Sound post patches, what shape?

[Conor Russell]

Hi all.

I have started on the repairs to a nice old cello. It's in fairly good shape, apart from some very poor old repairs to some damaged ribs, worn edges, a broken out button, and some minor cracks in the front.  It has a short  sound post crack in the front that will need a patch. 

When I first started, we learned to make a post patch oval, rather like a spoonful of wood removed and replaced. Later, in a good shop, I was shown to make a more square shape, with rounded corners obviously, but distinctly squared off. It never sat well with me, because of the amount of perfectly sound wood that had to be replaced, but especially because the patch would have a longer endgrain joint running east west at both ends.

Can anyone explain the rationale behind the square shape,  and change my mind?

[jacobsaunders]

2 hours ago, Conor Russell said:

 

Can anyone explain the rationale behind the square shape,  and change my mind?

No, I have never understood that either, and make oval ones, or in instruments that already have a patch, make it the same shape as the old one

[Brad Dorsey]

My teacher made SP patches rectangular with rounded corners, but I have shifted to the shape shown in the Weisshaar book -- straight parallel long sides running with the top grain and semi-circles on the ends.  I think I went to this shape just because it's easier to lay out.  Would oval or elliptical be better?

[Michael Darnton]

I like oval because it lacks stress risers.

[martin swan]

Given the strength of side grain glue joints, I'm not sure of the benefits of feathering the sides of a post patch. It seems to me that it could be fully inlaid at the sides, quite narrow, and only feathered at the ends (in other words giving the appearance of a long rectangle ...). I'm also not sure of the wisdom of offsetting the grain direction.

But I'm also very interested by something Michael said recently, echoed by an article by Christian Schabbon in this month's Strad, suggesting that table post cracks should not be patched if there's a clean new break. This makes a lot of sense to me.

[Don Noon]

12 minutes ago, martin swan said:

But I'm also very interested by something Michael said recently, echoed by an article by Christian Schabbon in this month's Strad, suggesting that table post cracks should not be patched if there's a clean new break. This makes a lot of sense to me.

If the glue is stronger than the crossgrain strength of the wood, and the glue doesn't creep, and you can get a full gluing of the crack, I don't see any theoretical problem.   I don't think it's possible to get absolutely all of the crack glued, as the ends of the crack taper down to zero and you can't get glue in there, but the ends of the crack are also where the stresses are low and unlikely to start cracking again.

[Michael Darnton]

The center seam is basically a crack with two unrelated pieces of wood on either side. A fresh crack is the same, except that all of the problems of unrelated wood--expansion, fit, etc.--are missing, and each side is intimately keyed to the other side, physically. What better glue joint could exist? So if you don't feel that center seams need studs, why should cracks. As Don says, glue is supposedly stronger than wood. So where's the problem with at least trying to let the glue do its job? The sound  post is a special case, but we don't have enough data to prove that a patch is actually needed. And patches have their own problems over time.

I'm not sure how poorly the glue gets to the ends of the cracks. That's a legitimate concern, and yet we've seen over centuries that well-glued cracks hold, so there's that fact to deal with.

I think the bigger problem is getting any crack well registered and well clamped, and that seems to be the biggest variable in how well the glue holds. Yet we all get to see poorly-aligned cracks that have been holding fine for decades, and even resist being re-opened to be properly glued with all their might.

[Jeff White]

1 hour ago, Michael Darnton said:

The center seam is basically a crack with two unrelated pieces of wood on either side. A fresh crack is the same, except that all of the problems of unrelated wood--expansion, fit, etc.--are missing, and each side is intimately keyed to the other side, physically. What better glue joint could exist? So if you don't feel that center seams need studs, why should cracks. As Don says, glue is supposedly stronger than wood. So where's the problem with at least trying to let the glue do its job? 

Well stated.  I have moved away from cleating, more, and more.  The caveat being "the first one to glue it" correctly, or am I "redoing" one.  In the latter, I almost always feel I have to cleat, as the joint is so compromised....usually.

[David Burgess]

1 hour ago, Jeff White said:

Well stated.  I have moved away from cleating, more, and more. 

I have not, because I don't see that cleating (within reason) does any harm. And it's completely reversible, should someone in the future come up with a better idea, or feel that the cleats are no longer necessary.

To me, a centerjoint during making is a different deal from repairing a crack, with far fewer variables and challenges. There is no problem getting glue throughout every part of the joint; alignment is not super-critical; and it pretty much goes the same way every time.

[jezzupe]

Oval or nothing at all.

[Conor Russell]

Thanks everyone!

[nathan slobodkin]

On March 21, 2020 at 3:25 AM, Conor Russell said:

Hi all.

I have started on the repairs to a nice old cello. It's in fairly good shape, apart from some very poor old repairs to some damaged ribs, worn edges, a broken out button, and some minor cracks in the front.  It has a short  sound post crack in the front that will need a patch. 

When I first started, we learned to make a post patch oval, rather like a spoonful of wood removed and replaced. Later, in a good shop, I was shown to make a more square shape, with rounded corners obviously, but distinctly squared off. It never sat well with me, because of the amount of perfectly sound wood that had to be replaced, but especially because the patch would have a longer endgrain joint running east west at both ends.

Can anyone explain the rationale behind the square shape,  and change my mind?

I have been thinking about this one and think it may be an advantage to have straight areas of the shape to line up positioning blocks and help with fitting. Can't be that much help and I think the minimilazation of removing original wood probably tips me toward the oval shape.

[Ganymede Piggot]

how do you make a patch that exactly matches the "hole" from removing wood?

 

p.s. if it involves chalk fitting and you had to do a lot of it, it seems like it would be a good idea to make matching cutters for the instrument and the patch.  spherical with some carefully chosen radius probably

[nathan slobodkin]

6 hours ago, Bill Merkel said:

how do you make a patch that exactly matches the "hole" from removing wood?

 

p.s. if it involves chalk fitting and you had to do a lot of it, it seems like it would be a good idea to make matching cutters for the instrument and the patch.  spherical with some carefully chosen radius probably

Yes traditiionally fitted with chalk .

Using any kind of machine cutting to thin the plate of an expensive or important fiddle down to 0.3 mm would be  a bit hairier than I would care for.

Some people now using computer operated routing to match the male portion to a hand cut patch bed.

[Jeffrey Holmes]

Done oval and straight sides... properly installed they both work quite well.  I'm most comfortable with the straight sided variety, but I'd do an oval patch if I thought it appropriate for the repair at hand.

Square with rounded corners never made sense to me.

Whatever you do, do it well.

[Don Noon]

5 minutes ago, nathan slobodkin said:

Some people now using computer operated routing to match the male portion to a hand cut patch bed.

Catching up to where dentistry has been for years now.  I enjoyed watching my dentist's CNC machine grind up a ceramic crown while-U-wait, after the bad part of the tooth was ground away by hand and imaged in-place.  No need for multiple visits.

[duane88]

I asked my dentist what he was doing with his CNC during his closed hours. He didn't seem open to my using it.

[Ganymede Piggot]

On 3/24/2020 at 10:28 PM, nathan slobodkin said:

Using any kind of machine cutting to thin the plate of an expensive or important fiddle down to 0.3 mm would be  a bit hairier than I would care for.

maybe not.  precision contraptions can be turned by hand.  is it really necessary to go that thin?

 

[jezzupe]

21 hours ago, Jeffrey Holmes said:

Done oval and straight sides... properly installed they both work quite well.  I'm most comfortable with the straight sided variety, but I'd do an oval patch if I thought it appropriate for the repair at hand.

Square with rounded corners never made sense to me.

Whatever you do, do it well.

Showoff! 

[Mark Norfleet]

On 3/24/2020 at 10:31 PM, Jeffrey Holmes said:

Done oval and straight sides... properly installed they both work quite well.  I'm most comfortable with the straight sided variety, but I'd do an oval patch if I thought it appropriate for the repair at hand.

Square with rounded corners never made sense to me.

Whatever you do, do it well.

 

The patch looks great Jeff, but the cleats hurt my head.  Why do you make them with such thick strong edges?  What am I missing?  I do see that they may be staggered along the grain.

[Jeffrey Holmes]

33 minutes ago, Mark Norfleet said:

The patch looks great Jeff, but the cleats hurt my head.  Why do you make them with such thick strong edges?  What am I missing?  I do see that they may be staggered along the grain.

Yup.  They're staggered.  

I think the direction the light is shining exaggerates the edges a little, but they were full when I snapped the photo. These cleats aren't a style I normally use for cracks (though others do and they work pretty well).  I was playing with the shape on this 'cello as I needed to cleat under a bar on another, and felt it might be easier to relieve the bar to fit to this shape.  I found it was.  I did reduce the edge in the end.

[jezzupe]

4 hours ago, Jeffrey Holmes said:

Yup.  They're staggered.  

I think the direction the light is shining exaggerates the edges a little, but they were full when I snapped the photo. These cleats aren't a style I normally use for cracks (though others do and they work pretty well).  I was playing with the shape on this 'cello as I needed to cleat under a bar on another, and felt it might be easier to relieve the bar to fit to this shape.  I found it was.  I did reduce the edge in the end.

When you do a repair like this, are you fussing over the piece of material you use for the patch? age, weight, that sort of thing. It is a very perfect looking repair.

[Michael Darnton]

Regarding cleats, I have been making the bevels very long and scooped so that only the middle is full strength, and the rest is like a graduated spring. That way, there's no sudden drop in support at the edge of the cleat. My cleats are twice as long as wide, and the scoops start in the center about 1 mm apart. I don't know if it matters, but it looks cool.

I tried pointed cleats a while ago under a bass bar, and discovered I like fitting round ones much better. Which is too bad, because I think the pyramid shape looks a lot cooler!

[Jeffrey Holmes]

1 hour ago, jezzupe said:

When you do a repair like this, are you fussing over the piece of material you use for the patch? age, weight, that sort of thing. It is a very perfect looking repair.

Past being old enough to be stable (which pretty much all my repair wood is), I'm not that fussy about the age.  I do try to get the grain close, and the color being close is a nice bonus.

[Jeffrey Holmes]

1 hour ago, Michael Darnton said:

Regarding cleats, I have been making the bevels very long and scooped so that only the middle is full strength, and the rest is like a graduated spring. That way, there's no sudden drop in support at the edge of the cleat. My cleats are twice as long as wide, and the scoops start in the center about 1 mm apart. I don't know if it matters, but it looks cool.

I tried pointed cleats a while ago under a bass bar, and discovered I like fitting round ones much better. Which is too bad, because I think the pyramid shape looks a lot cooler!

I've played around with lengthening the bevel as well... but haven't gone near 1 mm (I think I usually end up at about 3 or sometimes approaching 4 for violin).  I also played with shape and (for the time being) settled on a few I like.

Bass bar cleats: Yes...  like most things, some like 'em one way, others like then anther way. I found the pointy ones easier... especially on 'cellos.