Key pins and key bushings

[Ron Nossaman]

At 08:48 AM 5/28/99 -0400, you wrote:
>> When you turn a front rail pin you inevitably get a smaller contact
surface than if the pin "flats" were parallel to the bushings.
>
>Ron, think about this.
>
>Take a pin and cut it and look at it's cross section, you will see
>what I mean.
>
>The pin is shaped like a ball with the center section removed.  This
>is effective and inexpensive to produce.  Rotating the pin to take up
>some lost motion is perfectly acceptable unless the bushing is
>severely worn.


* Hi newton. Yea, I know. That was my first post, which I screwed up by not
saying what I was thinking. My apologies. If my second post didn't do any
better, I'll try again. You're right. With a new bushing, turning the pin
won't decrease the contact area, it just moves the contact point forward on
one bushing and backward on the other. With a worn bushing, turning the pin
decreases the contact area quickly, unless the key is very pully too. 



>The only time you will get a smaller contact area for the bushing is
>if the pin has a parabolic curve (absolutely useless and VERY
>expensive to produce) or a pin with a cheap rectangular cross section.
>
>The radius of the pin is constant on both sides so a rotation of the
>pin will present a constant tangent to the bushing _until_ _that_
>radius terminates.

* With a new bushing in a clean mortise, yes. With a worn bushing, the
contact area moves to the "point" at the edge of the pin wear track in the
bushing surface. 

>
>I prefer to rebush key myself because I do a better job than most but
>I still get some variation of fit which I will equalize by turning the
>pins slightly.
>
>Most pins do NOT have a consistent vertical shape so I test bushing
>and pin fit at the rest and depressed positions.
>
>		Newton

* I noticed just such a condition on the pin I measured to get that 0.010" @
20 degree figure in my second post. It's bigger at the top. 

I have a question. Why does that slop that can be taken up with a little pin
turn feel and look so much bigger than the mic says it is?

 Ron