Enlighten me on why drilling aluminum is done at such low RPM compared to milling or turning?

[Springy]

Can anyone shed some light on the material mechanics of SFM with drilling as compared to milling or turning in aluminum (specifically 6061)? With a 0.375 drill, recommended SFM is about 5x lower than that of turning or milling. Why is this? What specifically happens if you drill at too high of an RPM in aluminum? Is it a heat/chip evacuation issue, or something to do with the chips form/break?

 

[crossthread82]

For starters most drill speed charts are for HSS uncoated drills, and the milling charts are for carbide. That alone would make up the 5x difference. When drilling you have varying amounts of SFM b/c as you go in towards the point it decreases. At the very tip of the drill it really isn't cutting more like smearing the material out of the way. So if using a hss uncoated drill, if you smear the material at the tip to fast it will gaul onto the tool and weld up. Coated HSS drills meant for aluminum usually list speeds much faster.

 

[mhajicek]

Another thing with drilling, is it's hard for coolant to get in, and for chips and heat to get out. Through coolant carbide drills help with this a lot, and can be run much faster.

 

[john.k]

Its too easy to wreck the hole if chip weld happens .....one chopped out hole is a scrap component .......better to add a few seconds to the overall timing.

 

[eaglemike]

If you have a decent CNC mill look for the MA Ford 3 flute carbide drill. They fly, hole finish and size are awesome. I think it was gkoenig that shared them here. Big time saver for me.

 

[TAIWA NUMBA WAAN]

When I was doing production machining in VMCs, I would generally run M42 cobalt screw length drills at 6000 RPM with no problems. The few TSC carbide drills I had, I ran them at 12,000 RPM and poked deep holes in a matter of seconds.

I recently plunged a hole at 25% rapid with an HSS jobber length drill in aluminum. I'm not sure how it survived, because the hole looked fine despite the entire drill being wrapped in a very thick helical chip. Moral of the story; drilling parameters aren't as critical as people think, and drills can work anywhere in the range of "barely rubbing" to "corkscrewing downward" in aluminum.

 

[rjwalker1973]

I have always subscribed to the get in and get out as fast as safely possible in aluminum. Like everyone is saying a lot comes down to two things, tooling and coolant. Aluminum has a high friction coefficient, so more time spent in the cut more heat it creates, so the cooler you can keep it the better.

 

[michiganbuck]

And only two flutes could take too big of a bite of aluminum.