The inscribed circle (IC) is used as the insert size, and the clearance is the same 11-deg. The difference is only the thickness; however, the IC is measured from the top face of the insert. With a different height, the actual size where the insert fits the pocket will be slightly smaller for the TPMT 22_ insert than the TPMT 21.5_ insert. Correct? If the IC was measured from the bottom face of the insert, they would be the same. The taller insert would continue the 11-deg. edge to the taller thickness.

My debate: is the minor difference a practical problem for insert suitability? The bars, mostly, do not distinguish in the ANSI designation marked on them. As with the question in the other thread, a bar is often marked only "STFPR2." The manufacturer designation does not specify "STFPR 21.5" or "STFPR 22." Obviously, though, size 21.5_ is more common.

So can the two thicknesses suitably interchange?

01-16-2014, 11:27 AM
Fit correctly?, not really. Fit good enough that it will work just fine, yes.

01-16-2014, 02:20 PM
Bit off the thread but to me its that "more common" idea...not sure about boring bars but some of the turning tool holders in the 3/8" square and 1/2" square shank ranges use different sizes of inserts or, rather you can get a 1/2" shank for example using one size of insert and another 1/2" shank of nearly identical designation that uses the next size up, same shape. In some cases IIRC there can be some advantage to one over the other in terms of availability or more choice esp on EBay.