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In cases where it seems that the F&S are a bit higher than seems proper, is it better to reduce the feed rate or the RPM, or both?

Answered Closed

I used G-wizard to calculate speed and feed for drilling an 11/32 hole in 1/2″ CRS. I used cutting oil but no coolant, and the chips were a bit blue, plus I had a color change on the tip of the drill bit after drilling 4 such holes.

In cases where it seems that the F&S are a bit higher than seems proper, is it better to reduce the feed rate or the RPM, or both? I’m assuming that I’d adjust the feedrate down, as that’s what you’d do when drilling on a manual mill. I’m assuming peck drilling is more for chip relief than cooling, but perhaps that might be good too?

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Best Answer

Always preferable to reduce rpm before feed.

RPM (Surface Speed) is all about tool life, and especially heat, which is the problem you’d like to improve.

If feedrate is too high, you’re going to either break the cutter or perhaps chip it. You’re not seeing any of that.

In G-Wizard, if you dial back the rpm, it will adjust the feedrate to maintain the chipload, which is what you want. I would only reduce the chipload if you’re breaking/chipping cutters or perhaps if you’re having a tough time clearing chips.

BTW, when you are peck drilling, be careful that you don’t rise the tip of the bit above the hole top. It makes it easy for some chips to fall back in. When that happens, you’ll spin the tip of the bit against the chips trapped in the hole. It’s hard for the bit to cut the spinning chips, they rub, and things get hot. It’s a bad thing, LOL.

I’ve talked to some machinists who find it is useful on their peck cycle with tough materials to raise the tip slightly from hole bottom and dwell briefly while the flutes carry as many chips as possible up out of the whole. Then they continue to retract their normal peck amount.

One fellow said this dramatically improved his tool life on titanium (never an easy material). Of course, you’ll need a custom programmed peck cycle to accomplish this. Some CAM programs help with that, some can get it done with a special post, and for others it’s just hard to do.

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