Cutting sheet metal on the mill?

[AdeV]

I'm making some flat covers for a little project I'm working on (full write-up soon...). Whilst the perimeter tolerances are of the "hold thumb and forefinger just so and squint", i.e. eminently achievable with an angle grinder, I do have to drill 6 reasonably accurately positioned holes, so I can screw the top in place. So since I've got to put the metal on the mill anyway, I could also quickly run around it with an end mill to give me a nice rectangular cover of pretty much the exact right size....

The question is, what's the best way to hold it?

My first attempt was with double-sided sticky tape on a piece of wood. I'm sure you can all imagine how well that went.  I'll pause here, until you stop laughing.

Second version, I drilled a couple of holes in the middle, made a jig* with 2 bolts, the jig went in the vice & the sheet (rough cut to size) went on the jig. This works well - but it does leave a couple of unnecessary holes in the finished cover.

So - Is there another, better, less holey, way of holding a flat sheet (0.9mm thick) for both milling and drilling operations?

Cheers,
Ade.

*Calling it a "jig" is to massively exaggerate really: It's just a rectangular block drilled & tapped to take two M6 cap-head screws, and is deliberately smaller than the outline of the cover...

[Brass_Machine]

Sacrificial piece of aluminum. Bolt it down to the table or hold it in a vise. Super glue the thin sheet on. make your cuts, then some applied heat to pop it off.

Eric

[chipenter]

Drill your 6mm holes first and use those to clamp it down with a spreader bar on top , I have made sleeve  thread adaptors for my t-nuts 8,6 and 4mm for small stuff , that are very useful .

[Spurry]

For any of those awkward jobs, my favourite is a backing board of 19mm mdf. If it won't fit on a spare bit of mill bed, then, horror-of-horrors I have to remove the vice as a last resort. A bed-size piece of mdf pre-drilled 12mm at its ends to suit the slots secures it to the mill bed. The metal sheet can either use proper clamps through the mdf to the mill bed or just screw washers round the edge of the sheet to the mdf. All depends on the relative size of sheet to be machined and mill bed.

Pete

[John Rudd]

For any of those awkward jobs, my favourite is a backing board of 19mm mdf. If it won't fit on a spare

Pete

And......make sure you set the depth stop so you dont drill right thru the mdf.....should you get carried away with your success.....

[Lew_Merrick_PE]

AdeV  --  My approach is to use "Hold Down Battens on top of the "sheet" on top of similar "Battens" (with slightly larger screw holes) below the "sheet" such that the entire thing mounts to the t-slots and, if necessary, use "clamp bars" at the ends.  I probably have a half-dozen of such things in my "mill tooling" bin.  --  Lew

[PekkaNF]

Once I needed to make an swich/fuse panel for a boat. Pretty much only way I could figure out how to make square holes and make sides square was to draw the panel layout approxiamately to aluminium drill mounting holes to waste parts (two holes for inside of each square waste - where exact location is not that important and the applied two sided tape to mount it onto MDF and used plenty of screws to mount it securely and then used four flute slot drill to cut all the square openings. Previously tries failed because cuter packed too much swarf between the aluminoum plate and MDF.

[Spurry]

And......make sure you set the depth stop so you dont drill right thru the mdf.....should you get carried away with your success.....

John, the 19mm certainly helps in that respect,  although I usually have plenty of 12mm.

Pete

[WeldingRod]

+1 on the strap it to.wood...  put screw holes in the waste bits.

Sneaky hold:  blue tape a clamped down board, blue tape the bottom of your part, spead some super glue on the tape, and stick then together.  I've been milling 1/8" and 3/16" fiberglass this way.

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[rklopp]

+1 on the strap it to.wood...  put screw holes in the waste bits.

Sneaky hold:  blue tape a clamped down board, blue tape the bottom of your part, spead some super glue on the tape, and stick then together.  I've been milling 1/8" and 3/16" fiberglass this way.

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The robotics students where I mentor do the blue tape trick a lot while CNC milling aluminum plate up to 1/4” thick bonded to MDF. It works a treat as long as the coolant doesn’t get to the MDF too much. Stuffing in some short drywall screws halfway through the job is sometimes done for security.


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[RotarySMP]

Sacrificial piece of aluminum. Bolt it down to the table or hold it in a vise. Super glue the thin sheet on. make your cuts, then some applied heat to pop it off.

Eric

Unless the part is pretty small, heats up from the cutter, and releases its own superglue during the cut... or so I have heard. 

[AdeV]

Thanks for the ideas!

The part is pretty small - around 3" square give or take. The way I've been cutting them out, there is no sacrificial stock to drill holes in; and drilling holes in the part is what I wanted to avoid (but I'm not THAT fussy, it's not going to be visible, in use). I'll probably stick with the "jig", 2 holes & bolt it down. This has the slight advantage that I can mill all the way around it (to size) in one go AND put the screw holes in, without ever having to line the part up accurately, or edge find it.