How does a small negative rake boring bar retain its insert?

[ericc]

Hi. I acquired a couple of packages of Valenite TNFK 731 EL inserts in a grab box. They are negative rake small one sided triangles with ISO equivalent TNMM 09 02, which look like 7/32 IC or 9 mm nominal. I looked up the boring bars that hold these inserts (BHP-EC Valenite) and it is mysterious how they hold the inserts. The larger holders have a clamp, but the small holders seem to just have a screw without a head holding the inserts in. They don't have the lever attachment, and that would seem difficult anyway, because of their small size. Would these just be held in by friction, or some kind of clever wedging? Sorry if this is some kind of FAQ. I realize that these are orphaned inserts, and most of the small bars use positive rake inserts, but I'm just curious.

 

[mattthemuppet2]

most likely they use a screw with an eccentric pilot - I'm guessing the insert retaining screw screws in from the bottom of the bar? They work by having the eccentric push the insert back into the pocket.

 

[davidpbest]

The "K" designation in the TNFK spec is a straight cylindrical hole, and the insert is held in position via a rotating eccentric pin with a Torx head. See photo below:

 

[ericc]

David, that is very clear. Thanks for that picture and explanation.