Concrete with 8020

Hello all, I am slowly building a CNC Mill about 12x10x5" in size. I would like to use aluminum extrusions as they offer me the most flexibility because I intend to modify the build as I go, and change out linear rails, spindle, and ball screws later. I want more rigidity though so I would like to fill the frame with concrete where I can. I don't want o fork over the money for epoxy/sand/granite that's why I'd like to use concrete. Problem is that concrete reacts with aluminum, so I was wondering if anyone has tried coating their extrusions in clearcoat spray, thin epoxy layer, or something else to protect the concrete from the extrusions? Here's the rough sketch I have, but I am working on a more professional drawing as I get in parts and measure them correctly.

The blue is concrete and the gray is aluminum, I used 4" PVC pipe as a filler to reduce the concrete needed without reducing damping/strength much.

You show unsupported round linear rails. Very floppy.

Fix that before thinking about concrete fills etc.

Sent from my SM-G970F using Tapatalk

Quote:

---

Originally Posted by pippin88

You show unsupported round linear rails. Very floppy.

Fix that before thinking about concrete fills etc.

Sent from my SM-G970F using Tapatalk

---

Already fixed that, I have linear rail in hand now. that's just a layout drawing. The most important thing for any machine is a strong foundation. It makes no sense to spend hundreds on linear rails if they are bolted to something sloppy. That's why I'd like to see what can be done with concrete.

Here are the updated drawings using 6030 T-slot and 20x17mm rail with two carriage block each side (4 per axis). Shown are the usual Chinese ballscrews (400mm Z and 600mm X+Y) and some 4mm aluminum plate corner brackets. actual work volume is 350x320x150mm X-Y-Z. My plan is to fill this framing with concrete after clear coating the extrusions. I will have some 4" PVC pipe in the frame to reduce concrete volume and not have a million pound desktop machine. I would like people's opinion on the clear coating to protect the concrete from degradation from the aluminum.

Fix the area I've put a red line on. This bit was better in your original images.

Are you mounting rails directly to aluminium profiles?

Aluminium profiles are not precise or stiff.

They are good enough for a wood router. They are popular because they can easily be made as a kit for DIY assembly.

For a milling they are a poor choice.https://uploads.tapatalk-cdn.com/202...55ca3b0a6f.jpg

I appreciate you response, I recognize that 8030 is not ideal, but if you reference my original post I chose them because they give me needed flexibility. I intend to fill the frame with concrete so that the stiffness of the extrusions is not really important; as the concrete reinforced by the extrusions bears the cutting forces. I will look into improving the are you mentioned.

I'm not convinced the concrete will adhere to anything well enough to provide meaningful strength.

I think you should cast a concrete block, heavy on the rebar, and drill a series of holes for wedge anchors. These can support (shimmed) heavy plates that you can secure your rails to.

The reason people use epoxy/granite is because unlike concrete, it doesn't shrink when it sets. By trying to save a few bucks on material, you're going to waste it all and have to start over again.

Quote:

---

Originally Posted by strostkovy

I'm not convinced the concrete will adhere to anything well enough to provide meaningful strength.

I think you should cast a concrete block, heavy on the rebar, and drill a series of holes for wedge anchors. These can support (shimmed) heavy plates that you can secure your rails to.

---

This was my fear, I think I have talked myself out of the idea at this point.

- - - Updated - - -

Fair enough!