Plexiglass and Lexan

[dvmike]

I know that you experienced shop botters and sign guys must get a little frusterated by the simple questions that us "newbies" ask.
But after making simpler ,flat panel signs for the past year, I now have three straight customers that want something I'm new to.
While it is exciting,and a challenge it's also a bit frusterating !

Now I have a customer who wants a sign that employs a lot of Lexan or plexiglass.
The plexiglass will have mirrored lettering (which I have figured out and actually looks great ), but the profile toolpaths where I cut out the shapes leaves a white ,rough finish on the plexiglass and it looks bad. I have tried several suggestions I have received and had a limited success. But the cutout shapes are still not "good " around the edges.
I tried to offset outwards .020" and then do a finish pass . It works fairly well , but I still need a plan that will make the edges look good .
Any suggestions?

[dvmike]

I was just told to try a propane torch to remelt the edges. Anyone tried this?

[waynelocke]

Propane can work if the plexi is thin and you have a light touch. You can sand and polish the edge to whatever luster you want. Sanding also maintains the sharp edges whereas the torch seems to leave a kneaded looking edge.

[dvmike]

I've actually tried sanding the edges, but haven't gotten the luster I want. I will try some other grits. The time I tried was 320 grit.
Thanks Wayne !

[jerry_stanek]

we scrap the striaght edges with a utility knife blade and then polish with 480 sand paper. Some times we heat polish after the scraping.

[Brady Watson]

1) What plastic are you cutting exactly? Cast or extruded? There is a big difference between the two.

2) What tool are you using? Geometry (end mill, o-flute etc), How many flutes? Diameter? Length of cut?

3) How fast are you cutting it in XY?

4) What RPM?

-B

[dvmike]

1) What plastic are you cutting exactly? Cast or extruded? There is a big difference between the two.

2) What tool are you using? Geometry (end mill, o-flute etc), How many flutes? Diameter? Length of cut?

3) How fast are you cutting it in XY?

4) What RPM?

-B
Thank you for your reply.
I am cutting Polycarb in 4 x8 sheets. The thickness is 1/4 and 1/2 ".

I have ran tests at .5 inch per second all the way to 5 inches a second.
I have used a 1/4 ball nose and a 1/8 ball nose. I have tried 2 flute and 4 flute in both sizes.
The spindle RPM has been raised and lowered to pretty much cover the spectrum.

I've been cuttingthe peices out with a pass depth of .020".

When I use a higher RPM nad slower feed rate ,I tend to have the best results. I also seem to have better results with a 4 flute 1/8 ball nose.

[dvmike]

A little more info:

If any of you have ever seen an LED lit dry erase sign that shows only the marker on a clear background, that is simular to what I am doing.
I am making signs that have the lettering engraved into plexi. The letters are made with mirror image so that when you look at the uncut side ,you see the sign.
The LEDs are in a row at the bottom of the sign in a frame and unseen.
The LEDs shine up through the plexi and all you see is the sign . It is a really cool look that makes it seem as the letters are floating in space and lit.
That is the reason for needing the clearest edges possible.

I have a big intrest in this , and have made a few thus far. But They were square signs , and a freind cut them with a hot knife to give them clear edges. But the signs I'm making now have many different geometric cutout shapes and the hotkife is not an option.
Thanks

[cnc_works]

I generally cut cast plex with an Onsrud 63 series bit at 15K, 90ipm, depths dependent on thickness. For just a clear edge, go right to the buffer and polish. You will have clear ripples. For transparent, sand to 220 or even 400 depending on your buffing setup, then buff for a beautiful clear finish. I find that propane isn't hot enough to flash plex, at least mapp gas or acetylene to flash edges clear...takes a steady, careful hand though. I prefer the lesser risk of buffing.

[rcnewcomb]

Has anyone tried the 75 series bits from Onsrud for a polished edge?
http://www.onsrud.com/files/pdf/2009MonocrystallinePolishingTool.pdf

[Brady Watson]

Mike,
If you are engraving or 3D relief cutting the acrylic, then that is a whole different scenario than just cutting it. Stay tuned...I had some tools developed for me specifically for doing this type of work. I am hoping that they will be in next week.

If you are just profile cutting, I would recommend Onsrud's 2-flute spiral-O, with MS set at 1.0 to 1.5 IPS & RPM in the 15,000 to 18,000 range. You CAN cut this stuff with an end mill, but a spiral-o is the way to go. Single flute tools will severely limit your top speed, and are not as stiff (cross section) as a 2-flute tool, which can add chatter due to cutter deflection.

-B

[coach]

I cut mirrored acryllic a few weeks ago. 2.5 IPS at around 12,000 RPM.
It came off the table not needing to be cleaned up at all.
I will look up the bit number when I get to the shop. I can tell you it was a 1/4" and was from Onsrud. It was approximately 3/16" thick material. I cut in 2 passes.
David

[larry_r]

I think he said he was cutting Plexiglass, which is much tougher to cut than Acrylic.

Larry

[Brady Watson]

Generally speaking, cast acrylic et al, is much easier to machine than extruded varieties.

-B

[dvmike]

Are you guys using vacuum tables ?
I ordered 2 onsrud 75s today.
But I still think that I have an issue with the material flatness.
I put a new thread in the message board asking for ideas/designs on building a vacuum table. If anyone here has plans or build sheet, I could use some ideas.

[magic]

Buffing can get you a crystal clear edge but you have to have a very smooth surface or you'll take off too much material and make it wavy.

The 1942 guide from Rhoam And Hass shows sanding down to 400 then WET SANDING and the results are perfect

Also if you use a belt sander with a 90 degree table the edge surface will be square but of you buff with a wheel the surface will be rounded over.

For all thickness', you can flame polish with MAPP gas (recommended) as opposed to propane, The idea is to slowly get it up to almost melting point (test samples) but if it catches on fire you went too far. I usually know the speed to move the flame and polish a single piece in one turn.

I've polished thousands of feet of acrylic. Plexiglas is the trade name for acrylic. Then you have the various grades. G is the softest but also scratches the most. Perspex is the English name for acrylic.

When machining surfaces, use Cast but if you're only making boxes the extruded is less expensive and works.

Gluining (solventing) is another issue. #16 holds the best and takes several hours to set. #4 is water thin and works fast but you need very square edges with no gaps.

Hope that helps.

[chuckster]

Okay Mike, here is what I do. I cut a lot of both 3/8" polycarbonate as well as acrylic. After cutting the shapes with an Onsrud upcut bit, i take the parts to the sander and sand with 120 grit then 320 grit. My orbital sander does wonders. Once the parts are mostly smooth, I next want a glass finish on the edges. I dont use a torch because me and fire do not get along. I use Weld-On #3 solvent cement. www.ipscorp.com (http://www.ipscorp.com)
Seriously perfect edges all the time.

[bleeth]

Once you are past the rougher grits and into the 220 to 320 range if you use oil (like motor oil or even canola) instead of water you will get faster results. The commercial acrylic shop near me uses a buffing wheel and rouge. His work is fantastically beautiful.

[gabepari]

Has anyone tried the 75 series bits from Onsrud for a polished edge?
http://www.onsrud.com/files/pdf/2009MonocrystallinePolishingTool.pdf

Uh, did you look at the price? ~$1800 per each!?!?!

[dvmike]

I posted earlier that I had ordered 2 Onsrud 75 series bits . Actually these bits are ones I purchased for $66 each. The tool salesman got them for me as a trial. I don't pay unless they work.
I finally got them Thursday.
If all works out ,I'll post the info on where to get them .
I should know this week .

Thanks to everyone for their input on this subject !