Saturday, 28 September 2013

Because You Asked: Making a Lute Mold (3)

Okay, here we go--the final push to get this mold-in-progress glued together.

So far, we've made a base plate and a lot of blocks that screw up to it, and we've transferred the plan, long section and cross section profiles from our drawing onto them.  Before we glue all this up, though, we've got to make sure that these profiles will be visible and usable to us as we carve the mold.

So set up your bandsaw and sharpen your chisels... we've got some work to do.

Cutting the Blocks
With all the cross sections traced, you're ready to cut away the excess wood from the blocks.  You can cut very close to the line here--no more than 1mm.  Be brave--if you cut much farther away, you're just leaving more wood to hog off when carving begins.

When you eventually glue the blocks together on the base plate, you will need to clamp across the blocks as well as along their length.  To provide a flat clamping surface across, I mark and cut cleats into the blocks as I cut out the cross sections.
Mark and cut out all the cross section blocks--then meet me at the bench for the next step.

Trimming the Blocks
With all the sections now cut out, it's time to trim down to the long and cross section lines that you've traced onto the blocks.

Start with the cross sections first.  Place a block in your vise, and then with a chisel held at a 45 degree angle, trim down to your marked cross section line.  It's important to trim at a generous angle, because this angle will allow us to see and use (i.e., carve down to) the cross section after the mold has been glued up.  (When you get to a cleat, you'll have to saw away a bit of it to get close to the line with the chisel.)
Sawing away a bit of the cleat...
... so we can trim right to the line.

When you've trimmed down to the line, draw the rib joint lines--in pen--onto the bevelled surface you've just made.  This is to ensure you'll be able to locate the rib joint lines after the mold has been glued up, in order to lay out rib lines during carving.
For long section lines, proceed as with the cross sections--using a chisel, at a 45 degree angle.  (You may need to do a bit more chiseling if you find yourself having to contend with a cleat.)
Here's a look at one of our blocks--this would be from the front section on the bass side, showing the long section and cross section trimmed in.
Here's a look at all of the blocks on the bass side with long and cross sections trimmed in.  And like magic--there's the whole longitudinal section!

Maybe here you can get a better idea of what I was talking about in my last post about drawing in the cross sections only on the 'uphill' side of the blocks.  It's not necessary to draw them on the downhill side, because that cross section is actually the 'uphill' side of the next block in line. 

Preparation for Gluing
All the blocks are now cut and trimmed down to their lines; all blocks have cleats cut into them for clamping across the sections.  We are now almost ready to glue up the mold.

Before we do, though, it's a good idea to do a dry run, screwing all the blocks to the base plate, clamping across all the cross sections and along the long sections.  By the time we've finished gluing there will be a forest of clamps attached to this beast, so it's a very good idea to know how all of them will fit together around the mold at one time.

When you're certain all the clamps will fit harmoniously, remove and set them aside, close to hand, in the order that you'll use them.  Unscrew all the blocks from the base plate and set them aside also, in the order in which they'll be glued up.

Leave the screws in the base plate, backed out, ready to screw up into the blocks. 

Gluing up
Begin with one side of the bottom section, and apply glue to the bottom surface only.  I use Titebond, and spread the glue with a putty knife.  You don't actually need too much glue on any of the surfaces of the blocks; once all these pieces are glued together, clamped and dried, there's little chance of a glued joint failing.  On the other hand, too much glue will present problems--there will either be a lot of squeeze out to clear away, or excess glue will distort the alignment of the blocks.

Apply glue, then screw up to the base plate (save time and wrist fatigue by using a power driver.)  There will be some squeeze out, which should be cleared away with a sharpened stick.

Now go on to the other block of the bottom section.  Glue goes on the contact surfaces: the bottom (against the base plate) and the side (against the other block).  Screw it up, then clamp across the two blocks.
Once these two bottom section blocks have been glued, the mold can be set upright on the bench, allowing easy access for screwing up the remaining sections.

Go on to the next section, first one side and then the other, spreading glue on all the contact surfaces, screwing up tight to the base plate and clamping across.  At this point I also like to clamp--temporarily--along the long section too, just to make sure I get good squeeze out.  Clamp tight on both the treble and bass sides, then remove the clamps, and go on to the next pair of blocks.
Temporary clamping on the long section

I do this temporary clamping on the long section for each pair of blocks I glue in.  It makes for a fair bit of clamping and unclamping, but it's important to do it as you go along.  If you wait until all the blocks are in place before clamping the longitudinal sections, you'll find that the glue has already set in the first joints you glued up--and it will be impossible to get a good squeeze out (and a tight fit) in those joints.

Here's something like what the finished gluing-up job should look like.
(My darling wife Julia calls a photograph like this 'clamp porn'.  I prefer to think of it as 'clamp erotica'.)

That is all for now.  Let the glue dry; don't even think about working with it for a day or two.  Take the weekend off--you have my permission.  The really difficult part comes with the carving of this beast into something resembling a lute body, and we'll turn our attention to that in our next installment. 

No comments:

Post a Comment