What is the best plastic for cnc windows?

[1320feet]

What is the best plastic for cnc windows?

[txcncman]

I won't use anything except Lexan polycarbonate.

[1320feet]

I used Lexan about 3 months ago and it is cracking.

[boboxx]

Are you sure it's lexan (polycarbonate) and not plexyglass (methyl methacrylate)

Lexan is almost imposible to break!

[txcncman]

What thickness? Extruded or cast? If I recall correctly, I had some problems with extruded polycarbonate before with cracking issues and had to switch to cast. Or it might have been the other way around. On thickness, use the most that will fit into the application.

[Al_The_Man]

Are we talking operator viewing window, I pay a visit to the local glass cutting and supply and get them to cut the shape in tempered re-enforced glass.
It is not that expensive.
And it doesn't scratch and easy to clean.
Al.

[Geof]

Al_The_Man's suggestion is certainly the best; tempered laminated glass the same as is used for vehicle windows.

Polycarbonate is the best next choice but it must be plain polycarbonate, also called Lexan or Tuffak.

Polycarbonate is available in a coated version that is much more scratch resistant than plain polycarbonate but it cracks much more readily and with a strong impact will shatter just like acrylic (Plexiglas).

If you have used plain polycarbonate and it has cracked there are a few possible causes:

Some solvents can cause polycarbonate to crack. These are things like trichlorethylene and dichloromethane. I suspect some tapping fluids might cause it to crack but have not done the experiment.

Some thread locking compunds will cause polycarbonate to crack.

Most soluble coolants will not cause it to crack nor will most lube oils.

Drilling holes with a dull drill and/or a drill running too fast will severely heat around the hole and this will cause the polycarbonate to crack away from the hole. Doing this and tightening mounting really tight will make the problem worse.

Cutting the polycarbonate with a dull saw likewise will heat the edge and can cause cracks to start.

Always use sharp tools, drills and saws, with some lubrication. Soapy water is a good lubricant for both drilling and sawing. When drilling use a moderate speed something like you would you for stainless and feed the drill fast. Make sure the polycarbonate is well clamped because it will try to climb the drill and if the drill grabs it you will find how tough it is as it spins around.

Saw cut edges should be deburred with a scraper to take away the sharp edges.

My company makes polycarbonate parts and we have gone through the expense of learning all the above. Sometimes before the parts were shipped sometimes after which was embarassing and expensive.

[Geof]

What thickness? Extruded or cast? If I recall correctly, I had some problems with extruded polycarbonate before with cracking issues and had to switch to cast. Or it might have been the other way around. On thickness, use the most that will fit into the application.

Are you sure you were dealing with polycarbonate? To my knowledge polycarbonate only comes in one form which I think is extruded.

Acrylic (Plexiglas or Perspex) comes in cast or extruded and they are vastly different.

Cast acrylic will crack but with care if it is not overheated along saw cuts or in holes it is not likely to spontaneously crack. If it is annealed after machining again it is not going to crack readily.

Extruded acrylic is horrible. You only need to look at it and it will crack and drilling or machining it is almost a lost cause.

But both acrylics will crack and craze around bolts holes if much stress is applied and both will craze with many small cracks in the presence of solvent vapors.

[CNC Viking]

I have used cast Acrylic (Plexiglas) for almost two years now. It stands up against coolants and scratches remarkably well. And it does not turn yellowish as time goes either.

If you keep them clean, mostly by just flushing them with coolant, you hardly notice that there is a window between your eyes and work. I use 10mm thickness for the larger sliding side doors and 8mm for the front door windows. 5mm is useless, it bends and flexes. I also have 6mm acrylic as cabinet roof to keep any splash off my floor.

8mm cast Acrylic in front doors.


6mm cast Acrylic as cabinet roof.

[diyengineer]

Geof,

Thanks for this good info. Just ordered some 1/2" Lexan plate for my new window. I am also going to order some clear coating, similar to window tint (but clear) so it wont scratch.

(Lexan 9034 to be exact. A 1/2" thick 24"x36" plate cost me $125 shipped off ebay)

Al_The_Man's suggestion is certainly the best; tempered laminated glass the same as is used for vehicle windows.

Polycarbonate is the best next choice but it must be plain polycarbonate, also called Lexan or Tuffak.

Polycarbonate is available in a coated version that is much more scratch resistant than plain polycarbonate but it cracks much more readily and with a strong impact will shatter just like acrylic (Plexiglas).

If you have used plain polycarbonate and it has cracked there are a few possible causes:

Some solvents can cause polycarbonate to crack. These are things like trichlorethylene and dichloromethane. I suspect some tapping fluids might cause it to crack but have not done the experiment.

Some thread locking compunds will cause polycarbonate to crack.

Most soluble coolants will not cause it to crack nor will most lube oils.

Drilling holes with a dull drill and/or a drill running too fast will severely heat around the hole and this will cause the polycarbonate to crack away from the hole. Doing this and tightening mounting really tight will make the problem worse.

Cutting the polycarbonate with a dull saw likewise will heat the edge and can cause cracks to start.

Always use sharp tools, drills and saws, with some lubrication. Soapy water is a good lubricant for both drilling and sawing. When drilling use a moderate speed something like you would you for stainless and feed the drill fast. Make sure the polycarbonate is well clamped because it will try to climb the drill and if the drill grabs it you will find how tough it is as it spins around.

Saw cut edges should be deburred with a scraper to take away the sharp edges.

My company makes polycarbonate parts and we have gone through the expense of learning all the above. Sometimes before the parts were shipped sometimes after which was embarassing and expensive.

[CNCRebuilder]

Regardless of the material used (I use Lexan) construction of a door window is very important also. If you are doing a window replacement on a slant bed or a high speed lathe, a small part turning at 3500 rpm's + flying out of the chuck and hitting the window is a "hair-raising" experience. This issue has been a major concern of mine when re-manufacturing lathes. I'll use the Okuma Crown for an example here, the glass used in that door is contoured and very expensive to replace, last time I checked with Okuma it was over $1000.00 USD for the glass and gasket. The glass is a 2 part system with an air gap in between the 2 pieces. When we replace any high speed lathe door if it will permit we put "jail bars" like the old Mori's used, then a 2 piece Lexan with an air gap in between, we use the gooey windshield rope gasket as a sealer. With this 2 part system it allows the first layer to take the blunt of the impact and the second layer to buffer the first.
I have shot various caliber of rounds at this set up to test, and have been very satisfied with the results.

[diyengineer]

I only have a mini mill, but opted for a little overkill with the 1/2" i believe. I figure its money well spent if it saves me bodily harm. If i had a larger machine i think i would go with bullet resistant laminate glass haha. After watching some lathes blow up on youtube, scary!

Regardless of the material used (I use Lexan) construction of a door window is very important also. If you are doing a window replacement on a slant bed or a high speed lathe, a small part turning at 3500 rpm's + flying out of the chuck and hitting the window is a "hair-raising" experience. This issue has been a major concern of mine when re-manufacturing lathes. I'll use the Okuma Crown for an example here, the glass used in that door is contoured and very expensive to replace, last time I checked with Okuma it was over $1000.00 USD for the glass and gasket. The glass is a 2 part system with an air gap in between the 2 pieces. When we replace any high speed lathe door if it will permit we put "jail bars" like the old Mori's used, then a 2 piece Lexan with an air gap in between, we use the gooey windshield rope gasket as a sealer. With this 2 part system it allows the first layer to take the blunt of the impact and the second layer to buffer the first.
I have shot various caliber of rounds at this set up to test, and have been very satisfied with the results.