Mopus chapter 4: How Mopus Got Her Back

Mopus got her back!

Dealing with the back was quite a bit more challenging than the last time I did this. After all, the kit I had built came with the back already the correct thickness, joined into one piece, and even cut to the right shape. All I really had to do was glue on the pre-made braces. This time, all I had to work with was two book-matched pieces of rosewood about twice as thick as I needed, and some mahogany to make braces from.

The first challenge was to get the wood down to the right thickness. I used my vintage Safe-T-Planer (bought on eBay a couple of years ago) to get close, and then applied a lot of elbow grease, scrapers, and an random orbit sander to bring it home. I have dreams of one day owning a thickness sander. Wonderful glorious dreams. Like I used to have about band saws. But I digress.

Then came the fun part of making a perfect glue joint to join the two pieces. OK, maybe not so fun. The challenge is that it really has to be perfect. The litmus test is to hold the two pieces up together to a strong light, and make sure no light can be seen through the joint. This is not that easy, and it took me a while to get it right. I used a “shooting board”, and some of my larger vintage planes (nicely sharpened of course), to get close and then a piece of marble with fine sandpaper to get closer. It really took quite a while before I was happy.

And just to make things as complicated as possible, I decided to add a maple accent strip down the center, which makes the glue up a bit more challenging. I wound up making a sort of improvised jig to do the glue up, using standard ceder shims as wedges to apply even side pressure. I used plastic packing tape to make glue-proof surfaces, and I held down everything with heavy objects (which explains the planes and marble in the following picture). This all worked very well, and I can’t really see the point of buying a special “plate joining jig”. So I don’t need to dream about those any more.

The back came out pretty nice, I really like the center strip, and the joint seems quite strong. But I still had to apply quite a bit more elbow grease, scrapers, sanders, etc, to get the whole thing to a nice uniform thickness. Not terribly fun. Did I mention I sometimes dream of thickness sanders?

I then simply went to my trusty awesome band saw and cut the back to the approximate final size. I also added a reinforcement strip down the inside center of the back, and to clamp it,  I used the go-bar deck I made back in guitar #1 days.

Then came the brace project. I cut some brace blanks out of a piece of mahogany, and then used my 15′ radius dish to get the correct radius on the bottom edges.

To mount the braces, I had to cut notches in the reinforcement strip and then glue them in place using the go-bar deck. Which gave me the the opportunity to re-discover the design flaw of using driveway markers as the bars. They are just to thick at 5/16″. You put one in, and 3 pop out. Sometimes you put one in and all of them pop out. When you’re doing a critical glue up, and only have a few minutes to work with, this just sucks. And that’s sugar coating it. So that will be THE VERY LAST #@%#  TIME I use those bars. I immediately went to eBay and found someone selling inexpensive 3/16″ fiberglass rods, bought 25 of them, and I’m never looking back.

Once that was over, I spent some quality time with chisels and planes to shape the braces. The process involves holding the back up to your ear while tapping at certain points to make the back “musical”.  Really knowing what you’re listening for takes lots of experience, so I did the best I could, all things considered.

Then there was the tricky part of fitting the back to the sides.  You have to cut notches for the braces to fit through, and it has to pretty much be perfect. This part I had done before, so I kind of actually knew what I was doing.  Within no time I had a nice fit on the back.

Armed with my new improved go-bars, it was pretty easy to glue up and clamp the back. Last time around I used dozens of spool clamps, and it was an memorable and anxious ordeal. But this time, it was almost too easy. Having the right tool for the job is a thing of beauty. Bam!, back glued.

And that children is the story of how Mopus got her back! She’s a very happy Mopus, and is now looking forward to getting a top. But that’s a story for another day. Good night and sleep well!