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Tostra

Struggling with a cello top joint

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Hi all. I've been trying to join a cello top for a few days, but it's acting weird. I was hoping that some of you might be able to help me with it, or at least give me advice for future builds.
I have made two violins and one cello in the past. I don't remember the joining of the first violin, but for the cello and the second violin, I bought a beautiful (older) Stanley no 7 and joined the boards with it on a shooting board, and then rubbed the joint and left it with no clamps. The violin is perfect, the cello is almost perfect apart from one or two spots, which are good enough, but I want to do better.

I tried making this top joint in a similar fashion. Joined the pieces on the shooting board, checked that they were absolutely straight and flat, joining with no light seeping through. But then when I glued it, the pieces started opening up at the top of the wedge to a gap of about 0.75 mm. The wedge thickness is about 45 mm. The bottom was still joined perfectly, and when I took it apart, the glue indicated that the pieces were joined 2/3 of the way up. Not good enough though. I replaned, sized with a thin layer of weak-ish hide glue, planed off the surface just removing a bit of raised grain and glued again. I wanted to use clamps, but I couldn't find a position that would not open up the wedge as the bottoms aren't perfectly flat yet, so decided to just rub the joint together. It looked perfect that time, but this morning I noticed that the top of the wedge did actually open up again, even if it's just a tiny bit. I can feel with a thin strip of metal that the opened up bit goes somewhere between 2mm and 15mm (some very bad spots near the edge) into the top. I suppose there is still plenty of joined wood for a cello top left, unless I want it to be more than 35mm or so in height. I'm considering just using this joint as is, but can anyone tell me what I'm doing wrong? I'm up for one more attempt if I have a new thing to do, but just trying again the same way, expecting different results would be silly. My only clue is that maybe the wood swells when the glue is added, and that introduces a curve in the gluing surface, but how can I prevent that?

I still need to do the back joint as well as decide if I can accept this top joint or not. I really hope that someone can help me with it.
Thanks in advance

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8 minutes ago, Tostra said:

 

My only clue is that maybe the wood swells when the glue is added, and that introduces a curve in the gluing surface, but how can I prevent that?

 

It does. You need to sharpen your blade to take an ever-so-slightly hollow cut, so that it will become flat after moistening with glue.

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14 hours ago, David Burgess said:

It does. You need to sharpen your blade to take an ever-so-slightly hollow cut, so that it will become flat after moistening with glue.

Absolutely! There is at least one long discussion of this previously on Maestronet.

I Have made quite a few cellos and use a plane with a slightly convex blade to plane a tiny hollow across the joint and a slight, catenaric hollow length wise. I use a standard I- beam  bar clamp with the I-beam on the out side (roof shaped side) to close the joint with fingers only tightening the clamp. Then I take a strong light and  a magnifying loupe to look carefully at the joint to make sure it is perfectly closed all along the length on both sides.

Gluing is done with ample hide glue applied to both sides and three bar clamps on the outside of the plate tightening the center clamp first so that glue drips in tears from the joint on both sides. 

You mention that the shape of the wedges is still rough which may be giving you trouble as unless the flat sides are actually flat it is hard not to get a twist in the joint. 

Look up the previous discussion on this as there are more detailed descriptions of several methods used by experienced makers.

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9 hours ago, nathan slobodkin said:

 

You mention that the shape of the wedges is still rough which may be giving you trouble as unless the flat sides are actually flat it is hard not to get a twist in the joint. 

Yes! The underside (inside) should be dead flat and the planning surface should be totally square to it all the way along the surface. In addition, the top (outside) should have a flat area also square to the planning surface. I take my plates down to about 5mm of my desired height. I also cut out the excess wood from the upper and lower bouts at the corners of the plate and the c bout before I begin joining them. I clamp the joint using my bench vise and dogs at the c bout cut away, then bar clamps at either end. The plates are upside down on the bench (the top facing down).

Great info in the links above.

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Another consideration:

Unless special considerations are made, most of the clamping force will go through the bottom of the joint, with little or none at the top. You want the force to pass through the middle of the joint.

I do this by gluing on pieces of scrap wood, and trimming the ends to a height which puts the clamping force through the center of the joint.

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Thanks everyone, lots of good infi here. With the clamps I have, I don't have the opportunity to add a wedge underneath to push the top section together. That's part of the reason why I want to rub the joint and not clamp it. Should it still be slightly hollow lengthwise in case I don't clamp it?

I realize that the board shouldn't be twisted. I didn't flatten it because it's almost good, and I can clamp it at the points where it touches the shooting board, so that way it doesn't distort. At least it fit together perfectly off the board, so I don't think there is any distortion of the wedge while clamping. But somehow my shooting board was slightly off. The angle was just above 90 degrees, putting the force right at the edge of the seam where it really wanted to open up. I'll fix this issue somehow before continuing, of course, but I would still prefer a rubbed joint.

So far my plan is sharpening my plane micro convex, applying the glue with a bigger brush (I think it gelled up a little) and then rubbing it, maybe I'll moisten the top of the wedge so that it swells with the glued wood rather than curve it out. I'm not sure if that would work, but I noticed that wetting the top did close the gap back up, so I suppose it might be a way to help with the gluing. 

I'll have a look at all those links and see if I can find more tips before I go ahead and try it

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2 hours ago, Tostra said:

So far my plan is sharpening my plane micro convex, applying the glue with a bigger brush (I think it gelled up a little) and then rubbing it, maybe I'll moisten the top of the wedge so that it swells with the glued wood rather than curve it out. I'm not sure if that would work, but I noticed that wetting the top did close the gap back up, so I suppose it might be a way to help with the gluing.

To speed up the application of the glue and avoid jelling, I do not use the brush but pour the glue abundantly and quickly with a little jar. This way you will waste more glue, but it works perfectly. Of course, even using a very large brush capable of applying a large amount of glue in a single stroke would work fine, but you would need a lot of glue in the pot (wasting even more) and you would have the risk of losing parts of the brush hair that would create thickness during gluing spoiling the joint.

https://youtu.be/0JwbcnWLG0Y?t=189

Another suggestion I can give you is to wet the sides of the joint with hot water (once glued and clamped), but absolutely without passing over the joint so as not to wash off the glue inside the joint that has to do its job with the right viscosity. Instead, I put abundant glue over the joint (both inside and outside) so that it forms a thick bead that stays that way even once it has dried. This prevent the more peripheral part of the joint from drying out when the internal one is still damp and keeps the wood swollen, causing possible non-perfect contact of the peripheral parts.

PS Making a cello joint without using (appropriately) the clamps, for me it means relying a little too much on good luck, because the chances of the wood moving and distorting with the moistening and the heat of the glue are very high.

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Thanks! Davide, advice from you feels so surreal, since I have watched every single one of your youtube videos and admired your craftmanship like crazy ;-)

I think what you say about applying water to both sides is the same as I wanted to do. Not sure if that would be a bad idea if the joint is rubbed and not clamped though?
I have considered just pouring the glue. Seeing you do it, it looks much less messy than it was in my head, I think I'll attempt it. I'm sure it will be faster than the brushes I have at least.

And I know it sounds silly that I want to rub the joint and not clamp it... The reason, frankly, is that I don't have bar clamps, and the clamps I do have don't help much. They are too heavy and too deep, and I feel that they add much more distortion than there would be otherwise. I have succeeded with rubbing one cello, and from my previous two attempts at this particular top I could see that the pieces stayed nice and straight. Just the fact that the gap opened up evenly along the whole wedge to me means that there is one particular distortion happening evenly across the whole joint, which I attribute to the wood swelling, possibly before I get the pieces together. I think it should be fine, or at least I'll continue trying until it is. 
I've got the joint planed back to a pretty much perfect fit now, probably a lot better than before and with a micro convex blade, even though it is really very little. Tomorrow I'll mix up some more glue, try pouring the glue and then add some moisture along the seam. IF this doesn't work out, then yeah, I guess I should see if I can rent some clamps or something...

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You could put the glue into a squeeze bottle and keep it warm in a water bath until you are ready to apply it. 

It's also better to plane it immediately when you are ready to glue it up. The wood moves and changes slightly even over a short period of time passes.

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I did want to glue it last night for that reason, but I was too tired and didn't want to mess it up. I'll check it before I glue it and plane it again if necessary, of course. Squeeze bottle is a good idea, I'll see if I can find one

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Hi all, good news: The joint seems to be perfect at long last. I poured the glue like suggested, rubbed the joint and kept the surrounding wood moist for an hour, maybe two, by running a wet paper towel on both sides of the joint once in a while. Because I changed several parameters at once, I can't say which helped, but I feel like poruing the glue was the main thing, as my glue application to joints has always been too slow. Now it's been sitting for four hours and is still looking good, so I hope it'll stay like that.

Thanks a lot for the many great tips and links, I'll definitely go read a lot of it again when I'm ready to join the back. It needs some heavy planing firsst though, so it won't be right now

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1 hour ago, JacksonMaberry said:

If you decide to get more clamps, I strongly recommend the type of clamp pictured below

dubuquealuminumbbarclamp_1_2_2.png

I got some of these from Lee Valley.  Well made and very light.

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1 hour ago, David Burgess said:

This type attaches to standard iron pipe, so the clamp can be made to any length. Twelve bucks (although I don't know what the quality is on this particular brand).

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https://www.harborfreight.com/3-4-quarter-inch-pipe-clamp-with-base-94053.html

 

The nice thing about these type of clamps is you can have different lengths of pipes for different needs. I have some short 18" pipes for gluing violin plates, and longer for cello, bass and cabinet work.

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4 minutes ago, Bill Yacey said:

The nice thing about these type of clamps is you can have different lengths of pipes for different needs. I have some short 18" pipes for gluing violin plates, and longer for cello, bass and cabinet work.

Yup. If I lived in a hurricane zone, I might even to be able to use a 60 foot length of pipe to glue my house back together. :)

Great versatility, including the ability to easily reverse some parts, to enable minor spreading forces.

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Yeah, that's one of the many types of clamps I'd like to get and the one I felt like I was missing for this. They're called sash clamps, right? Wonder what they are in Danish so I can find some...

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11 hours ago, Tostra said:

Yeah, that's one of the many types of clamps I'd like to get and the one I felt like I was missing for this. They're called sash clamps, right? Wonder what they are in Danish so I can find some...

Around here we call them pipe clamps.

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17 hours ago, JacksonMaberry said:

If you decide to get more clamps, I strongly recommend the type of clamp pictured below

1528454069_BarClamp.jpg.3d36f745c89f791fc28056bef04a4766.jpg

These are also my choice, we call them bar clamps and they are lighter than pipe clamps.

2074562720_BarClamps.thumb.jpg.d3d90b2590963b6299bd2e649b8825d8.jpg

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13 hours ago, Davide Sora said:

1528454069_BarClamp.jpg.3d36f745c89f791fc28056bef04a4766.jpg

These are also my choice, we call them bar clamps and they are lighter than pipe clamps.

2074562720_BarClamps.thumb.jpg.d3d90b2590963b6299bd2e649b8825d8.jpg

Would a "real man" even notice the difference between a clamp which weighs three pounds, and one which weighs five? ;)

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13 hours ago, David Burgess said:

Would a "real man" even notice the difference between a clamp which weighs three pounds, and one which weighs five? ;)

:lol:

I confess that when I bought these clamps they were the only ones I found, it wasn't a choice dictated by weight:)

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I mean... I'd imagine a lighter clamp is better than a heavy one? Not so much because of me handling them but if a weight hangs off the side of the joint, that would pull the joint sideways to some extent? But maybe that just comes from my somewhat unsucceessful attempts to juggle very vintage and very heavy steel clamps.

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