Re: Milling Media Geometry


<steve.g.hall@xxxxxxxxxxx> wrote in message
news:1153118074.856956.287960@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
John,

The shape of the media is probably an important factor. With balls,
there are small points of contact between them. All of the weight of
the balls is concentrated on this smallpoint, which probably crushes
the BP more efficiently. Cylinder shaped media has much larger contact
areas, which spreads the force along a wider area. So, the force per
unit area would be higher with balls.

No, not really. The average dimension of the point of contact between two
radiused cylinders is a mathematical point, with one singularity of a
mathematical line, if the cylinders make contact when perfectly parallel.
Radiused-end cylinders offer a slight improvement in milling speed over
spherical media, in that they have slightly greater surface area per volume
than do spheres.

The higher density of the lead would also contribute to faster
grinding.

Also, I wonder if 3/4" media would be larger than optimum for a 3"
diameter ball mill jar. With larger media, there would be fewer contact
points to do the grinding. This is probably also a factor. What I don't
know, is what the optimum size media would be for a 3" jar.

This is the issue. Three-quarter-inch media is much too large for a 3" i.d.
jar. Media should be sized such that there are at least 10 media diameters
per jar diameter. Otherwise, the necessary 'cascading' action will not
occur, and impacts between balls will be infrequent.

3/4" media works well in 6" jars, but technically, it's a bit large. 1/2"
media works better, if the material to be ground is friable enough to crush
with that small a ball.

LLoyd


.

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