Has anyone used 1/2" HDPE as a fixture for cutting aluminum plate? I am going to be cutting a bunch of 1/2" aluminum tooling plate and I will need some sort of fixture. My plan is to edge clamp the plate on top of this fixture while I drill and thread mill a bunch of M6 holes. Then use M4 fasteners through those holes into the fixture below while I pocket a bunch of material. The plate size is about 10" x 17". I am looking at HDPE because I will need 8 different fixtures and they would be a bit expensive to make out of aluminum. I am trying to minimize machining time so I will be pretty aggressive with my speeds & feeds. The target roughing speed is 96IPM 24000RPM 0.101" DOC at up to full width using a 10mm Datron single flute 4 in 1 with air blast/IPA coolant. That works out to about 3.8 cu in/min removal rate. I have tested those speeds on the machine and it works great. I’m just not sure if HDPE would be up to the task as a fixture.
I’d pick delrin (Acetal) over HDPE. Much easier to machine and much stiffer.
Acetal (polyoxymethylene) is a lightweight and low-friction polymer commonly known as Delrin
Unfortunately, Acetal is more money than aluminum in the size I would need for a fixture. Doesn’t make sense if the point is to be cheaper than aluminum.
Nope. In a more helpful mode, I would ask several questions: What does “a bunch of 1/2” aluminum tooling plate" comprise? What grade of aluminium are you going to be using?
How many pieces of aluminium are you intending to machine?
Prima facie, that is a good plan. However… 1/2’ thick metal has very little flexibility. HDPE may not provide sufficient rigidity to hold the workpieces stable and in the orientation you need. The workpieces could potentially slip while being machined.
It may be a false economy. The expense could be passed on to your customer and you will feel happier if you quote the true cost of the job to yourself.
The foregoing sentence alone… tells me that HDPE (while it may work) may also cost you both time and money. Given your intention to get the maximum performance from your machine and the workpieces, it should be clear that using HDPE may well militate against that desired machining strategy.
0.101" equates to 2.54mm DOC and this DOC will take place at a feed rate of 2438.4 IPM. I know that Datron cutters are outstanding. I think your ambitious feed rate/DOC would potentially be managed more easily if you used aluminium for your fixture plate.
Since the job may be repeated in the future, especially if the customer likes your work, it would be helpful and wise to spend the money initially to assist you in getting it right in the first instance. This is just my opinion… please take it or reject it for what it is worth.
For whatever my experience is worth, I’ve used hdpe as soft jaws for milling aluminum but that was on the nomad…which I would consider virtually zero force compared to what you’re trying.
So my guess based on nothing but gut feeling is that the hdpe plate would be ~fine in terms of work holding but I’d somewhat expect the surface finish to suffer for it.
If it were my own project, I’d opt for trying the cheaper hdpe and except it to go well enough but if any other factors come into play, I’d probably lean towards the mic6 or similar.
Out of curiosity, what are the material prices you’re looking at? I’m going to get a bigger machine eventually so I figure I’ve got to prepare myself emotionally for the scaled up cost as well 
I am looking into getting some MIL-I-24768 for our SO5 Pro at work for the wasteboard and some fixtures as well. There are a bunch of sub-types, but the two I am interested in are:
/15 - phenolic linen (FBI)
/16 - phenolic canvas (FBM)
https://www.norplex-micarta.com/electrical-materials/electrical-industry-standards/mil/
The machinists say it cuts like butter and those two are pretty thermally stable and have very little dimensional change due to moisture absorption.
Not sure of the cost yet since I emailed the supplier and went on vacation. I know the largest sheets I found between 1/2" and 1" thick are 36" x 48". You can get it up to 2" thick, maybe thicker?
The stuff is for me and not a client so I decided to try some HDPE. I get most of my material from midweststeelsupply.com. In this case it is 30 plates of cast jig plate (ATP 5) which is similar to cutting 6061.
I think it would work well given you keep the heat to a minimum as it sounds like most of the forces will be minimal until you bolt it down and distribute the forces over more hardware. You might consider using Keenserts where you will be bolting a lot, as HDPE threads might wear/strip fast/easy.
I would also suggest using larger tooling pins in the HDPE as the X and Y locator instead of the HDPE itself. That makes the high wear and force areas more durable. The other option might be using something like the stops in the Crush-it Kit since you mentioned using side clamping anyway. 
That is a good suggestion, or use press fit drill bushings for plastic, depending on how many cycles you need the tools to last.
Not sure if this is applicable but 3dprint the holder? I do a lot of small metal work holding on 3dprinted jigs and holders. Super cheap, and quick to make. 3dprinter and Shapeoko are a great combo.