Having problems holding thin plastic down.

[knight_toolworks]

I thought it was the bit but today cutting some .09 polycarbanate with a onsrud 1/8" o flute. I had a 4'x4 sheet held down with my two fien vacs and the parts are not small the squares are 7x14 parts with 4 1/8" holes. but the parts are lifting up while getting cut. I would never have this problem cutting plywood or such. I am cutting at 10000 rpms and 1.5 ips. a few months ago I was cutting a large piece and had the same problem with I used a upcut bit.
I bid on this job with the cost of making my table a vac setup like brady does but I aonly doing two sheets so it is not worth the time to do any mods. I have cut smaller plastic parts that way with no issues.

[cmagro]

Steve,

Without any experience working with polycarbanate...if I had a two-sheet job to do like that I would sandwich the thin sheet between two sheets of mdf (maybe 3/8")..screw down the top one to the bottom and let the vacs hold the rest.

That's actually how I would do I thin sheet like that through the table saw or even a miter saw...

Just an old school thought..

Christian

[knight_toolworks]

with only two sheets i think using contact adhesive is the way to go on a sheet of hardboard. then use my vac to hold the hardboard down.

[Gary Campbell]

Steve..
Have you tried a straight or down spiral bit? I dont often cut anything as thin as .09, but have had some lifting in both 1/8 and 3/16 Lexan. As soon as the bit lifts a corner, the vac is all gone. I cut a Lexan valve slider last nite that was 3.5" by 20" with two 2.25 holes in it. I did drop the speed to .75ips @ 10K rpm with a 1/4" O flute. I try, but am not always successful, to not cut thru the lower layer of plastic wrap, which really helps.
Gary

[knight_toolworks]

yes I have used downcut bits. sometimes though you get all the shavings glued to the edges so you don't get a clean cut. but this stuff cuts well so I will try it.

[stevem]

Use a pressure foot, although it can scratch soft materials like Lexan.

[dana_swift]

Steve Knight- your method sounds a lot like like mine when you are talking about contact adhesive. Using vacuum only without gasketing is not going to hold a 4"x4" very well without a baseboard to amplify the vacuum holding forces.

I have had great results with super-77 for holding all sorts of tiny things like HDPE and UHMW to a base board that can be held by vacuum, screws, or whatever. I haven't used polycarbonate, so it may not work, but I would certainly give it a try.

For a base board I often use some scrap HDPE, since it forms a weak bond with the super-77. Cleanup is quick with a rag and naphtha, the super-77 goes away and I am left with nice clean parts. My cut depth usually only cuts the surface of the base material by 0.005 or 0.010 so it gets re-used for a lot of parts. It can be re-surfaced if need be.

I have made many parts less than 1"x1" this way.

D

[dana_swift]

Another thought.. I have made a vacuum fixture based on Brady's open grid idea, but I made mine a "closed grid" for gasketing. I use high vacuum, and this can conceptually hold a part as small as 2.5" x 2.5" by appropriate gasketing.


7367

I am getting 20"+ Hg vacuum on this fixture for generic one-off parts. So it might be a solution for 4x4 parts like you are talking about.

There is a "micro zone" shown here:


7368

This little dedicated area vacuum-holds the zzero plate flat against the grid surface to allow accurate zeroing. The rest of the grid is the generic hold down area. The double-purpose zzero gasket is permanent and keeps vacuum on either the outside or the inside of the region allowing parts to cover this part of the fixture also.

This shows the zzero plate on its dedicated zone.


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This shot shows the vacuum inlet connectors for both the main zone and the zzero zone. Also you can see this fixture bolts onto my BT32 table with recessed bolts so I can secure parts that overhang the fixture. Nothing protrudes into the working area.


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This fixture is made from 1/2" HDPE upper layer and 1/4" HDPE lower layer. The two layers are held together by 8x32 screws inserted through the bottom of the fixture into threaded inserts located in the bottom of the top plate.

Using this fixture to hold the baseboard which holds the smaller work peices keeps me from needing as many dedicated fixtures, one-off parts are practical with high-vacuum clamping.

The overall dimensions of this fixture are 24x24x.75 with 1 inch overhang where the vacuum inlets are.

Hope that is of use-

D

[erik_f]

go to the home depot and get some carpet tape...had then just run lines of the tape so it lines up with your pieces all the way down the board...worked great for me when I had to cut out 100 plastic pieces out of .125" material. The pieces were about 3" x 6". Cheap, easy and worked well.

[knight_toolworks]

the sheet was 4'x4' the parts are large really 17"x7" the smallest was about 7" x6"
I debated on making vac surface that sits on top of my vac table but like I said this job is too small and it would be a pain to store it.
I like the grid idea though I would need a large one.

[magic]

I make a jig that the vacuum can hold and put the part inside. Kind of like having a rail around the part. No vacuum holes are drilled through the bottom of the jig.

That's worked for me.

[scottcox]

I just use a preliminary toolpath to drill screw holes through the laminate, move my gantry and add screws to critical points. Then I follow the cutout toolpath with something similar to a yard stick holding down the material where needed. Not neccessarily automated, but quick.