Sunday, 6 October 2013

Because You Asked: Making a Lute Mold (5)

Last time we met, we had turned the very rough shape of the glued-up mold into something vaguely resembling a lute. Today we'll continue refining the overall shape, and then at a certain point, we'll mark out the rib lines and begin to think of it not just as a shape but as a mold--a sculpted object upon which separate pieces of wood will be glued together to make a lute back.

Refining the Shape
Two tools will make our work a lot easier.  First, a small curved finger plane is very useful for carving the top block; for most of this work, the plane cuts best working uphill, that is, from the tip of the block back toward the mold. 

The second tool is a Veritas low-angle spokeshave, available in Canada from Lee Valley Tools.  This  tool can be set closely to take very fine cuts, and it works well across the grain as well as on end grain--which we have to deal with down at the bottom end of the mold.
I've evolved a way of holding the tool so that the left hand is a guide, while the right hand supplies power.  I hold it lightly but firmly, and take fine, quick cuts.  Most of the time I'm working out and downward, from the middle of the back out toward the edges, cutting directly across the grain of the wood.  I try not to dwell on one area too long, but keep moving and refining the entire shape of the mold.

These next two photos show the kind of progress that you can make with these tools.  The first shows an overall smoothing both on the mold and on the top block; the second photo shows how I've used the spokeshave directly over the cross section to carve things down in a controlled way.
We've almost taken down the overall shape to the cross section lines, but not quite.  We need to go a little farther, and get a little closer.  Something like this:
That's close.  Notice, though, that you can still see the cross section, and you can still see the rib location points that I marked on the beveled edges of the cross section blocks before I glued up the mold.  Okay: I think we're almost ready to start laying out rib lines, and carving rib facets into our mold.

Once we start doing that, however, we will have to say goodbye to our cross section lines, because they are going to be (mostly) carved away.  We'll need to mark our rib location points so we can find them again as we work.  Here's how.

Take a sharp pin, and prick each rib point deeply.
Then take a felt-tipped pen, and press it into each pin hole.  By doing this, you're actually tattooing the rib points, ensuring that you can find them easily throughout the rest of the carving process.

Laying out the Rib Lines
To lay out the rib lines on your mold, you need to connect the dots--all of the rib points on each of the cross sections in your mold--from the very tip of your top block all the way back to the bottom end of the mold. 

The ribs are laid out on the face of the top block using a template derived from your cross section drawing. Notice how I've used a chisel to chamfer down to the line here, just as I did on the cross section blocks of the mold.
On the bottom end of the mold, you can use your cross-section drawing to lay out the orientation of the ribs as they disappear under the capping strip (in fact, you can lay out the location of the capping strip at the same time as you mark out the ribs.)

Use a pin prick through the drawing to show the location of each rib line below the capping strip.

Now, to connect the dots and make an accurate rib line, you'll want to make yourself this very handy tool.  Take a thin, flexible strip of plexiglass, between about 15-20 mm wide, and about 70 or 80mm long.  Scribe a straight line down the center of it, and every 10mm or so, drill a small hole for your marking pin to fit through.  Take this marking strap to your mold, and set the scribed line over the dots of one of the rib lines from the top to the bottom (you can fix the strap in place with pins).
When everything's aligned correctly, use your marking pin to prick through the holes into the mold.  Then remove the marking strap, and tattoo the pin holes with a felt-tip pen. 

All that's left to do now is to connect these tattooed dots, and I do this with a sharp, hard pencil and a flexible curve laid on its side.  (You'll need to cut away the inking 'lip' on the side of the flexible curve for it to lie close to the mold so you can mark an accurate line.)
Here are the rib lines, neatly laid out. 
I like using the marking strap and the flexible curve, rather than just the flexible curve, because the marking strap ensures that I'm creating a single, continuous line.  Because it's too short to reach over the entire length of the mold, the flexible curve by itself can only give segments of a line, with no real guarantee that the segments will align well.

Creating the Rib Facets
Now that the lines are laid out, the mold's facets can be carved.  This operation is pretty straightforward: all we're doing is removing material between the rib lines.  As long as you can plane, and scrape, between the lines, you'll be fine.

For removing the bulk of the material, you can use your small finger plane.
Just a word of caution though--you should leave the center rib, with our longitudinal section, unplaned for now.  We'll need the long section for reference for a little while longer.
Once you've removed most of the material between the rib lines, you can start working with a curved scraper to really flatten the facet.  You should scrape pretty much right out to the rib lines, and the facet should be quite flat between the rib lines.

Down at the top block we can really start to do some very accurate work as we shape up the rib lines and facets.  You might remove the top block and chamfer down to the cross section line on the front of the mold:
Then replace the top block, and chamfer the back edge of it to match.
As for the bottom end of the mold, you should stop flattening the facets of the ribs as you approach the top edge of what will be the capping strip.  Instead, use your finger plane, scrapers, files and sanding blocks to flatten the whole capping strip area.
I think this is a good place to stop for today.  We've taken the rough shape and refined it; we've laid our our rib lines and capping strip, and the project's actually started to look a lot like a lute mold.  Next time out, I'll discuss some ways we can refine the shape further--and further, and further still--until we reach a point where we can do nothing more, and we declare our mold finished.

The part of the process that we've just begun, of carving and refining, takes a long time, I've found, and a lot of concentration and stamina.  Have a good sleep and eat a good breakfast, and I'll meet you back in the shop in the morning.

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