Correct.

Theoretically if you plunge straight down the centre point has zero cutting speed, although it will cut if you have a positive feed rate. Likewise if you plunge down and then back up to create a blind hole the bottom of the hole will not be flat, but convex.

In my experience the bigger the slot drill the more likely it is to go walkies if you plunge straight down, and even more so if the cut is asymmetric. Even on a Bridgeport I wouldn't rely on an 18mm slot drill to cut a half slot accurately.

Andrew

Standard slot drills and end mills are ground with the ends slightly concave, about 2* if memory serves. The intent is to provide relief so only the periphery of the tool engages the bottom of a slot. If you need a dead flat bottom the tool needs to be re-ground. When plunging I find the 3 or 4 flute end mills are more stable than the 2 flute.

Most 2 flute cutters will be center cutting. Not all cutters with 3 or more flutes are center cutting. The vendor should state if it is center cutting.

In re-grinding some of my own cutters I do not always try to re-establish the center cutting configuration.

The concave end of end mills does nothing to keep them on line. You depend on the rigidity of your machine and your part clamping to hold to keep the cutter on line. Starting with a smaller slot drill should help by reducing cutting forces.

You can see the left edge is extended to cut to the centre in contrast to the right hand one.

Flat bottomed holes need a 'special' cutter.

In the old days not all slot drills were centre cutting.

That is the good thing about this method you can run a regular drills and if you don't have one big enough finish with a milling cutter or boring head, these were 16mm holes