21 replies to this topic

[Themolehole_9876]

Hey guys, i built my own ball mill and am just having trouble with grip. I cant seem to find anything half decent.. and when i do it breaks or wears off way too quick! Would anyone have any suggestions on what i could use? What do you use for grip on the container or on the 'rods' my container is only about 200mm wide and the rods are 20mm. Only one of the rods spins from the motor too. Thanks a lot guys!

[littlejohny]

Sometimes it can depend on which way the shaft turns, I think it's supposed to lift the container on the drive side, you could try silicone tubing if your container is made of PVC; what the hell Is only 200mm wide thats fairly large if you ask me.

[chris17]

You should try wrapping your jar with strips cut from the inner tubing of a bicycle tire. Inner tubing is commonly available and it works like a charm depending on the material of your rollers. Rubber against rubber gives good ammount of friction.

[Andrew]

There is a long answer to this predicament, involving inertias, torques rather than the simpler assumptions made on angular velocities which are easier to work with.

But in short and lay terms; if the driving wheel/bar is too small it will be spinning too fast and will not gain adequate purchase upon the jar. For various reasons the driving bar's diameter needs to be a reasonable fraction of the jar diameter, at least 1:6, more towards 1:4 (bar to jar), for a light ball mill jar (light means filled with ceramic media, (densities from 1 to 3 for the whole jar)) and when using standard contact materials. A jar that is heavy (i.e. Filled with lead media, densities of over 3 for the whole jar) needs a better ratio i.e. at least 1:3.

Higher ratios will work if very good grip is supplied, but at the cost of increased wear. The lower the ratio the better, also hard wearing contact material are a must, the higher the ratio the harder the contact materials need to be.





Example, well your case exactly;

Mill jar is 8? wide,
has a 6l capacity,
has 6kg of ceramic media in and 800g of bp ingredients
jar itself weighs 300g

Total weight = 7.1kg
Volume = 6l
Density = 1.2

The above ratios suggest that you would need a driving bar around 1.3? to 2? in diameter. This is a bare minimum.

So for this jar, say you want an angular velocity of 60rpm, you would have a 2? bar rotating at 240rpm.





A 20mm bar rotating at 600rpm would be of no use at all. You can use rubber with a very high tac, but it will shred itself to bits unless it is a very strong rubber. The slower the driving bar rotates the better. Car tyre inner tubes are made from a strong and hard rubber, and the tac is pretty good too, they are quite difficult to get hold of nowadays though, as all car tyres now are tubeless. The 8?, 6 litre jars I use have a rubber strip with teeth wrapped around them (bought from Inoxia, below is an image) and the bar has a hard silicone sleeve 3mm thick, its works a treat.

[lew]

Hi.

I have also had to modify my ball mill, but now it works very well indeed.

Here are Lew's top 3 tips on modifing your mill!


1. Thick rubber bands can be stretched around the milling jar to give better grip with the rollers, even better than this are those rubber fashion wrist bands that all the cool kids wear!

2. I Found my drum was turning too slowly, so to 'up the gearing', I simply wrapped the drive roller with some tightly wound cord, and then wrapped that with a layer of tight PVC tape. This had the effect of making the roller a good few mil wider, and therfore spinning the drum a bit faster.

3. Internal grip was poor. The smooth interior of the drum was allowing the powder and media to simply slide around inside, instead of 'tumbling'.
To stop this, I took some glue stick rods, and hot glued them inside the drum to make agitators. The inside of the milling drum now looks like the inside of a washing machine. The ridges throw everything around far better and this realy helps the whole milling process.

[Themolehole_9876]

my motor has been stopping too recently, only when there is too muh load on it though.. how can i slow it down without making it stop??? The rods spin fast.. too fast and the container with everything doesnt even weight that much.. but sometimes it just jams up and stops the motor

[littlejohny]

my motor has been stopping too recently, only when there is too muh load on it though.. how can i slow it down without making it stop??? The rods spin fast.. too fast and the container with everything doesnt even weight that much.. but sometimes it just jams up and stops the motor

Pulleys/gears.

[Themolehole_9876]

But if i gear it down.. wont it make it easier to jam up the motor??? cause it wont be as fast so it will be easier to stop with a heavier load

[pyrotrev]

No, it won't make it easier to stop it. If you gear something down, as the speed goes down, so the torque (= how heavy a load it will turn) increases.

[pyromaniac303]

What I use to increase grip on my mill jar is a type of rubber band that is used to hold the wings on model aircraft. They are over 1cm wide, quite thick and I've never had one snap during milling.

To attach it to the mill jar you will need a glue that doesnt make the rubber perish. Quite hard to find, but test several different glues on conventional cheap rubber bands before commiting it to your expensive mill jar bands.

I found a foam safe type of cyanoacrylate (superglue) worked well.

The type I have fits well over a 11cm outside diameter mill jar, but I think there are other sizes available from model shops.

[Themolehole_9876]

Okay, great thanks a lot guys! ill see what i can do over the next few days then report back!

[karlfoxman]

We built our mill with high torque in mind, the motor is an AC indution motor 3 phase (modified windings for single phase) that already has been geared down to 90rpm. The output shaft goes through a worm gear and this gives us huge amounts of torque. The drive is a direct coupling to one roller about 30mm diameter, we then have a free runninng roller parallel to it. The motor never stalls and will turn huge loads of 3 fully loaded mill jars. If you build one you may as well build one to last a long long time. Check ebay for AC motors, if they are not already geared down its easy to do. Good luck with your mill.

Edited by karlfoxman, 04 June 2006 - 06:53 AM.

[littlejohny]

We built our mill with high torque in mind, the motor is an AC indution motor 3 phase (modified windings for single phase) that already has been geared down to 90rpm. The output shaft goes through a worm gear and this gives us huge amounts of torque. The drive is a direct coupling to one roller about 30mm diameter, we then have a free runninng roller parallel to it. The motor never stalls and will turn huge loads of 3 fully loaded mill jars. If you build one you may as well build one to last a long long time. Check ebay for AC motors, if they are not already geared down its easy to do. Good luck with your mill.

So what is the speed of the mill jars, 90 rpm into a worm gear would have a very slow output speed would it not, I dunno i'm probably wrong

[karlfoxman]

So what is the speed of the mill jars, 90 rpm into a worm gear would have a very slow output speed would it not, I dunno i'm probably wrong

No 90rpm is the the speed of the output shaft, we get around 34rpm on the mill jar from what I remember.

[Phoenix]

That is pretty slow, unless it's a whopping great big jar!