A little help needed for milling plastic...

[macdarren]

Hi All,

(This is a cross post...I should probably have posted it here first...)


I generally work in metal, mostly AL, but of late I find myself cutting some things that are near the end of the production and fairly pricey to replace so I thought if I could find a less expensive practice material I could do some trial runs before committing an expensive piece of stock. In the past I have just used .1 inch AL plate to test my tool paths and it works ok, not the cheapest but not bad. However I always thought it would be nice if I could see through it (transparent AL not withstanding) I am often trying to fit a milled piece to something that exists already and being able to hold up a clear template would allow me to easily see where I might need to correct something.....

Anyway I found the 'perfect' cheap material that we all probably have laying around that being CD jewel cases, they are fairly stiff and transparent. My problem is so far my attempts to mill or drill them have been failures. I really don't know much about plastics or what type of plastic this might be but I need some pointers on the proper cutting tool and speeds. Generally I am using 3/32 end-mills and 1/16 drills for this work and so far the result is not good, and I am thinking the point of the mill must be getting hot. (I actually don't have the coolant running for the plastic parts so maybe that would help) What happens is I end up with a big glob of melted plastic on the mill or the drill. I am guessing I need to cut with a slower spindle speed or maybe I need some different tooling for plastics?

Any suggestions??

Thanks
Darren

[awerby]

Thermoplastics are all going to have this problem to one degree or another. They are designed to melt at a low temperature, so it's not surprising that this stuff is going gooey on you. You can try fighting it by using flood coolant, but the preferred coolant method is an air chiller. However, those aren't cheap, which defeats the whole point of this exercise.

If you want something transparent, try polycarbonate (Lexan). It's more resistant to melting, and cuts better than cast acrylic (Plexiglas). It's not very cheap, but you might be able to find scraps from someone who cuts a lot of it, like a window guy or a picture framer.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[chuck_hobby]

I've had great luck sourcing material like that as drops from my local sign shop (or plastic fabricator for boats, store displays, etc like TAP plastics). You will find they have tons of sheet (0.25in to 1in thick) in clear material...I raid the drop bin about once a month.

Smaller bits like the 1/16 drill need to go really slow for feed and speed. Chilled air might be spendy, but simple blowgun cooling works well for me when cutting dry...I use a pair of 26G syringe needles (blunted) on my airline for a fine stream of pressurized air (30psi).

I second the recommendation for Lexan...Makrolon seems to be a brand that is in the drops box pretty frequently. Smoke, tinted, mirror, and colors have all found special places in my shop.

Best of luck, and happy machining!
Chuck

[rschultz@asc]

ive recently started using delrin sheets in which i run with flood coolant. it is not transparable but it keeps dimensions very good and is much cheaper than aluminum or copper. great first work piece material.

[studioso]

hi,
cd cases are made usually of crystal polystyrene (HIPS). this styrene variant is very brittle and a pain in the neck to cut. consider switching to plain styrene, or hyps, which is white, not transparent.

plastic mills can be purchased from many company, I use onsrud, mostly. plastic bits usually have only one flute. so if your mill now has 2 flutes, you should at least double the feed rate. if you go to onsrud's website, somewhere buried there you will find reccomended feed rates etc.

good luck!

[amaymon]

point an air nozzle at your tool while its cutting to flush your chip away. 3/32 endmill should run about 4000 rpm at a feedrate of 20 - 30 ipm. the drill I would run at about 2000 rpm with a peck cycle of half the tool and air flush. this should work fine.