Machining magnesium

[shanej45]

Hello all

I have an interesting aero space part I need to machine. Sorry I cant send any pictures, prints, ext.
The part starts as a 2-1/2 x 20-3/4 x 21-1/4. It has large pockets on both the flat sides and some on all sides as well. Obviously they are making this thing as light as possible. My question to the community when machining Magnesium AZ31B-H24. Is this stuff really stable when removing large amounts from both side? I have a flatness callout of .005 that I need to hit. The end product will be 2 inches thick and very hollow. I am worried about it warping.
I only get one shot at this......no stress....

Thanks

 

[Larry Dickman]

I would rough both sides, then finish.

But, I would check with my insurance company first.

 

[Mtndew]

If you haven't cut this stuff before, make damn sure you have the correct type of fire extinguishers on hand because this stuff can catch fire easily and the regular ones just make it angry.

 

[rk9268vc]

It depends how it is made. If extruded, it will want to warp, if cast, it will generally be more stable

I would rough the majority of it out, release the vise, then gently re tighten and finish.
Check out Titans of CNC's video on "Machining the impossible plastic part" on youtube. they give some good pointers for dealing with material that wants to warp.


Isnt magnesium pretty flammable? Sounds like asking for trouble, but im sure you already know that haha

 

[MaxPrairie]

Guy close to my shop used to machine quite a bit of magnesium. Make sure you have cost of chip disposal accounted for in your quote. I would not go right to finish on something that big. Also if you flip and ding up a surface you have material to clean it up.

 

[shanej45]

We have been machining this stuff for awhile. But nothing like this shape. So we do have the right fire extinguisher on hand. I will check if its extruded. They don't always tell me. Material should be here tomorrow.

 

[DouglasJRizzo]

Dad used to cut a lot of that in his place. The advise given is sound. Rough it out first, then finish when both sides have been roughed. Temperature can play a factor in tolerances too (like most other materials).

 

[CITIZEN F16]

Guy close to my shop used to machine quite a bit of magnesium. Make sure you have cost of chip disposal accounted for in your quote. I would not go right to finish on something that big. Also if you flip and ding up a surface you have material to clean it up.

He is in Idaho, he could probably just throw the chips in the dumpster. I machined it before many moons ago, feeds and speeds the same as 7075 aluminum as I recall. I rolled the dice and did not get a class D extinguisher, but I did not do any heavy high speed hogging. Those class D extinguishers are $700+. Will the customer pay for it?

 

[Kingbob]

Good advice here that it is never wise to finish a warp risk part on the first clamp.
While not a directly analogous to your case; I machine a lot of thin walled rings that can't be allowed to warp.
Typically we will rough and then rest the material at least overnight up to several days. Other examples from the greatest hits collection include leaving the parts in directly sunlight to maximize temperature swings or even heat cycling the parts a few times in the breakroom oven/freezer.
For what you are describing I'd bake and freeze that part a couple of times after roughing.

Edit: when I say bake we usually only go to about 250f so as not to alter the temper, only to stress relieve.

 

[shanej45]

Does magnesium move a lot with small temp changes? we have done alot in magnesium. All our parts up to now, by design you wouldn't ever know. My tolerance is .005 for flatness.

 

[Scottl]

If you haven't cut this stuff before, make damn sure you have the correct type of fire extinguishers on hand because this stuff can catch fire easily and the regular ones just make it angry.

That's for sure. Back in the 70s Wentworth Institute in Boston used to rent space to the Air Force and part of the work involved machining Magnesium. Apparently one day a tool bit sparked and ignited the shavings in the tray. BFD responded and put it out but if they didn't have a station right up the road the building probably would have been lost.

 

[CITIZEN F16]

That's for sure. Back in the 70s Wentworth Institute in Boston used to rent space to the Air Force and part of the work involved machining Magnesium. Apparently one day a tool bit sparked and ignited the shavings in the tray. BFD responded and put it out but if they didn't have a station right up the road the building probably would have been lost.

I admittedly machined magnesium without a class D extinguisher as sometimes I like to live on the edge. I ran it at 7075 feeds and speeds on a screw machine. I even tried lighting the dry chips with a fire place lighter, I could not get them to burn. I was disappointed.

 

[DouglasJRizzo]

I admittedly machined magnesium without a class D extinguisher as sometimes I like to live on the edge. I ran it at 7075 feeds and speeds on a screw machine. I even tried lighting the dry chips with a fire place lighter, I could not get them to burn. I was disappointed.

Dad lit a few on the shop floor - burned a hole thru the concrete.

 

[excello]

The University of Iowa used to machine those type of magnesium "boxes" in their Physics and Engineering shop. The boxes held electronic science experiment equipment for use in satellites.

Yes, lots of deep pockets with very thin walls, small corner radii, and lots of little tapped holes . As I recall they received gold plating after machining - can't remember the reason for that.

The shop I worked in put a bid on making one but we never did. I think our bid was high because of uncertainties just as you mention.

The University guys had already made a bunch of them in their shop, so had things well in hand.

I do recall the University's head machinist saying that the warpage was not bad at all considering the vast amount of material removed.

interesting job - Good Luck !

excello

 

[mhajicek]

As I recall they received gold plating after machining - can't remember the reason for that.

Metal Plating on Satellites: The Benefits of Light Reflection | SPC