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Spool clamp force
How much pressure does a well-applied spool clamp assert?
Topic Rating: 5 Topic Rating: 5 Topic Rating: 5 Topic Rating: 5 Topic Rating: 5 Topic Rating: 5 (2 votes) 
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Peter
West Sussex, England UK
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October 14, 2019 - 7:31 am
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I am a few days away from my first rebuild of a violin, and I need some advice on the technical aspects of clamping the table.

The usual method for securing the table is to apply a set of spool clamps to maintain pressure on the joint until the glue has cooled and set. I am following a different route, and will be using bespoke 12 mm plywood clamp plates to both support the back's edges and to apply pressure to the table edges, with a 3 mm closed-cell foam load-spreading layer around the edge. The pressure will be applied to the upper plate's centroid with a weight. The motivation is simple: this is my first rebuild, and I'm supremely nervous about the open-time of the glue, and the proposed method affords a near-instant close-up.

What weight should I apply? Logically, it's the sum of the set of spool clamps' individual contributions, and I was wondering if anyone knew what the single spool clamp pressure is.

Peter

"It is vain to do with more that which can be done with less"  - William of Ockham

"A crown is merely a hat that lets the rain in" - Frederick the Great

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Peter
West Sussex, England UK
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October 15, 2019 - 8:04 am
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Please accept my apologies for such an impertinent request.

I have since found out how to replace the table without any panic, and with complete control, using spool clamps:

I shall humbly slink away and behave myself.

Best wishes to all,

Peter

Peter

"It is vain to do with more that which can be done with less"  - William of Ockham

"A crown is merely a hat that lets the rain in" - Frederick the Great

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Irv
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October 15, 2019 - 8:53 am
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I would have cautioned against using a central clamp.  Unless the ribs were secured in a mold, there is a fair amount of flex and multiple clamps are needed to assist alignment.  

Several foot pounds of torque is applied to each clamp, which is of course multiplied based on the pitch angle on the screw (an inclined plane).  Summing that over the number of clamps, I suspect the total is several tons.  The violin is a rugged structure.

Without deviation from the norm, progress is not possible.         —Frank Zappa

It is unpleasant to be thought so uncleverly unclean and capable of poisoning a whole city.—Sir Walter Scott

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Peter
West Sussex, England UK
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October 15, 2019 - 9:44 am
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Hi @Irv,

Thank you for your reply. I did the mathematics this morning, using standard engineering formulae, and the figure I arrived at was 7,500 N (around 3/4 tonne). You're probably right about the strength of the violin in that plane, but I am unwilling to take a punt on my antique fiddle taking that pressure.

I watched the chap in the video this lunchtime (UK; BST) carving a scroll, and noticed he had other violin-making videos. The linked video answered my questions, and apart from initially feeling foolish about the idea of using two single clamp plates (essentially moulds supporting the joint directly over the ribs all round), it'd probably work.

However, I've cut and drilled some spools, and I have some M8 bolts and wing-nuts. I also have my glue-pot ready (home-made, of course), a bag of glue granules and enough cleat material to splint those cracks.

In terms of 'spool clamp force', the man in Cremona says, "Always tighten the clamps slightly, applying as little pressure as possible, as not to cause damage to the edges. If the seam is well made and fits, the force required to hold in place the plate is minimal". I guess the idea is to just close the joint, and provide a sensible joint thickness rather than extruding all of that lovely stuff.

Cheers,

Peter

Peter

"It is vain to do with more that which can be done with less"  - William of Ockham

"A crown is merely a hat that lets the rain in" - Frederick the Great

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Irv
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October 15, 2019 - 4:27 pm
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I protected the ribs from the threads of the clamps with plastic straws cut to length.  You can extend the working time of hide glue by placing incandescent infrared lamps over the work (or use conventional reflector lamps at a reduced voltage, which produces the same effect).  

Fracture of the glue joint is preferred over a wood crack.  Tertis had his luthiers glue a strip of newsprint between the plates and the ribs to lessen the inherent strength of the joint.  That always made sense to me.

Check out my thread on making a diy case humidifier.  I will include some goretex cloth in with the care package.

Without deviation from the norm, progress is not possible.         —Frank Zappa

It is unpleasant to be thought so uncleverly unclean and capable of poisoning a whole city.—Sir Walter Scott

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Fiddlerman
Fort Lauderdale
October 16, 2019 - 5:14 am
Member Since: September 26, 2010
Forum Posts: 15166

I guess you figured it out already. I just wanted to say that you would probably have a mess if you tried to quickly apply glue to the whole surface and then quickly clamp it together. The thought was good but you don't want to be rushed when doing this. It's better to have the time to do it perfectly.

"The richest person is not the one who has the most,
but the one who needs the least."

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Peter
West Sussex, England UK
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October 30, 2019 - 9:13 am
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Hi all,

Just to close-out this thread, I'd like to report on the success of the repair, and the clamps.

The clamps worked just fine, and I found that applying enough torque to them to maintain their purchase on the plates and intervening rib was enough to give a good joint. Removing groups of clamps and wiping the glue in with a thin, rounded blade as shown in the video above was a relaxed and straightforward process, and the result is a tight joint all round without excessive ooze within the body. On the matter of the precise torque value, I'd say two white knuckles, like assembling an aluminium greenhouse with aluminium nuts & bolts. Yes, it's a 'feel' thing.

I've bagged the clamps and stored them for whatever I need them for next (an itch to make a Gretch-style guitar needs scratching), and I now have a playable fiddle once more.

Peter

Peter

"It is vain to do with more that which can be done with less"  - William of Ockham

"A crown is merely a hat that lets the rain in" - Frederick the Great

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Fiddlerman
Fort Lauderdale
November 4, 2019 - 10:01 pm
Member Since: September 26, 2010
Forum Posts: 15166

Congrats Peter. 😁

"The richest person is not the one who has the most,
but the one who needs the least."

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