Rakes?

[PixMan]

There is one way that the term "rake" is often used even though it's really a misnomer. That is when referring to carbide turning inserts as "negative rake" or "positive rake".

In reality, they're almost ALL positive rake. What I mean by that is most negative rake inserts have straight sides, so the holders tip the insert to a -5º angle in two planes as a means of offering front and side clearance angles. That would mean the TOP RAKE would be at a negative 5º. In reality, the vast majority of negative rake inserts have top form geometry (a.k.a. chipbreaker) which compensates for the tilt and usually adds more positive rake. Only flat-top inserts, primarily used for cast iron, would be true negative rake though the moniker will go on forever.

Inesrts referred to as "positive rake" are single-sided ones that have side clearance angles either moulded-in or ground. Many modern ones have a countersunk srew hole in the middle (making them "screw down inserts), though older designs were and still are meant to be used in holders with a top clamp. These inserts sit in the holders flat, usually giving a 0º top rake angle. Nowadays there are a great many positive rake inserts that have chipbreakers moulded-in, adding to the amount of effective top rake angle. Those are typically weaker than negative rake inserts of similar geometry, though modern grades help tremendously to solve that problem.

Too much information? If you've got questions about carbide insert technology, always feel free to ask me. If I don't know, I'll make a up a believable story...er...I mean....find out the correct answer.

High positive milling insert, SEKN43. Top clamp, has chipbreaker.


Negative rake CNGG431-SR turning insert with two upsharp chibreakers on the 80º included angles. The obtuse 100º angles would be considered true negative rake.


Two SPEB09T308 screw-down positive rake milling inserts. One flat-top, one with chipbreaker making it high positive.


Negative rake WNMG542 insert in holder. This one is a strong edge chipbreaker design for high feed rate roughing operations. You need lots of HP to run one of these!