Interested to know what you all have actually used for your builds. I’ve see a few topics on Qmax and also adding sugar but looks like that user just went the standard direction. Has any used anything other that the standard mix? Qmax looks like the best option with early strength gains, 6500psi and corrosion inhibitor. Slightly more expensive. Profinish 5000 also looks like a good option with early strength and 5000psi. Working times are 45-60minutes. Also seems to be instructions to leave surface wet/ covered or using a surface cure/ sealer for every type. No mention in the video.
I was the one that used Qmax with sugar. Turned out great…
MR-1 Build Three
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Awesome just ran past your build thread, I’m not farm from you here in long beach. Going to grab a bag or two tomorrow of qmax and test my concrete skills
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Are any of you at all worried about concrete longevity over time?
My garage here in WA has no insulation to speak of, and easily drops down to freezing or below at night. Obviously temperature variability is bad for machining accuracy in general so I will eventually have to fix the insulation issues, but meanwhile I’m debating whether to use normal concrete, or fill the base with an epoxy-bonded mineral composite.
I mixed up some test batches of polymer concrete with 12% mass fraction of low viscosity epoxy binder. The solids component was a 50/50 mixture of 125 micron silica sand and microsilica flour. It took more effort to homogenize than normal concrete, but it is extremely hard and has surprisingly high impact strength. I was able to put the “puck” I cast into my bench vise and hit it hard with a hammer, and it completely shrugged it off.
I’m not sure what the rigidity profile of this formula has, but I’d ballpark it has a flexural modulus somewhere between 15 and 30 GPa.
Pros I can see are:
- Not temperature sensitive, won’t crack when it freezes
- I’d expect better longevity / fracture resistance in general, but the Colosseum is still standing, so clearly concrete can have excellent longevity. Though, they didn’t build the Colosseum north of the 45th parallel.
- Better vibrational damping
Downsides:
- Order of magnitude higher cost: about $3-400 for the binder, $150 for the silica.
- More difficult to mix. Will need to see if I can get the mixture to a reasonably flowable viscosity, right now it’s more of a spread-and-compress putty.
- More potentially variable terms of flexural stiffness (could be lower than basic portland cement concrete if formulated poorly, though possibly higher if done correctly).
My biggest question is, how much does the concrete slab contribute to the flexural rigidity of the machine, and to what extent was it selected for rigidity vs damping?
FWIW the pan and base flexes all over the place before concrete, and the concrete surrounds the axises to secure it all together with the base plate. So it plays a huge role in rigidity.
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That’s valuable info, thanks!
Another thought I had was to get a chunk or two of 1" thick granite countertop offcuts from a local salvage yard (probably 20 bucks each), rough cut it down to a ~31 x 37" rectangle to fit at the bottom of the chip tray with notches for the base pillars, and then embed it (or a 2" epoxy-glued stack of them), at the bottom the epoxy mineral casting.
That would give the whole thing a known baseline flexural rigidity, and cut down on the cost of the epoxy significantly.
Qmax and some of the other more specialty concrete has air entrained that helps prevent freeze/ thaw cracking issues. The concrete is also not under constant stress and fully supported so less likely to crack. QMax is also fiber reinforced further increasing durability. Langmuir has reported no issues whatsoever and I think their oldest concrete unit is 2 years old? With I believe completely standard concrete.
From what I’ve seen a 50% by weight epoxy silica mix will have a 2-7 multiple of thermal expansion compared to concrete. With a 27x10^6K expansion coefficient and a 0-100F temp swing you would see a .069" change in 50 inches length which is pretty substantial. With concrete you’d see a max of .033" and a min of .015" which is much better. Not sure if your mix would fare better but id stick with the concrete. All of our huge gantry machines at work are all framed in concrete and we hold .0004 across yards
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A few things in response:
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I’m not proposing using a machine at 0 C, just that it could experience those temps without issue.
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A 50% epoxy silica mixture is waaay too much epoxy.
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Epoxy based mineral castings have a mass fraction of around 10 to 15%. While epoxy does have a fairly high CTE, silica has an extremely low CTE (0.55e-6 m/mK). I think we can ballpark estimate CTE for the composite using the rule of mixtures, which would put the CTE of a 12% epoxy-silica material at around ~13e-6, which is on par with Portland cement concrete.
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polymer mineral castings are also commonly used in high precision CNC router based, for eg Datron machines.
I think Youd have some real issues getting the table to sit in with no air gaps and the Rail stabilizers with that content, putty is way too stiff.
It flows, just very slowly. Luckily the epoxy has a 5+ hour pot life to gel time, so plenty of time to settle.
Here is the recipe for the concrete that I used in my base pour. I’m from Canada so measurements are metric.
1 litre water
1 litre latex admixture
1 bag premixed concrete (25kg) standard 27 mpa - 4000 psi
500 ml CSA cement
1 handful of glass fibre
20 ml plasticizer
This mix is almost self-leveling, and flows like crazy. Mix and dump, a little tap-a-tap with a rubber mallet, repeat. I didn’t have to use a trowel other than to push things around a bit. Covered with poly after a bit. 8 hours later, I could barely scratch the surface. 24 hours after pouring, concrete was probably 70 MPA (10000 psi). Continued with my build and was up and making chips in about 6 days.
CSA cement is awesome! 28 day Portland cement strength in 24 hours, even with only 30% of cementitious weight. Latex admixture adds flexural strength and reduces shrinkage. Glass fibre adds reinforcement. Using the recommended water volume, the mix is horrible to work with, and you still end up with bleed water on the surface after a while. Adding the plasticizer makes it super easy to work without adding additional water. The CSA cement uses more water in its reaction than Portland so covering with poly helps keep things moist.
This is not what LS recommended, so don’t do it.
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