Making a Cello #4: Finally a Joint!

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Blog entry by PhiltheLuthier posted 06-08-2012 04:18 PM 8615 reads 0 times favorited 1 comment Add to Favorites Watch
« Part 3: Working on flattening the plates. Part 4 of Making a Cello series Part 5: The 'C' Ribs »

It has been a little longer than expected but here I am. I won’t bore you with my excuses, rather I’ll show you the little progress I’ve made over that time.

Yesterday I set up to do the back joint. This joint is roughly a meter long, by 32mm wide, and is vital because in the end, after all the carving, it’ll lose about 98% percent of it’s glued area while still required to hold two boards each about 23 cm wide, together ad that under about 200lbs (oops where’d the metrics go!?) of string tension exerted mostly at a spot 1cm sq. Okay, maybe it sounds worse than it is, after all a lot of these joints last hundreds of years.

So first things’ first the set-up!:

What I have here one very long straight edge, an engineer’s square, a block plane, a workbench with an end vise, and a twobyfour with a wedge cut out of one end and a wingnut/carriage bolt for tightening. The latter can of course be made more fancy (and probably should be), and frankly in this case the wingnut isn’t strong enough to pull the thing closed, so I had to use a wedge to tighten the board to the support. As you can see, the board is clamped to the bench and supported on the other end, allowing me to work the joint with my block plane. You may say, “but isn’t a block plane a little small for making a joint of that size?” Well I don’t know any different, this is how I was taught to do it, and so this is how I do it! It’s like I heard a story once of a Christmas ham, this woman always cut 2 inches off the ham because that’s what her mother showed her, later in life she got curious and asked why here mom always did that, and her mom said “because our stove in those days wasn’t big enough to fit the whole ham.” So it may just be that it’s the only plane someone had, and that got passed on through the ages…

The Process:

I lay the straight edge on the joint to see if I have any high spots or whatnot. Also (not pictured) I check the squareness of the joint against the flat I made with the planner. While fixing the squareness I bring the joint to be flat and true, it can not have any twist to it or the joint will fail.

After this is accomplished it’s time to put a slight hollow in the joint length wise. I check the gap with the straight edge for now, it should be quite small, as it will double in size when the joint is made.

Here it is too big, so I had to re-flatten a little.

Once I feel that one board is nicely prepared I put it aside and start on the other in the same manner as the first. When that’s done I put the halves together to check the joint. I’m looking to see several things:

1- that there is no wobble, the top board should sit flat, if you touch it it should stay immobile.
2- that the gap is not too large or too small and that it is gradual, starting smoothly coming to a high point in the center and reducing smoothly.
3- and when test clamping that the gap closes evenly, that the ends don’t open up, and that both sides of the joint close.

I forgot to show the scraps glued on where the clamp will go, they are glued to the edge of the wedge and then made square to the back. This gives more surface to put the clamp on, and evens out the load so that one side of the joint doesn’t receive more clamping pressure than the other.

Congratulations! The joint is ready to glue!

Gluing the joint is rather interesting too, I should have filmed it I guess, but there’s not really enough room in my shop for that, nor am I well organized enough to accomplish such a feat. So the tools I use to glue are a P&G Hot Pot, a bristle brush, hot hide glue that is at least 6 hrs old, a big clamp (a parallel jaw clamp would be ideal, but I haven’t got one yet) and a hair dryer (again, a heat gun would probably work much better, but you’d have to watch out not to scorch the wood). With the one board still in position from jointing, I mount/hold the other one as close to the same position as possible. Using the hair dryer, I warm the joint surfaces, then slather on a ton of glue to both surfaces. Curly maple ‘drinks’ a lot of glue, European less than American for some reason but still significant. Then I slam the joint together because it’s fun, and rub the board with the other for a bit, though not until it grabs since I’m using a clamp. Get the alignment nice so I don’t have a lot of planing to do afterwards, and put the clamp on exactly the same way as when I did the test clamping. Now I run a strip of hot water either side of the joint, and some fresh glue on the joint (both sides of the board) and heat the area with the hair dryer. After all that excitement I get this:

A messy workshop with glue all over and a glued up joint drying nicely. The clamp will stay on for about 24hrs, and I won’t stress the joint for a while, though I’d say 48hrs minimum before working with a glued up plate.

That’s it’s for now, thanks for reading.

1 comment so far

View HamS's profile


1832 posts in 2625 days

#1 posted 06-09-2012 01:21 AM

Thanks for posting. I am interested in seeing the carved back.

-- Haming it up in the 'bash.

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