[pianotech] Design Hammer Bore Angles; was: Simulated Nose Bolt

[Terry Farrell]

Thanks for your input Ron. Comments below:

Terry Farrell

On Nov 2, 2012, at 9:10 AM, Ron Nossaman wrote:

> On 11/2/2012 7:30 AM, Terry Farrell wrote:
> 
>> To the best of my knowledge, it seems to me that the hammer boring
>> for an American square grand piano (or any piano) should result in
>> the diameter axis of each hammer either being parallel to its
>> respective string(s), or if not parallel, then purposely angling the
>> hammer diameter axis a few degrees toward parallel with the hammer
>> shank. Is that correct? Should hammers be bored so that there is
>> always some appropriate bias toward the hammer diameter axis being
>> more parallel with the hammer shank?
>> 
>> That's what I observe on all vertical and modern grand pianos, and
>> makes sense for clearance. I believe the same applies to a square
>> grand.
> 
> Well, I'm not an action designer, but I'm a fair to middlin' mechanic. Look at the difference between the angles of the strings to the shanks between modern pianos and that square. You have a whole different world there, and nothing you do will make it work like a modern piano, or even an intelligently designed mechanism.

Absolutely. I'm in full agreement there.

> Compare the old hammers with the new and see how the angles match. If they do, they aren't mis-bored.

Well, therein lies part of the problem. Hammers were replaced some decades ago with a set of hammers that were bored and shaped by a kindergarten arts & crafts class. The new ones are pretty close to the bad old replacements, but I assumed (yeah, I know) that the boring would be based on the string grooves, not the replacement boring. That appears to be where the improperly bored angles come from.

> If they aren't mis-bored, rejoice that you don't have to learn the hard way from scratch what they need to be to work.

So, it appears that I do have to learn the hard way from scratch what they need to be to work. And hence, the question.

>> On the (new) set I have, the bass hammers are close to parallel with their
>> respective bass strings, but starting in the low tenor and going up
>> the scale, the hammers start to diverge from being parallel with
>> their respective strings, but the deviation from parallel with the
>> respective strings is toward the diameter axis of the hammer being
>> perpendicular to the hammer shank - not parallel with the hammer
>> shank. I'm sure you can easily imagine what kind of hammer clearance
>> problems that produces.
> 
> The clearance problems should be worst in the bass and low to mid tenor.

Right. But that's not what's happening because the bass are bored so the hammers are parallel with the bass strings (no deviation), but in the mid tenor, where the strings are more parallel with the shanks, the hammer boring makes the hammers not parallel with the strings, but the hammer angles deviate toward being more perpendicular to the shanks than the string angles.

> Depending on the bore angle, your choice is between only hitting one unison with the hammer, or having them clear one another. Theoretically, there is a compromise where each is possible, if you squint just right and are good with a sharp knife and sanding paddle.
> 
> At the absolute very best, the hammers BARELY clear and BARELY miss adjacent unisons. Plan on being there for a while. Except for certain pathological cases, aversion therapy works!

I agree wholeheartedly with all that. But I still have the basic question:  Assuming I am thinking clearly here, what kind of maximum bias toward parallel with shank from parallel with strings is appropriate? I realize you can't go too far because you want the hammer contact point to be close to a line at the apex of the hammer, but how far is too far (assuming the hammer doesn't end up hitting something else)?

I guess I'm looking for a number, if there is any, that would apply to a normal piano action (not a square). I know I will have to use that kind of information and see how it might apply to my square grand. I know that no one is going to give me a number for the square. But maybe my most basic question is this: the existing hammers in the tenor are bored so the hammers are about 10 degrees toward perpendicular to the shanks from parallel with the respective strings. My common sense tells me this is WRONG - plain flat wrong. My common sense tells me hammers should be parallel with the strings where clearance allows, or angled slightly toward parallel with respective strings to improve clearance. I'm hoping that some background on how designers choose hammer boring angles in normal pianos will help guide me in my quest to make some intelligent decisions on my square.

Thanks so much for sharing your thoughts.

Terry Farrell
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