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Getting the block out of the mold

A project log for Onix - Yet another concrete Lathe

An upgradable, scalable 200$ Lathe for everyone* (*with some basic tools)

moritz-wenzelMoritz Wenzel 10/27/2018 at 18:461 Comment

Here we go, to be honest i'm quite disappointed with the result. I kinda hoped i could avoid the "problem" of having huge gabs and holes in the block by compressing the concrete layer by layer, probably it helped but still, there were some huge holes in the bottom section of the block, since i couldn't get the concrete mix 100% under the 40mm cutout tube, and the white 100mm*100mm bearing cutout board. There were also quite a few smaller holes and air bubbles at different places. SOO What does that mean?  i HIGHLY recommend to use concrete liquifier and a liiiiittle bit more water than i did, since my mix was pretty dry (25kg concrete mix to 2,5 litres water). Don't simply add more water until it's moist enough to flow into position, there is special concrete liquifier which doesn't affect the structure of the concrete, while adding more water will weaken the concrete later. After using a more liquid mix don't forget to vibrate it a little so the air bubbles will come out. My mix didn't really flow, you should read the instructions on the concrete liquifier but i guess it would even help if it's barely floating into position, but it don't has to be like maple sirup though.. 

I filled the holes with the same concrete mix i used before, but i ran it throug a sieve so only the fine bits and the cement would remain, which makes it much easier to fill in all the holes. It helps to rub it in by hand if you want to fill the small cheese-like air holes on the surface.. 

Discussions

Andrew Smart wrote 12/11/2018 at 04:05 point

I have not worked much with concrete, but working with plaster of paris molds I place the mold on top of an electric shaver for vibration. Important to do this in parts/layers, not fill the whole mold at once then vibrate. Maybe put this on top of an idling tractor (or other heavy vibrating equipment) to get the bubbles to come out?

Did you see this project? https://blog.opensourceecology.org/?p=1408 See they had a problem with their base vibrating, claimed unsatisfactory for lathing operations. They propose casting a metal base as the way to go- they didn't consider concrete to be acceptable?

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