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Cutting a slot in copper

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jheinen

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#1
So I've got my new PM-25 all set up with the x-axis power feed, mist cooler, etc. For my first "real" milling job I need to cut a .180" slot into a 3/8" bar of copper. The slot will be about 1" long. I've got a 1/8" two-flute HSS endmill. When I use G wizard to calculate feeds and speeds it says that if I cut .07" deep, it would have a deflection of 94%. That's at 1 ipm.

Is that the right way to do this? Total it would be 12 passes at 1 ipm to cut the whole slot? Should I cut the slot all the way through at 1/8", and then cut the rest of the slot after? Should I cut the slot to the right dimension on each pass?

Thanks!
 

P. Waller

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#2
You have no experience with copper?
Makes for difficult chip control in a slot, chip evacuation will be more important then feed speed.
 

tq60

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#3
Copper is nasty stuff.

If through slot consider drilling with smaller drill through then finish with end mill.

Plenty lubricant to avoid sticking.

Shortest end mill you have too.

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jheinen

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#4
You have no experience with copper?
Makes for difficult chip control in a slot, chip evacuation will be more important then feed speed.
LOL. I have no experience with ANYTHING. I've been a blacksmith 'til now, so if I needed I slot I'd just hot punch it. :) I've got a mist cooler, and I've heard that helps with blowing the chips out.

Copper is nasty stuff.

If through slot consider drilling with smaller drill through then finish with end mill.

Plenty lubricant to avoid sticking.

Shortest end mill you have too.
I've heard that that the start and stop cuts you make when milling through pre-drilled holes risks breaking the endmill. Is that not a problem with copper?
 

gradient

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#5
Did a lot of copper milling recently. Really tough on the mill bits. You would be better off with a 0.035 depth and 2 IPM for HSS at 5100 RPM. Use lots of coolant and blow out the chips often. My Gwizard shows that if you have coated carbide bits you could take a 0.07 cut at 2.5 IPM at 5100 RPM. Gwizard just updated as I was checking this out. It warns to keep the IPM up or you will risk rubbing the tool.
 

jheinen

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#6
Unfortunately my mill maxes out at 2500 RPM. I have a four-flute carbide bit, would that be a better choice than the two-flute HSS?
 

GL

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#7
You didn't say if slot way all the way through, but based on the blacksmith comment, I would assume so. You might consider plunging all the way through as essentially a series of holes, then milling the sides smooth and to width. Allows chips to fall through the bottom and reduces the side load on the cutter at the furthest point away from the collet. Copper is sticky. 1/8" mills will break if you look at them hard (staring at the wall while milling does not help by the way).
 

jheinen

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#8
Yes, it's all the way through, so that technique might be worth a try.
 

gradient

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#9
My experience is that two flute are better for copper. As mentioned, it is pretty sticky so it's better to have fewer flutes to leave room for the chips to clear.
 

tq60

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#10
Push cutter fully in so cutting at shortest bit length.

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jheinen

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Push cutter fully in so cutting at shortest bit length.

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Do you mean push it fully up into the quill so as to leave as little as possible out?
 

GL

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#12
Hate to answer for tq60, but maybe to clarify, insertion depth should be no further than the shank depth, even if you can poke it deeper in the collet. My experience with small diameter end mills is they break off at the transition from the, say 1/4" shank to the 1/8" cutter about where the cutter flutes start. Then you turn it over, since small mills are often double ended (which reduces the cursing volume by about half, the first time). Then you grind it into some other cutter at a later date and compliment your cleverness/thriftiness. You are looking to get the tool as rigid as you can in the collet, but in this case the most flex you get is in the cutter - too much and it fatigues, in the bend the paper clip too many times kind of way. You indicated before that the Gwizard tool said you would have a bunch of deflection, breaking cutters is the result of leaning on it too hard.
 
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