Thread: Designing a better bed
Here are some thoughts
- Epoxy adheres well to steel but may not stay bonded due to differences in thermal expansion. That would make the steel tub a bit overkill
- If you use that much epoxy it is likely to form a layer on top of the casting.
- Using milled carbon as well as steel tubing is a bit like suspenders and belt to keep your pants up. And still, 20.-- seems a small amount.
- using tubing instead of bars lowers the mass/volume which I think is a good idea.
- I would use round tubing though as I think it would leave for a stronger EG mass.
- You can probably get away with using pvc piping instead of steel if the plate is thick enough
Last edited by Sven; 29-07-2015 at 07:25 PM.
Thanks Sven, after Robin's comment about having a big chunk of ALU that can be sold if things go horribly wrong I have to admit that I'm rethinking things a bit...
I'm not too concerned about thermal expansion because I'm building a CNC machine in a garage that is attached to a house and the temperature doesn't won't really fluctuate more than 15C over the course of a year.
I was banking on the epoxy getting vibrated to the top and selected the epoxy especially to do that as it will self level.
The milled carbon amount is based on the best practice mix on the easycomposites website, 10% by weight compared to the epoxy. Obviously the quartz/granite/etc in the mix doesn't count, this results in a surprising small amount of milled carbon fibres that add greatly to stiffness and dimensional stability.
I'm not sure exactly where the optimum lies between tube and square, I selected square so that I could weld it together easily, circular tube would be much more difficult to fabricate.
I've been arguing the toss with a structural engineer and insurance company about the fact that my house has giant cracks in the extension so for the next couple of weeks/month I need to back burner the CNC which is very annoying, I am relying on it to do a lot house upgrades. :(
I' ve been reading up on EG as my own ideas are taking shape.
As far as I' read, vibrating EG does not work unless you use a mix with too much epoxy for a small size casting. Too much meaning it will yield an end result that has some shrinkage and maybe some warp.
The best seems to be a mix that is "very dry" compared to what most people on forums seem to be using: in the 4% area, or a bit wetter where it needs to adhere to another material.
Also, best to use various grain sizes, the next size 1/5th of the previous.
Put a measured amount in a jar, then add the smaller size and shake until the volume increases and you know the ratio.
A dry mix like that will need stamping.
And off course, make sure to do trials :)
Last edited by Sven; 15-08-2015 at 05:59 AM.
If all you want is to flow a flat surface, why not use Wood's Metal? That goes very runny and sets hard. You could even reflow it if you moved the machine.
Have you ever tried it if you have some lying around?
For the smaller sizes I think it would have to be blasting grits:
Then Aluminium Oxide powder and milled CF.
Linear or volumetric thermal expansion coefficient is irrelevant in constucting a CNC.
First of all cement, steel and concrete have very similar thermal expansion. Even if the epoxy has higher thermal expansion its irrelevant at 1m and temperature changes. Plus the epoxy cement or epoxy granite will more similar expansion to the concrete
EG is same like all, do it properly and it will work well.
There are better options of course and not so expensive- purpose made epoxy concrete, especially made for the purpose. Problem is finding them close to you or in your country at all.
But those who search will find :-).
epoxy concrete cast machine bed, ultra-high performance concrete
These guys http://durcrete.de/ can do machine bed design or cast inhouse for you or as far as i remember 1 ton of the stuff was like ~400euro
Gues what i will be bringing home if i go to Germany :-)
I may have had my expansion reasons mixed up...
A guy I know built an epoxy-concrete machine with steel bars at the surface to connect the rails to.
They were flat when casting started but the end result was not.
If you cast EC in a steel tub, it may end up not being part of the construction but just a liner and not a reinforcement.
There are many ways to not do it right. Apart from choosing the best epoxy for deep casting, is good idea to read the documents of the said epoxy and follow them instructions to the letter. best is to call the epoxy manufacturer and speak with them for the most suitable epoxy for the purpose.
Cause at the end of the day they know their stuff and even can mix a custom epoxy for your purpose. I have talked with one company technician and he knew exactly about that situation and explained me the differences between the 3 suitable epoxies they had.
The http://durcrete.de is quite interesting.
What I was aiming for was finding a sweet spot between price/performance and ease of construction given that I only have not very accurate tools to work with.
The Durcrete site has some vibration damping data available and a demo:
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