Case tumbler turning speed

[rrawhide]

IS there a recommended RPM for turning/polishing brass? I have an old BBQ spit motor which turns approx 5-6 rpm. I am using corn or walnut media. I am in my first turning - 4-5 hours but it seems slow. Any recommendations?
thanx

rrawhide

[LE-28]

I don't think you will live long enough to polish shells with a rotisary motor.

My home made tumbler is running around 700 rpms. It looks slow even at that speed. It has done a really good job for me since I built it about 25 years ago.

I used a 1725 rpm motor and redused the speed through pulley size to 700. I put a 2"pully on the motor and a 5.25 on the first drive shaft to get this speed. It may be a little fast but it still takes around 2 hours to fully polish a load of 9mm shells.

[rrawhide]

Hi LE-28 and thanx.

I do have a 1/3 hp motor 1725 rpm too. Also have a 2" and a 5" pulley. What do you think the speed would be with this setup? Would this be workable?
Appreciate your input.

Also, how full do you fill your container? And do you have wood or ? inside to help mix the media?

rrawhide

[LE-28]

I'll round up some pictures and pm you with them to give you some ideas. With a 2"pulley on the motor and a 5" on the driven shaft, the ratio would be 2/5 of 1750. 3450/5=700 rpms.

[rrawhide]

Thanx LE-28

Look forward to the pictures.

[email protected]

[Bart B.]

rrawhide asks if there a recommended RPM for turning/polishing brass?

Yes, about 60 to 120 rpm for the tumbler; at least 1 revolution per second to no more than 2 revolutions per second. This is for tumblers of 10 to 12 inches in diameter.

Any faster with media and cases spinning around starts getting into the accident range.

A tumbler rotating at 700 rpm turns almost 12 revolutions per second. Way too fast. Is someone getting a vibrator mixed up with a tumbler for what to call it?

[B.L.E.]

If you spin it too fast, it becomes a centrifuge instead of a tumbler and no polishing action happens. Think of a front loading washer during it's spin cycle.

[ScottRiqui]

If you spin it too fast, it becomes a centrifuge instead of a tumbler and no polishing action happens. Think of a front loading washer during it's spin cycle.

Very good point. With a 12-inch diameter tumbler, a rotational speed of 240 rpm gives an apparent centrifugal acceleration of 9.81 meters per second squared (one "g"), which means that cases and media won't ever "fall" as the tumbler rotates - they'll just stick to the tumbler walls.

[LE-28]

I explained this very poorly.

The can spinning in my tumbler is spinning on a shaft turning 700 rpms, the can is 6.5"in diameter and that in itself is a speed reduction.

The can itself would be spinning at a speed of around 100rpms.

sorry for the confusion.

Rawhide, email sent

[jmorris]

I use a 40 rpm gear motor on mine, works as well as the thumblers, just holds a lot more.

[maggys drawers]

I use a rock tumbler I bought at a yard sale. 8 inch drum and a small/large pulley system turns it at about 45 rpm. Brass cleans up in 2-3 hours.

[Unclenick]

LE-28,

So you are saying you've got a can about 6.6 times the diameter of the drive shaft being turned by the pulley, giving you a second reduction.

5.25/2×6.6=17.325 so 1725/17.325 = 99.6 rpm (close enough to 100)


ScottRiqui,

I get just 76.6 RPM for 1 g in a 12 inch diameter drum. You can prove this to yourself by swinging any object on a 6 inch string in circles in the vertical plane. It takes just over 1 circle per second to keep the object from starting to fall at its apogee.

Where length is in inches:

ac = 1g = 32.17 ft/s² = 386 in/s² = v²/r = (2πr/s)²/r = r(2π/s)² = r4π²/s² = 39.48r/s²

s = √(39.48r/g) = √(39.48r/386 in/s²) = √(0.1023r) = √0.6137s² = 0.783 s/rev

rpm = 60 s/min / 0.783 s/rev = 76.6 rev/min

I've made up a table here:


I don't really know where greatest cleaning speed could be predicted to land. I assume the commercial tumbler makers have figured this out, perhaps by trial and error. My Thumbler B liners are 7" across in the inside corners, and 6⅜" across the flats. That's 100 rpm for 1g across the corners and 105 rpm across the flats to hit 1g. Thumbler makes them with a standard motor for 30 rpm and a high speed motor that turns them at 40 rpm. So I think that suggests Maybe 1/4 to 1/2 the rpm on my table will be the range you normally want to fall into.

If you have a smooth sided drum with no ribs or flats, you are more likely to want to go closer to the numbers on the table to get the content raised before it just slips down the sides.

[ScottRiqui]

Thanks for the correction, Unclenick - when I was balancing the gravitational force against the centrifugal "force", I switched units from Newtons to kilogram-force midway through the calculations. So my final answer was off by a factor of sqrt (9.81).

Serves me right for not "stink checking" my answer - 240 rpm should have seemed excessive to me.

[Unclenick]

No problem. Lots of opportunity for a unit slip in there. On my first run through I managed to substitute the fraction of a minute for seconds and after squaring got a number off by more than three orders of magnitude. Fortunately that stuck out like a sore thumb.

[jib]

I run my wet SS pin tumbler around 72 RPM. I started around 44, but it was too slow. I'm done, with clean flash tubes too, in about 2 1/2 hours.

[LE-28]

I have an old frequency inverter at work that has been pretty much discarded, I've been thinking about putting it on my shell tumbler and playing with the speeds. I don't know if it'll make it work any better than it does now, but it will be fun to play with it.

[LE-28]

Jmorris,

If I needed one that size I would probably just buy one of Grizzley's cheap plastic cement mixers.

I know! It's more fun to invent your own. That's why I built mine 20 some years ago. I've been going to build one out of fancy oak to have something really unique but I can't wear out the first one I built.

[rrawhide]

Just wanted to say thanx for all your responses and ideas. I did, however, end up making a vertical vibatory brass polisher. I took an idea from 'homemade brass tumbler' off u-tube and made one. It works perfectly and is very fast and I cannot believe how 'new' the brass looks. Wish i could post a picture for you. Thanx again.

rrawhide

[Unclenick]

Put up a link to the Video if you can. Others may be interested. I have a vertical vibratory tumbler myself called a Vibra-Tek. I don't think they've been made for years. Basically a kind of spring diving board that is vibrated by an AC magnet. The board holds a 3 gallon plastic ice cream tub that the brass is shaken up and down in. Works fine, but very loud.

[BumbleBug]

Thanks Unclenick for updating the RPM info.

Originally Posted by jmorris

I use a 40 rpm gear motor on mine, works as well as the thumblers, just holds a lot more.

Could you post/attach more pictures? I'm especially interested in how you attached the bucket to the gear-box shaft. What does the lid end looks like? I was surprised that the lid end is not supported on rollers?

TIA....

...bug

[jmorris]

All the photos of it that I have uploaded are here.

http://s121.photobucket.com/user/jmo...art=all&page=1

I used 100# chlorine buckets that are much stronger (and larger) than a normal 5 gallon bucket. One is bolted to a 3/8" steel face plate. The bucket that holds everything just nests inside that one.

Makes it so you just pull out the bucket to fill or empty it, don't have to mess with the entire contraption.

Thought about other rollers on the end but the bearings are good for the better part of a ton each and have just never needed it.

[maggys drawers]

I use an old rock tumbler I got at a garage sale. It turns at 42-43 rpm, and gets a load of cases done in 2-3 hours with ss pins.