I use a tumbler I built that is the bottom 1/3 of a 45 Imperial (55
US) poly barrel attached to a shaft at about a 45 degree angle (much
like a cement mixer). My use is for rounding off the edges of slabs
and taking the saw marks off - generally to give them a more natural
look. It holds approximately 50 lbs per load.
The critical aspect of tumbler design is the speed the barrel
rotates. Too fast you will either get too aggressive cutting action
and damage from the rocks chipping each other or you will have the
rocks held against the outside of the barrel and nothing happening. To
get a rough idea of how fast the barrel should rotate - time a hobby
type tumbler (Lortone etc.) - then calculate the surface speed that
the outside of the barrel travels at. For the barrel I run my
reduction is approximately 30 to one - getting the barrel turning
about 60 rpm - which is good for the rough stage but may be a bit fast
for the polish cycles.
Also you may want to include either a bunch of small stones in your
mix or plastic pellets - helps cut down on the impact of the rocks
against each other. You also have to only load the tumbler about 3/4
full or there will be no room for the material to tumble.
How loud is my contraption when it is running? Well lets put it this
way, it lives at the end of 100 foot extension cord and is not run
after dark, and that is with a plastic barrel.
I was finding I could round the corners of the slabs (Moh hardness of
about 4.5) and remove any saw marks in less that 16 hours. It would
take a bit longer to get rough pebbles to smooth out. I just add a bit
more grit to the load when the cutting action seems to reduce.
The rest of your questions are not too specific to the process I use
my tumbler for but drop me a note if you would like more details on
the design or some of the research I did in building it. I am not
quite happy with the longevity of the poly barrels so I may resort to
building some neoprene lined steel ones.
Thanks, Cameron Speedie Island Gem and Rock