Building a banjo, part 3: building the rim
Building the rim
A block rim is basically a stacked set of “laminations.” Each of these is an octagon (though you also see 12 or 16 segments). You can mismatch pieces to get a checkerboard pattern or use different woods for different laminations for stripes, rim caps, wooden tone rings, etc. There are a bunch of videos and examples around the internet, this is a distillation that I’ve used.
Cutting rim segments (the blocks)
Splitting a circle into segments involves some math for angles. Look back at that layout above, that’s where these numbers below come from. You’re dividing 180° by the number of segments. I found it useful to draw up a plan: this is for a 16-segment rim that I hope to do sometime soon. There’s a smaller one in there for a banjo ukelele that I want to make for my kiddos. It’s hard to read, since I drew this out on brown paper, but it might be worth the time to sketch out what you’re looking for, especially if you are not doing an 11- or 12-inch rim. The measurements for those are below.
|Segments||Angles||Long side (11”)||Long side (12”)|
|8||22.5°||4 3/4”||5 3/16”|
|12||15°||3 1/16”||3 3/8”|
|16||11.25°||2 9/32”||2 1/2”|
These are the angles for your trapezoids. The lengths are really specific. When all is said and done, it’s most important that the angles are right and that the segments are all the same size. If they are a little bigger or smaller, it just means that your hoop will be a little bigger or smaller. That doesn’t matter at all.
The easiest way to cut them is using a chop saw, mitre saw, or radial arm saw. Basically, a tool that makes repeated identical cuts easy. Do this once with a test piece of long scrap, if you have it.
First, set the angle and make one cut to get the end off. Then, you can use the angled side of that end piece as a block so that each other piece is identical in length. Do this by flipping it and clamping it to the saw itself. The piece I cut for this photo was a little short, so I clamped another scrap as a stopper.
From here, you just flip the stock (the long wood you are cutting) to align with your block, slide it down, and make the next cut. Take it off, flip the stock, slide it down, cut.
When you get half of your segments for one of the laminations, lay them out on the floor. They should make a half circle. You can check by lining up a straight edge (a ruler or the long piece of wood that you’re cutting). If it’s way off, you’ll want to adjust the angles and redo them. You can cut these a bit shorter (shave off the angle) and make a smaller rim if necessary. Since you’re hopefully working with scrap, that’s not a problem. A small half circle will give you the same angles as a large one.
Once you’re set, get your regular stock (or put it back on) and keep cutting. If you are doing 3 laminations of 8 segments, you’ll need 24 of these segments. Lay out and check your octagons as you go. The description of this process at Bluestemstrings.com suggests leaving the last segment a little long so that you can adjust it to make everything else fit. I had not seen this before and will probably try it out the next time around, I bet it will save some headaches. (This next photo shows the segments glued and taped, which comes in a moment, but it shows the general idea of how the alignment works.)
If they are off by a little, you can shave off the difference in a few ways. One that I remember from the many YouTube videos I watched involved a table saw, where you basically lay out the two half circles and cut their ends flat so that they mate well. Drawing a line with a straight edge and cutting with a hand saw should work, too. Be careful of sanding since you can end up with rounded edges. This would mess with your gluing surface.
Gluing the laminations
Once the blocks are cut and you know that they fit together well, you can glue them together. One easy way to do this is with blue painters tape.
Lay the blocks for one lamination (one layer) out on a table on their long edge, end to end. This gives you a chance to practice lining them up as closely as possible. Rip off a piece of painters tape that is a few inches longer than the line of blocks. Starting at one edge of the tape, stick the blocks end to end as close as possible. You’ll use the few inches of tape to hold the octagon (or whatever) together after gluing.
Practice rolling it up and taping it shut. The blocks should line up perfectly. If they are jagged at all, fix it. You can use one edge of the tape as a guide. If any are out of “true” or straight, you’ll have to sand the entire layer of rim down to accommodate the gap. It’s an easy thing to get just right and it’s worth the time.
Once you get them exactly right, you add glue into both sides of each glue surface (where adjacent blocks will touch). Use your finger to spread the glue across the end grain of the wood so that it completely covers both sides of the gap. This may mean you have to lift the blocks a bit by grabbing the tape so that you can get right into the edge of the corner between the trapezoids.
Then, roll it up, use the end of the tape to hold it nice and tight, wipe off any excess glue, and leave it to dry overnight. Do it all again for each layer. If you are going to do a cap on the rim (a layer that’s a different material), this is the time to do it. The second picture below is from a time when I wanted to add a layer later on. It turned out to be a tone of awkward extra work since I couldn’t get the trapezoids to align exactly correctly.
Roughing out the laminations
Most rims are around a half inch thick and round, not octagons. It’s time to get a rough cut of each lamination so that they are easier to glue together and sand evenly.
Before you can cut, you have to mark. There are plenty of ways to do this, but I found it easiest to simply make a guide out of cardboard (Amazon boxes). Use a compass to cut a circle with your outer diameter and another with the inner diameter. Leave some give, so plan on a 3/4” rim for now. You’ll slowly work it down to size after they’re glued together. The last thing you want is a dip in one lamination.
The two here are 10 1/2” and 8 3/4”, leaving 1 3/4”. Remember that this divides in half (there are two widths of rim within the diameter), so they make a 7/8” rim. It’s pretty easy to get this down using power sanders, but not so much by hand, so you’ll have to balance the reward of starting closer to the target thickness against the risk of ending up things not lining up quite right. Remember, too, that you’ll be gluing a skin on, so it doesn’t need to align with a standard size measurement.
The easiest way to do this (by far) is to use a spindle sander for the inside and band saw for the outside, but those are expensive. (Lots of videos use a lathe with a giant chuck or big old piece of wood glued to paper to hold it on, though.)
You just run the outside through the bandsaw to get a rough circle, then pop each octagon over the spindle and hog out wood until you have a circle. No worries about cleaning things up yet. That will come after gluing.
I can think of a few ways to do it without those specific power tools. Sawing the angles off until you get close to the outer ring size isn’t a big deal. A coping saw should do the trick for the inside. You could also use a hand saw to “kerf” the inside with little cuts right up to the line, then use a chisel to knock out the waste wood.
Gluing up the rim
Once you have the rough shapes cut, it’s time to glue the laminations together. This is pretty straightforward. First, do lots and lots of sanding to make sure that the glue surfaces are clean and even. Then do more. And again. And some more.
Stack the laminations up and think about how you want to align them. If you’re doing a fancy checkerboard, this is the time to get it set. You want the segments to overlap in each layer so that they provide extra strength for each other. Go ahead and mark a few lines so that you know where the layers intersect once you put glue on.
Then, spread some glue on each side of the glue surface and set them up. Put the clamps on loosely, to hold, then tighten them little by little, moving around the rim as you do it. Glue slides, so getting one cranked down may mean that you’re off a bit and can’t move it. Take your time and get them tight. Wipe off the excess as it bleeds out and leave it overnight. (If I knew that I were writing this, I would have taken more pictures of each of these steps!)
Pull out the belt sander, if you have one, and start shaping the rim. You want a nice clean circle. The inside is a pain without a spindle sander, but way-too-much hand sanding will get you there. Start with the coarsest sandpaper and don’t step it up until you’re pretty much where you want to be. (I go from 80 to 220.) It’s definitely worth using a sanding block here: either wrap a piece of sand paper around a scrap piece of wood or glue it right onto a scrap. This keeps your sanding straight and efficient.
Once the walls are even, smooth, and feel really nice, you can think about the top and bottom. The bottom is flat. A few runs on the edges with sandpaper will pull off the sharp edge, but if you’re hand sanding, you probably knocked that corner off already.
The top is different. You want a banjo rim to come in contact with the skin evenly and only in one spot. For this reason, you need to be absolutely sure that the top is flat. The easiest way to do this is to put it on a flat surface (a piece of glass is good, but a trustworthy table works, too). If it rocks, you need to sand off the high spots that make it rock.
Then, you only want the rim to hold the skin at it’s outermost part. Most banjos use tone rings to do this: a brass hoop or something similar that sits on the rim and holds the skin. We’ll do a “wooden tone ring” by angling the top down as you move from the outer to the inner edge.
I’ve found it to be pretty quick work on my mounted belt sander. You can hold the rim at an angle and just wind it through to “cut” (sand) the bevel. Hand sanding (by finger or with a block) would do the job too. The bevel doesn’t matter so much; you’re just trying to get the wood out of the way of the skin so that it can ring nicely. Once you’re done, run the sand paper over the top a few more times to wear away any sharp points that could tear the skin and flip it upside down again to ensure that it’s perfectly flat.
The rim is good for now. We’ll come back to it when it’s time to cut some holes for the dowel stick, but that’s not until after the neck is done.