[CAUT] Fw: capo bar reshaping

[Alan McCoy]

Hi Paul,

The email came through the first time. But the issue is a can o worms. The
proper shape of the capo depends on who you ask. If it is too pointy and not
hard enough, you quickly get grooves, which gives the capo a too flat
bearing surface. If it is too flat (as it is when grooves have been cut into
an otherwise sharp profile) then you will probably have noise (string
sizzle) at the capo. Also part of the puzzle is the angle between the string
plane and the duplex segment up to the counterbearing. If it is shallow it
won't cut grooves as quickly, but it may not terminate the string properly.
If it is steep, there is a tendency for cutting grooves. So what you need is
a decent counterbearing angle (anywhere from 12 deg to 20 deg though you'll
get arguments aplenty about what's the best angle) and you can get away with
a shallower angle if the counterbearing segment is shorter (long and shallow
will often give duplex noise especially if the length of that segment
happens to be harmonically related to the speaking length). Also you need
the capo to be properly hardened. And it needs to be profiled to some
radius. Talk to Ed McMorrow and he will advocate a very sharp (0.5mm radius,
I think). While others will advocate more like 1.5mm (about 1/16"). Me? I
use a Stewart MacDonald diamond fret file and file it to probably around 1
to 1.5mm radius. As a rebuilder (of sorts) and somewhat pragmatic I worry
less about the capo itself and more about the counterbearing angle and
length of duplex segment. That is really the only easily changeable part of
the equation. I know that some will harden the capo, but I don't have that
skill yet. But I do alter the front duplex when I think it will be some
benefit. Just finished grinding off the counterbearing bars of the piano in
my shop and will replace the bars with half-round brass. The angle won't
change much at all, but the segments will be significantly shorter (less
than 30mm my eyeballs tell me, though I haven't measured yet).

Like I said, can o worms.

Alan


-- Alan McCoy, RPT
Eastern Washington University
amccoy at mail.ewu.edu
509-359-4627
509-999-9512


> From: Paul T Williams <pwilliams4 at unlnotes.unl.edu>
> Reply-To: "College and University Technicians <caut at ptg.org>" <caut at ptg.org>
> Date: Wed, 10 Sep 2008 12:29:47 -0500
> To: <caut at ptg.org>
> Subject: [CAUT] Fw: capo bar reshaping
> 
> I'm not sure if this went through yesterday.  I'm trying again.
> ----- Forwarded by Paul T Williams/Music/UNL/UNEBR on 09/10/2008 12:28 PM
> -----
> 
> Paul T Williams/Music/UNL/UNEBR
> 09/09/2008 01:23 PM
> 
> To
> caut at ptg.org
> cc
> 
> Subject
> capo bar reshaping
> 
> 
> 
> 
> 
> Hi List,
> 
> I'm working on a plate from a 1926 Steinway M that had been breaking lots
> of string along the capo.  Both sections broke about the same number of
> strings per semester.  Upon pulling the plate and looking carefully at
> both sections, the top section was really flat (plus grooved and crusty)
> and the lower section was more pointed like we like, but way deeper
> grooved (and just as crusty). So, my conclusion is that both too deep of
> string groove and too flat a capo surface experience the same amount of
> string breakage.
> 
> Now, I can't remember the width of the "peak" of the capo, but seem to
> think along the lines of 0.5mm-1.0mm (top to bottom of whole capo)
> 
>  Is this a bit too sharp? Could someone please remind me the proper
> "shape" before I begin filing?  Do all the string grooves need to be 100%
> gone?
> 
> Thanks for the input!
> 
> Paul