Epoxy Granite Machine Frame || How To
Updated: May 25, 2019
Epoxy granite makes an amazing machine base, but there is a bit of work that goes into it. This is a full tutorial on how to achieve fantastic results. It's broken into 4 parts. 1. Theory 2. Experimentation 3. Build
Building a solid base for machinery can be a daunting task. The base needs to be strong, stiff, absorb vibrations, and be mechanically accurate. This is a lot to ask.
An metal frame would satisfy strength, stiffness and mechanical accuracy, but would perform very poorly in absorbing vibrations. This is quantified by a material specification called the loss coefficient. Loss coefficient is the internal friction of a material. Every wonder why cymbals are made of a copper alloy? Incredibly low internal friction, so they will "ring" for a very long time.
Below is a loss coefficient chart with a few common machine frame metals called out. It's plotted against Young's Modulus (Stiffness).
The higher the loss coefficient, the less it will vibrate under impact. See copper (Cu) way down there at the bottom? Three popular metals are highlighted; aluminum (Al), steels and cast iron. All have very poor loss coefficients, but are very stiff.
The high loss materials, elastomers, foams, and polymers all have low stiffness. This means you can't have both a high internal loss material, and a stiff material... right...?
Combine two materials, one with stiffness properties and one with high internal friction. This is where epoxy granite takes over as a superb machine base material.
For my designs, I use an aluminum frame to provide the stiff skeleton, and the epoxy granite to provide the shape and vibration damping properties. Best of both worlds, high stiffness, and incredible vibration damping:
This looks composite frame is shown below. The reference surfaces are all made of aluminum, and machined to exact dimensions. However, the epoxy granite material fills in all of the rest of the space, providing the vibration damping, and also looking pretty good.
The aluminum reference surfaces are all joined by an internal aluminum skeleton frame. The epoxy granite is then poured around the aluminum skeleton to form the complete machine base.
Next up is to find the best ratio to create the epoxy granite mixture. The goal here is to minimize the amount of epoxy needed since it is expensive. For my experimentation, I played around with the proportions of 4 different ingredients looking for the best ratio.
1. Play Sand. Nice and simple, can be purchased from home depot, gravel supplier, pretty much anywhere.
2. Rocks/gravel. Can be purchased from most landscaping yards. Any variety is fine, round or smooth. You want to make sure the largest rocks in the gravel are less than 1/3rd the width of the thinnest section of your casting, otherwise you may have flow issues.
3. Dye. This is used to turn the mixture to a darker color of your choice in order to improve aesthetics. Black iron oxide can be used (Fe3O4, purchased online), epoxy dye, or any paint pigment could also be used.
4. Two part epoxy. This is binder that holds everything together. Any brand will work, the slowest cure time you can find is the best. Thick molds can take up to 48 hours to cure. The slow cure time helps trapped air rise to the surface letting you achieve a smooth wall finish.
A few basic tools help make things easier. Here is what you'll need
Epoxy granite can make a bit of a mess, and will most likely ruin whatever you are going to put it in. The high top 5 gallon buckets help contain the mess and splashing during mixing
Drill and Mixing Paddle
You could do the mixing by hand, but in large quantities, it just won't do a satisfactory job. The key to a good machine base is a solid mix job. Using a drill and mixing paddle takes the guess work out of it and ensures a thorough mix.
Drill Link: https://amzn.to/2YIDu1l
Mixing Paddle Link: https://amzn.to/2VWJIxE
I'd also highly recommend some gloves, makes clean-up a lot easier.
A shaker table is a nice to have, but not necessary. What it will do is help remove more of the trapped air from the molds, especially if they're more than an inch or two deep. You can get by without one, but if you want the best results, I'd recommend one. There are two options
Option 1: Buy it https://amzn.to/2JXI6N7
Option 2: DIY
A DIY version can be quite a bit cheaper, and you can customize the table size to meet the project. There are 3 simple parts you need to buy, totaling around 100$.
Rubber feet, a plywood deck, and a shaker motor. Simply bolt the feet and shaker motor to the plywood deck. The motor will generate the needed vibrations, and the feet help isolate the vibrations to just the platform.
The deck thickness may need to be re-enforced depending on the weight of the machine base. Start with 1/2" thick plywood, and you can always double it up if needed.
Rubber feet: https://amzn.to/2VXPFKS
Plywood: Home Depot
Shaker motor: https://amzn.to/2YIHE9t
Ok, let’s get into the experimentation. I’ll go through all my failed attempts and discuss what worked, and what didn’t. In total I ended up experimenting with 7 different samples, changing the blend ratios of the ingredient before I finally settled on a solid contender.
59% Sand || 29% Rock || 12% Epoxy
Decent result, although there was a bit too much embedded air that resulted in a lot of cavities. Vibrated for first 30 minutes during cure.
58% Sand || 29% Rock || 13% Epoxy || Few drops of blue dye
Blue dye worked really well, but there are still too many embedded air pockets. Vibrated for first 30 minutes during cure.
57% Sand || 29% Rock || 14% Epoxy || Black Dye
This time a vacuum pump was used to try and pull air out of the epoxy test piece. However, what this ended up going was fluffing up the mixture into Swiss cheese, since the vacuum pump was unable to fully pull the air out of the mixture. This did reduce the density by about 25%.
With more experimentation, and perhpas a pressure chamber, this probably could have had a more successful result, but the final machine base was too large to vacuum pump, so this idea was abandoned. Sample was also vibrated for the first 30 minutes during cure.
45% Sand || 17% Fine Sand || 24% Rock || 14% Epoxy || Black Dye
Sand was put through a sieve to separate out the smallest particles to form the 17% fine sand. This resulted in a much smoother test piece, but there was still voids on the surface. Vibrated for first 30 minutes during cure.
69% Sand || 17% Iron Oxide || 14% Epoxy
This sample came out great (the white on the ends is the wax from the mold release). However, I couldn't find large quantities of iron oxide for cheap, so the prototyping continued. Rocks were eliminated from this sample, since they were causing a lot of voids, and the aluminum skeleton would the stiffness needed. Vibrated for first 30 minutes during cure.
85% Sand || 15% Epoxy || Black Dye
Now down to just 3 simple ingredients. Starting to get really close here, but needs a bit more epoxy to fill in the voids. Vibrated for first 30 minutes during cure.
80% Sand || 20% Epoxy || Black Dye
A beautiful result with a perfect surface finish. A solution was found. Vibrated for first 30 minutes during cure. It turns out keeping things simple works the best.
3. The Build
Layout all the pieces of the mold and skeleton, and wash with soapy water to clean
Assemble the mold by fastening all of the pieces together. Epoxy will stick to almost everything, but it will not stick to wax. For this reason, a wax mold release was used on the mold. Apply the mold release to the entire surface, make sure to let it dry. Wipe off the excess mold release, and then repeat the application. It is key to not miss any spots.
Mold Release: https://amzn.to/2HCAq1b
I also used wax fillet sticks, and carefully molded them into the sharp corners to create the internal radii that will produce smooth transitions between all of the surfaces. Sharp 90 degree corners are a recipe for disaster, filling them in will be hard, and they will be prone to chipping.
Insert the assembled skeleton into the mold. Use tape to cover any screw holes that may be needed later. Make sure the skeleton frame doesn't get any wax on it. (Note, side panel of mold wall removed here for photo).
Now the mold is built and waxed with internal skeleton frame in place. Double check that all mold surfaces are waxed, and any threaded holes that will be needed later are covered with tape.
Prepare the ingredients and the mixing tool. Here is the order I went about mixing:
Pour in 80% sand by weight into bucket. You can use a bathroom scale to do this.
Pour in part A of epoxy, and thoroughly mix for 2-3 minutes, the epoxy won't cure until part B is added, so no need to panic
Pour in the dye (I used 0.25% by weight, but your portion will depend on strength of dye)
Pour in part B of epoxy, and mix thoroughly for 5-10 minutes. You need to make sure it's thoroughly mixed.
Very slowly pour mixture into the mold. You really want to minimize the amount of air that is mixed in at this point.
Place the mold on the shaker table to help agitate the air to the surface.
Vibrate the entire mixture for the full working time of the epoxy you purchased. In my case, that was 40 minutes of vibration. It takes about 5 minutes, but then bubbles start coming to the surface, and will continue to do so for the whole time the table is vibrating.
When the working time is up, turn off the vibration table, and use fast passes with a heat gun on the surface of the epoxy to pop any remaining bubbles.
The result of putting in all of this work. You're left with an incredibly clean and smooth molded epoxy base. This machine base will be incredible solid, dampen all the vibrations and will impress anyone that works with the equipment sitting on top.
Here is a write up on the machine this base was build for
Closeup look at the base
Thanks for reading! Happy building!
Any questions send me an email at firstname.lastname@example.org