"George" wrote in message
. ..

"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.

I've been following this thread and think this is a comparison of apples and
oranges.

I feel that a good point to start is an empirical test and then come up with
an explanation to fit the results.

Test 1. Rest your hand on a rotating smooth spindle of wood at a low speed
and at a high speed and decide on which your hand get hotter.

Test 2. Run the spindle at 500 rpm and grip lightly then tighten your grip
on the wood.

For those cautious ones who are concerned about health and safety, loosing
their hands, stripping the skin from their palms or suing me. The answer is
the faster it goes and the harder you grip it the hotter it gets (but you
knew that already).

If we are talking about sanding then the concern is kinetic friction which
increases with the force applied (as has been stated) and presumably varies
with the grade of sandpaper but is not dependant on the area of contact. It
is also a dimensionless quantity (it is just a number). So friction does not
generate heat, something else has to be happening also.

Checkout 'Energy of friction' at the bottom of the page on:-
http://www.answers.com/topic/friction-1

The energy (heat) = the normal reaction force x coefficient of kinetic
friction x distance travelled.

'The normal reaction force' = how much you press on the sandpaper
'Coefficient of kinetic friction' = depends on the wood and sandpaper
'Distance travelled' = in any unit of time, say 1 min = rpm x Pi x diameter
of the piece being sanded.

So the more you press on and the faster the lathe goes the more heat is
generated.

This would seem to tie the theory to empirical test.

OK finished now for those still reading this and I'm going to find my flak
jacket.

BillR