Sometimes I forget just how much I don't know - SLOTTING

I've got to make these little boxes that clamp together with tapered slots and tabs to bull into the "exact dimensions. Within .003 or so.
Seems easy right? I've got thousands and thousand and more thousands of hours of machining aluminum. Some 4140 should be no big deal. Just go slower right? Well, aluminum is pretty forgiving. Steel fights back. LOL. I figured I'd just slot the cut, and then come back with a taper mill to profile the other side of the slot that fits the tab. It came out like crap. Several times. I tried conventional, climb, mixed, finally I moved the slot over about 30 thou and then took several finish passes. It came out straight and clean. With the final pass as a cleanup or spring pass its within .0015 of net dimension between the slots, or on the mark .00075 within .00075. The other wall still has a hook to it, but its getting cleaned up with the taper mill anyway.
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I can slot aluminum on the little high speed machines and its pretty darn close to the size of the endmill and straight. Doesn't work on the big mill. I even turned down the speed and acceleration and cranked up the tightness of the gibbs. The motors are drawing close to peak current for adjustment. (not peak for cutting) Nope. Its just gonna throw that little end mill all over the place and make a sloppy over size crooked slot (1/4" end mill). It might be a little better if I had a proper 1/4 end mill holder. My setup probably has a bit too much flex. I had to mount the 1/4 end mil in a 1/2" shank collet chuck mounted in a 1/2" tool holder. Every thing is as tight as I can make it, but the more there is the more it flexes.
What frustrates me is I ruined 4 pieces of steel before I figured out how to get it to come out right. I actually planned to ruin some work pieces figuring out how to get the tapered and straight sides of the slot just right, but I sure didn't plan for the headache I had. Here's hoping the taper cuts pretty close. I hate throwing metal on the scrap pile.
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I use an undersized reground endmill to rough out slots.
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Without heavy coolant, the Al will expand into the cutter due to heat and you undercut. Then it cools off and the slot is wider.
I turned a 6061 bar and forgot to keep it cold. It heated and cooled and the end result was a bumpy rod. Junk.
Martin
On 2/24/2018 4:44 PM, Jim Wilkins wrote:

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There isn't much heat coming off my little Clausing mill. Its problem is rigidity. -jsw
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wrote:

Well fluted? Does that mean more flutes, or better shape for chip removal, or ?
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wrote:

Nice, thanks.
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On 02/24/2018 02:25 PM, Bob La Londe wrote:
You weren't trying to use a 4-flute end mill to do that, were you?
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Bob Nichols AT comcast.net I am "RNichols42"

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Bob La Londe wrote:

Small diameter end mills work a lot better in carbide, it is several times stiffer than steel. So, the small mills deflect less. There's a happy medium between too little depth of cut (work hardening of surface) and too much (tool deflection). Full-width slotting will always cause the cutter to wander, leading to rough or wavy edges. So, you use an undersize cutter, plow at full width down the middle and then climb-cut the slot to final size.
And, of course, if you can use a slotting cutter (side mill) on an arbor, you can slot very smooth edges in one pass, but you get a curved bottom at each end of the slot, unless the slot goes off the end of the workpiece.
Jon
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