Designing DIY CNC Milling Machine

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.

Not looking for any 'official' resources, this is hobby project after all.

This is extremely valuable insight, thank you so much. I know very little about _engineering_ cements and concretes, need to read more about this.

Here in Poland it's not really possible to purchase RapidSet CEMENT, only RapidSet CEMENT ALL which is a grout, so cement mixed with some mineral filler if I understand correctly. Not sure how much difference this is going to make.

Another question is whether CSA with steel or basalt filler will have enough tensile strength for concrete-only base with internal steel frame (with major structural connections working in tension, so tower will be bolted to base with 4 long through-bolts).

If you can get RapidSet cement-all that's fine.  It's grout, which makes it easier to handle, mix, and pour/place.

I don't think fibers add much to tensile strength, but I'm not an expert. If I understand the theory, the fibers are put in UHPC for crack control, and the added stiffness is a bonus we in the DIY-CNC world can make use of.  Steel fibers have the highest Young's modulus, but basalt aren't far behind and they're easier to deal with (lighter, wet out nicely).  Nylon or other 'crack-control' fibers would be useless in this application.

Tensile strength above and beyond the basic cement UTS comes from reinforcement - think of pre-stressed civil engineering structures.  Bridges and other structures have huge cables running inside the concrete.  Those cables are stretched prior to placing/casting the concrete and then the strain is released after the concrete is cured - putting the concrete in compression.

In the case of, say, a mill column, the same effect could be duplicated (sort-of) by casting hollow vertical tubes through the column and base.  Once assembled, steel plates are put on the top of the column (and below the base) and long rods are passed through the whole assembly and then tightened down.  The column and base would then be put in compression, but you'd need some pretty heavy-duty rods to get meaningful forces acting favorably on the assembly.

Not sure how to reinforce a whole column/base/saddle of a typical C-frame mill to withstand twisting forces, but maybe someone clever could figure it out.

I think it'd be easier, and the concrete less likely to get damaged, to use hollow steel tubes/forms and fill them.  But that's obviously dependent on one's skills and available resources.

Attached is a photo of my mill right after I filled the castings with Rapidset Cement-all.  After it cured, I bedded ~8mm plates in epoxy paste to box-in the cement and bolted the plates to the cast iron before the epoxy cured. (a 5-sided box isn't torsionally stiff, but the 6th side makes all the difference)

The result is, essentially, a monolithic lump of concrete & cast iron.  Massive difference in rigidity and vibration damping - deflection at the spindle was cut by more than 90% (sideways force applied to spindle box and deflection measured before/after the modifications).

If you (or someone else) do use Rapid-set, I highly recommend using as much fluidizer as possible, and mix with ice/ice-water to slow down the reaction.  That crap sets up FAST.

For anyone into building a nice, heavy, small, again heavy, mill, here is a tip:
-find some shop near you that does CNC plasma cutting
-go inside, find the boss
-make him/her/it? an offer that can not be refused, as follows
--if you let me clean your plasma table for free, can i take 2 or 3 buckets of slag for free
After some strange looks, you will most probably get a yes, or have the police escort you
Clean that slag just a bit with water, use it instead of sand, add epoxy and mix thoroughly.
Be warned, that stuff is heavy, it is very close to the weight of metal at 7850 KG/M3, or over 3 times as heavy as sand!
Also be warned, cleaning a plasma table is no fun.

Using a filled epoxy product made for the process, you use a flat reference surface to cast a thin precision mating surface on the underlying imprecise component.  i.e. blubber up epoxy on a steel beam, and press a precision straightedge in to the curing epoxy.  Assuming you applied mold release to the straightedge, your warped, chunky steel beam now has a nice flat surface for a linear rail (or other precision component).

What is an example of an epoxy made for this? I have seen lots of people do this but never found an epoxy that is made for it specifically.

Epoxies from Diamant (DWH, Moglice) for example.