Copper Help

rustyh · Jun 25, 2019

Good Day All,

I wonder if I can get some help.

Currently running as copper job using 3/8" thick, 1 1/2" wide and 75mm long C101 copper. Having to mill the part down (pretty simple features) with a thickness tolerence of +/- 0.05mm.

I run op 1 doing a face op with a 6mm 3 flute cutter (face off 0.5mm followed by a 0.1mm finsish pass), followed by milling the outside rectange profile with the same tool, and then a small shallow pocket in the middle of the part (around 1mm deep) with a 2mm end mill (it has internal rads that need the 2mm cutter size). Also a couple of tapped holed.

I then flip the part over to simply face off the opposite side to achieve the thickness. Im facing off about 3.5mm, hitting it in one roughing pass of 3.4mm then a finish pass of 0.1mm.

Im finding that when I take the parts off to inspect, I see them bowing. This in turn throws the part out of tolerence.

Can anyone help with the reasons to why it is doing this, and any ways to prevent it or improve it to hit the tolerences required?

I am using Aluminum 3 flute, uncoated cutters by the way.

Thanks

Rusty

broke · Jun 25, 2019

Whats the tolerance?

Try skimming the skin off one side then put that down and proceed as you are. Sometimes the skin seems to hold stress where the rest of the material is pretty unresponsive.

good luck

rustyh · Jun 25, 2019

Thanks Yoke,

The thickness tolerence is + / - 0.05mm (2 thou). Using a Doosan DNM

So take something like 0.3mm off one side, then flip it over and do Op 1, then flip it back over and do op 2?

litlerob1 · Jun 25, 2019

Were it me (gratefully it's not). I would do the Thickness within .5mm and Finish the Profile. Then do the rest as subsequent ops.

R

Timfevs · Jun 25, 2019

I agree with yoke. Always try to remove the skin first on opposite side.

Tichy · Jun 25, 2019

3.4mm out of 3.5 is way too much for op2. What's your tool radius?

I'd try to go to 3.0-3.2mm depending on your sharpness and do the rest in one go as the finish.

Obviously you need to measure dimensions before the final cut.

litlerob1 · Jun 25, 2019

I didn't know Copper had "skin".

R

guest · Jun 25, 2019

My sense is that most of the bowing is due to all the material being removed from one side.

Rather than taking .5mm on one side and 3.5mm on the other, I'd balance the material removal more evenly.

Establish when it is moving. Run OP1 and check flatness. I think the bowing is happening on OP2.

Skimcut one side so it's flat, flip it over. Machine 2mm away, flip back and finish.

.002" on thickness is not the same as flat to .002". If there's no flatness callout, they might not care. If it's attached in the assembly with the 2 tapped holes, it's gonna suck down and be as flat as whatever it's attached to.

Vancbiker · Jun 25, 2019

litlerob1 said:
I didn't know Copper had "skin".

R

Cold working copper will work harden it, so it may be possible that removing the outer work hardened rolled surface will release internal stresses causing warpage. One interesting fact about copper is that to anneal it, you quench the heated material in water. Just the opposite of steel.

Tichy · Jun 26, 2019

Actually never mind.

Street · Jun 26, 2019

Try annealing it as stated above, most likely it has internal stresses due to processing.

I have waterjet cut copper and if its thin such as a residual bit of metal it will warp, thankfully whole shape came out fine in regard to cutting.

aj · Jun 27, 2019

Vancbiker said:
One interesting fact about copper is that to anneal it, you quench the heated material in water. Just the opposite of steel.

I used to own an old Triumph 650 and that was the recommended service procedure for re-using the copper head gasket. Heat it red then quench it in water.

3t3d · Jun 27, 2019

My customer would kill me if I tried to anneal the copper!

They specified the hardness, NOT dead soft, annealed copper.

Buttpoop · Jun 27, 2019

Clamp it lightly and DO NOT hammer it down. If you hammed it down, you will clamp it flat but it will return to its bowed state when released. Skim ~.01 .015" off one side. Flip it, now you can hammer it down. Machine this side, see if you can take it to finish thickness. Program your feeds/speeds in a manner that generates as little heat as possible. Lower RPM's & moderate feed. For jobs like this you need to think about introducing as little stresses to the material as possible.

Bobw · Jun 27, 2019

My 2 cents, for what its worth.

Can copper be 'stress relieved' without effecting the heat/hardness of it? I really don't know.

The other thing, I'm assuming this is bar stock.. Anytime I have something that I am afraid might
warp, or bow, or twist, or *SPROING*... I try and use sawn plate instead of bar stock, it has
a ton less internal stresses.

rustyh · Jul 2, 2019

Thank you to all for your comments, appreciate the advice.

I dont have the equipment in house to get it up to red hot, the best I could do was shove it in a 250c (480f) oven for an hour, then put in water. Probably not hot enough to do a fat lot.

I then done as suggested and took a fine cut of 0.5mm (0.02") off one side, using a 6mm cutter. They were coming off the vice nice and flat when checked against a parellel.
I then fliped it over and maching the op one, facing 0.5mm off the top as per the previous skim.
Finaly third op to remove the clamping material of around 3mm (0.12") in 0.8mm (0.031") passes to keep stress down. Seems to work a dream.

Thanks you everyone. I never knew Copper was such a fine art to machine. Huge Learning curve

ONe thing I am quite surprised about, unless I am doing something wrong, is copper seems to be quite aggressive to wearing carbide tools!!

broke · Jul 2, 2019

Glad to hear its working for you.

DouglasJRizzo · Jul 3, 2019

I had a job where I machined thousands of 1" square rods of C110 copper. I used a Zirconium carbide endmill that had a funky tooth form that would shear away the copper, throwing the heat into the chip instead of the part. Worked amazingly good.

3t3d · Jul 8, 2019

I was making a lot of .375" thick parts..
At first I was profiling cutting down in steps in the Z direction.
That would wear out the end of the end mill in a step pattern.
Went to a tool path with al full depth passes, and the tool wore evenly.
Easy to comp out the wear of the tool and parts came out great.
FWIW, I used 37 degree helix, three flute Zircon coated end mills.
Very good life when used as above. A fraction of aluminum cutting tool life, for sure.

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