What's new
What's new

Milling Thin Copper (Warping)

WakelessFoil

Aluminum
Joined
Aug 18, 2020
We are attempting to mill a "T" profile on 24" long copper bars that are roughly 3/8" thick and 1-1/4" wide. You can see our set up in the attached images.

I knew milling copper was a pain from what other machinists have told me. I followed all the suggestions; use a 3-flute cutter, use high spindle speed, conventional milling, etc.
Everything went smoothly but when I untightened the vise the bar popped out and appeared substantially bowed. I read that when milling copper, some material is forced back down into the stock causing uneven tension along the Z-axis. Any tips on remedying or minimizing the deformation? This is my first time milling copper on the CNC machine.

Thanks,
JustinIMG_1454.JPG
 
My guy, a former Yugoslavian, would straighten it post machining, with a few blocks, perhaps a dead blow hammer and by channeling the spirit of his father, who kept the family afloat through the 70's and 80's rebuilding wrecked cars with hand tools, by eye. So that's an option.
The more fun thing would be to think about how to redesign it to avoid this problem. Can you divulge what it's for? You often want monolithic copper for thermal conductivity reasons but then why would you have the T-Slot. Could you bend it from sheet like a piece of unistrut? This has the feel of a physics research thing.
 
You could turn autofocus on your camera on, that'd be a good first step.

Any time you pull metal off a bar that's been cold finished, it's going to have a lot of stresses that go cattywampus. Before you straighten it in the press, you could try stretching it, that's what they do to extrusions. And there's also some pretty cool machines with a bunch of rollers that will straighten warped pieces of flat stuff, you just run your part through. But probably more money than it's worth for a few copper thingamajigs.

Sorry, no magic answer, going to have to straighten it.
 
You will get that issue in all sorts of materials, cold or hot rolled. But cold rolled being worse.

If possible, take only light cuts to rough the form, then flip the bar a few times to take the warp out from the stress releasing.

Then take finish cuts that are even smaller. Also change your tool often to only cut with the sharpest of tools. That will lower the stresses put in the bar from cutting.
 
You could always anneal the copper first. A couple or hours at 800C in a reducing atmosphere will render it dead soft. Of course in that state your machining fun will be even greater.
 
You can get it dull red then lightly clamp it flat. It will be soft and straighter afterwards. Not a full quench anealling, more 3/4 soft, stress relieving in desired shape. Bend it with your hands after it cools if needed. Copper is one of my favorite metals, it will work easily.
 
How straight does it have to be? Eyeball or is there a number you have to hit? I'd have a hard time getting a 24" part hot to anneal. And maybe you have a temper requirement?

Probably easy to straighten in an arbor press. Two supports maybe 6" apart with adjustable stop underneath in between the supports. Deflect to the support, move and repeat. Adjust the support as required.

If you've got a bunch to make you can use a shop made straightener with three rollers. Two on top. One underneath. Put a crank on one of top rollers. Crank it through and adjust gap until it's straight. A place I worked made gear racks for those stair elevator seat things. The stock was 3/4 square cold finished bar. It would curl up after the teeth were cut. They made a deal and cranked it through. Worked great and very fast. If you go too far flip it over.
 
What type of cutter are you using? If it is an insert cutter, their will be more stress than if cut with a sharp high speed cutter, and I assume you are keeping it cool with coolant.
JH
 
How straight does it have to be?
Just within maybe .010" end to end, would be ideal.
What type of cutter are you using?
We are using a 3-flute HSS Ti coated endmill with plenty of coolant.

Thank you all for the replies. I found an easy way to get the bar reasonably straight with the use of a makeshift bender constructed from a table vise. See video below.

I have realized that some of my dimensions are off a significant amount in certain spots. I am usually very accurate and consistent with metals like steel and aluminum. Has anyone experienced inconsistent machining when working with copper? I suspect that the flexibility of the bar was working against me in this process.

Thanks,
Justin
 
Just within maybe .010" end to end, would be ideal.

We are using a 3-flute HSS Ti coated endmill with plenty of coolant.

Thank you all for the replies. I found an easy way to get the bar reasonably straight with the use of a makeshift bender constructed from a table vise. See video below.

I have realized that some of my dimensions are off a significant amount in certain spots. I am usually very accurate and consistent with metals like steel and aluminum. Has anyone experienced inconsistent machining when working with copper? I suspect that the flexibility of the bar was working against me in this process.

Thanks,
Justin
I would suggest from my own experience that your issues are coming from proportional issues rather than material types.

Even the best machinist/ toolmakers have those moments that make them question thier own abilities, regardless of their own years of experience.
 
Anneal your warped finished part after machining. Then you will find it is very easy to straighten. Should you not get it perfect before it starts to work harden, anneal it again and have another go.
Copper is like Play-Doh when it is soft - but probably best machined when half hard.
 








 
Back
Top