Flame-Polishing Acrylic Edges

A router may leave scalloped chatter marks on the edge of acrylic pieces, no matter what bit you choose. But a quickly-applied hot blue flame can leave the edge glass-smooth. June 26, 2006

Is it possible to create a glass finish using only a router bit? I have a set of acrylic (1/4") screens that I need to cut, but the problem is that my customer wants a glass-like or see-through finish on the edge. I am using "O" flute bits from Onsrud but the problem is that I always will have some amount of scalloping or marks on the edge.

When I run the bit extremely slow I get an incredibly good finish but this is not good enough for this particular job. What technique or bit will give me a finish such as this? Do I need to heat the edge or use some sort of chemical solution to achieve this level of finish?

Forum Responses
(CNC Forum)
From contributor W:
Use a torch with a fine welding tip, adjust for a small blue flame then quickly run the flame over the edge. I used to work at a sign company and we used this method every time. Also, make sure to leave the paper on while doing this

From contributor M:
Contributor W hit the mark right away. It is a process called Flame Polishing. After you do a few pieces you will get a good feel for it and know just when and how fast to move your flame tip along the edge of the part. This will result in a crystal clear finish. Practice on the scrap before you work on the real parts. You can use a welding torch or a simple hand held propane torch. A heat gun might work too but I've never tried that.

From contributor R:
Be careful flame polishing in cold climates unless you can anneal the plastic. Flame polishing creates uneven tension between the surface and interior of the plastic. Loading out on a frigid day can release this tension and cause crazing. The same goes for cleaners with alcohol in them.

From contributor W:
Contributor R - I agree the acrylic can craze but I find it does it more with the cheaper FF grade. We always used Acrylite GP cast acrylic and never had a problem with crazing and I live in Canada where it's almost always colder.

From contributor G:
You should not be receiving any marks. We manufacture a single flute tool to machine this material and our customers love the edge finish. We also had some acrylic shelf units built, using this tool and have a perfect edge, with no flaming.

From the original questioner:
I have never seen or used a bit that would create a glass like finish. I actually tried the flaming technique mentioned above and it works great. I have talked to one tooling specialist who told me about using a bit made from real diamonds but this tool would be very expensive.

From contributor G:
The best cut again for your material is a CP design, a single fluted tool vs. a 2 or 3 flute. A 3 flute will also provide you with a better finish vs. a 2 flute. What you may have is your feeds could be incorrect. I wouldn’t suggest using a PCD tool in a straight cut on the diameter of 3/8" or under. A 1/2" or greater may do the job, at under $300 with a cut length of 7mm.

From contributor R:
You should be getting a great edge using an "O" flute cutter. If you are getting scallops or ridges in your cut edge it’s because of vibration. Your part could be vibrating as you cut giving the scalloped look. More often it's either your machine vibrating or old collets/bearings in your motor. If your machine is an aluminum gantry type, it will be hard to eliminate the vibration.

That's just the nature of that type of machine. A little vibration on wood is not really noticeable but on acrylic, every little mark will be magnified. I have both aluminum and heavy iron machines so I've seen it first hand under one roof. Now as far as glass clear edges, you need a secondary operation mentioned above. A hydrogen/oxygen mix works best. Small blue flame and quick movement is the key.

There are also special units out there that make a hot flame with the use of H2O. It’s a fairly good sized investment however. For even larger investment you can by an edge finisher with a diamond wheel. This however limits you to straight edges and an open pocketbook. I think minimum of about $5000.00 gets you started. Go with the torch is what I say. By the way, propane only is not a good idea as the flame just isn't hot enough and may make your parts warp before the edges polish.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor T:
Hydrogen/Oxygen acrylic flame polishing yields the highest quality professional finish available with almost no time and effort. It is perfect for creating flawless edges and scratch removal yielding perfectly clear surfaces without introducing contaminants or discoloration. Flame polishing is also a great way to restore and improve the looks of older plastic parts. I bought my kit from Acrylic Flame Polishing and it’s very worth it.

Comment from contributor A:
In order to achieve a glass edge finish after routing or a table saw (with a tcg (triple chip grind blade)) you have to sand each edge through 120 to 400 plus grit all the way to "0000" wool. Only after the edges have been properly sanded then you can flame polish with MAPP gas without any scratch marks in the finish.

To flame polish, do a slow pass across the edge, a quicker second pass, and a third quicker pass for assurance. If you flame it too long in any one place it will catch on fire and create bubbles that cannot be removed. Practice on scrap first.

Comment from contributor B:
All of the above are accurate. You can get diamond tip router cutters and these cut and polish at the same time. They are mighty expensive though. If the piece is square/rectangular, a diamond polishing machine is best. Again these are expensive bits of kit, but they give a full square edge finish. Flame polishing always rounds the corners - or softens the edges.