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#634159 - 07/25/03 07:19 PM Caul sizes
JIMBOB Offline
1000 Post Club Member

Registered: 11/18/02
Posts: 1326
Loc: South Carolina
I am rebushing the keys on a Grand Piano spinet ( great play on words). I am pretty sure that I measured .142 on my mic but misplaced the paper . I'm going back before I rebush but here is the question . What size cauls should I use ? I have 146 but obviously they are the wrong size if it turns out the pins are 142 or 141 . I bought the 146 Spurlock cauls at a PTG convention in May because they are a "standard size". I haven't seen anything close to a 141 or 142. It is also quite humid here now in SC so I am concerned about the size of the felt as well as the cauls. Any advice would help.I have been taught that the pin size determines the caul size
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#634160 - 07/25/03 08:48 PM Re: Caul sizes
Rick Clark Offline
1000 Post Club Member

Registered: 01/04/03
Posts: 1810
Loc: North County San Diego CA
Yes the pin size determines the caul size. For center rail bushings I like a caul just a couple thousandths bigger than the pin. For the front rail holes more like 3 thousandths bigger or so. I keep a wide assortment of sizes.

Then you will need a few sizes of felt to take up the gap, because the holes tend to be unevenly sized on spinets.

I use brass cauls (hot) and hot hide glue for it's gap-filling qualities, and because it doesn't soak into the felt.

Regards,

Rick Clark
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#634161 - 07/26/03 01:27 AM Re: Caul sizes
JIMBOB Offline
1000 Post Club Member

Registered: 11/18/02
Posts: 1326
Loc: South Carolina
Rick... according to the notes I have the size of the bushed mortise will be exactly the same size as the caul used to bush it and the size of the caul used to bush is always based on the size of the pin. At the balance pin we are trying for a free fit with no play so ideally the caul would be the same size as the pin. When rebushing in the summer in high humidity the guidance is to leave less clearance at the pins because the keys will loosen up in the winter. The accucaul chart I have shows stock sizes .088; .131; 136; 139;; 145; 152 and 160.
Spurlock stock single shoulder cauls sre fitted for pins sizes .087; 125;129;131;133;137;146;152 or 162. It seems that I need a size smaller than the 146's or I will be asking for loose fit. Do I need to get some custom cauls or will 139's work ?
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#634162 - 07/26/03 11:02 AM Re: Caul sizes
Rick Clark Offline
1000 Post Club Member

Registered: 01/04/03
Posts: 1810
Loc: North County San Diego CA
If I had 142 pins I would go the next size up with a caul, so 145s. The bushing hole doesn't size exactly the same as the caul because as long as you don't botch the gluing, it's cloth and it's squishy and has nap. If you select the right size cloth it comes out just right.

A .003" difference will give a much better tolerance than the piano had originally from the factory.

I am basing this on brass hot cauls and hot hide glue. If things would work out differently with the Spurlock method, I can't say.

Regards,

Rick Clark
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#634163 - 07/26/03 05:44 PM Re: Caul sizes
JIMBOB Offline
1000 Post Club Member

Registered: 11/18/02
Posts: 1326
Loc: South Carolina
Thanks Rick.... Bill Spurlock called me back
(should I say cauled ?) . Anyway he said to try the 146 cauls with thicker cloth and keep them in
for about 10 minutes. He suggested bushing about an octave and then testing them out. If you leave the cauls in too long they will make a compression set in the wood. I'll try it out but before doing anything I'm going to look for the measurements or return and re-measure.
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#634164 - 07/27/03 03:28 PM Re: Caul sizes
BDB Offline
Yikes! 10000 Post Club Member

Registered: 06/07/03
Posts: 22453
Loc: Oakland
Compression set in the wood from the cauls? If that is the slightest possibility, the felt is too thick.

This is not rocket science. You are talking about the thickness of a piece of paper, in a piano that undoubtedly doesn't even have the mortises lined up within those tolerances, in a climate where the humidity will probably cause a bigger swing in the mortise width. The unequal forces due to the bend in the keys will probably affect the result more.

Don't worry so much about it. You will probably do a much better job than was done in the piano in the first place.
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