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[machinist] machining thin plate on magnetic chuck -is three point suspension best practice?
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] machining thin plate on magnetic chuck -is three point suspension best practice?
- Date: Thu, 20 Feb 2014 15:09:36 -0500
http://www.practicalmachinist.com/vb/general/machining-thin-plate-magnetic-chuck-three-point-suspension-best-practice-280524/
machining thin plate on magnetic chuck -is three point suspension best practice?
I'm face milling thin wide stainless plate 10mm x 1000mm x 1500mm
(0.787"x39.370"x59.055")
in a vertical machining center with magnetic platen chuck of similar size.
The plates call to be flat within .002", but it's tough because the magnetic chucks deflect the plate. These plates are coming in rolled and/or forged.
Is the 'three point suspension' principle my best bet for keeping these flat? Use 1"-2"-3" blocks or something?
Is that what industry does when milling thin and wide objects on magnetic chucks in VMCs? There are no stupid questions, right?
Thanks--
02-19-2014, 10:20 PM #2
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- Oct 2010
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- Oregon, USA
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Three point suspension is just going to make your problem worse.
Standard practice (for smaller parts) is to shim under the raised areas of the part so the mag chuck can't suck the part down. This gets you a flat top surface which is stable when the chuck releases (well, stable up to released surface stresses). You can then flip the part over, and mill/grind the second surface parallel with the first. Since these are rolled, you probably should do this as a roughing pass on each side followed by a finish pass on each side, because you're going to release some surface stress.
As a practical matter, finding raised spots and placing shims under a 1 meter by 1.5 meter plate is going to be a pain in the posterior. Maybe you are lucky and the plates always have a well-defined convex side, which you could place down on the chuck and then insert shims from the edges to block it up. If it didn't have its own drawbacks, I'd suggest bedding the plate into a thin sheet of patternmaker's wax on top of the mag chuck for the rough pass(es).
And if the plates are coming in with significant bow, you may have issues with them cleaning up to the desired thickness. If that's the case, try to have them pre-straightened. -
02-20-2014, 05:00 AM #3
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yes, standard procedure is to shim and flip as necessary until flat. however, when the plate gets really thin, shimming needs to be 100% correct, or the plate will be deflected by the magnet, 'sucked down'. when the plate is machined, and the magnet is turned off, the plate pops up, and it's not flat.. by a mile. (km)
i'm thinking about getting it close to size as near as possible with shimming, and THEN suspending it on 3 points for a few finish passes. Otherwise i'll have to resort to a clamping fixture rather than magnets. -
02-20-2014, 05:11 AM #4
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- May 2011
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Variable power chuck and a surface grinder.
Can you use a grinding wheel in your mill? -
02-20-2014, 06:17 AM #5
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i didn't know that they made variable power chucks. I'm taking finish cuts with a 6" cutter w/ 2 wiper inserts. Grinding would be moot, we'd still have the same problem.
-
A couple of dumb questions..
What sort of stainless is it? My first impulse is to say ferritic or martensitic since you are holding it on a mag chuck? But you didn't mention it.
Is it possible to relieve the rolling/forging stresses with heat treatment prior to machining, the reason I ask, is that it seems to me that to hold 2 thou tolerance over that size sheet, it's going to move and warp every pass through the mill. Flip do one side, warps flip again do other side warps again...
The traditional way of stopping initial flexing on the mag chuck when surface grinding is to use double stick tape, grind one side flat, flip and grind other side, repeat until flat.
I don't think that's going to work on this scale or this job, but maybe, with some research you could adapt the idea..
Maybe a big blanchard grinder would do the job... plenty of people here with experience that could advise you on that option..
RayRichard King and tdmidget like this. -
02-20-2014, 09:56 AM #7
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You may want to check with a shop who has a 60" Blanchard or Madison Rotary surface grinder. They can block in the plate and grind it slowly with downward pressure. If it warps a bit they can flip it. I haven't seen one in years but we used to also use vacuum chucks. The 3 points would be a mistake as the plate would bend. 3 points works good on thick material and if you use small support jacks around the edges and blocked it in. But your material is to thin in my opinion.
-
So this is 400 series or (less likely 17-4ph), as 300 series would have to be extensively cold worked to get it magnetic enough to stick, and then the uneven internal stresses induced by and revealed by machining or grinding will give you a potato chip.
A plate that is 400 series, that is a meter by a meter and a half and a cm thick, will weigh about 112 kg. And you want it flat within 0.002? It seems to me that if bolted to some sort of machine or assembly by the edges, the center will sag more than 0.002 inch. If this is bolted to a machine in a way that the center is supported, then the plate will conform to the supports and will run out about the same degree as the support structure.
If this must be done, I was wondering if the following had ever been done. Formulate a magnetostrictive (right word? I want stuff that solidifies in a mag field) liquid that is about as dense as the plate. Build a plastic tub around the mag chuck, and fill the tub with a small depth of the fluid. "Float" the plate on top. The fluid should be deep enough to just come up to the top surface of the plate. Won't be perfect, but it will minimize forces on the plate. Turn on the chuck. The fluid solidifies and holds the plate in its "floating" unstressed conformation. Machine (or better, grind) Turn off chuck and invert plate. Repeat. Same thing might be tired with low melting point metals. Melt, float the plate, let solidify. Aside from the small stress induced by the 175°F or so melting point, the plate should have floated and should reflect its natural shape, with support under the high spots. Just a goofy thought.
I agree with the others, the three point support (even at Airy points) subjects the metal to its 250lbs weight, and will preclude an accurate flat surface.
Now, if you had a fixture that duplicated the support structure in which the plate will ultimately be installed, and took care to not allow the tool or grinding wheel to press down too much, this might work. -
I must point out that .787" is 20 mm, not 10. Even so this is not a realistic call out because the workpiece will bend more than that of it's own wgt. How will you measure that flatness? The usual method is to support on 3 points and check with an indicator on the bottom side. This piece is too limber for that. I would consult the customer as you have, for all practical purpose an impossible and unreasonable call out.
And I would NEVER mill on a magnetic chuck. -
Good catch, td. +1 on all you say.
-
Another dumb question, once ground to a .002" tolerance, would it not flex if mishandled during shipping and moving around?
Thus be out of tolerance by the time the customer get to use it. -
02-20-2014, 12:23 PM #12
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"Sucked down" and pushed away by the cutting force made by the cutter. An area not supported will push away during cut then spring back to make increased out of flat condition. if the mag is not true flat that will be reflected in the part also.
Originally Posted by Thomas Paine
yes, standard procedure is to shim and flip as necessary until flat. however, when the plate gets really thin, shimming needs to be 100% correct, or the plate will be deflected by the magnet, 'sucked down'. when the plate is machined, and the magnet is turned off, the plate pops up, and it's not flat.. by a mile. (km)
i'm thinking about getting it close to size as near as possible with shimming, and THEN suspending it on 3 points for a few finish passes. Otherwise i'll have to resort to a clamping fixture rather than magnets. -
02-20-2014, 01:19 PM #13
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- SE PA
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to answer some questions:
yes, it's 20mm, not 10mm thick (
)
its 416 series stainless. martensitic and magnetic
-floating the plate with a foam of some sort is a great idea, but not feasible for production.
-yes, i beleive supporting on three points is letting the plate sag in the middle.
-my workholding magnets call for 3" pole extensions MAXIMUM, and i already have 3" extensions on there, so the plate moves slightly anyway
-flipping and shimming a million times has already proved not to work. The shimming must be PERFECT.
-we tried Blanchard grinding, but the heat caused the center to expand, and be undersize when cooled. (yes we used coolant)
-yes, the plate probably will warp anyway when exposed in a cold truck and shipped 1000 miles away
-in the past, the customer never cared about flatness much with very thin plates, maybe they are having problems too.
tdmidget is correct that magnets are not the way to go, but it's a production shop. you know how that goes.
Maybe i'll try self shimming pole extensions before i make up a clamping rig.
trying to upload some pics here, but my pc isn't cooperating. give me a few minutes. -
I looked up 416SS on cricible's website, and it shows 416ss to weight in at .280 lbs per cu/in. At those dimensions, the plate should weigh in around 500 pounds.
If this were my shop, I'd lose the magnets, and lay it down flat to the table. I'd let the plate float, without clamps, and try to use maybe a 15* lead-angle face mill and take the smallest cuts that I could. If the plate tries to slide around, block up around it - maybe use some mitee-bite clamps (very lightly, or even just use Socket-head bolts, screwed into the T-nuts/slots, and let the heads trap the part from skidding around. This would be your best bet a getting a big flimsy plate flat.
Flip and repeat as necessary...
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- [machinist] machining thin plate on magnetic chuck -is three point suspension best practice?, Lawrence London, 02/20/2014
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