Designing DIY CNC Milling Machine

Yeah, I even tried putting couple of numbers quickly but I'm getting unexpectedly low values. Need to be doing something wrong.

Y axis will be the heaviest with X stacked on top, granite chunk I'm going to use for X weights around 18kg. I tried putting very pessimistic numbers but still getting 1.4Nm top. What am I missing? (see attached PDF)

spumco wrote:

Good score!

 
Not really.
 

 
My mistake, first slab he proposed was nice granite, two other on the same pile were quite dirty. The dimensions were the same, we loaded it into my car. Right now I pulled it out, dusted off and it's marble.

Just want to double check, marble won't do to serve as a baseplate for Y or Z? It sounds spindly, so I doubt it. And even though after I cleaned it it turned out to have some surface defects.
In that the case, I'll probably imprint Y and Z with the granite slab and then make X (longest, ~100cm) axis out of it as a solid. But cement is significantly less dense than concrete, makes little sense to make movable axis the heaviest. That's a bummer. I need to think about it. 

Bummer about the marble.  Maybe try a different stone mason, or try to find a quarry nearby?

Marble and granite have about the same stiffness (Youngs modulus ~50), but marble is less than half as hard on the Mohs scale, and about 1/3 the compressive strength.

So I wouldn't use marble anywhere in a machine tool - not even as aggregate in concrete.

About the CSA - is this possible to add any stone fillter (like granite sand/rocks) to RapidSet Cement ALL to reduce the amount of cement needed? From what I see it does not seem like it should be possible, but wanted to make sure.

I don't see why not.  CTA sells other CSA-based concrete products, some with aggregate already mixed in.  The "DOT Concrete Mix" appears to be a mix of Rapid-set, fluidizers, fibers, and 3/8" aggregate.

Makes it harder to mix, of course.  Maybe mix up 1/3 of bag with maximum plasticizer and ice/water to get a feel for how it flows/works, and then do another test with using some  ~6mm to ~10mm angular gravel aggregate.  Last 1/3 of the bag, try fibers and aggregate together.

Cast yourself a few stepping stones for the backyard, or a couple of doorstops in the name of research.

Definitely won't go straight into final casting. But it's also great not to waste expensive cement on something dumb.

Thanks for all the tips!

Ok... for a 'not dumb' test, cast a simple beam.

Calculate the volume that 1/4 of a bag will fill, and make a mold for a rectangular beam with the same approximate volume.

Test the resulting cast beams for:

• Shrinkage (should be close to nothing)
• Deflection
  • Stiffness should improve if fibers with high Youngs modulus are added
  • Might do a couple tests with different aggregate loadings
• Edge stability (holes/features close to edge)
• Surface finish (may impact your design if you feel the need to 'finish' the casting cosmetically)

For the price of one bag, you should be able to get four different samples that will help you decide on fiber & aggregate loading, plus workability and mechanical properties.

You may find that the fibers are a huge pain to work with using limited tools, but that a 50% aggregate load will extend the cement without affecting the performance negatively. 

Or you may decide you hate the whole idea entirely because it's a mess.  

spumco wrote:

Ok... for a 'not dumb' test, cast a simple beam.

 

I might have made myself unclear. By 'dumb test' I meant that I wanted to ask whether it even makes sense trying to add aggregate so I won't wast a bag of cement on attempts if it turned out to be bad idea in first place. I was doubtful because I failed to find anything about mixing aggregate in in Cement All datasheet.

I never meant to criticise your suggestion about casting test pieces. Sorry If I sounded this way

And thanks for even more tips on constructive testing!

 

No offense taken, but thank you.

I think the fact that CTA sells aggregate-filled CSA concrete indicates it's worth a try.  If you look at some of the other products on their site, one of them indicates up to a 100% aggregate loading can be used for deep pours.  i.e. 50-50 grout to aggregate by volume.

So if you have access to some cheap aggregate with a diameter less than 1/3 your casting's smallest cross section, it should be a good way to extend the cement.

On the down side... if you do have to drill the casting later, it'll make it MUCH more difficult.