Re: Milling Media Geometry
- From: steve.g.hall@xxxxxxxxxxx
- Date: 16 Jul 2006 23:34:34 -0700
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.
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.
Steve
John wrote:
For making black powder and Chrysanthemum 6 I use two types of milling
media in my 3" ID jars. One type is 3/4 " brass bar stock cut into 3/4"
lengths - small cylinders of brass. The second type is .5" caliber lead
shot. In each case I charge the jar half full of milling media and a
further quarter of the jar of composition.
I prefer the brass bar stock because I can put in charcoal just as it
comes out of the retort - no pre-grinding required - the cylinder shape
of the brass, it seems, grinds up the charcoal for me and saves me one
step. As well, the black powder doesn't cake onto the sides of the jar
when I use the brass bar stock. Because the media is large it's also a
bit easier to handle when separating out the media.
The one down side to the bar stock is that I have to mill longer.
Starting with coarsely ground charcoal I'll have hot powder in about 4
hours with the lead media. It takes more like 12 hours with the brass
cylinder media. This is not due to grinding up the charcoal - that
happens fairly quickly - less then an hour.
What I am wondering is what is causing the difference in the milling
times. There are three variables - the brass media is in cylinders
while the lead is in the form of spheres, the brass is quite a bit
larger then the lead and the brass is somewhat less dense than lead. At
this point I'm willing to discount the differences between densities
since brass is about 75% as dense as lead and the difference in milling
times is much more then that.
Is it the size of the brass media or the shape which contributes to the
longer milling time?
If it's the size I can probably hang out at my favourite scrap yard
until I find a smaller diameter brass rod. If it is the shape then I
guess I'm out of luck because I like how the cylinders cut up the
charcoal - it takes longer to have the cylinders grind up the charcoal
but it's a lot easier for me.
Thanks in advance.
.
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