Machining plastics with CNC

[Ben Mitchell]

I already ran a seach for this topic, and while there is some info there, I'd like to ask my specific question in the hopes that someone has experience machining plastics and can help me.

First, let me explain what I'm trying to do. I need to machine a ridged surface into a 1 foot square piece of plastic. Depth of the surface is at least 15mm - so the plastic needs to be about 3/4 of an inch to one inch thick. I'm doing this using an old boxford CNC 3 axis router.

I've tried using wood, including regular wood, MDF, and particle board, but all woods warp a lot under either the stress of being machined or the intense heat being produced. I've attempted to control the heat of the motor and the bit by running vacuums into the machine (one that travels with the machine pulling air away from the bit and one on the other side of the machine) and opening side panels.

Since wood doesn't work, I've moved on to our synthetic friend, the plastic. I don't have any experience working with plastics. I've done some homework and read the other thread regarding plastics, and i've also checked out wikipedia and other web sites. I've also consulted some catalogues.

I see three main options:

1. UHMW polyethylene
2. PVC
3. Some other plastic such as ABS

Does anyone have any experience working with these materials in quantity, and would know if they warp and melt and machine poorly? Or, does anyone have any suggestions for a plastic I could use? I'd like to spend less than 200 dollars on 8 square feet of this stuff, and I can't have the pieces warp as they need to mesh with one another with some precision.

Different sources say different things, and I don't really know what I'm looking at when sites give me statistics. I need something that won't melt or make burrs to a large degree.

I can buy PVC and UHMWP relatively easily (both cost about the same, ~20 a square foot)

Any suggestions/ideas/condolences?


Thanks,

Ben


Edit #1 - Forgot to mention, the CNC machine I'm using was pretty slick about a decade a go - so it's a little old. I cannot change the speed of the motor, which is (no joke) a hand router motor someone bolted into the machine.

I'm machining this in two stages - a rough pass and a finishing pass. The rough pass I'm running with a .25" straight bit (2 flute) and the finishing pass I'm running with a .125 inch ball-end router bit (also 2 flute). I can adjust the feed rates, to some extent.

[sanddrag]

Delrin machines quite nicely with a 3 flute endmill at 4500 rpm and 5 IPM or so. Might be a bit expensive though. I know the shop here does some gray PVC as well.

[Andrew Blair]

I think you'll find that UHMW will machine very easily and consistently on a router- though you may find you have to deburr edges with a knife. Sanders create this problem sometimes, though I'm not sure what a router would do. PVC I have no experience with, but I do know that ABS machines easily as well- but I doubt you can get the material you need for your budget.

In the same line of thinking, Lexan is an excellent material choice and machines wonderfully on a CNC router, but is expensive as well.

I think you'll find CNC with plastic is a very easy and forgiving thing most of the time.

[Ben Mitchell]

My main concern is the thermal expansion - I've heard that with UHMWP this is a real problem. Is that true?

Lexan I've considered, but is well above my budget.

The heat from this router is my biggest concern - it gets *really* hot in there and the system is designed for small modeling jobs - it simply cannot dissipate the heat a large job like this produces. In the past, I've let the router cool between rough and finishing passes, but it still warped wood. Machining time is roughly 5 hours per square foot. Stepover rates for the roughing pass is 85% and 15% for finishing passes. I could cut down the heat for the finishing cut by upping my stepover but that makes a poorer finish.

UHMWP was my first choice, and I had the phone in my hand to order the materials, but I read an article about it's rate of thermal expansion and decided to ask around.

After machining it will not be under stress, just getting walked on a bit. PVC might be an option but I'm afraid that might melt too much.

[Jim Schaddelee]

Delrin machines very well ,but can be problem if you you are going paint it.
You might try high density foam board (renshape ) not sure of the spelling.
I would also suggest buy a roll of 3M carpet tap .When machining plastics they tend to warp badly.Two sided carpet tap work great to hold the material .Make sure everything is very clean.cut light and fast.

You should post a picture of what you want to cut ,I some different material around my shop.If you have the size I might be able to send you the material.

jim schaddelee

[Ben Mitchell]

Well, I appreciate the offer but I need 8 square feet of the stuff (sized into .75/1" x12x12 squares) - a bit much for me to ask anyone. I'd rather just spend the 200 bucks and buy whatever I need.

Carpet tape I have and it works great - in the past I've used it to mount my workpiece onto a piece of plywood. The plywood, however, warps.

I could, of course, tape it right to the bed of the router - that's a possibility that for some reason I never thought of. Duh.

The amount of material I'm moving is not much (a few millimeters at a time) - the problem is thermal expansion and warping due to heat buildup - I'm almost certain it is heat buildup because the problem does not occur on smaller pieces.

Also, would tape stick to UHMWP?

As far as machinability goes - will UHMWP warp under the heat, will PVC? What is the better of the two options? How fast will they kill my router bits?

I feel like I have a million questions - I have never used this stuff before and want as much advice as I can get before I start working. I'm also on a time crunch in that I need to get this done by mid-april, so prototyping and testing time is limited.

[Jim Schaddelee]

I have may that in scrap.Do you have any color you prefer?( if you want to part with 200 buck give it to you first team )Also what will I do sometimes on my mill is cut the material over size. clamp the material around the outside,do all of your inside cutting .The last thing you do is cut the outer boundries leaving about .005 material. This will keep your part from moving.Use a knife carfully to remove your part from the area tht is clamped.I will look at work tomorrow for material.
I hope this helps.

jim

[Gdeaver]

This summer I replaced allot of trim on my house using expanded PVC boards. You can buy this material at most lumber yards. I had no problems with the router. You didn't mention the application. Expanded PVC is relatively fragile compared to other solid plastics. In the future you may want to explore molding processes. It takes time to master the process, but really isn't that hard. My Wife gets very upset when I use the oven for vacuum forming.

[dtengineering]

I don't know what sort of tolerances you are looking for, but we have had great success machining MDF (although I hate the dust) and Baltic Birch Plywood on our CNC router. The baltic birch plywood held tolerances down to a few thou... great for press-fitting bearings for the shooter on last year's robot.

Not knowing the design and tolerances or application, however, it is difficult to suggest any alternatives. Hopefully you find something that works.

Jason

[techtiger1]

Uhmw machines great but you have to watch the heat a lot and it also likes to flex this is also common with delrin. Uhmw also has the tendancy to burr up pretty good as before mentioned with this material a light finishing pass is generallly required. Lexan machines well but it needs to be used in the right application. I have experience with all three of these materials uhmw is the probablly the most versatile. 1251 started using a new material this year that you might want to look into called ACM or alumnium composite material. This material is a wax based plastic with two thin sheets of alumnium around it. I would be happy to answer anymore specifi questions about these materials.

[Lisa Perez]

I've machined an iPod dock out of PVC without a problem; I'm pretty sure that the item you are trying to machine is feasible. You'll have to look up the speeds at which the CNC can machine PVC, of course, but as for thermal expansion, I didn't have too much of an issue.

(It did have to be sanded/Dremeled quite a bit, but more or less with any plastic, you'll encounter the same problem).

[John Gutmann]

I have machined delrin, ABS and PVC before and they all work great. I would have to say the easy thing to machine was the PVC. I don't remember the spindle or feed rate, but I do remember I cranked up the feed rate and I didn't even have to debur any edges. It was almost as easy as machining those wax blocks. Like everyone else is saying the material greatly depends on your application.

I have heard a ton of great things about Baltic Birch Plywood. I would say your best bet with that is to take off a few millimeters at a time with a larger bit and a fast feed. I would say you should have no problem using a 1/2" or 3/4" bit. Heck if you can find it a 1" bit should be fine. I have taken huge passes with a 3/4" bit through aluminum and never had a problem.

-John

[Kirk]

When machining plastics try hooking up an air nozzle to the machine and point it at the cutter. That will help with the heat buildup on the cutter. You might also have to stop and let the tool cool down.

[Dad1279]

You may want to ty Azek, available in building supply houses. A 4x8 sheet costs about $120, and it routs easily, and can be glued with pvc cements. We have been using it on the robot for bearing/bushing blocks and wheels.

I'm suprised about the heat problems, especially with MDF. I think you need to address that problem, by either taking breaks, or perhaps using a different bit.

Are you using a stright-cut bit? I believe CNC routers recommend an upcut spiral to clear the material away.

[Ben Mitchell]

Application could best be described as a floor mat. Tolerance is as small as possible - less than a millimeter would be fine.

I've actually ran samples of Azek through, and it worked ok, but the texture is filled with little bubbles that gives it a mottled surface, I'm looking for more of a smooth surface.

To give some more detail - I'm making round bumps on a surface. The surface material does not need to support people.

Between roughing and finishing passes I let the machine cool down. In addition, I had two vacuums running - one pulling air away from the bit, another pulling air out of the machine (it's shaped like a closed box, which makes it essentially designed to trap heat.