Hold-down of thin material to CNC mill

Hi, all:-
What's a good way to hold down material such as thin aluminum panels (eg. 1/16" or 1/8") while through-CNC milling cutouts, holes etc.?
Up to maybe 8" x 19" but probably more like 4 x 6" typically.
Presumably some kind of sacrificial material underneath.
I see one company has a material that is porous and allows a vacuum table, not sure if that's worth it.
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Spehro Pefhany wrote:

I machine 1/16" aluminum all the time with just edge support, but maybe your pieces are bigger than mine. I have various bits of sacrificial scrap that I can lay under the edges, then use various clamps to hold the edges down onto the strips of stuff used for the edge support.
I made a 13 x 25" piece os 3/8" aluminum with a 1 x 1" grid of 10-32 holes tapped in it, and 4 big countersunk holes for bolts to clamp it to the machine table.
Wood gets chopped up into dust that clogs up my coolant system, so I don't use that as a sacrificial support.
Jon
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Spehro Pefhany wrote:

As does Jon, I machine thin sheets frequently.
I have prepared a (somewhat) sacrificial baseplate from a faced-off slab of aluminum. Through that, I drilled quite a number of holes (more on that later).
I use a very aggressive commercial double-stick tape to secure the plates. They - and the baseplate - must be immaculately clean before the tape is applied. I lay it on close together, but with no overlaps or bubbles, then press down the plate onto the tape. I then weigh it down with a lot of weight (maybe 200lb per sq. ft.) overnight to secure it, and ensure any trapped air has been pressed out.
Except when using an oil-emulsion coolant, I've not had anything peel loose, not even when cutting profiles that leave a slug. It is essential to make the last cut through a slug very thin, so as not to apply much lifting force to the waste material.
After machining, I spray an oily solvent (PBlaster) into all those holes (told you I'd mention them) to help loosen up the adhesive. After soaking for a while, gentle prying will slowly release the sheet.
It's a chore, but for stuff with slugs left over, it works neat. For "ordinary" work on thin sheets, I just use clamps and plan my work in stages so I can move the clamps around as necessary between machining operations.
One can also CAD the work to leave "holding tabs" around slugs, and remove them with secondary finishing work.
LLoyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message

I have not had to deal with this myself, but I was wondering, could you use a steel baseplate and put a few strong magnets on top? Of course you would have to program the tool path to avoid the magnets.
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You'd still need some sort of scheme to prevent the work from sliding side-to-side under cutting thrust. Magnet-to-magnet will resist sliding due to the coupling of lines of force, but the only thing keeping a sheet of magnetic material from sliding on a magnet is the friction due to the attractive force.
If the magnets were strong enough... eh... yes. But then removing the material would be pretty hard, too.
Lloyd
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On Tue, 24 Apr 2012 16:07:45 -0500, Lloyd E. Sponenburgh wrote:

...
...
Have you tried heat for removing the tape? I've frequently used a heat gun for removing tape, and sometimes then a solvent to remove sticky patches left behind. For the large-area baseplate you mentioned, an oven might work better than a heat gun. The main advantages of heat vs. PBlaster would be faster turnaround and less need for "gentle" when separating the sheet and baseplate.
--
jiw

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I've never tried with this particular tape, but I will! <G>
Thanks, Lloyd
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On Tue, 24 Apr 2012 16:07:45 -0500, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:> Spehro Pefhany wrote:

Lloyd: Can you share with us what tape you are using?? Thanks!! rgentry at oz dot net
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yep... made in Canada.
Tape-Rite Co., Inc. 1-800-882-7348
It's a flame retardant high-performance carpet tape, part # 108FR DC Carpet Tape, Black. (the FR stands for flame-retardant)
It comes on 25yd rolls, and surplus, it cost me $7.00 per roll.
Sticks like IRON.
LLoyd
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On Thu, 26 Apr 2012 11:43:03 -0500, "Lloyd E. Sponenburgh"

Thank You :) rgentry at oz dot net
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On Tue, 24 Apr 2012 16:09:01 -0400, Spehro Pefhany

For thin stuff I make a sandwich, sacrificial above and below.
Karl
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wrote:

Agreed.
Except the sandwich plates won't really be sacrificial, as much as comprising a *fixture*, as the sandwich plates will simply have the part geometry milled right into it.
I would also have the material oversized, for screw holes coincident with the sandwich screws, but if the plates are rigid enough with enough friction, you might not need these. Also, the bottom plate could be milled out slightly, to "capture" the thin sheets, avoiding extra peripheral material, etc.
PMT's technique is good, but requires a fair tolerance in Z. The sandwich strategy is more forgiving.
--
EA


>
> Karl
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I guess it depends on the type of cuts and how often you have to do it. I always thought a pair of edge clamps that put a sheet under tension would be good if you are just milling holes in it, but if you have stuff that cuts through the edge it would distort. I usually epoxy aluminum to a milled flat piece of MDF myself. Usually its one piece, and I have to do all kinds of weird things to it. Sometimes the epoxy will break loose if you get the work piece to warm or put to much stress on it.
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Once upon a time, I used some "medium density sawdust wood", ie mdf, in the construction of a vacuum chamber. Air went through the meduim density stuff too quickly to even build a vacuum. The mdf might serve the dual purpose of porous/throw-away support with a vacuum box underneath. Secure the aluminum to the mdf with tape and seal the remainder of the mdf with tape too. If you give it a try, post the results.
Hul

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snipped-for-privacy@kbrx.com fired this volley in

FWIW, even high-density fiberboard can be used as a vacuum base. I work periodically with a guy who owns a ShopSabre CNC router. His "waste sheet" is HDF with the sealed surfaces milled away (which is necessary to "true" the sheet to the bed, anyway).
Of course, for a 4x8 sheet, he has a 20HP 3-phase "vacuum cleaner" doing the suction! (regenerative impeller-style vacuum pump)<G>
Lloyd
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Generally I leave excess all around the periphery and screw it to the table, also adding screws at any holes or cut-outs....
Then I machine the entire part staying appx 020 above the fixture, following up with a second pass at about .005 above....essentially, this leaves me with a strip of aluminum foil attached around the edges etc which peels away from the finished part with minimal effort.
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I'd consider 3M No. 77 spray glue and a fiber board underneath.
After cutting, solvent or heat will release the glue.
Something like PCB making in a lab for one of a kind prototypes.
Martin
On 4/24/2012 3:09 PM, Spehro Pefhany wrote:

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On Tue, 24 Apr 2012 16:09:01 -0400, Spehro Pefhany

Greetings Spehro, I have used double sided tape for the kind of work you describe but it's a pain to remove even when it comes off easily. For doing several of the same parts I like to machine channels into an aluminum plate in a grid with a channel around the periphery of the grid that holds a seal. The seal can be O-ring material. But there is also available round cord for seals that is made from closed cell foam and has a smooth outside surface. I don't remember where I got it now but I could find out. It is what I use now for vacuum chucks. Anyway, when I machine the channels I place them so that all the cuts on the part won't be where the channels are so that the vacuum won't be lost. I also use locating pins on two sides of the part and MITEE BITE clamps to to push the part into the pins so that the part is secured against side to side motion. Sometimes I also use cutters with lefthand flutes to provide downward pressure if there is not enough surface area for the vacuum to hold well enough. When I do that I machine the plate deep enough for the chips from the lefthand fluted cutter to have someplace to go. Eric
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    One approach is a double-sided tape to hold it down.
    Or hot shellac allowed to cool to hold it, and heated back up to remove it after machining.

    Yes -- bolt it down to a tooling plate and replace it every time the pattern changes -- or just start out with it quite thick and mill it flat before each new pattern.

    Vacuum hold-downs can be good if the ratio of remaining metal to hole area is good, and the overall area is large relative to cutter load. The more holes you cut, or the larger the holes, the weaker the vacuum hold-down gets. But you can do it with a shop vac (and hearing protection) and a box with a maze of paths milled in it to route air to all the holes for the hold-down. You will need more material surrounding a smaller workpiece to keep from losing the vacuum.
    Good Luck,         DoN.
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