How to get a smooth finish in a 3D pocket.

[missionrat]

Hi folks,

I have a part that needs a pocket/nest for a 1" diameter ball bearing to rest in. The diameter of the nest will be about 1.025 diameter and .5 deep. I want the part to be made out of aluminum and I need the finish to be smooth, with little visible tool marks. Am I better off to rough the nest out with a flat bottom endmill and finish with a ball end mill OR rough it out with a flat bottom endmill and finish it with a form cutter?

I worry the large bearing surface of a form cutter will create a lot of chatter, even with slow rpm and lots of coolant. I'm not sure if finishing the nest with a ball endmill, taking several passes, will leave a smooth enough finish.

Any help would be greatly appreciated.

[mcphill]

If you want a flat bottom, do NOT use a ball mill. That will put "scallops" on the surface. Ball mills are for 3D surfaces. You would likely be best with a roughing flat end mill, and leave 1/16". Then do a final finish pass with a smaller end mill with 25% stepover. Don't use too small a finish end mill either, but make sure it is good and sharp.

[missionrat]

The profile of the nest will be the same as the ball bearing, meaning no flat bottom. Picture profiling the letter U in 3D.

Thanks,
Steve

[mcphill]

So you mean looking down the "legs" of the U, and the bottom is curved. If that is the case, then yes. You can rough it what whatever you want. To get the best surface finish in the "U" section, use the smallest ball end mill you have, with the smallest stepover you are willing to wait for the machining to complete. A 1/32" ball with a 0.002 stepover would end up with a very nice finish. Keep in mind the very corners of the bottom of the pocket won't be square - if that matters...

The bottom set of pictures in this linked post are finished that way:

http://www.cnczone.com/forums/bobcad...successes.html

[missionrat]

That is just what I was looking for. The part was being made out of plastic using a cnc router at 18000 rpm. The mill we can afford to get to make it out of aluminum runs at 4200 rpm. I'm guessing it's going to take a while to cut the pocket.

BobCad is what was used to design the part.

Thanks for the help.
Steve