help with cutting hdpe plastic

[jellyam]

hi folks, I recently got a new job where i'm going to be CNC machining a number of new materials,i was machining ice before, and I had a machining job come down the pipeline where i have to cut 1/4 in black hdpe plastic.

I'm sure i got a few things wrong here, For the job i'm using two bits an 1/8 inch ball nose bit (i think it came with the shopbot starter bits) and a 1/4 inch bit for drilling some of the holes. when i ran both of these bits i had the hdpe chips tangle up in the router bit, causing me to stop every so often to clean it off.

the 1/8 inch bit also broke on me when the job was nearly done, i'm running the spindle speed at 12000 rpm and the cutting speed for the 1/8 inch bit was 2 inches per second (I'm using v carve pro).

if you look in the picture the hdpe is sitting on top of a foam sheet and is taped down with a strong double sided tape but i don't think im going to continue doing that in the future

any suggestions would be greatly appreciated

25702

[steve_g]

Josh…
A bit designed for cutting plastics will help… The “O” flute is designed to roll the chip into a ball and throw it clear of the tool.
Here’s some quotes from some manufacturers web sites…

SG

“The highly polished open flute not only allows for easier chip formation and evacuation, but also provides for a sharper cutting edge”

"O" flute tools create the perfect curled chip that allows the softer plastic to be cut without chip rewelding. Ideally suited for plastic materials that form curled chips as opposed to fractured chips.

[bleeth]

For drilling only sometimes the best answer is to chuck up a drill bit and run spindle spin as slow as it can go and make your plunge max at .50"/sec.
There are drill bits made for plastic also that have a bit of the edge filed so they don't grab and pull the material as you go through.
You didn't say what you are using the ball nose for. If you broke it at those speeds you were probably cutting too deep although a slower spin is likely called for.
Second on using an o-flute for cutting plastic.

[knight_toolworks]

I just cut 12 sheets of 3/4" hdpe. man that stuff is heavy. 0-flues work well I used this o-flute that was or phenolics because they did not work on that material. I drilled and poked it and cut through wit that bit. the d=trick with hope is keeping it in place. ˆ found anything under about 14x14 may come loose. so I leave a couple hundredths onion skin. the material is somewhat grabby I don't cut it fast maybe 1.5" a sec with the 1/4" bit.

[jellyam]

thanks everybody for the advice, I'll definitely be taking it.

the only reason I was using a ball nose is because it was the only thing i had on hand to cut 1/8 in