I seem to be at odds with how to proceed with 3/4" HDPE.
In order to cut to depth, I need to use a long cutter which introduces flex meaning I have to slow my feed rate. However if I go too slow on feed rate, the material will melt. This creates an interesting issue when I’m having to use 1/8" mills.
I guess what I’m really trying to decide is if I just cut my parts out of 1/2" plywood or MDF and use those as router templates and just take everything to my router table with a template bit to cut the final products and then drill press all the various holes I need.
Would your design allow rest tool pathing, I had same issue and found the 1/8th passes were ok after the 1/4 had done the heavy cutting, as the doc was max then looked to use hsm toolpath
Going to 1/4" bit would be ideal but I’d have to up size my stock which adds a noticeable amount to the materials cost with a lot of waste. I only have about 4.5mm between some of my parts now which is enough for the 1/8" bit but not enough for a 1/4" bit.
Not familiar with the term rest tool pathing. I’ll have to look that one up.
Of course that also means I can’t cut my drill holes. I can cut the pockets for the heads and maybe do a touch off to mark them for drill on the drill press but can’t go too deep since they’re 1/4-20 and 1/4" drill would be too big.
I work in the marine industry and make use of HDPE, Acrylic, and Aluminum.
I use a uncoated .25" single flute cutter for almost everything I do. And make as little use of .125" cutters as possible. Standard flute length is .75" on the .25" cutter, and handily cuts .75" HDPE. I’d also suggest that rubbing is not much of a concern with plastics if taken into consideration. In fact, the cutters I use, rub to some degree by design. So, I’d imagine you will be fine using a .125" mill with typical .5" cutter length. I’ve personally never had an issue with HDPE or Acrylic melting, and while I have airblast/mister, I never use it. I do use a vacuum. Outside of holes that can be helical milled, I spot everything on the machine, and drill on a press or with hand drill.
I do not consider the Shapeoko particularly capable of producing an excellent edge finish on HDPE, and when a superior finish is required, and the geometry allows, I prefer the finish I get from a router table, handheld router, or jointer. I use guide bushings and templates whenever I can. For parts with straight edges, I will machine oversize, then joint the straight edges to specification. The radius corners, with their minor extrapolations, typically hide tools marks well enough.
Depends a lot on what your time is going to cost you vs material. If I was making a bunch of these, I’d do the material vs time calculation first. If time is cheaper, clearly a set of templates and a router is going to get you there.
If you can cut it with a router, you should be able to achieve the same finish with the shapeoko, but it will be harder to home in on. Template or not, if you’re trying to cut ¾" and not able to get the speed/feed right you’re going to get gummed up tools and melted material - that really should be a wash. Remember, you can use router bits in a shapeoko, you just can’t effectively use router bits with a bearing.
Going down the template route would be far too much effort in my opinion - you have a CNC router, why not use it?
I wouldn’t worry about flex too much - I regularly cut 32mm thick hardwood right through. Of course you need to be sensible about feed rates.
Can’t you do most of your work with a 1/4" bit, then mill your 1/4-20 holes with an 1/8" bit?
I have a Bitsetter and the task of changing bits is brilliantly easy - some jobs I use 3 or 4 different bits.
If you are going to thread for 1/4"-20, have you considered getting a thread mill? Do the whole job on CNC! I just purchased a little tiny M3 thread mill and have been having fun with that in aluminium. Check out my first go at threading M3 holes:
