What speed to run the mill at?

[snub]

I'm using a 3/8" ball end mill to cut a .187" deep groove in a piece of 6061 aluminum flat bar. I usually run my mill at 600 rpm but it was making a hell of a racket even with a shallow cut of .005". I turned the speed up to 900 rpm and the results were no better. My mill will go way faster, like 3000 rpm but that seems really scary. The slowest speed is 120 rpm. So I had to make 40 passes to cut the groove and it was still not very smooth. Not bad on the right side but very rough on the left.

So, I'm wondering if anyone has any knowledge of basic guidelines for what speed to use, and how to get a better finish.

P.S. All my end mills are your standard Busy Bee/Grizzly cheapest they have. Some are made in Poland, mostly made in China.

[AdeV]

In my - limited - experience... the smaller the cutter, the faster it should be turned. A 3/8" cutter is small enough that you should be able to dial up the speed to 2000+rpm. With the higher cutter speed comes better rigidity of the tool, so you can cut deeper & faster - you should be able to cut full depth in one pass at that speed; but even if you wanted to go carefully & take say 0.050" passes, it would be fine.

A ball-end mill is a funny beast, because it barely cuts at the bottom, it sort of "smears" the metal; With your 0.005" cuts, you'd be trying to use the least able surface of the ball mill to do the bulk of the cutting. IMHO, you'd be better off using a thinner (say 1/4") roughing mill & go "almost" to depth (say 0.150"), then use the ball end mill to finish off. As Kenneth suggests, if you can use a smaller ball mill (i.e. cutting on one side only), then you'll get a better finish at the expense of taking a bit longer over it.

[kvom]

With aluminum, it's almost impossible to have two much RPM.  I'd follow the prior advice and mill a slot 1/4" wide at full depth, the finish with the ball mill.

[snub]

Da_Kengineer_Meister, I'm unsure as to what "using a 0.035-in step over between passes" is refering to. Could you enlighten me?

[snub]

^..thanks, I got it now. And thanks to all that replied. I cranked the mill up to 2000 rpm and am having much better results.

[Bogstandard]

Just another couple of tips, you now have the speed, so reduce the feed and give it a little squirt of WD40 if you have some, or paraffin (kerosene).

You should be able to get mirror like finishes if you get your technique correct.


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