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Monthly Archives: August 2009

Last week I made a bridge for the guitar that I’m building at the moment. Here’s a photograph taken while it was being French polished. It’s in Rio rosewood and the tie block is inlaid with a strip of spalted beech to echo the rosette that I wrote about a little while ago.

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To cut the channel for the saddle and for the recess behind the tie block, I used this very simple router table. The idea came from an article in Fine Woodworking (No 182, February 2006) where Doug Stowe described how he made something rather similar for a full size router. There’s a brief description of his table here where there’s also a link to a full explanation and downloadable free plans.

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In my table, the Dremmel is mounted overhead on a cantilever. The table itself is a board of mdf. The adjustable fence is simply a straight strip of wood that pivots at one end and that is clamped at the other – an arrangement that allows a remarkable degree of precision. Depth of cut is controlled by the position of the router bit in the collet. The Dremmel isn’t powerful enough to cut slots to their full depth in one pass so, to avoid the fiddly business of repeatedly having to change the position of the router bit in its collet, I place a shim of 1.5mm thick plywood under the workpiece for each subsequent pass.

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The only bit about making the table that’s not straightforward is how to mount the Dremmel firmly and vertically in the cantilever in a way that allows removal. I solved the problem by buying a 3/4 inch diameter 12tpi tap, which matches the thread on the nose of the Dremel when the collar above the collet is removed. Then it was only a matter of drilling an undersized hole and tapping it out.

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The table is quick and easy to set up and it doesn’t take up much room in the cupboard when it’s not being used. It isn’t big enough to deal with anything very large, of course, but for making guitar bridges it works fine.

In Roy Courtnall’s book, Making Master Guitars, there’s an interview with José Romanillos in which he talked about some of techniques he uses. To attach the ribs to the foot of the neck, he prefers a wedged joint over the usual 2mm wide slot cut at the 12th fret line. Apparently, he got the idea from seeing such a joint in a 17th or 18th century French guitar. He gives some rudimentary instructions about how to make it:

‘You cut a wide tapering slot, then fit the rib tight up against the front end. Then you drive a wedge down, which matches the taper exactly. It is very strong.’

Well, I haven’t had any problems with strength of the joint when the ribs are housed in conventional narrow slots. But I’ve never found it easy to cut these slots to exactly the right width with a hand saw. If you want to do it with a single cut, you need to adjust the set of a back saw so that it cuts a kerf 2mm wide. Quite apart from the fact that it’s hard to do this without breaking the teeth, it makes the saw almost useless for any other purpose. The alternative is to do it by making two cuts. After the first cut, you can place a piece of plastic or plywood in the kerf to guide the saw for the second cut. But it’s not a very satisfactory solution because it’s too easy to cut into the plastic or wood and end up with a slot that’s too narrow near the bottom. You can get around that problem by substituting a sheet of metal, such as a cabinet scraper, but it doesn’t do the saw much good. Things get even more difficult if you want the slot to be 2.5 or 3.0mm wide to accommodate laminated ribs.

So I was interested to learn about Romanillos’ wedge technique and decided to try it out in the guitar that I’m making at the moment, which does have laminated ribs – zebrano lined with maple with a finished thickness of about 3mm.

The 2 photographs below show the wide tapering slots cut and chiselled out in the foot of the neck before the heel has been shaped.

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Here, I’ve roughly shaped the heel and lower part of the neck.

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Then I cut the wedges and adjusted them to fit. Obviously, it’s particularly important that they draw everything up tight before the narrow end of the wedge reaches the soundboard end of the slot. I deliberately made them too long initially to give plenty of room for error.

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This is a dry run before gluing to make sure that everything fits perfectly. I discovered that another advantage of making the wedges too long at the beginning was that it provided something to grip when wriggling them out.

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And this is the finished joint, glued and cleaned up. As you can see, I’ve already started attaching the ribs to the soundboard with tentellones.

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Altogether, this turned out to be a useful experiment. The wide slot presented no problems to saw or chisel out. Indeed, it was significantly easier than cutting the conventional narrow slot. There’s a bit of extra time and trouble preparing the wedges but, as long as you have the right jig (see here) it’s not difficult. Gluing up was easy: plenty of room to coat all the surfaces before putting them together and sliding in the wedge. A couple of taps with a light hammer and it’s done. I’m fairly sure that I shall be using this technique again.

A while ago, I wrote about using a Millers Falls scraper plane to cope with some highly figured cocobolo that I was using for the back of a guitar. It’s an excellent tool for finalising the thickness and it leaves a clean finish even on the most awkward wood. The disadvantage however, is that it takes only the thinnest of shavings so if you’re starting with wood that’s way too thick, you’re in for a lot of time and effort to get to the right final dimensions.

Of course, the usual way to get around the problem is to run the wood through a drum sander. But I haven’t got one, partly because there isn’t room for it in my small workshop and partly because I’m allergic to sandpaper. I don’t mean it literally – I don’t come out in a rash if I touch the stuff – but I do think that there are nicer and quieter ways of shaping wood than grinding it into dust.

Another solution is to use a plane with a toothed blade. This won’t eliminate tear out completely but, should it happen, it’s limited and shallow and can easily be dealt with by a scraper later. Toothed blades work because the individual teeth are too small to grab enough fibres running in the wrong direction to rip out a large lump.

I use a No 4 Record bench plane fitted with a standard blade that I modified to look like this. Put the blade in the vice, cutting edge upward. Take a cold chisel and, against all your instincts, hammer a small gap into the cutting edge every 3 or 4 mm. Then sharpen the blade in the usual way.

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Another way of cutting the teeth is to use a thin grinding wheel in a Dremmel.

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Here are a couple of pictures of a guitar back in zebrano being thicknessed with the toothed blade. If you’ve ever used this wood, you’ll know that the interlocked grain structure makes it very hard to work. With a toothed blade and a wipe of wax on the bottom of the plane, the task becomes a pleasure.

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The marks left by the toothed blade are just visible running diagonally from bottom right to top left. And you can see the linguine-like shavings that are produced.

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Switching over to the scraper plane for final adjustment of the thickness and to remove the corrugations left by the toothed blade.

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