Milling a large 6061 plate FLAT

CNCBurnout · Jan 3, 2018

Hey everyone!
I've this 6061 plate 1.25x24x40 (final size) I have to machine but the engineer wants me to hold the 1.25 flat and parallel within .005". Seems pretty tight to me for a milling op with a plate so large. What is the best way to try to mill this flat? I'm sure this plate will start out 1.5" thick so lots of material to cut and flip and cut and flip and cut...... But should i try to three point it or bolt it flat to the table?? Can the aluminum be stressed relived? I saw this on a old thread here with 304 stainless being the materiel of discussion. Thanks for any input!

-Ed

HuFlungDung · Jan 3, 2018

That seems like a pretty generous allowance. The part still can look bad though, particularly if your machine is badly out of tram. If you don't know its tram condition, check before you begin. Absolute 'relaxed flatness' doesn't matter that much if the thing gets bolted down to a flat table in several places. Vibrations and ringing of the part is likely a possibility, so if you place it up on 123 blocks, you'll need provision for damping the thing. So if you have plans for counterbored capscrew holes somewhere near the centerline (or put them in if they won't hurt anything), they could be utilized during the facing operation.

Forrest Addy · Jan 3, 2018

0.005 flatness in a 1 1/4 thick 24 x 49 rolled aluminum plate. Rolled plate usually has locked in compressive stresses from the cold rolling process. I sugges you skim about the same amount from both sides to equalize the movement. You may have to flip it more than once. Clamping it flat on the table after the first flip may be tricky..

You may have to resort to setting the plate on three points on a surface plate and using transfer gage and dial indicator map the error to aid your set up on the machine table.

Expect movement hours or days after machining. Aluminum plate is great stuff but it can crawl around if you're trying to machine large flat surfaces in thinnish sections.

DMF_TomB · Jan 3, 2018

CNCBurnout said:
Hey everyone!
I've this 6061 plate 1.25x24x40 (final size) I have to machine but the engineer wants me to hold the 1.25 flat and parallel within .005". Seems pretty tight to me for a milling op with a plate so large. What is the best way to try to mill this flat? I'm sure this plate will start out 1.5" thick so lots of material to cut and flip and cut and flip and cut...... But should i try to three point it or bolt it flat to the table?? Can the aluminum be stressed relived? I saw this on a old thread here with 304 stainless being the materiel of discussion. Thanks for any input!

-Ed

.
.
goes by different names but jig plate is annealed aluminum (6061 T0 ?). 6061 T6 is heat treated and wouldnt surprise me if it warps over .050" or 10x your tolerance
.
even using jig plate usually best to rough it out and leave .020 for finish cuts after a rechuck at lower torque. but you need to watch it warping more than the .020 and not having enough for finish cuts.
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jig plate is not as strong but its sold cause it warps less
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6061 T6 i have seen after .020 cut it warped .010" more, really annealed jig plate is much easier to work with

Larry Dickman · Jan 3, 2018

send it out to be Blanchard ground. It's not worth screwing with.

Forrest Addy · Jan 3, 2018

DMF_TomB said:
.
.
goes by different names but jig plate is annealed aluminum (6061 T0 ?). 6061 T6 is heat treated and wouldnt surprise me if it warps over .050" or 10x your tolerance
.
even using jig plate usually best to rough it out and leave .020 for finish cuts after a rechuck at lower torque. but you need to watch it warping more than the .020 and not having enough for finish cuts.
.
jig plate is not as strong but its sold cause it warps less
.
6061 T6 i have seen after .020 cut it warped .010" more, really annealed jig plate is much easier to work with

Yeah, I should have remembered. In my neck of the woods we call it "cast tooling plate" or "tooling plate." MUCH more stable. Use keen edge inserts, shallow depth of,cut, high feed rates, mist coolant, and have your totem handy.

DMF_TomB · Jan 3, 2018

Forrest Addy said:
Yeah, I should have remembered. In my neck of the woods we call it "cast tooling plate" or "tooling plate." MUCH more stable. Use keen edge inserts, shallow depth of,cut, high feed rates, mist coolant, and have your totem handy.

had a job 6 foot long aluminum part, engineer wanted flat to .001". didnt get it annealed and it was .100 curved when clamps released. shop bent it back with press to .002" but not unusual that after a few days it warped back .005 or .010" metal had to have time to distort some more. really if not annealed it can be 10 to 100x harder to hold a flatness straightness tolerance

DMF_TomB · Jan 3, 2018

Forrest Addy said:
Yeah, I should have remembered. In my neck of the woods we call it "cast tooling plate" or "tooling plate." MUCH more stable. Use keen edge inserts, shallow depth of,cut, high feed rates, mist coolant, and have your totem handy.

.
jig plate, fixture plate, tooling plate, goes by different names

scadvice · Jan 3, 2018

Ask the engineer for a material change to Mic-6 aluminum. Here’s one link to spec’s ect… It's already to your needed spec's
Product Catalog: Mic 6(R) Aluminum Cast Plate | Arconic

Wild West · Jan 3, 2018

Larry Dickman said:
send it out to be Blanchard ground. It's not worth screwing with.

Best advise yet!

CNCBurnout · Jan 3, 2018

Thanks for the advice everyone! I'll have to ask the engineer if its T0 or T6 aluminium that was ordered.

scadvice
Mic-6 would be nice to order right to size Ill also ask the engineer for a material change if possible.

Larry Dickman
If it was up to me that's what I would've done but I don't get to make those calls.

HuFlungDung · Jan 3, 2018

Larry Dickman said:
send it out to be Blanchard ground. It's not worth screwing with.

Serious question: I've got no such service nearby, but have wondered who on earth would want to grind aluminum? Maybe it grinds well when wet, does it?

gkoenig · Jan 3, 2018

HuFlungDung said:
Serious question: I've got no such service nearby, but have wondered who on earth would want to grind aluminum? Maybe it grinds well when wet, does it?

Lots of shops Blanchard grind aluminum all day long, primary to get plate stock flat similar to what you're after, or centerless grinding of stock for something like a Swiss lathe. To be clear, we aren't talking about doing precision detail grinding like a toolmaker or anything, though I'm sure some nutty bastard has done such over the years.

CNCBurnout · Jan 3, 2018

HuFlungDung said:
Serious question: I've got no such service nearby, but have wondered who on earth would want to grind aluminum? Maybe it grinds well when wet, does it?

Yea that's a thought I had as well. I don't know who would even have a vacuum plate that big to grind the aluminum plate i was talking about anyways.

Larry Dickman · Jan 3, 2018

CNCBurnout said:
Yea that's a thought I had as well. I don't know who would even have a vacuum plate that big to grind the aluminum plate i was talking about anyways.

for a Blanchard grinding shop, the plate you described is a small one.

stephen thomas · Jan 3, 2018

I used to grind Al on the DoALL before getting my planer. Never knew it (aluminum) was supposed to be a problem?
46I or J (or even 36), 10" x 3/4" or 1" wheel, through the wheel + flood coolant. You're supposed to use oil for Al, but I just ran it with the Cimcool 95 that is in the tank for everything else. ( going to imagine that being basic, that mix would tend to corrode Al if left in crevices, I always washed with detergent and then clear water). But it worked fine for grinding. I tend to run everything with the side/corner of the wheel - deep passes, small step over, machine runs while I do something else. That said, it's been a few years. Most stuff, the planer is nicer.

As far as holding it, no experience with a Blanchard, but on the recip, I just shimmed & blocked it really well, and used the flexy finger/comb style magnetic clamps judiciously.

For the OP buying mic6 already made seems a no brainer, though.

smt

AARONT · Jan 3, 2018

Depending on the purpose of this part you may not be able to use jig plate. The reason being, these plates are cast aluminum and are fairly brittle.

I agree with having it blanchard ground. This is much more cost effective then flip, cut, flip, shim, cut, flip shim cut, etc.

B-Mathews · Jan 3, 2018

AARONT said:
Depending on the purpose of this part you may not be able to use jig plate. The reason being, these plates are cast aluminum and are fairly brittle.

I agree with having it blanchard ground. This is much more cost effective then flip, cut, flip, shim, cut, flip shim cut, etc.

TO add to the mic-6 issues some time had would be it does not anodize well from what I have seen. Not sure if you part would be getting that though. I would just send 6061 t6 out to be ground myself. Will it save lots of time/money

CNCBurnout · Jan 3, 2018

Well what we have in stock is T6. Not an option to reorder T0, MIC-6 or farm out Blanchard grinding. My pleas to the engineer and manager that T6 is not the correct material for this application if they don't want to send out for grinding fell on deaf ears. I guess they don't have a problem blaming me if I can't hit the tolerance lol. Just have to give it a go I guess.

This isn't they type of machining I'm use to so forgive my ignorance but some people have recommended flipping and shimming? If i was to shim the low spots wouldn't that take the plate out of its relaxed state and only make it harder to chase flatness/parallelism once i release the clamps?

HuFlungDung · Jan 3, 2018

You place shims wherever you have a gap between the work and the table (or 123 blocks) so as to not spring the plate when clamping down. Then you rough the first side. It may still have a bow when released, but it should be a smooth curve, and not wavy. Then you turn it and shim the gaps again before clamping down and take another rough cut. Then you release the clamping and see what it does for springing. If it barely moves, then perhaps adjust your shim packs slightly, reclamp and finish that second side. Then flip it over and clamp it down with no shimming at all (unless you have machine leveling or tram issues) and finish side 1.

Milling a large 6061 plate FLAT

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