holding down larger flat plates

[awetmore]

I find myself making many objects out of 3/16" flat plate. Usually I like to setup the machine to make a few objects in the same run, because I can pack them together to save material and spend less time setting up raw stock on the machine.

I'm having a hard time finding a good way to hold down larger sheets of plate. Yesterday I run an operation on a 3.5" x 9" x 3/16" piece of 6061. I held it down with this sort of clamp:
http://www.a2zcorp.us/store/ProductDetail.asp?Cguid={CEC11431-195C-4D3A-90E4-1779F697DB5C}&ProductID=5811

The clamps are 9" apart, at the far ends of the stock. This makes the stock bow up in the middle, making it hard to get accurate depth of cut. The middle is also not well supported and vibrates easily.

I'm wondering if using two toolmakers vises side by side would be a better option? I have one of them, but could get another. That would work well if I didn't use the full width of the material (I often do).

What other options work well?

[Jeff-Birt]

While it depends on the parts your making, one common way to handle a problem like this is to use the stock itself as a clamp. In other words can you place a hole between two parts that you can use to bolt through to a T-nut? Sometimes parts will already have such holes so you can set up your code to cut a few holes in the parts, then you bolt through those holes before continuing.

[awetmore]

While it depends on the parts your making, one common way to handle a problem like this is to use the stock itself as a clamp. In other words can you place a hole between two parts that you can use to bolt through to a T-nut? Sometimes parts will already have such holes so you can set up your code to cut a few holes in the parts, then you bolt through those holes before continuing.

My parts usually do have holes and I had thought about that approach. The problem is that I don't precisely know where the hole will be relative to the table when I'm first setting things up. I would need to know that to place the T-nut in the right place or to make sure that the hole will be over a tapped hole on my fixture plate.

If I were making dozens or hundreds of the same item I see how I'd make a fixture plate to line everything up properly. When I'm doing them a couple at a time it seems like it adds a few extra time consuming steps.

[Arich0908]

When I find my self having trouble holding down a large piece of stock . I most of the time make an adapter.
I have a few IE i Have a round piece with 3 small steps on it for when Im turning large pieces in my 3 jaws .
I have a few strips different sizes that I mount on the stock usually with a M5 socket bolt.
I usually mount it where a hole will be so it dose not show up. If there isnt a hole I make a pattern there to make it look like it belongs.
I just do what ever works for me .

[japroach]

Assuming its aluminum or plastic, could try double sided tape.

You would need a spoil board of some suitably smooth material underneath (plastic, alu, etc.).
If you zero your final depth just above the material, and use some relatively thick tape, you can reuse it indefinitely.

[themedulla]

I use loctite black max on a sacrificial plate. If double sided tape works, then I would go with that.

The key with the loctite is to clamp it together well and give it long enough to set. To release it, acetone works great (and the horrible fumes come free).

[ImanCarrot]

There's plenty of heat activated adhesive out there that'd do the job. With double sided tape on such a thin part you might bend the part in removing it from the sacrificial plate... just a thought? I used to use 50% parafin wax and 50% Rosin on my blanchard to hold down glass.

If you got into large production quantities I'd recommend making a vacuum fixture.

[Arich0908]

I use loctite black max on a sacrificial plate. If double sided tape works, then I would go with that.

The key with the loctite is to clamp it together well and give it long enough to set. To release it, acetone works great (and the horrible fumes come free).

Dose that work with aluminum also?

[pblack]

I use cheap white PVA glue for aluminium on an MDF backing board, it seems to bond very tightly after around an hour clamped up, and is not affected by the wd40 i use for cutting lubricant.

Double sided tape and solvent-based contact adhesives seem to be badly degraded by WD40, while the PVA holds on tight.

Removal from backing board is acheived by simply soaking the board/plate in hot water. the cut parts drop off after about 10 minutes. The MDF is completely ruined of course.

[noisillator]

I work with panels too, usually between .062" and 0.5". In fact, cutting complex holes in panels is the primary reason I bought a CNC mill. When I first set everything up, I built a clamp like the one shown at this site (Click "Projects," then "Taig CNC Vise/Clamp"):

The Hobbyist's Machine Shop

In my case, there's no fixture plate, so I use T-nuts to hold the clamps directly to the table. Each of my clamps also has a small shelf cut into the edge facing the work. That keeps the work elevated above the table.

Lately I've been thinking about a system that would clamp the work along the width of the table, rather than its length. That would allow working with pieces longer than the table. It's just a rough concept at the moment though.

None of this resolves the issues of bowing and vibration, and I'm afraid I don't have many suggestions for that. Working with thinner panels, I just try not to apply any more clamping force than is really necessary to secure the part. Sometimes, a smaller cutter and slower speeds can help. I've never tried to clamp multiple parts together for machining.

Jack

Ha, ha, I just realized the original message was posted back in May. Hope it's all been resolved by now!

I find myself making many objects out of 3/16" flat plate. Usually I like to setup the machine to make a few objects in the same run, because I can pack them together to save material and spend less time setting up raw stock on the machine.

I'm having a hard time finding a good way to hold down larger sheets of plate. Yesterday I run an operation on a 3.5" x 9" x 3/16" piece of 6061. I held it down with this sort of clamp:
http://www.a2zcorp.us/store/ProductDetail.asp?Cguid={CEC11431-195C-4D3A-90E4-1779F697DB5C}&ProductID=5811

The clamps are 9" apart, at the far ends of the stock. This makes the stock bow up in the middle, making it hard to get accurate depth of cut. The middle is also not well supported and vibrates easily.

I'm wondering if using two toolmakers vises side by side would be a better option? I have one of them, but could get another. That would work well if I didn't use the full width of the material (I often do).

What other options work well?

[noisillator]

Thanks for posting this idea. It hadn't occured to me to attach panels to a sacrificial piece of wood. I sometimes engrave my panels, usually no more than about .006" deep, and maintaining that depth across the work can be a significant problem if it's bowed. Given the small quantity I build (I'm not in a commercial production environment), this sounds like a near-perfect solution!

I use cheap white PVA glue for aluminium on an MDF backing board, it seems to bond very tightly after around an hour clamped up, and is not affected by the wd40 i use for cutting lubricant.

Double sided tape and solvent-based contact adhesives seem to be badly degraded by WD40, while the PVA holds on tight.

Removal from backing board is acheived by simply soaking the board/plate in hot water. the cut parts drop off after about 10 minutes. The MDF is completely ruined of course.

[pblack]

I've done engraving too, and like you have struggled to maintain consistent depth across large areas - in fact the last batch of items i did (small dog-tag type things), i decided to just go one-by-one.

I cut out my blanks using my 'PVA glue the plate to mdf' technique, and soaked them off. As these pieces needed to be fully profiled, any glue adhesion problems quickly become a nightmare, as if one piece is pushed loose by the cutter it tends to affect neighbouring parts. PVA is the best thing i have found for this, better than superglue, double-sided tapes, or flooring contact adhesive.

Then I used another piece of mdf and cut a pocket in it which was a loose press-fit for the blanks. the pieces had some edge variation after cutting so some were shimmed in tight with a piece of paper. I also hand-cut a small slot in the front-side of the holding jig so i could get a lever under the piece to pop it out.

This let me do one at a time, which was time consuming, but it got the job done with consistent quality, as all the engraving was done in one part of the table, and flex and vibration wasn't a big issue. It was also possible to stop the job and re-zero the z axis down or up by a tiny amount if i saw the cut being made too light or too deep at the start of each tag engraving, which would have been impossible if i was doing it all in a batch. My engraving was done with a sharp-pointed dremel carbide burr, which isn't great but worked fine for my purposes.

Personally i think the way to do this would be a springloaded cutter with a depth-stop - i started making one out of a pin-chuck and some steel on the lathe, but ran out of time. Once you had this item, your setup would be super-easy, and slight bowing wouldn't be a problem either.