Thank you saxplayer, I probably have more question than most people have time for. I'm in the process of making barrel from 6" PVC right now. I have many question around that topic! Do you know the best rpms for rough grind and are there better rpms for the polish?
6" diameter barrels are my favorite diameter. Most rocks we tumble are 1 to 2 inches in size.
The barrel diameter to rock size 'ratio' has a bigger effect on the(dynamic) conditions in the tumbler than I ever imagined.
Rotation speed is also important.
The difference in grind rate and bruising potential going from a 4" diameter to an 8" diameter is giant.
IMO 4" barrels are way too small to grind effectively in step 1. I find them useless. And they don't hold crap, no capacity.
An 8" barrel can bruise rocks in a hurry. The fall within can get too violent without slurry thickeners.
Conditions in a 6" barrel are perfect. Build it 20+" long and it holds a bunch of rocks. Nothing wrong with it being 30" long.
For 6 inch barrels I like 20 to 25 rpm for 220-500-1000-polish steps. 35 to 40 for coarse grind. I use slurry additives in step 1 that allow 60 to 70 rpm on step 1 without one iota of damage to my rocks(or even delicate glass).
At 60 to 70 I get some real fast step 1 shaping. Hate that step, it takes so long.
PVC is fine for 220-500-1000-polish steps. They will last forever.
If running coarser SiC 30 or even coarser in step 1 you will find PVC barrels to wear out in 1 to 2 years.
Especially if rotating them at higher speeds like 60 to 70 rpm with slurry thickener and SiC 30 or coarser.
I welded up 6" HDPE barrels for step 1 using super coarse SiC with slurry thickener. They don't hardly wear.