We've been making tons of little machines for the creation of little objects. What about creating big stuff like furniture, houses, and Mount Rushmore? Not going to fit on your Makerbot, sorry.
With that in mind, I've starting playing with concrete as a material for the creation of much bigger making machines. Here's a bit of experimenting I did the other afternoon.
The raw materials---a 50 pound bag of fancy concrete, and some canola oil as a mold release. [Edit: Spray Canola oil should have been called mold glue, don't use it.]
Speaking of molds, here is a test mold for a concrete machine base. I machined it in pink foam insulation on a ShopBot. The part itself is a little bigger than 1x2 feet.
Concrete is very strong in compression. It can handle 5,000 - 20,000 pounds per square inch. Unfortunately though, it really stinks in tension where it can only handle 500 - 1000 psi. To help with this issue and it's tendency to crack while curing, reinforcing steel (rebar) is embedded in the concrete. You can see a thin grid of steel rebar in this image.
For the rebar to do its job, it needs to be in the middle of the concrete rather than sitting at the bottom. I made these little chairs to lift it off the bottom of the mold.
To use "ready-mix" concrete like I'm using, you have to add water to the mixture. Here I have a 5 gallon bucket with a disposable bucket liner. The bucket is McMaster-Carr 4269T342 and the liner 9772T86. Using the bucket liners makes this process MUCH cleaner. If you can't get it clean enough to reuse, just toss it.
I know that some dislike McMaster-Carr because they don't ship internationally. I too think that policy is unfortunate. However, in the US they are very convenient and usually reasonably priced which greatly accelerates my work. Outside of the US, you shouldn't have too much trouble finding equivalent products since most of their items are pretty generic.
Using the drill press as a cement mixer was a complete failure. It spins way too fast (500 rpm in this case). Instead I ended up using a cordless drill and a much smaller mixing bar. This combination *barely* worked, so I have now ordered this guy. Its from Amazon.
Because I had to work pretty quickly, I didn't take any pictures during the mixing and pouring process. Its pretty simple though.
- Liberally apply mold release to mold.
- Add about 1/3rd of the concrete mix to the bucket.
- Look at bag to determine recommended water dosage. Add first 1/3rd.
- Start mixing
- When things are starting to mix, add 1/3rd more mix and water.
- Add last 1/3rd of mix and water. Do not cheat and add more water. The concrete will be significantly weakened.
- Mix some more for good measure. Are you glad you're not doing this by hand?
- Slowly pour concrete into mold. Take care not to dislodge rebar or create air pockets.
- Once mold is full, use a spare piece of wood or similar to smooth upper surface.
- Cover mold with plastic to slow water loss.
This is what my mold looked like prior to covering. I'm going to let it cure for a few days and we'll see what happens!