[greybeard]

Greybeard,

I'd think about using water instead of weights in you Mk2 jig. Everybody can measure water but weight takes a scale. You need to go all of the way to failure to find flexural strength while you only need a regular set of weights vs. displacements to find flexural modulus. I'm a bit vague form the diagram about how the needle suspension of the mirror and the adjustments work.

Good point. Perhaps the Mark 3 will have a plastic measuring jug on the beam.

The beam has a flat end(might be a piece of glass glued on) and the needle is held horizontally against it by the flat leaf spring. At one end of the needle, a small mirror is glued to the needle. As the beam is deflected downwards, the needle rolls and turns the mirror.
It's a device I've used for small deflection measurements before, and works a treat.
The zeroing screw allows you to fine tune the angle of the mirror.
Once you have placed the sample in position, drop the beam on to it.
The leaf spring is then eased away from the beam, and the needle/mirror placed in position. It's roughly positioned by hand, then the reflected laser spot can be "zeroed" with the screw.

As for the bubble in a vacuum question, like many of your questions it sounds simple but it isn't. These are the questions we learn from as I have from all of your other questions......

I would assume that to first order, you could model the vacuum contribution initially as 14.7psi of pressure equating to a force on a circle the same diameter of the bubble creating an upward force opposed by the weight of the bubble and the weight of the epoxy above it.

Yes, this was all I was looking for, because I did the calculation just like that.
Have I made a mistake here ?
I was considering the case before the air was removed, and took the air pressure to be exerting a force downwards on the bubble proportionate to its diameter.

EDIT
I now realize that as this figure is far greater than the buoyancy figure, all bubbles should sink. (chair) ...Help :drowning:

When I multiplied the buoyancy of the bubble (ie the volume of the bubble times the density of the epoxy) by the 60G gravity the spinning gave me, it didn't amount to anything like the air pressure component opposing it.
Yes of course it helps, but I was expecting the spinning to produce a force an order of magnitude greater than the air pressure.

All I can say is that I think I'd rather have something under vacuum even with the morass I see above than 5kg of epoxy granite spinning at 600 rpms. The latter should make some nice molded in place architectural molding however.
--Cameron

OK, so now, trying to turn(sorry) a problem into an opportunity, I go back to another idea of spin casting the mix inside plastic pipes, and taking a space frame approach to the machine structure.

Pack the wettish mix into plastic pipe and spin it around its long axis till pre-cured. This, when pushed out of the pipe (could be round or square I suppose), will produce a beam that has air/excess epoxy along the central, neutral axis. I'm not sure how a mixed aggregate will behave, if there will be any exchange of particle distribution, but I think it will give a skin effect of highest density on the outside. Useful ?
It wouldn't take too much ingenuity to fix inserts in place either.

Regards
John