CNC: Indexable insert drills – why and how you use them
Insert drills have a hardened steel body (hardness approx. 50 HRC) with inserts at the end that do the cutting. There are two overlapping inserts, a central insert and a peripheral. Large diameter drills will have more than two inserts. Hole diameter tolerance achievable is typically +0.25 mm.
Coolant is delivered through the drill body to the point of the drill, to reduce the temperature at the cutting edge and flush out chips through the flutes. Because of the high metal removal rates, coolant plays a great role in evacuating the chips. The coolant pressure should ideally be more than 8 bar, pressure increasing as the hole gets deeper.
The cutting speed at the periphery is more than the cutting speed at the centre. At the centre the cutting is very poor because of the low cutting speed, and the drill presses and scrapes the material instead of cutting it. The qualities of the central and peripheral insert are therefore different, and
1. The inserts may be of different shapes.
2. The inserts may be of the same shape but of different grades and chip breaker geometries.
Like in insert selection for any operation, an insert with the grade and chip breaker geometry recommended by the tool manufacturer must be used.
Text and pics. source: CADEM NCyclopedia multimedia CNC training software.
