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Bill[_47_]

wrote:
On Sat, 02 Aug 2014 01:47:10 -0400, Bill
wrote:

Grant Edwards wrote:
On 2014-08-02, Ed Pawlowski wrote:
On 8/1/2014 8:25 PM, Grant Edwards wrote:
On 2014-08-01, -MIKE- wrote:
On 8/1/14, 3:45 PM, whit3rd wrote:
But when you want to REMOVE a screw, you don't
want to apply a push (this hurts you because it increases friction).

I don't get this. Why not push when removing?
Because the objective is to _raise_ the screw. Pushing down on it
makes that harder to do and thus requires more torque to raise it. To
get more torque without cam-out you have to push harder, which then
requires more torque to raise the screw, which requires you to push
harder...

Theory aside, when the head is buggered, pushing gets it out, not
pushing makes the bit slip. One law of physics out does the other.
True. But the point is you're better off using a screw head and bit
design that minimizes the amount of pushing required -- especially
when removing a screw since the pushing is forcing the screw in the
"wrong" direction.
Since the amount that you wish to raise the screw head with a twist of
the wrist is very, very, small, I think the
effect you describe may be negligible. Of course, the wider the
threads, the greater the effect. I could probably
design a screw that would be difficult to unscrew by pushing down on
it! ; )

It's not. Friction is proportional to the normal force applied, so
force applied to the head of the screw does increase the friction of
the screw threads against the material.
I agree with you. I think there are several factors at play here.