A tale of coasters, epoxy, user errors, and edge finders

A few days ago I went in the shop with the “let’s make something to beat procrastination” mindset, and thought I would try to customize some coasters I bought that were collecting dust. I also had about 200g of epoxy left (i.e. not much), so I ordered more and proceeded to make epoxy coasters with that leftover.

And then yesterday @MikeG posted about his great see-through epoxy coasters, and he could not have done a better job at documenting it for other people, so go read that thread if you haven’t already to know how to do this the right way, it happens to be EXACTLY what I wanted to try :+1:

While my version did not go so well, I thought it would still be interesting to share the process and my mistakes (and maybe ask for tips from seasoned epoxy users)

So, I started with this set of 4 coasters I had bought from a store, it’s Acacia wood.

Tape & glue for workholding as usual,

Since my main goal was to test the “epoxy all the way through” approach, I settled for the simplest pattern, concentric square slots, 1/8" wide, in an outer square matching the measured dimensions of the coasters:

I measured the coasters to be 10.35mm thick, so I set the toolpath total depth to 9.5mm, leaving a bit less than 1mm of wood on the bottom, which I would later remove after flipping the pieces, to reveal the epoxy on the second side. Anyway, that part of the cut worked fine, 1/8" 2-flute endmill, 24000RPM, 80ipm, 0.03" per pass to be super cautious:

Light sanding on the top surfaces to clean the tearout here and there before pouring the epoxy:

Then I prepared little epoxy dams with the intent to not let the epoxy overflow and mess up the side walls (spoiler alert: it did not quite work):

For some reason my initial thought of going for translucent epoxy turned into “let’s make it pitch black for contrast”…which is really, really silly considering the whole point was showing that epoxy was going all the way through, and opaque epoxy is not a good way to convey that. ANYWAY…epoxy was poured:

At least I remembered a tip from @fenrus to come back multiple times to pop air bubbles in the epoxy. In the “torch vs heatgun” debate I chose a side, and went to the kitchen to borrow the Creme Brulee torching device. I had to torch the epoxy surface 4 or 5 times, 5 minutes apart.

And now for the easy part: running a surfacing pass to remove the extra epoxy. 6mm 2-flute, 10k RPM, 70ipm (or faster), 1mm-deep pass, that I run as many times as required, each time lowering the Z zero by the required amount to shave off just enough material.

But you know what happens when you touch off to set Z0, but forget that ONE click to actually reset it in the G-code sender in the heat of things ? MAYHEM! :sob:

At least I know I can easily cut 10mm deep at 3mm stepover in acacia wood…the machine did not even flinch during the time it took me to hit the E-stop. But how good is a set of THREE coasters?..sigh…

I used that messed up coaster to at least check that my surfacing pass was correct (when I set zeroes correctly, that is):

And proceeded to surface the three remaining ones with no additional drama.

But there was the other issue that epoxy had found its way between the painter’s tape dams and the coaster walls, leaving epoxy stains around all edges. Now than I think about it, the heat from the torch as well as the heat generated by the epoxy when setting must have softened the glue on the painter’s tape, just enough to let the epoxy overflow a bit :man_facepalming:

I really, really did not feel like manually sanding that off, so I went to create a contour cut to shave off material, but I had no precise reference for where the center of each piece was located. This was the perfect opportunity to finally test this edge finder I had bought a few months ago:

I’m an edge-finder newbie, but love them already. The way it works (well this type at least) is that when you spin it (slooowly) and it doesn’t touch anything, the bottom part of the edge finder will wobble (wiggle?), then when you move them (very carefully) against an edge, at some point the wobble will stop, and when moving a tiny bit further, there will be a sudden visible shift of the bottom part: that’s when the edge finder is exactly [half the diameter of the edge finder] from the edge, and one can then make a note of the absolute machine coordinate there.

Here’s a video from the side, during X-finding of a right edge. The final shift is not very visible as it is in the front/back direction:

Here’s another view while finding a Y edge, with a better view of the final shift to the left:

In both videos I was making 0.1mm jog increments (I did not need more precision, and my edge surface was messy anyway). I will note that edge finders usually have a very low max RPM limit, this proxxon model is to be used at 500 RPM. I could not have pulled this off with a router, so I’m thankful I have a water-cooled spindle I can run at arbitrary slow RPM. A little M3S500 command in the console is all it took.

After (tediously) finding the left, right, top abd bottom edges of each coaster, I determined the coordinates of the center for each of them, and CAMed several profile cuts: a first one following the outer size of the coaster, and then smaller ones by 0.1mm increments, as I did not know exactly how much material would need to be removed to get rid of the epoxy stains.

I ended up requiring shaving off 0.5mm, and I got clean walls and no visible X/Y shift that could have made one side thinner that the others, a good sign that the center-finding exercize worked.

Time to flip the pieces, to remove 1mm of material (0.85mm of wood + margin) and reveal the epoxy from the other side:

Did I mention I had forgotten to seal the wood with clear epoxy before pouring the black epoxy ? Lucky for me that particular acacia wood does not seem to be prone to bleeding, so I dodged that bullet (which would have completely ruined the project and my mood)

Finally, some manual sanding down to 1000 grit, a little oiling, and I called it done.

While these won’t win any contest, I’m still happy I did it:

  • I definitely want to make more of those pieces where the epoxy is visible on both sides. As @MikeG showed in his thread, this can be used to produce very cool looking objects with tiny wooden parts “floating” in the epoxy.
  • @MikeG or others using masking tape to contain the epoxy: did you ever have that issue of epoxy still finding its way between the wood and the tape ? Masking works just fine when it’s done on a flat surface, but I have yet to find a method that works reliably when one needs to make a “vertical” dam as I tried to do here.
  • I discovered the value of edge finders, to salvage a piece. That proxxon device was well worth the 35 euros it cost me.

Don’t hesitate to share your tips and tricks, and I’ll try to not make as many mistakes next time!


Edge finder note: perhaps no one will take notice of my super secret method for using an edge finder with a high speed router: a low speed motor with a belt. :smirk: You’re welcome.


Awesome end result @Julien and great write-up adding a few pointers that I had missed!

Did I conveniently forget to mention that one of my coasters didn’t turn out so well because I forgot to change the outside profile cut from inside left to outside right? D’OH!

In other words, your version turned out great!

One question about using the PROXXON edge finder…why not just use the C3D BitZero to find the edges?

As far as masking the edges, I’ve had the same experience as you’ve documented here which is caused, I think, by the porosity of the material. That darn epoxy is going to seek and find any void, gap or low spot. And it does so so agonizingly slowly that you think “This looks great!” only to return 30 minutes later and see a low spot in the epoxy and the dreaded outflow. So I only use the masking on the flat horizontal surface and rub it (or use an art roller) to get the edges down tight. My experience is that the surface tension of the epoxy mostly keeps the containment manageable using this method. It looks like your pieces had room on the edges to do this?

Optionally, sealing the edges really well before fastening the pieces to the spoilboard might give the masking tape a solid grip area, maybe worth a try?

Also, if you’re using enough heat to melt the masking tape, that’s way too much in my experience. Very short bursts, or in your case with the Creme Brulee torch, pass over the material very quickly to prevent scorching. I also have had much better success holding the flame straight down and being patient checking and popping bubbles every 10-15 minutes. Ambient temperature will dictate this of course.

The really satisfying part is when somebody picks the finished piece up and you catch them wondering “How’d they do that?” :slight_smile:


Also really like the reflection after your epoxy pour of one of your previous projects “Keep Calm and Machine On”

And where did these nice acacia coasters come from, another Ikea item?


Because I needed a reason to use my new edge finder ? :slight_smile:
With a square stock like that I could indeed have used the probe!

I concur. Works great on horizontal surfaces.

Indeed. In any case, starting from a larger piece of stock and doing the contour cut after the fact is much, much easier and efficient and makes this problem irrelevant too. It just so happens that I wanted to cut those preexisting coasters.

Yeah I may not have been very light handed with that torch. I did not burn the tape but might have heated it enough that it did not hold so well.

Exactly what I am aiming for :slight_smile:
And the new delivery of epoxy showed up in my mailbox this morning, so I think I’ll start another run soon and put what I learned to good use. I have lots of pigments I have been wanting to try too.

Not this time, they came from “Habitat”, a local home furniture & decoration store. Those four coasters were sold for around $15, not dirt cheap but affordable enough to allow for silly mistakes and support my learning experience :slight_smile:


Julien you risk making your significant other mad using the kitchen torch in the shop. I thought you were smarter than that rookie mistake.


A lot of people who do resin work use pretty much everything and anything to make walls around their molds. Everything from strips cut from plastic bottles to modeling clay to posterboard, foam core and even rigid insulation. Scrap wood strips set up as a box around the casting also works. Hot melt glue to hold those structures together is a must have. I usually use Alumilite products, which have some very good tutorials on their website and they are very responsive if you need help or advice.

@gdon_2003: nah I’m fine, I’m the one doing the cooking anyway :slight_smile:
@crokinoleworld: thanks, they have nice tutorial videos indeed. In their video about making casting boxes, they talk about using a variety of materials held with hot glue, it makes a lot of sense but only works when the box walls are a little distance from the piece. In this case here I was aiming for setting up a containment that would hold the epoxy on the top surface of the piece only, protecting the sides. But it’s probably more effort than is worth, I have a CNC after all and milling the sides after the epoxy hardened is much easier.

Doesn’t cutting epoxy dull the bit faster? I use routers/cnc for flattening it, but cutting groves has got to get some serious heat going on in there…have you found your bits survive “trimming” well?

I did not observe the cutting edges up close, no. I would not expect epoxy to be extremely abrasive but I honestly never gave this any thought. I naively went with “this must be similar to cutting something between acrylic and HDPE”, which cut like butter when going fast enough. Have you had issues with premature wearout of bits while surfacing epoxy ?

No…but I’ve been advised against planing it, as I’ve been told it dulls the knives - and so I extrapolated to CNC cutting. I haven’t done either based on being told this would happen…so if it’s not true, that would be very interesting.

1 Like

I see. I never stumbled upon that warning, but I know one thing for a fact: I’d rather wear a cutter or two than ever going back to manually sanding epoxy off, which was the worst experience ever :smiley:


I believe epoxy is just plastic and I’ve cut quite a bit treating it as plastic with no issues. However, if you use any paint with glitter, that’s a different story as it is metal. Aluminum as near as I can tell by my highly scientific method (not) of placing a strong magnet nearby with no affect pulling the glitter to the side or top.

1 Like

Gary - I’ve run pieces through the planer with the only negative being the mess the plastic shavings make. But I don’t have great dust extraction setup and only do that outdoors.

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.