Community challenge #10, 2019-2020 edition

Dear members of the Fellowship of the Chip,

Have you cast your vote for challenge #9 entries yet? Interesting race going on there. In the meantime, let’s start the next challenge.

Challenge #10 is: Milling plastics

This time there is no specific object theme, the idea is for you to try your hand at milling plastics, no matter what for. Plastics are both easy to mill and treacherous (keep moving that cutter fast or you will be making molten plastic and you might break some bits, I know I did), and it’s not so easy to get a perfect finish quality, so in that sense it’s a challenge, especially if you never tried it before.

The rules for this 10th challenge are:

  • submit your entry in this thread:

    • the project must be made on a Shapeoko or Nomad
    • the project must involve mainly milled plastics. Since I may not be aware of all subtleties of the English language, I’ll say it another way: no engraving plastics, let’s keep those diamond drag bits ready for another contest.
    • you must include pics of the finished piece.
    • you must include the design file (so watch out for any licensed vectors you might use, that might not be shareable)
    • tell us about your mistakes, tips and tricks, etc…
    • posting your project to CutRocket will get you a +2 bonus on the vote tally.
  • you can post multiple entries if you want.

  • timeline:

    • deadline is set to July 3rd, midnight PST
    • there will then be 7 days for voting.
      • voting will be open to legit community members only, and the jury reserves the right to remove votes from “outsiders”, and will also break any tie.

Here are the prizes for this challenge:

Plus some world-famous Carbide3D swag


Let’s get to work, I want to see plastic chips flying!


Is it acceptable to submit an older project? (It is not one that will be a competitor, probably, but might seed the field)


It’s something like 1:30AM for Julien currently, but I’m sure he would say yes as he has before. :crossed_fingers:
Looking forward to seeing an early entry!


Sooo… the sign i submitted for challenge 9 is plastic… haha


Don’t have any plastics in my stockpile and am interested in doing something more 3D.

A cursory look online at the usual suspects for anything thicker than a sheet (thicker than~0.25"-0.75") is looking quite expensive.

What is everyone’s favorite vendor to purchase machining plastics? Specifically block form factors.

I just buy inexpensive plastic cutting boards from the restaurant store.

If you have a feed supply they may have 4x8 sheets of HDPE, and some big box stores have PVC lumber and various sheet plastics stocked.

One material I’ve considered using is old CD cases — I believe they’re polystyrene, cut may be cutable with suitable feeds and speeds.

Another option is PVC pipe — source a large scrap piece, cut it open lengthwise, then use a heat gun to soften it and flatten it.

Or, collect a bunch of crinkly plastic grocery store bags — melt them in a bit of cooking oil, then pour into a mold to make a block for cutting.

List of vendors at:


@enl_public: yep like @MikeG said it’s fine, I have no way to tell anyway :slight_smile:
Seriously though, as you know the point is to showcase a piece while also explaining the process for designing and cutting it, which are the most valuable aspects for other community members, so as long as you do that it’s fine.

@Sbedow7885: don’t make me add a new rule for next time :slight_smile:

@duexx: one of my usual sources for HDPE is Ikea (cutting boards), they come in thicknesses up to 1". No “blocks” there though, I agree those are more difficult to procure. There are 1.5" thick small blocks of HDPE on the C3D store.


Thanks for the info, Will. Those are some creative solutions. I can’t imagine what sauteed plastic bags smell like!

I maybe wasn’t specific enough so I updated my post, but I was looking for plastic in a block form factor. Specifically, I’m hoping to find a 4"x4"x4" cube of HDPE for the idea I have for this challenge

Generally there seems to be a big disconnect in material availability and price online as you move away from sheet stock to something more block like.

Probably silly, but how about surfacing then stacking/glueing several 3x5x1.5" thick blocks? I have no idea whether the glue layers/marks would show after milling. Wait, maybe that’s a…challenge!

I can imagine how a 4"x4"x4" form factor would be really uncommon and hard to find unless placing a custom order (but then the price will not be low)

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Would round be workable?

a 4" diameter Delrin rod 4" long is $39.03 at McMaster-Carr:

Other plastics are less, but available in differing lengths.

Also check Fastenal and Grainger.


They have the 4"dia HDPE by the foot for $US37.04/ft, as well.

You can get 4" thick sheet from them, too. At these sizes, expect issues with material movement due to trapped strains.

Note that the price goes up out of proportion to weight for most larger sizes, except for a few special ones.


If you’re fortunate enough to have a plastics supplier nearby, they almost always have a scrap bin for odd sized leftovers and cutoffs that are attractively priced in my experience.


My go-to for HPDE.


Submission (old project, but why not get the ball rolling) part 1:

Description: Several years ago, I CNC’d my Atlas lathe, just because. This was a very large project, as I rolled my own controller. I dabbled at it for several years, so by the time I heard of GRBL, or got my Nomad, I was better than half way.

Last year, I made a few changes based on my experience, and two of those are presented here:

The ones I will show here are a mount used for stepper drivers to control cooling (Big Easy Driver replaced the original for the leadscrew) and a decent mount for the leadscrew stepper to replace the cranked up sheet metal mount I threw together for development, and left in place since it worked. Noisy like crazy, though, and limited quality of work on the machine. No damping, poor stiffness, and lots of surface area is all bad choices for for mounting steppers.

The main panel has the operator controls for manual use (feed and threading) and holds the electronics.

This is the interior when I was part way through refitting last year. There are a number of parts visible that came out of the Nomad, including a couple circuit boards (the rotary quad encoder control board is not visible in the housing) and most of the parts for the rotary encoder (rough cast on a $US200 3D printer, then machined on the Nomad to tolerance).

First, the airflow guide:

You can see the fan on the left, underneath the leads to the terminal block.

HDPE, milled on the Nomad. The CAD design shows what it looks like better:

Note the form of the channel. The driver is cooled by the copper mass and vias on the circuit board, so the goal was to maintain good flow over the board surface. There are a couple copper pins I added that reach out into the airflow under there. This took the operating temp from at the limit to about 20C above ambient running full current.

The work was done in Inventor, though Fusion360 will do everything I did here, but slower. The machining steps:

An adaptive clearing for the majority of the material removal with a 1/8" two flute, 10KRPM, 1000mm/min feed

A ball end was used for the fancy surfaces. Morphed for the slope, which makes any scalloping be in line with the airflow, followed by Scallop for the more vertical sides and fillet blend:

Then the drilling. The drill operation here was to locate the holes, and they were finished on another machine for practical reasons. A Nomad is not a drill press. The torque isn’t really there for deep drilling, so if it can be finished on a more suitable machine, why torture the poor beast?

(Part two with the stepper mount to follow)


The stepper mount is a different animal. For a number of reasons, I wanted to make it easy to disengage the leadscrew drive mechanically. I did this using a face-pinned coupling (think a finger coupling) so the stepper could be slid off easily. The mount facilitates this.

The upper portion is slotted and can slide axially on the lower riser. The upper part (the mount) is 6.3mm acrylic cemented together and the riser is HDPE.

The mount was modeled as several parts but was cut as a single assembly:

Machining it started by drilling the holes that would be threaded for the stepper mount screws.

followed by windowing the parts

1/8" two flute square end tool, 10KRPM, 1200mm/min. Really should’ve gone a bit faster, but I used a vacuum to flow the chips out so it was fine. The tool here was TiCN coated. Nice and slippery.

Then the majority of the isolation cutting was done with an adaptive. Slotting is annoying in acrylic. Use a controlled engagement strategy if you don’t want to destroy tools and work.

This was done with 0.5mm axial leave, and 0.1mm radial. The axial holds the parts together, and the radial allows for a nice final finish pass. Glue-up is easier if the surface has a clean finish. THe adaptive strategy leaves a lot of entry points as it gnaws away.

Then the separation cut, with tabs to control breakout. The material was taped down to avoid the cuts, so there was less hold that desirable. Doing it this way keeps good support until the end, where these are very light cuts.

This was going to be the test part, followed by a final one with alignment tabs for assembly correcting any errors, but it turned out fine. I had to cut back the tops of the gussets a little due to interference with the stepper, but that was it. Yes, the stepper is up in a flexible region, but there is no load or vibration out of the plane of the sheet.

The riser block worked out really well:

Three guides for the slots, drilled for the hold-down screws, and counterbored holes to mount to the bench.The height here is critical, as it provides one dimension of axial alignment. This got fixture mounted to a bed-of-holes plate, and rerun to get the height dead on.

First the perimeter and top were roughed:

Then the drilling started. All holes were piloted with 0.85mm drill first. Fast peck cycle to get the chips out.

Then a larger bit to clear room for the tool to do the counter bore. Not truly needed, but makes chip control a lot easier. Note that the holes to this point only go part way through. The bits are short and small, so no desire to push it

The counterbores

There clear drill the rest of the way. The bits are long enough to make it the rest of the way and there is room for the shank

and bore out to size for the mounting bolts

The three holes for the hold-down screws on top will be tapped

Last is the finish sizing of the locating nubs and the counterbores.

I don’t have any shots of machining these, unfortunately.

The Inventor models, for anyone interested: (1.6 MB)


Epic write up, thank you !

1 Like

So I will start this contest entry with the following quote and i think it will set the scene for my entry.

"A person who never made a mistake has never tried something new" - Albert Einstein

If this is not a valid entry happy to have it moved :slight_smile: and will take no offence as it is not complete. However i wanted to show my sons, that even in failure we can still achieve allot !! and that our failures need to be celebrated as equally as my successes.

My design was simple in my mind at least, i wanted to make something for my office that was useful, so i liked the idea of lit LED Plexi signs, and ended up on a small Calendar with removable months and days that i could swap out.

These would then sit in a wooden base that has a hollow bottom for the electronics and slots for the LED light to shine up and illuminate the tombs.

These are the two tool paths to hollow out and then to chamfer both the top and bottom to give the base a nice uniform (almost oval) appearance

so far so good!

Now things go badly wrong i purchased 2 x Plexiglas (one sheet as backup) 3mm thick. Worked with the feeds and speeks and looked at at Winston’s C3D video on suggested cutting speeds and came up with

Started the cuts and things looked really good ( i must admit i gave a little fist pump)

as you can see bottom left is nice and clean, top left we have a problem, this boils down to the cut building up heat i believe.

then after a few more seconds of running started to to encounter molten issues

and molten issues on my bit

tried new Feeds and speeds after some research, in an attempt to clean up the cuts but this only ended in further disaster lol

followed by “OH SNAP!”

So i got a new bit and second sheet but things just turned out the same again - Heat is defo my problem but i cant figure it out, below shows the cuts i am getting. Though interestingly the contour cuts are amazingly clear, and looked good whilst cutting, but towards the end of the cutting job it was starting to get a buildup on the bit again!! argh!!!

Close up of the edge - really happy with that for sure, but same F&S as the rest so confused why different results

Another issue was cutting integrity something i have not really encountered, by this i mean i obviously did not leave a big enough gap between the tombs to allow the sheet to retain rigidity

It started to pull up the pieces when cutting, i used a bit of scrap wood to hold it down and shift while cutting. (really i should have stopped but was worried that the Plexiglas might shatter) .

So i took some of the best pieces and tried a simple test to see if my design principles at least would work and to be honest i was very happy with the results - if not the finish. the best was number 3!

so in short - i am glad i tried this and stuck with it over the two days, even tough i really struggled with this process. I think if i could get more stock in time i would try again, but will miss the deadline at present, and therefore i humbly submit my entry, even though like my wider journey in CNCing, it is not complete.

Again and for clarity will not take any offence if this is not permitted, i would though ask if it could be moved over to main shapeoko thread.

Thanks and thanks for this challenge it has for me at least pushed me into an area i would not even consider!



Used some of my wastage as suggested by @enl_public, I think I have found the spot, it just feels the feeds and speeds are way to fast but below shows second try on left original on right

so try a few more things today.

Even a poor plan executed with ferocity can provide results - my new motto


I was worried I would be the only entry.

This looks like a really good start. Feeds/speeds for acrylic can be a bit squirrely to find. Plan on test material. A summary is: aggressive enough to get the chips out and not heat the tool too much. Engraving is tough because you have no choice but bury the tool, but there are a few threads dealing specifically with engraving acrylic.

Curiosity- feel free to say none of my business- Why can you not get more stock?


I’ll add that:

  • when things do not work out in plastics, feed and plunge faster.
  • single flute endmills are your best friends in plastics
    In your case I would try and plunge twice as fast.
    Thanks a lot @Sherpa, this is exactly the spirit of those contests: learning together

EDIT: to make you feel good, here’s one from my first cuts in plastics:

To elaborate on what I was saying above, I found the hard way that you can mess up a cut in plastics even with good feedrate/RPM if you stick to the usual (very conservative) guidelines of slow plunge rates. And then you end up melting plastic during that first second when the tool enters the material, and after that all bets are off.

Best case scenario, you just get some plastic strings wrapped around the shaft:

which is usually no big deal (unless your job has a lot of retract/plunges, and these strings start to accumulate), and worst case scenario, well c.f. first pic.

Single-flute endmills are cool because they allow for much better chip evacuation, hence fewer chances to have chips accumulating, being recut, getting pissed off at not being able to enforce social distancing, and then melting out of pure rage.


Is there a way one can tell something about the quality of plexi?
I read on another forum (dont know where anymore) that high quality plexi has a better heat resistance whereas low quality (from the hardware store around the corner) tends to melt quickly.

And thanks for that summary! I’ll be really careful if I try plexi in the future!