Thursday, 3 October 2013

Because You Asked: Making a Lute Mold (4)

When last we spoke we had glued up all the pieces of the mold-in-progress, shut the lights off, closed the shop door, and with a very satisfied feeling gone home for the weekend.  Now it's Monday morning and we're back at the shop, ready to get carving!  Well... not quite so fast.  There are a few technical 'housekeeping' items that we need to get done before the real shaping of the mold can begin.

The Clamping Channel
After removing all the clamps and screws, the very first thing I do is to cut a channel in the lower end of the underside of the mold.  This is the channel used for clamping the capping strip, when it's glued on late in the process of building the lute back.  You can cut this channel at practically at any time in the mold-carving process; I prefer to do it now, when the back's not yet been carved into a slippery, roundy shape, and it's much easier for me to clamp it securely upside down on my bench.
 I draw the outlines of the channel--
And then I cut it with a few passes of a plunge router.  The channel will eventually be between about an inch and a half to two inches deep, depending on how wide your capping strip is.
Trimming the Excess
Once that's done, I can start working with the shape of the mold.  First, I take it to the band saw and cut away most of the wood that's overhanging the base plate.  You can't really mark a line on top that you can cut to with the saw, but it's easy enough for you to see where the saw blade is in relation to the base plate so that you don't go sawing into it.
There we are: our basic shape.  Let's take a quick look at some of our glued joints.

This is the result of all that marking and carving we did--our cross sections and long sections are plainly visible and ready for us to carve to, and the rib lines are marked out too.

The Belly Scoop
If you want to integrate the belly scoop into the edges of the mold, now's a good time to do it.

If you aren't familiar with the term, the belly scoop is the gentle curve that's planed into the edge ribs of the lute back, so that the belly (when it's glued in) dips a little toward the middle and rises toward the ends.   A common feature of historical as well as modern lutes, the belly scoop serves a couple of purposes: it creates a bit more space between the strings and the belly out in front of the bridge (where most of the plucking gets done), but more importantly, it also stabilizes the belly to help guard against distortion caused by string tension (like the kind of 'potato chip' profile one sometimes sees on the soundboards of guitars.)

Depending on the size and type of lute, the belly scoop will probably be somewhere around 3 to 3.5mm at its deepest (the center of the body), tapering out to zero at both ends.  I first draw the curve on paper, laying it out on a grid; this allows me to measure the depth of the belly scoop at specific points, and to transfer that depth to the edge of the mold, using my sharp dividers.  I can then plane a bevel into the edge of the mold, connecting all these points and creating a gentle dip in the outline of the mold.  (When I've assembled the lute back, I can then mark the belly scoop accurately on the inside of the outer ribs.)

The Mold-Holder
In order to carve the mold efficiently, we need to mount it on some kind of vise or block system.  Whatever we use, the hold needs to be firm, but it also needs to release quickly so that we can tilt the mold and work on different areas.  I've seen some very slick-looking universal-joint carving vises people have made (or bought) to hold their work; I use a very low-tech system of a block screwed securely to the underside of the mold, held by a large, heavy clamp, which is itself set into a built-up plywood block and held in my bench vise.  Have a look.

(By the way, these are photos of a different, finished mold.)

Note the bolt that I've inserted into the clamp's wooden handle.  This is my tightener and quick-release, and this system would not work without it.

I like this setup because it allows the mold to readily tilt toward and away from you, while the tip of the mold can be raised or lowered simply by releasing the bench vise.  (To spin the whole thing end-for-end, you actually need to lift it out of the vise, turn it around in your hands, and set it back into the vise.  Not the most convenient manoeuvre, but I've learned to live with it.)

The Front Section
There's one cross section that we haven't drawn in and carved down to yet, and that's the section at the front of the mold.  In fact, we've left a little extra material here on purpose, so that we can plane the face of the mold after everything is assembled.  Let's do that now.

You can chisel a rough profile first--
Then flatten off the front with a low-angle block plane.  This surface should be dead flat and perpendicular to the bottom of the mold.  When it is, you can use your drawing to lay out the section profile and rib lines.
Notice how the cross section matches exactly the long section point that's already carved into the mold: that's a very good sign that we've been doing accurate work, and that all our profiles are matching up closely.

Rough Carving
Before too long, we'll need to fit up a top block to the front of this mold.  However, at this point we can begin to do some rough carving to whittle this big battleship down to something more closely resembling a lute.

Begin with a large, sharp chisel, one inch wide or more, and take fairly gentle paring cuts, either across or slightly askew the grain of the wood.  The cleats you left on the sides of the blocks should come away easily, and in a while you'll start to see a rough shape emerging. 

As we carve, we're keeping in mind our cross sections and long sections, using them as our guides.   We can begin to cut down to them a little more purposefully by using the chisel directly over the sections, carving directly across the grain.
You'll notice that as I'm carving down to the lines, I'm allowing the wood in between the lines to remain a little humped up.  It's a natural tendency as you carve, and you should go with it--as we continue to refine our carving we'll be taking down these humps in a very controlled way. 

The Top Block
This is the 'nose' of the mold, the block onto which the ribs will be glued as the lute back is assembled.  It therefore needs to be separate from the mold, held securely but detachable from it.

I use poplar for a top block, and I attach it to the mold by means of a plate screwed up to the underside of the mold.  The plate is birch plywood, and it's held to the mold by #10 screws.  I screw the top block to the plate with 1" #8 screws.

Of course, the bottom surface of the block needs to be perfectly flat, and perpendicular to the back surface--the one that sits against the mold--which is also perfectly flat (the fit between these two surfaces needs to be very close).  Once the oversize block's been fitted, you can use templates derived from your drawing to lay out the shape of the block, and then go to the band saw to cut away the excess. 
And there's the result.  Now we've got a lute-mold-in-the-making that looks a little less like a battleship, but a little more like a duck-billed platypus.  Still, it's progress, and a little progress every day is all I need to keep me coming back for more.

Next time we'll start refining the shape, and laying out rib lines.  Hope to talk to you again soon.

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