Most of my carbide holders, which are cheap Chinese copies of industry standards, have negative rake. e.g.:
Screen Shot 2021-04-18 at 2.50.34 pm.jpg
That one is designed for TNMG inserts, which are reversible so have no relief. The 6° down rake is to stop the side of the insert from rubbing under the cut - basically 6° relief.


Unless I get very well moulded inserts which have ridiculous top angles:
Screen Shot 2021-04-18 at 2.58.33 pm.jpg
then this tool will never cut well - it will always roughly tear chips off instead of machining nice spirals.


Boring bars are a more interesting example:

s-l1600.jpg






So, given that this one takes CCMT inserts, which have 7° relief, why the hell do they grind the flats on the thing to have an additional 10+° of negative rake??????



(Threading tools have flats similarly ground, but their inserts are always aggressively raked, so they seem to zero out. The better shaped inserts even have nice shapes to carve metal nicely:
Screen Shot 2021-04-18 at 3.20.41 pm.jpg
)




Anyone worked out how to grind/pack them into a toolholder so that they are closer to zero?

(I was thinking of just grinding the top flat around, and mounting it in the V groove in the bottom of a "boring bar QCTP holder", but thinking further, that wouldn't really help.)
Attached Images




Read the full thread at metalworkforums.com...