Perhaps the reason for "rake" is this:

[gordon stelter]

When a key is gently struck the hammer butt moves
slowly, and the shank and hammer move accordingly with
little flex to the shank. But when a key is hit hard,
the shank precedes the lagging hammer a bit, and
flexes in the process.
    By designing "rake" into the system, it is assured
that the apex of the hammer crown, upon a harder blow,
hits the prime striking point: not above or below. It
also imparts extra strength to the system, as more
strain would be put on the shank were a hammer bored
at exactly 90 degrees. Then, as the hammer shank
flexed on a very hard blow, the hammer would hit the
string from beneath the crown area, imparting a
"prying" force to the hammer head/shank juncture and
more likely snapping it.
    I believe that these two factors, working in
tandem, are the prime consideration for using "rake"
in hammer boring.
     Thump

--- Richard Brekne <Richard.Brekne@grieg.uib.no>
wrote:
> 
> Jim, Keith, and Roy.
> 
> Thanks for prodding my brain a bit. I actually saw
> what it was I misunderstood a
> little while after writing my last. And by doing
> much the same as you both
> suggest. I had thought that Jim was outlining some
> requirement for the momentum
> of the hammer crown to be perpendicular to the
> string at impact at the same time
> as a hammer with a 90 degree rake  was to be
> perpendicular to the string. Of
> course I couldnt make this add up :)
> 
> But I do still  have a little itch on my left
> temple. Jim mentioned that it
> would be desirable to put the center pin on the
> string plane for both grand and
> uprights. That would indicate to me he's got
> something else in mind then just
> the gravity assist on rebound for an upright.
> 
> I think the diagram below shows what I am confused
> about here. While the tip of
> the hammer would certainly have its momentum
> orientented horizontally, the angle
> of the hammer itself is far from perpendicular to
> the string at impact. And the
> longer the hammer bore the larger that angle would
> be. Isnt that going to cause
> several other problems ? It would have an increased
> tendancy to stress the joint
> of hammer and shank. The hammers center of gravity
> wouldnt be perpendicular
> either... or what ?
> 
> Perhaps it is a misconception, but it has always
> been my understanding that the
> maximum amount of energy that the hammer can impart
> to the string occurs when
> the shank is paralle to the string, and the hammer
> perpendicular to both. Jims
> post seems to be saying something else. Or what ?
> 
> 
> Cheers, and thanks
> 
> RicB
> 
> [Image]
> 
> Keith Roberts wrote:
> 
> > Try this. Take a hammer/shank/butt and with a pin,
> affix it through the
> > centerpin to a piece of paper. Scribe the arc made
> by the end of the shank,
> > Then drill a hole through the center of mass of
> the hammer and scribe that
> > arc. You can see that the center of mass is moving
> down at an angle to the
> > strings when the bore distance is set up at the
> center pin distance. If you
> > cut loose this missile (which is sort of what
> happens when let off occurs)
> > it would immediately begin to tumble......
> > Keith Roberts
> 
> >
> > Ric:
> > The crown of the hammer would move in a different
> arc than that of the shank.
> > When the hammer shank is vertical, and the arc it
> travels in is at it's apex,
> > the tip of the hammer is on a downward descent.  
> Draw a straight line between
> > between the tip of the hammer and the hammer
> center pin.  When that line is
> > vertical, the tip of the hammer is moving straight
> into the string.   The
> > shank would be tilted back.   Thus, putting the
> center pin closer to the
> > string means that the tip is hitting the string on
> a
> > more horizontal plane.  It makes sense.  Maybe
> it's a question of which arc
> > you want to worry about.
> >
> > Roy Peters
> 
> --
> Richard Brekne
> RPT, N.P.T.F.
> UiB, Bergen, Norway
> mailto:rbrekne@broadpark.no
> http://home.broadpark.no/~rbrekne/ricmain.html
> 
> 


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