[dudz]

I have an alpha 025 hss cse-25 countersink bit. (3 flutes) maybe not the best countersink bit to use ??

I tried for the first time today, pushing it into a 12mm dia through hole in mild steel on the cnc mill. I used 250rpm (my cnc mill lowest rpm). Surface speed of 20m/min - plunge rate of 64mm/min . That's what fusion through out. I'm using flood coolant

I pulled it up half way on each hole to remove swarf and continued. Ok until I got 2/3rds of the way in. It sounded like it was rubbing - then pushed up further into the ER32 collet and span in the collet. Now got to order a new collet.

Was I way out on the feeds/speeds ? (remember this is a 1980s machine with 2500rpm max)

[Superman]

You may have pushed too fast a feed, as the clearance angle is not large.
How deep were you going into the 12mm hole ?

I generally use them to create the angled bases, shallow chamfers etc, & leave the heavier cutting to solid carbide/ tipped tools

HSS tooling only stays sharp for a short time, you need to get the best use for that tool in that time.

[dudz]

I was going full depth. around 10mm I think. They are cobalt HSS, but they were the only type I could buy locally. Today I'm going to try boring the holes with 0.5mm stepdown with a 10mm endmill and hope the collet will grab the shank enough. Then when I have another collet I will repeat the countersink. This way I suppose the countersink wont have so much material to chew on. Someone told me they use a 6mm chamfering tool and use a 2d profile cut rather than a plunge. I guess this prob a much better way to go.

[metalworkz]

Maybe try a different countersink. I think 3 flutes is too many, perhaps better chip removal and less heat with 2 flutes. I have done a lot of manual countersinking in drill press and mill with Weldon style 0 flute csinks, they cut clean, rarely any chatter but with any csink you might experiment with spindle rpm! Rotate too fast without removing material and the material heats up usually getting hard. Hit enough holes when material hardens and tool will get dull. I would suggest try single holes with reduced rpm and increase gradually until you find sweet spot. Then try several to verify it is good.

[dudz]

I bored the countersink with a 0.2mm clearance with a carbide 10mm straight flat endmill (3 flute). Then I done a circular path in fusion with a carbide 6mm ballnose. Turned out like glass and perfect thanks

[metalmayhem]

The 3 fluke are ok for drill presses but I don't find them suitable for cnc's .
If you want to use countersinks in the future then I suggest getting the severance chatterless countersinks or better yet go with carbide spot drills which you can also interpolate with .