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Help determine knuckle-center pin distance #2920854 12/08/19 04:37 AM
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Apache Offline OP
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Hi all,

I have a piano that have old type of hammer shank where the knuckle is part of the hammer shank.
I want to replace the knuckle by machining: remove the old knuckle and create a slot for new knuckle. However I have difficulty determine the knuckle center-pin distance. I know if I get the distance wrong by a little can cause a wrong touchweight (Source: touchweight).
But the distance is really ambiguous as the knuckles are out of shape.
Please see the pictures attached:
[img]https://www.dropbox.com/s/jup8qqjanvf6z32/2019-12-07%2014.54.33.jpg?dl=0[/img]
[img]https://www.dropbox.com/s/qyk9pseif8o9pd9/2019-12-07%2015.00.31.jpg?dl=0[/img]
The wood one is the hammer shank. The white(plastic) one is a model I created using common 17mm slot for comparing geometry. The second picture clearly shows a difference.
Because the model I made has a different hammer and weight (the shank itself, too), I couldn't confirm the knuckle distance by comparing the touchweight.
I would like to ask how is the knuckle-centerpin distance is determined in this kind of hammer shank. Please see an enlarged picture below:
[img]https://www.dropbox.com/s/csmuswj9bz21jqr/2019-12-07%2014.56.31.jpg?dl=0[/img]

The second question is what size of knuckle slot does modern hammer shank have? I don't have any to measure on my hand. The knuckle tongue I measured is 2mm width and 2.5mm depth. However, with a 2mm x 2.5mm slot, the knuckle is a bit loose sideways and the felt (buckskin) does not completely touch the hammer shank (a small gap, see below). The question boils down to whether the tongue end or the buckskin should transfer the force (from the jack) to the shank?[img]https://www.dropbox.com/s/cemfgbx4iaqvy3i/2019-12-07%2015.08.50.jpg?dl=0[/img]

Piano model: 1921 A.B.Chase Baby grand (~170mm)
Thanks ahead!

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Re: Help determine knuckle-center pin distance [Re: Apache] #2920913 12/08/19 10:01 AM
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Bill McKaig,RPT Offline
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Your second picture shows the knuckle lines up pretty well front to back, but not in height. You may need a smaller knuckle.

For alignment here's what I would do. A set of knuckles consists of 90 pieces. I would take on of the extra ones and sand off the tongue, take one of the original shanks and machine off the knuckle. Re-install the shank and use a straight edge between the neighboring original knuckles to align the new knuckle with its neighbors. Mark where the tongue should go on the shank.

Regarding your second question. The knuckle is supported by the tongue. However, I like to use enough glue to support the buckskin. You can cut the slot deeper, but with those old shanks I would try to leave as much wood as practical.

Last edited by Bill McKaig,RPT; 12/08/19 10:02 AM.

Professional Piano Technician serving the Tampa bay area. website: mckaigpianoservice.com
Re: Help determine knuckle-center pin distance [Re: Bill McKaig,RPT] #2921312 12/09/19 01:59 PM
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Thank you Bill. I need to double check the knuckle diameter first. Your suggestion sounds great. I'll make a sample shank to find the slot position.

Re: Help determine knuckle-center pin distance [Re: Apache] #2921343 12/09/19 03:38 PM
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Ralphiano Offline
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It seems that you might be making a lot of extra, unnecessary work for yourself by converting the piano to modern knuckle design.

Why not just replace the worn knuckle leather, and if needed, the underlying felt? The new leather and felt replacement would be massively cheaper than buying all those knuckles, and would require far less time, and risk, than your re-engineering idea. And, the piano's original design of jack-against-hammer leather felt is a time proven design as used in upright pianos.

Also, if you are unable to find knuckles that exactly reproduce the radius and height of the original hammer butt leather, you will have to engage in serious adjustments elsewhere to compensate for the increased jack-to-leather/repetition lever gap. Might this open a whole other can of worms for you to additionally investigate and solve?

Unless you have a very good reason for adding knuckles, I would stay with the original, simpler design.

Last edited by Ralphiano; 12/09/19 03:40 PM.

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Re: Help determine knuckle-center pin distance [Re: Ralphiano] #2921618 12/10/19 01:53 PM
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Hi Ralph, I wasn't aware I could replacing just the knuckle leather and felt. Could you point out the source?

The reason for replacing knuckles is that I believe the old knuckle is too much compressed height-wise, and expanded sideways. When I set a 1mm (.040") standard aftertouch (that is, 1mm key travel after letoff), the jack tip is still under the knuckle. The "letoff to clear jack" key travel almost exceeds 2mm. The jack-knuckle alignment is already set as far (out) as possible without cheating. This makes me believe the knuckle is in a wrong shape. The model shank with new knuckle has a very crisp letoff/drop and I have much room to set aftertouch. I tried bolstering the old ones but it doesn't help reducing the aftertouch key travel. Maybe I did it wrong, but I just feel it not accurate at all.

Yes it will be a lot of work. I'm going to use a small CNC engraving machine to cut each shank accurately, using maybe some 3d printed mounting jigs. Still planing.

Re: Help determine knuckle-center pin distance [Re: Apache] #2922046 12/11/19 05:12 PM
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Ralphiano Offline
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There are a couple of youtube videos showing the replacement of hammer butt leather and felt on uprights. This one seems best to me: Hammer butt leather replacement

Any piano supply house will have the leather and felt you need. A helpful, and regular contributor on this forum, kpembrook supplies materials, and there is a link at the bottom of his posts.

I have a baby grand piano from the same era that I am about to start on. It has the same configuration, using hammer butts instead of knuckles.

Last edited by Ralphiano; 12/11/19 05:13 PM.

Ralph

Kawai VPC1
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Re: Help determine knuckle-center pin distance [Re: Apache] #2922125 12/12/19 02:13 AM
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The knuckle on my shank looks more like the round knuckle on grand pianos (except non-detachable) than the upright knuckles. That is saying, the underneath wood core doesn't have round/curved shape.
[img]https://www.dropbox.com/s/jup8qqjanvf6z32/2019-12-07%2014.54.33.jpg?dl=0[/img]
I don't think it is easy to replace the felt and leather and recreate the round shape on mine.

Re: Help determine knuckle-center pin distance [Re: Apache] #2922235 12/12/19 12:17 PM
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You can work out the leverages by measurements and ratios. But what you want is for the front of the knuckle to be at the same point that it is on the old shank.


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