My Friend that tumbles sez he thinks his goes 40rpm & thought mine was fast, but then I noticed he spends more time tumbling, over a week on every bottle, I've never spent over a week on any bottle but just figured my faster speed got them done faster then him. LEON.
If you want something cheap and easy to build, try an old electric ice cream maker. It turns around 40 rpm and does a great job cleaning the inside of the bottle. I use cut copper, comet and water for regular stains. Soda ( sick stains ) will take more time and a little more abrasive. I put the bottle in a sock and pack it tight with newspaper. I use those rubber floor protectors that you use on furniture legs to cap off the bottle.
I have another one with a longer can and I can do two or three small bottles, just have to pack them very carefully. Yes, not being able to do the outside is one drawback, and don't think I haven't tried to figure out to do it with this setup. I do have some aluminum oxide, but haven't had much success with really sick bottles, might be the slower rpm's. If all you have is regular stain, rust, dirt and such on the inside usually an overnight spin is all it needs to have you a nice clean bottle.
Speaking of the Jar Doctor, I met him at the national show in Memphis a couple years ago. Very nice guy.