Mopus Chapter 3 – The rim’s the thing

After a bit of an adventure, the rim of the Mopus guitar is done. I made some missteps, some which I’ll mention and some that I won’t. One thing I find very rewarding about guitar building is that it seems like every mistake is an opportunity to get creative, and so far that’s been true of every guitar I’ve built. Mopus, being built completely from scratch, provides me far more opportunities to mess up get creative.

To make a rim from the sides, the first step was to cut blocks of mahogany to serve as heel and end blocks. When I built the Stew-Mac guitar kit, these came already made, but this time all I have is hunks of wood. Luckily, I now have a band saw, so making these blocks is a no brainer.

I decided to use white glue for the blocks, I was planning to use hot hide glue for everything, but I kinda chickened out here.  The main reason was that I somehow managed to cut the sides a little short, and I needed extra gluing time to make sure the rim hugged the mold. The sides therefore do not meet each other, but that’s not a problem because later on you wind up covering these areas anyway.  From here on out it’s hide glue….. I think.

I then got to try out my new radius dishes for the first time. The guitar back and front are not flat, they instead have a gentle radius. A radius dish is a sanding surface which conforms to the desired radius and using one ensures that everything will fit together nicely. Last time around, I made a “sanding stick” to do this, but this time I invested in some dishes from Canadian Luthier’s Supply . Here I’m putting a radius on the back of the rim:

Then I installed the kerfing. This is the slotted wood strips that line the rim and provide the gluing surface for the front and back. Just to make things extra complicated, I used mahogany “reverse kerfing” which is very fragile and requires pre-bending. And to really kick it up a notch, I used hide glue for the first time. Before I was done, I had several moments of panic, I even made the crazy mistake of bending the kerfing upside down and applying glue before realizing my mistake. I was all set to punt and order new kerfing, when Lucas said – “hey I thought the whole point of hide glue is that it’s reversible”. Truth. So armed with a heat gun and tiny scrapers, I removed the glue from each kerf until it looked like new. I managed to do that without breaking it, which is a miracle. Then I bent it the correct way, reapplied the glue, and continued with my life like nothing ever happened.

Next project was to cut strips of leftover rosewood to act as side braces. Not much to tell here, but i did get to build some clamp mouse traps. I keep re-discovering that you simply can’t have too many clamps. It’s just not possible.

Then I made an end graft for the butt end of the guitar. Since I made a mistake and cut the sides short, it’s only appropriate that I over-compensate by making a nice end graft. I used a bit of curly maple and some perfling, and made it a bit wider than usual, to cover my blunder:

To finish off the rim, I made a sound port. You don’t find sound ports on production guitars, but are common on high quality hand made instruments. They vent the sound so that the player can hear better, and actually improve the overall sound of the guitar. Since this is Mopus, named after a kitten, I decided to go with a paw print. To prevent cracking the sides, I first made a few thin strips of veneer on the bandsaw, one in maple and one in rosewood. I pre-bent them slightly to prevent cracking:

To make a clamping caul, I used Friendly Plastic – which becomes pliable when heated and then rigid when cooled.

Clamping the veneers in place:

After a couple of attempts, I settled on the graphic and position and glued on a template:

To cut the soundport, I used a Dremel and a cutting bit:

After some clean up with various files:

And now the rim is officially done!  

I might as well confess one other blunder innovative feature of Mopus. When clamping the guitar I discovered that the rim is thinner on one side than the other, by nearly 1/2″ in the lower bout. For the life of me I can’t figure out how this happened, I cut both sides together, and there really is no obvious way I could have done this. It’s much too late to do anything about it, so from now on I’m sticking with the story that this is an intentional feature. The idea is that when held in a playing position, the guitar will sit on your body more comfortably, since it’s thinner on the upper side. Yeah, now that I think about it, I definitely meant to do that.

Now… on to the back.

Mopus chapter 2: The Sides get Seriously Bent

A banner day in guitar building land! I got the sides bent, and although it wasn’t exactly smooth sailing, in the end it did work out. This was big for me, I’ve been wondering about how this would work out for quite a while. In fact, I did quite a lot of planning over several months, originally I was going to build a side bender which used silicone heating blankets, a temperature controler, and custom forms. I can’t even count how many times I almost ordered the parts, and it’s still likely I’ll get around to completing the thought one of these days. But what I did instead was buy a heating iron and bend the sides freehand. The main advantage in doing it this way is that I can bend any shape without making a special bending form each time. The down side is that quite a bit of practice is needed to master the technique.  In this case it’s trial by fire since rosewood is not cheap, and I’ve only got one set of sides. This way of bending is the old school approach and many many guitar sides have been bent using a hot pipe heated with fire or propane. There are commercially available electric bending irons available, but I was put off because most of them have cheap construction and are known to burn out easily. And they aren’t cheap. I ultimately wound up getting a custom iron made by Caramillo in the UK. The guy who made it teaches at the Newark School of Violin Making at Lincoln College and was a fun guy to deal with. The iron is made from solid milled aluminum and has two heating elements and a thermostat. 

 

But of course, before I get on with the bending, there’s always one last thing to do. I first needed to finish the mold, adding a bottom hinge, a latch, and 4 spreaders. That all was pretty easy, and I finished all that in no time. Here it is, finally ready for action:

 
Now on the bending. It took a while for the iron to heat up to close to 400 degrees, and as I never did this before it took me a good long time to get even the slightest bend. For a while I didn’t even think this was going to work out. But eventually, I got the idea, and managed to bend one side and then the other. It’s not perfect, but considering how many times I’ve done this, it actually came out pretty well.  

 

  

And voila, the sides are bent and chilling out in the mold! Next stop, making the rim. Since this is a rare weekend where I really have no other obligations, I might actually get that done!

 

  

Mopus chapter 1 : The story so far…

At this point, I can say that Mopus is underway. For a while there, the project was stuck in a murky swamp called “just one more thing before I really start”. There was some major prep work to be done first, the most significant was the purchase and setup of a bandsaw. That alone took a few weekends before it was ready to go, outfitting it with a moble base and a riser block, and fiddling with various blade setups. Still more to do there – no dust collection yet, and I need to rig up  some better fence options.   

Then there was the whole hide glue extravaganza. I’ve decided to use hide glue on Mopus for various reasons – bottom line being it really is the perfect glue. It’s super easy to clean, it’s reverseable if a repair is necessary, it’s natural and non-toxic, and it sets so fast that clamps are often not needed. However – and this is a big however – it needs to be heated to a very precise temperature and kept at that temperature. Even a 5 degree difference matters, so you can’t just wing it. I eventually rigged up a temperature controlled water bath for this purpose. I’ve also prepared enough glue for Mopus, and have made frozen glue ice cubes for quick use. Hide glue has a short shelf life if not refrigerated or frozen.  

  

I also mixed batches of shellac by using ground dewaxed flakes mixed with Everclear, but I’ve done all this before so at this point it’s no biggie. But I did use nicer bottles this time. I made 2 pound cuts of various tints, and will dilute to 1 pound or less when I get to the French polishing stage.

 

The true start of Mopus was when I started working on the guitar mold. The mold determines the shape of the guitar body, in this case a standard OM. It is important to me that I make the mold myself,  rather than buying one, giving me a lot of flexibility in the future. Besides, as they say, “if you can’t make the mold, you can’t make the guitar”. In the process I did run into issues using a router to trim plywood. I wound up making an impromptu router table, otherwise I could have easily lost some fingers. I need those fingers.

  
Here’s what the mold looks like so far, a little more to go before it’s ready for action.

        

Finally it’s time to work on some real wood! The first step is to prep the rosewood sides. These come as a book-matched set, but they are way too thick to use as is. Making them a uniform ~0.1″ thickness poses some real challenges (unless you have a thickness sander, which I don’t). I did some rough thicknessing with a Safe-T-Planer and a jig I made for this purpose almost 2 years ago. At the time I thought I was preparing to make another acoustic, but then wound up taking a 3 electric guitar detour. And now two years later, my Safe-T-Planer setup is finally being taking out for a spin. 

  

After rough thicknessing, quality time was spent with scrapers. This takes a great deal of effort, and I still have more to go. And after this, I have to do the same thing (only more so) for the back. It does help to have good and correctly sharpened scrapers,  and I’m getting better prepping and using them. So I do have some hope.

 

I tried out a new random orbit sander to do some additional smoothing.  I love this thing! I should have gotten one of these long ago.

 That’s where things stand today. I’m getting very close to bending the sides and starting the rim.  I may even get to some of that this weekend. 

Stay tuned to see a guitar take form out of (almost) thin air!