Monthly Archives: February 2011

Bracing the Back Plate

Like the cat in the hat I’m stuck inside with all this rain today so I might as well move right along with the next post; Bracing the back plate.

Strike your center line with a pencil. Lay out the shape with the half template. Flip it over and do the same.

Take me to the river…. Oh sorry meant to say take it to the band saw and cut it out leaving about 1/4″ around your line.

I put it through the drum sander and took it down to around 2.6mm and then did the final thicknessing to 2.5mm by hand up to 240 grit.

Now I will glue some cross cut strips on the seam to strengthen it. Cross cut strips ensure it will never come apart.

Close up of cross cut strips. These are cut from the waste of the top plate and fit in between the braces so they only need to be around 6″ long.

Mark the top and bottom of the body and then mark the center point of where your brace will cross.

Cut the cross cuts to size and then I tape them together along a straight edge.

I then glue the strip on the back using my straight edge rule to keep it in line.

After it is all glued up I sand it and begin laying out the position of the braces.

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Gluing the Back Plate

The back plate is done in the same procedure as the top plate. Sorry folks no pics for this step as it replicates the top plate glue-up.

1. Thickness the panels down to 2.8 mm. And then 2.5mm for the final thicknessing after glue-up.

2. Joint the mating edges in the bookmatched position with the panels taped together at the ends. This is something I like to always do if I am working with bookmatched stock. That way if the joint is even slight off 90 degrees when you open it up it will always mate perfectly.

3. Lay the panels down on a board still taped together, hanging over the edge and sand with a straight edge to 240 grit.

4. For this standard butt joint put the two plates together, raised in the center with a 6mm thick board and some wax paper.

5. Screw some blocks down to the work board keeping the plates snug together.

6. Apply glue to the jointed edge, pull out your stick in the center and gently press the two plates flat down onto the work board. Lay some more wax paper over top and clamp it down with some long reach clamps.

Here is the glued plate. In this picture I was contemplating putting a decorative seam (some of that curly maple I have left over from the kitchen counter) down the center but opted to not do it this time. I decided I just want solid rosewood this time.

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Gluing the X-braces

Well I got a little more work done on the top plate. I left off last time with just the x-braces glued on after shaping them. I also pre-shaped the tone braces.

In the next step I glued up the rest of the braces and did some final shaping. On this guitar I am using scalloped braces as opposed to non-scalloped. Scalloped braces offer a little more vibration in the top plate given more overall volume. Non-scalloped give a more even, balanced range between the highs and lows. The scalloped style is more common with the vintage guitars. Finger picking guitarists tend to prefer the scalloped bracing for its range and volume.

Gluing on the six tone braces. Notice they are notched in under the x-braces for added structural strength.

The bracing around the sound hole. These are only 2mm thick.

I then drilled a hole in the brace for the upper bout for access to the truss rod and then glued it on flat. This brace is not arched. The top plate will transition from a dome to flat in the upper section. Some do this and other don’t. I am trying it this way to make the finger board, which extends down past the 14th fret easier to attach to the sound board. I also glued on the bridge patch, which protects the top plate from the guitar string ends digging in under tension. I used some 2mm quatersawn ebony.

Finally I took a scraper and rounded off the edges, did some hand sanding and added a half-lap top crown on the x-brace intersection.

When I do some tapping on the sound board it has a nice tone. I am not sure what to listen for as I have no experience here but I can say I hear the wood singing. That is to say there is a resonant tone.

This sound board is finished. Next I will move on to the back.

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Gluing the X-brace

Now before I glue the braces down on to the back of the top plate I have to shape them to the desired radius. As I mentioned in the last post I am using a 25′ radius.

The radius dish I made will be used for the glue-up but it will also be used for putting the radius onto the braces. I have seen some make a special jig just for putting a curve on the braces but I like the idea of using the radius dish. So in the next step I tack some 120 grit sandpaper down on to the inside of the dish where the brace will lay and start sanding by moving the brace back and forth.

Sanding the correct radius into the x-brace

Checking the fit. I will also do the same for the other braces excluding the braces in the upper bout.

Before I glue the x-braces down I need to notch them so the other 6 tonal braces can tuck in under them. The braces in the upper section stay flat.

I pre-shaped the tonal braces so I could find my position to mark and notch out on the x-braces. I also marked my end points and maximum depth for the scalloping on the braces. I used a French curve template and a thin strip of hard maple to trace curves on to the braces.

I pre-shaped the x-braces before gluing them down. You can see my end point and maximum depth pencil marks. It seems many luthiers shape the braces after gluing them down but many luthier supply shops sell pre-shaped brace kits so I decided to shaped them before gluing to avoid any nasty slips I am prone to making.

All the braces pre-shaped and ready to go.

A close up of the scalloped braces.

Gluing down the x-brace with my makeshift go-bar. The go-bar is a clamping system many luthiers use to get clamp pressure down on top of the piece. In my case I didn’t actually make one I just used 1.4 meter x 10mm dowels in between the braces and my workshop ceiling. I even put rubber booties on them cause I get rain leaking in when it pours.

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