[pianotech] snugging up an agraffe

[Ed Foote]

I wrote: 

 >I sense torque with my hands when 
> installing agraffes,and when the feel goes soft,(threads beginnng to 
> bend away from the skirt), well before what I know to be a trustworthy 
> and solid seating has occurred, I measure. 
 >And yes, the undersize 
> contributes to this problem, a lot. 

Ron asks:>>I don't see how. I've yet to strip an agraffe thread, nor see how it's possible with the fit mismatches reported. How does the thread fit interfere with alignment? <<

    Not an alignment problem, a strength problem, as in an agraffe that jumps up a thread when the strings are pulled up to pitch. Upon measuring, the threads were too small for the hole.  I finally put an old one back in and it held just fine.  The differences between them were something like .008" at the thread.  It may have been a slightly oversized hole and the combo just didn't work, but all new agraffes seem to be considerably undersize.  
 
> I like to put a small touch of grease or wax on the threads, so that the 
> top of the stem isn't being stressed by friction as I flatten the skirt 
> to order. 
 
>>I use a drop of oil, but that's for the threads running into the iron, so they spin in and out more easily. I can't see how that alleviates stress at the top of the stem as the skirt contacts the counter bore. 

     As the agraffe is being cranked the last few degrees, the stem is being stretched, lengthwise. It is also having to supply the path for whatever force is required to turn the threads as they engage. I reduce the latter because I want to feel the seating stress, not the threads. Lubed threads for more accurate torque reading is not a new concept. 

> When the agraffe comes to its final position, it needs to be as tight as 
> possible without damaging the stem. 
 
>>This seems like a good time to ask a question I've been carrying around for a very long time waiting for an opportune time for insertion. Has anyone reading here ever actually found an agraffe that made noise that was verifiably found to result from not having been tightened enough? I've found agraffes that were finger movable without string tension, that didn't make objectionable noises, and have NEVER found an agraffe noise attributable to having not been tightened down enough. 
 So what's the real story? Any rationally evidential contributions out there.  

           There may be reasons for tightness beyond noise.   I can't see the benefit of an agraffe that can move during the stringing process, so there is an argument there for tight.   I have seen pianos with agraffes that were crooked, and upon unstringing, found them to be loose.  Did they turn by themselves, over the years?  The offset string angles to them may have had enough force, I don't know, but I am not going to make a case for anything loose in the terminations. 

Ed FooteRPT







 
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