Cutting Ribs, the Easy Way.

Let’s get building again!

Now that the holidays are over, it is time to get back to the workshop and make some progress on that new building project you’ve been planning for a while. Sure, there are a lot of great ARFs out there these days, but have you ever considered the possibility of actually cutting wood and building a model of your own?

Starting with a kit or even just a set of plans and a stack of wood from the local hobby shop is a time-honored tradition in our hobby and if you haven’t tried it before, I’ll let you in on a little secret. Building your own model is immensely gratifying and really isn’t hard. Yes, it may take a few more evenings that assembling an ARF, but with a few simple tools, you can create a unique project different from the model-of-the-week that everyone else is flying.

I’m going to jump over a few basic steps here and go right to the one area that many people tell me is a real stumbling block for them when building from plans or older kits-cutting ribs. And to make the discussion even less intimidating this time, I am only going to talk about cutting ribs for a constant chord wing.

Long before today’s fancy laser cutters, model airplane kits commonly included “print wood” ““ sheets of balsa that had the were printed with the part outlines. The instructions told you to carefully cut out the parts and then assemble them. Well, if your skills with one of dad’s worn out razor blades were typical of most beginners, you soon found that it was hard to cut a straight line, let alone follow the smooth curve of a rib or former. Accurately cutting snug spar notches is also a challenge, until you know a few secrets.

Secret Number One? Don’t Do It Freehand.

Even with the best printwood or plans, hand cutting ribs without some sort of a guide is nearly impossible if you want consistent results. Start by making a simple template from any reasonably thin, easily-worked material. My favorite is 1/16 plywood. It is thick enough to guide the blade well, yet flexible and easy to work. 1/32 ply works well too and can be readily cut with shop scissors. I have also used formica and, in a pinch, thick cardstock such as cardboard from a new shirt or a cereal box.

Print out or make a photocopy of the plan area showing the rib shape, rough-cut it out with scissors and glue it to your template material. You don’t need a great bond here, so some spray glue, some glue stick or even a finger’s smear of Elmers will do just fine. Thin or soft stock can be cut close to the line with a pair of heavy-duty scissors, than sanded to shape. With thicker and harder stock I reach for a jeweler’s or coping saw or turn on a scroll or band saw. Be sure to cut just outside the outline, then sand to final shape with a sanding block.

Don’t forget to trim the spar pockets to fit the intended stock, along with the leading and trailing edge areas. I often use a small file to accurately shape the notches in plywood templates. Check these notches against stock of the proper dimension. While you many not use the notches now as you cut a full set, they come in handy if you need just one or two for repairs. At this point I harden the edges of my template with thin CA. Once it cures, a few final swipes with 220-grit will smooth it nicely.

Time or Money?

Before cutting the first rib you have a choice to make. You must decide whether your balsa stash is more valuable than your time. If wood is precious, you can layout your cutting pattern with the ribs close-spaced, rotating the template to fit the ribs together with minimal waste. When I opt for this approach, I use a soft pencil to trace the shapes on the sheet stock so that I don’t lose track of my intended pattern.

You can also do the layout on a piece of paper if you don’t want to draw on the wood. Now take the template, hold it in place on the sheet and trace around it with a new blade in your hobby knife. Take care to keep the blade vertical and tight against the template. The bevel and CA along the edge should be enough to keep the blade edge from cutting into the template.

Don’t try to cut out the notches at this point. Notching the ribs with your hobby knife often splits the edge of the rib and maintaining consistency is a challenge. In a few paragraphs I will describe an easy-to-make, simple sanding tool that quickly gives accurate notches. It also allows you to notch all the ribs at once, ensuring that they are properly aligned.

If you want to cut the notches by hand anyway, work carefully with a light touch on the knife. Pressing hard while you cut can split the grain near the edge of the ribs. I find that my best results come from pressing the blade down through through the balsa when notching, rather that drawing the blade on a pull stroke as used when cutting the outlines.

Mass Production

Working tightly on the material does slow the process a little though. More often than not, I measure the max height of the rib then cut strips to this width. While you can use just a straight edge and knife to cut these strips, I prefer a bit more consistency. I have several “balsa strippers” designed to make quick work of cutting consistent strips. The least expensive is the Master Airscrew Balsa Stripper. It is often available for a few dollars at your local hobby shop. Faster and more accurate cuts come from my Byrnes Model Machines Table Saw, along with a much higher tool cost and wood wasted from its kerf. Regardless of which tool I am using, I use the rib template to set the fence spacing. This “gang-cutting” technique does leave more wood in the bin, but it is much faster when cutting a larger number of ribs.

Now take these strips and cut them to the exact rib length. These will be the ribs for my McGuire Quadruplane and, despite what it says on my template, I needed 40 of them. Setting up the miter stop block gave me consistent, repeatable results with my table saw. I’ve achieved similar results with a few scraps of hardwood or plywood tacked to a piece of light ply to reference a steel ruler and the rib blanks so I could cut them accurately by hand. Yes, cutting by hand is a bit slower, but it works very well if that is your only option.

Once I have enough pieces, I stack them all together and cut the spar notches. Here I am again using my bench-top Byrnes table saw. The saw blade cuts a narrow kerf, so I have set the rip fence to reference one side of the spar notch and clamped my shop-made stop block to the miter fence for the other. I carefully set the blade height based on the spar thickness. Ganging the rib blanks as shown helps minimize variation between ribs. I like to run a sharpie along one corner to help maintain orientation over the next few steps.

Make A Sanding Stick

Don’t think you have to have a table saw to notch ribs though. You can also use a spar sanding stick that only takes a few minutes to make and will repay you many times over with clean, consistent spar notches. To make a custom sanding stick, start with a piece of ply, rock-hard balsa or even hardwood that is the same thickness as the intended spar. Glue a piece of 120-grit to the edge using thick CA. I put the paper face down on my bench, and then apply glue to the wood and press it down on the paper. A quick spritz of kicker will quickly cure the glue once you have the stick in position. Now carefully trim it to the width of the wood with a razor blade. Don’t worry if the blade wanders a bit. A few swipes with a sanding block will quickly trim any excess right to the edge of your stock. This tool could be used as is, or given increased accuracy with another simple step.

Controlling the depth of your spar slot is a lot easier if you affix guide rails made of hardwood or ply to the sides of the sander. You could add just one, but I usually add one to each side. I like to give them a quick smoothing on the lower edge where they will contact the ribs. Set a scrap of wood the same dimension as the intended spar depth on your workbench and use it as a shim as you glue on the rails. You can protect the shim with a bit of waxed paper or just be careful as you glue the rails in place. In all honesty, I often can’t be bothered to reach for the box of waxed paper and just work carefully as I position the stop blocks. As soon as you have the rail in place, check the underside for glue squeeze out. A few swipes with a scrap of wood will smooth any excess glue and prevent damaging the ribs. Now mark the spar dimensions on your new sander for future reference.

In use I clamp a number of rib blanks together, then just pencil on some guide lines at the spar location. One helpful option is to make another line on the blanks, spaced a bit from the intended notch. While I didn’t draw such a line on this stack of ribs, doing so gives a great visual reference for your guide rail and lets you align the sanding tool very accurately. It only takes a few seconds to notch the stack of rib blanks. The bit about 120 grit isn’t cast in stone either. I have used paper as fine as 180 grit when making these tools and while it works well, it is definitely slower going than with a coarser grit. On the other hand, you don’t want to go so coarse that you have to worry about tear out. I have used anything between 80 and 180. It really comes down to the size of the spar and what sandpaper is most handy.

Whether you use a small table saw or a custom sanding stick really doesn’t matter. Both work well and give similar results. To be honest, Once you have a few of these sticks made up and close at hand, I am not even sure that the table saw is significantly faster due to the extended setup time involved.

Alignment Fence

Cutting the notches before shaping flat bottom ribs also lets you index the blank and rib template against a fence. Glue a small tab that fits the spar notch to the straight edge of a scrap of ply or hardwood. This guide needs to be at least as thick as the rib stock and template combined. You will use the stub and straight edge to align the stock and template. I clamp this fence over a self-healing cutting mat on the edge of my workbench simply so it doesn’t move around as I work with it.

Take care when you “load” each blank and the template to ensure they are both firmly seated against the edge of the fence and spar stub. Similarly, hold both securely when it comes time to cut the rib outline. These simple measures can yield a rib set that doesn’t need sanding.

Results To Be Proud Of

Once I had made the template, jig and cut the rib blanks, it took just 10 minutes to cut the outline of all 40 ribs for my McGuire Quadruplane, and I then lined them up on a scrap of spar stock to admire my handiwork. The freshly sliced ribs were more consistent than many from kits I built before laser cutting was common and I could have used them as they were. Instead I added just a couple of swipes with my sanding block to finish the stack off smoothly.

Next time I’ll move on to assembling the wings and share a few more tips there.

Tools Needed
Hobby knife
Cutting matt
Straight edge

Supplies Needed
Balsa sheet with the proper thickness for the design
Thin plywood, non-corrugated cardboard or formica