What Can I Expect From A Long Hss End Mill--

[TommyD]

Chip buildup can cause a hole to go oversize, plunge rate too.

 

[Bob V]

TommyD,
As I think about it-- it might have been plunge rate too fast-- I was cranking the knee up and I don't tend to do that very evenly-- thanks for the tip. I was however clearing chips very frequently.
Bob

 

[4GSR]

I would have drilled a series of 15/32 holes to near point depth to remove the bulk of the material from the slot. Then go in with your end mill again to near depth and clean out and straighten out the slot. Last go to depth and start working the sides .010" to the side, then .005", then .001-.002" at a time until final size is reached.
I'm not a real big fan of 2-flute end mills with extra long flutes in HSS. Three flute HSS would give better stiffness. Ideal would be to use a 3-flute carbide EM with high helix to help remove chips from the pocket. A spray mister is a big plus if you have one.

 

[Bob V]

Thanks Ken.
To me, 2 flute end mills have always looked less stiff- good to get that confirmed-- I don't mind clearing chips and going slowly--so maybe now I'll buy fewer 2 flute and more 4 flute end mills-- but for now- gotta use what I have.
Still not sure in my own mind whether it is better to clean up by going down .100 then cleaning up a side, then down .100 again and clean up along one side of the slot; OR drill then plunge to final depth as you describe, then work along each side .010, .005, etc as you describe.--Maybe both techniques will work?

If you go over it a few times you should get a good clean up. I am just fond of the stiffest set up I can get! You may have a bit of a smaller slot in the bottom from 'push off', but should be in your tolerance range no problem.

I would have drilled a series of 15/32 holes to near point depth to remove the bulk of the material from the slot. Then go in with your end mill again to near depth and clean out and straighten out the slot. Last go to depth and start working the sides .010" to the side, then .005", then .001-.002" at a time until final size is reached.

 

[4GSR]

Bob,

One word of caution, don't use 4-flute end mills on aluminum. They don't have enough flute clearance to let the chips evacuate from the cut. Don't ask me how I know it. Dad about killed me messing up several four flute end mills cutting slots in a piece of aluminum. The chips kind of "glued" to the flutes of the end mill. And the end mills were 3/8' in size. That was when I was learning at a very early age in life.

Ken

 

[mksj]

One word of caution, don't use 4-flute end mills on aluminum

Couldn't agree more on this. There are specific end mills for aluminum, I usually use a 3 flute high rake/variable rake which does well. Chip evacuation is very important, so use air or vacuum to remove the chips. A little lubricant helps significantly, and in aluminum you should use climb cutting direction as opposed to regular feed. There will be a significant difference in the final cut finish and dimension. Try cutting the edge of some aluminum in one direction, and then the other and look at the finish, with climb cutting the chips are kicked away as opposed to getting recut again be the end mill.

A 3/8" end mill would work fine for a 1/2" slot, the key is to not use an end mill that is too long, so a 1.5" depth of cut end mill would be much better than a longer one. I prefer to use cobalt or carbide end mills, even though they cost a bit more, they last a long time. They are more brittle, but stiffer, you can feed/cut at higher rates if your mill has that capacity. But a 4 flute makes a mess in aluminum.

I often take a series of passes going down in increments of depth, try to widen the the slot a bit so the chips aren't regrinding on themselves. I then do a final dimensional pass using climb cutting and taking something like 0.005" for the finish cut. Having a DRO makes this much easier. I learned early on that to get a dimensional slot you cannot use an end mill of the same size, especially in aluminum.