How to Hold Plastic sheet's to table - Multi-Mat?

Hello,

Very New to CNC. I cut the Plastic called Kydex (Mixture of PVC & Acrylic) that is .118" thick just under a 1/8". I cut all the way through with a 1/8" UpCut Carbide Bit from MLCS woodworking.

I tried Double stick tape & Carpet tape on top of my MDF sacrificial table & it sticks to the plastic just fine, but doesn't stick to the MDF very well.

Mat Material Question:
What do you use to hold your plastic pieces down... There is No way I can use clamps as the shapes come out to the edge of the material on all sides.

Speed Question:
I ran it at 100in/min 16,000 RPM with Double Stick tape = No melting or clogging, but had Chatter
I ran it at 80in/min 16,000 RPM with Carpet Tape = No Melting, But had Clogging, no Chatter

Rotation Question:
Climb cut Clockwise = chattered looking edges on the outside edges?
Climb for a slot was Counter Clockwise = Smooth edges ?



Thank you for any & all help


Sincerely,

Matthew
CNC Shark 60th Anniversary Edition

Get a piece of hdpe or polypro to cover your top. 2 sided tape will hold well to it and can easily be removed.

Benn

I imagine that what you're cutting should be reasonably clean cutting since it sounds a bit on the harder side for a plastic. When you say that you are getting chatter, what is causing the chatter in your estimation? Is it that the material is vibrating because it isn't held down well enough or is the chatter a result of the feedrate? It's hard to decipher what the material thickness is, but it sounds like the typical .235" type sheet? If so, 100mm/min at 16,000rpms is a pretty good combination. I tend to slow everything down a little and go at around 70in/min with a spindle speed of 10-12k rpms, so very similar ratio of rpms to feedrate. 1/8" bits seem to generate a little more heat than I'd like at 16,000rpms and they break more often even though the chipload is the same. You'll get a little less of the clogging that way as well.

I find that plastics don't seem to climb mill well, so I typically do a conventional mill (counter-clockwise around outside, clockwise for interior cutouts). Also, you should look for bits that have a very deep gullet to the flutes. The plastic expands a bit when cut, and needs extra clearance to get out in my experience. For hold down, it depends on what the part surface area is like. Sheets just under 1/4" should be pretty rigid, so they should stay held down well with a vacuum table so long as the part is large enough (about 1 square foot of surface area).

If your part is smaller than that, there are two options. First is double stick tape as you have already suggested you tried. However, I think you're using tape that isn't ideal for the job. I use a 2" wide double stick tape I find at golf stores which is used for gripping golf clubs. The tape is like standard tan paper masking tape, but with two sticky sides. I've held aluminum blocks down to my MDF wasteboard with that tape and made finishing passes using a full 2 1/2" long flute engagement as well as pretty heavy roughing passes. Anytime I use that tape, I have to be careful not to push it down too hard or you have to pry things off with a spatula it sticks so well. The nice thing is that it comes right off the part once you separate the part from the table, and leaves very little residue. Again, it's exactly like standard masking tape, but with sticky on two sides, so if you go to a golf store and they have something else, I can't say how great it would work if it's different. The tape I use is #592 at this link:

Double-Coated Tape / Double-Sided Tape from IPG

The other option that I have used is gluing the plastic to a Melamine coated 1/4" panel and then separating the plastic from the panel after cutting is done. This is for especially difficult to hold down parts where there just isn't any surface area. I use "Roo Glue", which sticks to both the Melamine and the plastic just well enough to hold it for machining, but not well enough that I can't get them apart later. There is usually some residue left from the glue, which may not be easy to remove, so I use this method for parts that are not a finished part. If you have to have the part stay pretty, a vacuum table or fixture is best, followed by double stick adhesive.

He states the material is .118". That would be 3mm or 1/8" thick.

Ben

He also says it's "just under 1/4", so really we don't know what it is. He may have meant to say it's just under 1/8"? If that's the thickness, I expect that the chatter is probably the material moving.

I agree that it's all a guess with just one post to go on. I would think chatter is being caused by sticking 2 sided tape to mdf. Since it doesn't hold very well it would cause it to move a lot. That's why I suggest hdpe as a top for sticking the tape to. Kydex is very similar to abs but doesn't melt as bad. It is not particularly hard though. I also suspect the clogging he experienced with the slower feedrates was not due to the material or chatter.I believe it was from cutting through into the tape and pulling it up onto the bit causing it to gum up.

Ben

Quote:

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Originally Posted by mmoe

He also says it's "just under 1/4", so really we don't know what it is. He may have meant to say it's just under 1/8"? If that's the thickness, I expect that the chatter is probably the material moving.

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You caught me, my bad. It is .118" just under 1/8" ....Not 1/4".

I am also cutting all the way through in two passes. .06" for a total of .120" so it cut into the Carpet tape & got a bit sticky & maybe the reason it chattered. My cut starts from the exterior of the material & then goes into the material for an interior cut. So my cuts are essentially all interior cuts eventually. I'll try the clockwise method on both cuts & let you know.

I'm thinking that the material is moving a bit as well with the MDF as the surface I am mounting the material through.

I'd use a Vacuum chuck, but I don't think it will work. I am making a trapezoid shape with some slots in the middle of it so I would need a gasket around the outside shape then around the inside slots so I don't loose vacuum when I cut the slots out. I cut the slots first then the outside trapezoid shape.

I'll try slowing down the speeds as well to 12,000rpm & 70in/min (What does the 70in/min convert to in mm?)



Has anyone used the Multi-Mat holder? It's $329 for a 24"x24" seems very expensive, but if it helps in my production runs, it may be easier than Double sided tape, especially if the tape has to be applied each time.

Thanks a bunch for your help so far.

Sincerely,

Matthew

Quote:

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Originally Posted by bhurts

I agree that it's all a guess with just one post to go on. I would think chatter is being caused by sticking 2 sided tape to mdf. Since it doesn't hold very well it would cause it to move a lot. That's why I suggest hdpe as a top for sticking the tape to. Kydex is very similar to abs but doesn't melt as bad. It is not particularly hard though. I also suspect the clogging he experienced with the slower feedrates was not due to the material or chatter.I believe it was from cutting through into the tape and pulling it up onto the bit causing it to gum up.

Ben

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Ben,

Hit the nail on the head, Thanks I am thinking I did the same thing, with the tape issue. I will try to do two passes equaling .1175" so it is close to the .118" thickness, if you think that would be best.

I was using this bit as well: MLCS the 5141 carbide bit & does well so far.

I just received some Onsrud 63-710 made for cutting plastics here: Zoro cheapest I have found with free shipping. It also has a larger chip extractor.

What speed rate do you suggest for two passes in the material?


Sincerely,

Matthew

Awhile back I was having the melting and clogging issue cutting acrylic. I read on a post that many folks put some water and dish soap in a spray bottle and spray it on the material while cutting. I tried this and have been a big fan ever since. You can run your feed rate slower eliminating chatter, get much better cuts and not encounter the melting ot clogging problems at all. It acts as a coolant and makes your bit non-stick at the same time. I do acrylic cutting with 1/16" bits and like to run the feed rete slow because I really hate snapping $40 bits in half.

Quote:

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Originally Posted by The Meerkat

Awhile back I was having the melting and clogging issue cutting acrylic. I read on a post that many folks put some water and dish soap in a spray bottle and spray it on the material while cutting. I tried this and have been a big fan ever since. You can run your feed rate slower eliminating chatter, get much better cuts and not encounter the melting ot clogging problems at all. It acts as a coolant and makes your bit non-stick at the same time. I do acrylic cutting with 1/16" bits and like to run the feed rete slow because I really hate snapping $40 bits in half.

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I read that same thing today, Thanks, So just on the material itself, as the cutter is going over that area? I'm using my Porter Cable 890 variable speed router & wouldn't want to damage the router with water if in fact it would.

What speed rates do you run?

I am very new at this CNC awesomeness~! A couple days setting up, a couple days installing software, one day cutting, Now asking questions. So I am at 6 days with the CNC.


Sincerely,

Matthew

Meerkat check out drillman1 on eBay for your bits. They are Kyocera us made solid carbide and top quality. 1/16 actually .063 upcut in .312 cut length are 6 dollars. They also have single flute and down cut. Best deal I've found on small cutters ever.

Ben

Hi Matthew,
I have a Bosch 1617 variable speed router on mine. It's almost identical to your porter. When I first experimented with the soapy water I found a few of little tricks that work well for me. Use a spray bottle that can be adjusted to stream mode. In a pinch an old 409 or Fantastik bottle will do really well. When you start cutting spray a little about an inch or two in front of the bit. Magically the extremely fast rotation of the bit along with the downdraft of air fron your porter will literally draw the soapy liquid in towards the bit.. There the liquid will form a little spinning foamy tornado around the bit. The sight of this little tornado at work is really interesting. It seems to travel absolutely in sync with the rotation of the bit, keeps the bit cool and absolutely nothing sticks to it. When the little tornado starts to fail, I give it another shot or two of soapy water near the bit. The liquid draws itself into the vortex all by itself and it just keeps working. I use my shop vac nozzle to keep things clean at the same time. I also put a flattened out trash bag under my part when possible only because I have a leveled 5'X5' piece of birch plywood underneath. In reality it doesn't produce enough moisture to warp anything, but better safe than sorry.
Hope this helps and best of luck to you. Half the fun is in the tinkering.

Bhurts, thanks for the tip. I actually saw and ordered those exact bits a couple weeks ago on eBay. They arrived but I have not had a chance to use one yet. Wife has kept me busy engraving Cheese boards for Christmas presents. :violin: Thanks again.

Quote:

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Originally Posted by MatthewsFab

You caught me, my bad. It is .118" just under 1/8" ....Not 1/4".

I am also cutting all the way through in two passes. .06" for a total of .120" so it cut into the Carpet tape & got a bit sticky & maybe the reason it chattered. My cut starts from the exterior of the material & then goes into the material for an interior cut. So my cuts are essentially all interior cuts eventually. I'll try the clockwise method on both cuts & let you know.

I'm thinking that the material is moving a bit as well with the MDF as the surface I am mounting the material through.

I'd use a Vacuum chuck, but I don't think it will work. I am making a trapezoid shape with some slots in the middle of it so I would need a gasket around the outside shape then around the inside slots so I don't loose vacuum when I cut the slots out. I cut the slots first then the outside trapezoid shape.

I'll try slowing down the speeds as well to 12,000rpm & 70in/min (What does the 70in/min convert to in mm?)



Has anyone used the Multi-Mat holder? It's $329 for a 24"x24" seems very expensive, but if it helps in my production runs, it may be easier than Double sided tape, especially if the tape has to be applied each time.

Thanks a bunch for your help so far.

Sincerely,

Matthew

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70in/min ends up about 1800mm/min. I run everything in metric and do all my drawing/model work in metric, which is why it came out to a weird number in inches. I'm cutting a job tomorrow in ABS that will be run at 12000 rpms using a 1/8" upcut bit conventional milling at 1800-2000mm/min. That's a single pass through 1/4" material, but if I were cutting acrylic or something harder, I probably would do two passes or go to a larger bit if possible. The main thing that I've found in machining a lot of plastic is that most of the time slower spindle speeds and relatively higher feedrates are best if you have to take a full width cut. If you want to do a clean up pass, make it really light and keep the spindle speeds down, but also drop the feedrate to get the smoother finish.

Thanks for all the replies. For some reason I wasn't getting any messages saying anyone replied.

I am grateful for all that have replied.

Also the more I think about it, the sticky mat would get stuff on it too much and when I went to put another piece on it may hay chips underneath my new piece. So back to the drawing board.

I'm thinking either rare earth magnets underneath MDF in recessed holes & magnets on top of the material to hold it down.

Or a vacuum type table using a shop vac

Or some type of rabbited clamp & screws to hold the material down.