Vacuum Hold Down Chucks

[awemawson]

Several years ago I bough an 18" x 12" vacuum hold down chuck for use in engraving on my Interact. Made by a firm called Debece it comprises a sandwich of an aluminium core with milled airways, a lower mounting layer of some white plastic material, and an upper layer of the same white plastic with a quadzillion tiny holes in it each at the base of a 4mm drilling.

I never used it in anger as it was warped when I got it - about a 4 mm 'hump' from side to side so pretty useless - I honestly cannot remember if they refunded me or what, but it's sat in a box until just now when I dug it out when looking for the Tapmatic Drill Speeder. I think that they are out of business as they don't pop up on a Google search. Measuring the hump again it seems to have settled down to about 0.5 mm but as it 'humps' rather than 'sags' the hold down clamps don't straighten it to the machine bed.

So I though that it was time for some ingenuity. I set it up on spacers on the bed of the manual Bridgeport so that the extreme edges of the short sides were suspended on bars. I set up a dial gauge below it to measure deflection, and put a beam parallel to the bars and lowered the mill ram onto it until I'd bent the sandwich 3 mm in the opposite direction. Left it there and had lunch.

Releasing the pressure it sprung back, initially to about 0.5 mm in the opposite direction to the original hump, then slowly over the next hour crept back to dead nuts on flat!

No doubt it will creep again over time, and I may try bonding it to a thick alloy tooling plate, but at least it's now usable 

Andrew

[John Stevenson]

Aahh, a memory CNC vacuum table.

Hope you had more luck than I did, bought one of these new memory foam mattresses but trust me to get the only one made with Alzheimer's............................

[Brass_Machine]

How strong of a vacuum do these things actually make? Can you do heavy work with them?

[awemawson]

Well given a perfect vacuum you've got nearly 15 pounds per square inch of down thrust. So over (say) a 12" square piece of flat plate there's 12 x 12 x 15 lbs = 2160 lbs down thrust. I put a sheet of aluminium 10" x 15" on the vice, and using my workshop vacuum cleaner as a source of suck couldn't pull it off by hand.

Andrew

[bp]

I once visited a firm who used vacuum chucks extensively for making thin walled panels for electronics enclosures.  They used al. alloy 6061 T651 exclusively.  A vacuum chuck was made for each panel, but they were very simple, consisting of a piece of al. alloy plate about 6mm bigger all round than the workpiece to be clamped, this chuck panel was clamped direct to the table, and skimmed flat.  A recess or pocket was cut maybe 1.5mm to 2mm deep to accept the workpiece.  A few mm in from the wall of the pocket an "O" ring groove was milled, then using the same cutter a system of tracks was milled out to within 4 or 5 mm of the "O" ring groove, radiating out from the centre of the panel, where a largish (say 10mm diameter) hole was drilled 75% of the way through the plate thickness.  The panel was tipped on one edge and a hole drilled to meet up with the 10mm hole, tapped to suit the connection from the vacuum pump.  The "O" ring was cut to length and fitted.  Bingo a vacuum chuck.
Using this it was possible to routinely achieve wall thickness' of (slightly) under 1mm on a panel up to about 400mm square.
Don't try and use a vacuum cleaner as a vacuum source, they stall and do not provide a constant vacuum.  A positive displacement pump (a.k.a. a "vacuum pump") is required.
cheers
Bill

[awemawson]

Bill that's very interesting.
For my applications a vacuum pump isn't suitable as there are many 'through' holes produced as the work progresses meaning the pump has to cope with a very large flow rate and be able to swallow coolant and swarf. I have a dedicated 'wet &dry' cleaner that copes well and gives a remarkably low pressure. Neither of my two high vacuum pumps can give the flow.

Andrew

[bp]

On the examples I saw, wherever there was a through hole they simply put an "O" ring in the vacuum plate, obviously bigger than the hole, remember these things were planned!!.  Another thing I've seen, if you have a compressor, is a little box shaped thing which might be called a "vacuum generator" (in shipbuilding terms it would be an educter...sp?) where you plug a compressor in one side, and it creates a vacuum in another port.  Using a decent vacuum chuck that doesn't have too many extra, non functioning holes, you don't need a huge flow rate.  The type of little vacuum pump I use for vacuum bagging composites has been used for a small (150mm x 150mm) vacuum chuck and it worked very well.
Its a really interesting process/device
cheers
Bill

[awemawson]

Bill, the ones you have seen were production clamps. My stuff is all one offs so not feasible to mask off the holes really. I do have a venturi vacuum generator like the one you mention, but it, and they in general, use an awful lot of compressed air and are noisy.

My Earlex shop vac works very well for me (though it can't be called quiet   )

Andrew

[NeoTech]

I've made makeshift vacuum chucks out of round stock.. just a hole in the center with a "tube" running off to the side and then a groove for a oring, you leave a bit of space to pull on.. can easyy be bolted down to the milling table and quite large pieces will stay on there with no problem.

[Doc]

My real job I work at I work as a tooling design engineer. I have designed several vacuum fixtures (I do mostly hydraulic fixtures). One of the recent fixtures was designed for a part that is approx 6 feet long and 8 inches wide and the material is 2.5 inches thick and weighs in at about 145 lbs. When the part is finished and removed from fixture it weighs in at approx 6LBS. The part is 6061 t6 aluminum and the material is removed using high speed machining. SO the answer to can you machine aggressively yes if you have enough square inch coverage as Andrew stated at a perfect vacuum pull you would have approx 15 LBS per square inch but that would be a perfect pull I usually use a 12 lb per square in my calcs. So if you have parts with enough planer surface and lots of parts to run vacuum works great.

[NeoTech]

I get around 8-9lbs and it works fine for aggressive manual milling on smaller areas as well.

[Brass_Machine]

I have been debating methods for work holding for a project down the road. I think making\designing a vacuum chuck and fixture will be best.

[NeoTech]

Making the fixture out of a solid plate with some indexing pins and a rubber gasket works well if you wanna clamp really thin parts. Like knifeblades (what i use em for primarily) so i can surface mill the raw RWT steel before i take it to the belt grinder.

yeah a surface grinder would be better, but i dont have one and they are big and expensive. =)