"Dan Bollinger" wrote in message
...
Hmmmm, my book says sand with less pressure at whatever speed you care
and you'll get less heat. Coefficient of friction being basically the
same, (might actually be less at low pressure) it all depends on
pressure, not speed.

George, what you say is true, but only for limited ranges of pressure and
speeds. Few things in nature are linear. My book, The Machinist's
Handbook, says this about friction:

For low pressures the friction is directly proportional to the normal
pressure between the two surfaces (Just as you say), but goes on to say:

At very low velocities the friction is independent of the velocity
(similar to what you say)

For high velocities the friction (and coeficient of friction) decreases
(the opposite of what you say)

The friction is independent of the areas in contact.

What this means for heat buildup is that pressing with the same force with
three fingers instead of one yields the same friction (and stock removal)
but it generates less heat per surface area since it is spread out over a
larger contact area. Going faster reduces the friction (and therefore
heat buildup), leading to longer abrasive life, and at the same time the
increased speed will have a cooling effect upon the surfaces, too.1

Well, no. Five pounds per square inch pressure is still five pounds per
square inch whether you have six square inches in contact or three. If you
had a constant weight (pressure) what you said about reducing the force per
unit of area would be true. That's why the corrugated sole on the jack I'm
using to level some panels just now generates essentially the same friction
as a solid one, less a bit of suction on its own, but different depending on
how hard I press. I am assuming, of course, that 200 pounds of Polack is
not capable of exerting the consistent pressure of his entire bulk - a
limiting factor - in this case or with the sandpaper.

Factor in the next obvious step, Dan. As you press, you dig in deeper,
burying the grit more to the backing, increasing the coefficient of friction
and resistance. If you don't press, your depth of "cut" is less,
coefficient of friction less, and so forth to the obvious conclusion.