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Making a Mold for the Geometric Pattern

November 14, 2010

I’m getting tired of waiting for my Strad poster to come in the mail…  Still n0 poster in the Saturday mail, so since I had the whole weekend to kill and nothing to do…

I had some aromatic cedar that at one time I thought I might make the mold out of, but later changed my mind to use something more conventional, instead (plywood or at least something without knots in it).  So, while I’m still waiting, I decided to “waste” the aromatic cedar on making a mold for the geometric violin pattern and save the plywood/knotless wood for a different pattern–assuming I find/make a pattern I like better.  And if I decide to go with the geometric version in the end (likely, at this point), then it won’t have been wasted (assuming the knots aren’t a problem…)

So, it’s probably unconventional, but it smells so nice!  :D

Cutting out the half-pattern…

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I started using a scissors to cut the pattern out.  (it’s some thick cardboard from an art store)  Then I switched to an exacto knife.  That made a nicer/cleaner cut.  I wanted both sides of the pattern to be preserved.

First I traced around the edge of the inside of the pattern to make the outside outline (the outline for the front and back plates), just for reference.  Then I needed to reduce the outline by 4mm for the mold itself (3mm for the plates’ overhang, and another mm for the rib thickness).

I used the old “washer trick” and the outside of the pattern, as seen in the picture below, to reduce the outline by a constant amount.  It took a while to find a “washer” the right size.  This is some unknown piece of plastic left over from some project or another.

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You put the pencil tip inside and roll it around the edge of whatever you’re reducing or enlarging.  This is why I wanted the outside of the outline well preserved and not cut up into several waste pieces.

I decided that if I make the geometric version, in the end, I wanted the middle to be a little bit narrower than the geometric pattern resulted in.  So I reduced that section by an extra not-quite-one-milimeter.  –or, to be more accurate, by the width of two cardboard credit-card shaped coupons from Lowes, which I held between the outline and the “washer”.  You can sort of see this in the picture above on the “C” bout.

Below is a picture of the outline for the plates (the outer pencil-line) and the mold (the inner pencil-line) copied onto my aromatic cedar, as well as some of the construction lines that were used in generating the template  (the lines which divide the various curves of differing radii.  I used those lines in deciding where to place the 6 blocks, which you can see sketched onto the mold as well (you can click the picture to see the full-sized version)).

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You can also see that I joined two boards together to get the right width of wood for the mold.  This created a nice centerline, as well.  It was initially done as a practice project in joining wood.

Also, if you look closely enough, you can also see the pin holes (two on the top, two on the center line at the upper and lower bout width lines, and two at the bottom of the wood) that I made to pin the cardboard templates onto the board for steadiness while tracing and while “washer-reducing”.  For the inside template, you flip the template around and stick the pins into the same place.  Then, with the inside template pinned in place, put the outside template in position, and pin it down as well (anywhere.  I chose at the top and bottom).  Then unpin them both, and flip them around together and pin the inside down again.  Then finally place the outside template’s pins into the same holes in the cardboard, which makes new holes in the wood.  If that makes sense…  Or if it doesn’t, then whatever.  I’m sure you could figure it out if you needed to.  ;)

In this next picture, I am cutting on the inside line with my coping saw.  Aromatic cedar seems to be very easy to saw.  I positioned the outline so that I wouldn’t have to cut through any knots…  We’ll see if they become a problem, later, though.  I think they might be ok…

imageAs you can see, I cut right through the places marked for the corners, because eventually I’ll make recesses in the mold for the 6 blocks.

Below, you can see the “finished” product.

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I rough-cut it first, then went back and sawed it more closely to the line (which is what the random thin pieces of wood in the picture are from.  I’m particularly fond of the curved backwards-S shaped sliver on the right side of the mold.  Some places of it are less than 1mm wide, and it didn’t break.).  Then I sanded the edges (with the mini-sander you can see I made from a chunk of wood that I cut from the waste material near the bottom.

Ok, so it’s not actually finished, but it’s getting there.  I think I need to actually sand off another 1/2mm all around (achoo!) and more like 1mm in the middle C bouts (achoo-achoo!).  We seem to have a case of disappearing millimeters, here…  The measurements all around are less than 4mm reduced than the outline.  I suspect the washer wasn’t as wide as I thought it was…  Or maybe my cutting was too conservative (I doubt it, though, because I cut the original line away…)  Oh well, I’d rather have to cut more than wish I had cut less.  :)

I also need to decide the final block sizes and positions and cut the recesses for them.  Then I need to decide where/how I’m going to clamp the ribs on when I glue them and make the right holes in the mold so that I can do so.

And after all this, I won’t be surprised if my Strad poster comes tomorrow.  ;)  (although I won’t be surprised if it doesn’t, either…)

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