Understanding the best bit for plastic/HDPE

Confession first: I’m a wood guy and cutting plastic is way new to me. I recently cut some dividers for my RamBoxes (it’s a Ram Truck thing) and they worked great so I agreed to help a neighbor with a project. He needs some 1/5" diameter holes cut into 1" thick plastic (I think it’s HDPE - basically a really thick cutting board) so I did my first one and it came out really nice but I was using the 1/8" single flute straight bit and it took almost three hours.

So, now the next one is 4 times as big which makes me rethink the 1/8" bit. I’m searching for a larger bit - would a double flute straight bit work? I’ve got a Freud 1/4" one but I’m not sure if the double flute is a good thing or if I need to stick to the single flute. Or - is there something else that would work better/quicker.
Thanks - John

I’m used to cutting PMMA (plexiglass) and I gained my experience the hard way with broken bits and bald spots on my head. Cut speed affects results. The math is how much material needs to be removed per bite (called chips) to keep things cool so you don’t melt the material and create swarf which will gum up your bit.

Linear speed and RPM will control how much bite your bit will cut. No enough movement or too much RPM and heat builds up on the bit because your chip size is too small. Too much movement or slow RPM and your are biting more then you should.

Double flute vs. single effectively doubles your RPMs reducing your chip size if you don’t increase your rate of travel. It took me a while to really get it in my head that I wasn’t going fast enough. (I had a 10k RPM router and double flutes.)

Another thing I might suggest if you are cutting through holes, just cut the outline of the hole. If you are cutting pockets (as in your picture) it’s gonna take a while.

Grab a test piece and start playing with the numbers (speed & RPM) until you have problems. Use a notebook and record results, you will appreciate it in the future. Make note of the Router, Speed setting, Material, Bit size and type, and even it’s sharpness. Wood you will want to note the season or humidity/moisture level.

Good luck!


Wow. Thanks for a ton of information to process. I’ll have to do some thinking on the speed vs. size relationship but it makes sense that those would be paired in the formula of success. I also need to do the notes like you mentioned to track success and not so much success. Thanks

Even with a single flute (1F) you may wish to go pretty fast, and keeping the chip thickness the same with 2F you either half the RPM or double the feed rate. But - since this is a tight radii carve any real feed rate may be reduced by acceleration value due to the constant change of direction.

Since you need to carve 1/5" holes in 1" plastic I would stick with an upcut, ideally running a helical ramp manouver.
Easel however dont support that.

Haldor, Thanks for the info. The holes are 1.5" diameter and 1/4" deep and the first test cut well but I just need to reduce the overall cutting time as the next board is about 4 times the size! I’ll play with adjusting the feed rate. Thanks again.