Designing DIY CNC Milling Machine

Again, concrete and sand weigh in at about 2.5T per M3, while metal is 7.85T per M3, so 3 times heavier.
And buying 400KG of metal for a small mill is roughly 500 Euro where i live, forgetting the fact that full bar metal in those sizes can not be found here, while using epoxy and hollow frames comes to about 100 with everything.
Anything bigger and the math goes through the roof!
Also, i will bet that the same machine filled with those two materials will have very, very different capabilities.
And i use water tables, so cleaning is very easy as the slag does not stick.

Yes, but there is a fundamental problem. It is not clear how he will mix the epoxy with the steel....maybe the end result is not what he expected. On the other side epoxy and stones has already been played and there is information about it.

I do know from personal experience that adding aggregate to both epoxy and concrete/cement makes mixing a pain.  Fibers are even worse.

When I did the Rapidset + basalt fibers, I used the maximum permitted water/ice ratio and as much superplasticizer & retarder as the directions allowed.  Looked like soup.

Added about 3% fibers and it turned to peanut butter - my mixer drill stalled and I had to get out the big corded drill just to get the last bit of fibers to wet out before it all set up in the bucket.

Similar issues when mixing epoxy, even through I used super-low viscosity deep-pour casting resin.

I can't imagine trying to mix in 20 pounds of steel fibers in a bucket with a cordless drill.

spumco wrote:

I can't imagine trying to mix in 20 pounds of steel fibers in a bucket with a cordless drill.

I have a Bosch that can do that easily, it does 63NM!
When i converted a Kawasaki 600cc engine to compressor, that was the only one able to turn it, out of 7 or 8 i have, including 2 corded ones of these one is Bosch concrete drill. Well all but one are Bosch.
And no, that is not a hammer drill, it is a drill for concrete, has no hammer as the usual ones, uses a cylinder and it is named "electro-pneumatic drill"
How is "grin" done in HTML?

I don't know how to make the right proportion. Adding too much epoxy will result in a soft construction. If it is very little, it will be weak. My MV junior is 3.5t but the base is weak. For better results, a foundation must be made.

 


If it has to be steel, why not make a welded construction?  It can be much stronger than epoxy (if they are the same weight).  The old chirons have a welded construction.
 

spumco wrote:

No 'official' resources I've found (like peer-reviewed journal studies) specifically on DIY machine building.

There has been discussion of CSA properties, as well as a couple of build logs of moderately informative content, on CNCZone.  The epoxy-granite and DIY sub-forums are littered with lots of back-and-forth debates, but there were a few threads in the past couple of years which included discussion of CSA concrete and attempts to quantify improvements in stiffness.

A number of concrete-only, or concrete-filled machines have been built by people over the years - there are even some YT vids available.  I think the fundamental problem with using concrete is that Portland-cement based concrete shrinks and cracks.  Using it to fill structures doesn't work well (far as I know) as the shrinkage decouples it from the outer structure and it just becomes a weight without adding any strength or stiffness to the assembly.  Even if cast around internal steel reinforcement it still cracks and shrinks.  Weight is certainly good, but it tends to be a by-product of a massive structure rather than the end goal.

Epoxy doesn't appreciably shrink when it cures, allowing it to stay in contact with the structure (or remain stable if it is the entire structure).   But epoxy isn't stiff - compared to metals, ceramics, or rocks.  So you add stiff fillers (which also reduce the cost) and, in theory, you get an improved bulk stiffness for the whole casting.  Problem is that without some trial and error and fairly tight control of the mix and placement, you will wind up with a heavy, but not extremely stiff casting.  Plus casting in really thick sections is a challenge without fairly specialized epoxy (i.e. overheating).

The deal with CSA cement is that it doesn't shrink like Portland cement.  It can also achieve extremely high compressive strengths very rapidly.  So can Portland-based UHPC, but those take rather a long time to achieve the high strengths and they still have the shrinkage problem.

CSA isn't extremely stiff (compared to steel), but adding stiff fibers to it improves things dramatically.  Fibers are typically added to UHPC for crack control, but a side benefit is that steel or basalt fibers increase the average stiffness... something desirable for a machine tool frame.

So the 'ideal' frame uses a high tensile strength outer skin, and a high compression strength internal filler... and design the geometry so that the skin is in tension and the core is in compression.  Then make sure the connection between the skin and core is intimate and will not dis-bond (or is mechanically coupled).  Make sure both the filler and skin are very stiff and you've got a solid assembly.

I've got two questions:
- Do you have any suggestions about any other products like Rapid-Set? In Poland I was able to find only one place where they sell it, but no way to purchase the Flow Control additive.
- With basalt fibers, is this possible to later drill in that? I'm curious if I definitely need to put inserts everywhere I might need these, or will this be possible to attach something later (I'm thinking stuff like chip guards, bellows, etc). What kind of inserts will work best?