ISON Tandem AirBike Builder's Log
Perfectionist's Guide to Great Gussets -- Fast
Theory of the Three P's:
I'll start this off by admitting that I am a perfectionist. However, I run out of patience easily, so I am also a fan of quick-and-dirty construction when it doesn't matter what something looks like.
The thing is, I have this hang-up about airplane construction. To me, it is a very high form of art, and I find few places in this art form where it doesn't matter what things look like. That's my hang-up, and I'm OK with it. If you don't suffer from this, that's OK too!
Despite the truth present in the "theory" above in blue, I generally find that doing something "right," or with an aesthetic flair that isn't necessary, doesn't have to take any longer than not doing it "right." Rib gussets are a good case in point; with a few good tools and some basic knowledge of the use of jigs, parts can be made that are precise and consistent, and even in less time than with other methods. This is especially useful when you need to crank out hundreds of identical parts, such as in our topic here.
The Tandem AirBike's wing rib gussets are cut from 1/16" (1.5mm) birch plywood. The best tool I have found for cleanly cutting this material is a 6" carbide-tipped circular blade made by Makita for their battery-powered trim saws. The blade is very narrow, cutting only a .050" kerf, and has a 5/8" arbor which fits my table saw. It will make really decent cuts in unsupported material (such as when there is lots of space between the blade and the table saw's insert), but it will make perfect razor-sharp cuts in even these difficult-to-cut thin laminates with the simple embellishment shown in the photos below.
The trick is to fully support the edges around the cut. Lower the blade below the table top, lightly attach a thin auxiliary surface over the blade opening with double-sided tape, then turn the motor on and raise the blade up through the new surface only as high as needed. In this case I'm using a 1/4" white-faced masonite panel for the auxiliary surface, and you can see the saw blade cutting through the birch ply part above. The other black line is a blade kerf from a previous setup. I'm using a knife to hold the gusset blank down and against a sliding jig which orients the part at the correct angle to the rip fence. The knife also pushes both parts as one unit past the blade. In this operation the part starts as a rectangular blank, and two quick passes over the blade produces the finished part with perfect edges needing no further finishing.
The gusset above starts out with a parallelogram-shaped blank (it may look rectangular in this view but it isn't). The blanks were cut to length at the appropriate angle using the table saw's miter gauge -- whenever you can save another trimming operation it is a Good Thing! Now, one pass over the blade with a different sliding jig (masonite again) and two perfect mirrored parts are created from the blank.
I should note here that I have used sharp metallic pushers (knifes, awls, rulers, etc.) around table saw blades for years, knowing full well that this practice isn't endorsed by those who write safety manuals. It requires the operator to be ever vigilant and alert, as it could be very dangerous if the pusher were to contact the blade. I accept the risk for doing this myself, and the description of this practice here is not a suggestion that others do the same. Always do whatever is safe for YOU. A safer alternative might be a stick with some coarse sandpaper glued to the end.
Parts with inside corners will require the use of a bandsaw or scroll saw. It's easy to set up cutting jigs on these tools also.
Here we have a sliding auxiliary table top (1/2" birch ply) attached to a wood runner which slides in the band saw's miter gauge groove at far right. The block visible at top center is clamped to the table top and stops the forward motion of the jig. Three positioning blocks have been screwed to the sliding surface, with drywall screws installed in the ends of the blocks for the blanks to rest against. Turning these screws allows you to minutely adjust the orientation of the blank for a perfect cut. Note that the blade will cut into the sliding jig as well as through the part -- this supports the cut just like we did on the table saw, creating a clean edge with no burr that will need to be finished. The first cut for our inside corner is shown above.
Here is the second pass which completes the inside corner on this gusset. I found it easy to cut three blanks at once with this fixture, so this operation was noticeably faster than the others.
Now we're back to the table saw to complete this gusset:
In two quick passes with the same sliding jig against the saw's rip fence, another gusset is completed. Once you're in the groove with these procedures you can make each pass in about 5 seconds. Compared to tracing all of the gussets with a template and then cutting them out by eye, this system has a lot to offer in both time savings and precision. Plus you end up with clean parts that won't need any further finishing before adding them to the rib assembly.
The result of a weekend's work -- over 900 ready-to-glue gussets in 13 different shapes. A lot of tedious work that doesn't feel much like airplane building, and very nice to have out of the way! I figure I spent about 45 seconds average on each gusset, including setup time.
First, there's a fresh strip of wood attached to the miter gauge to help support the cut to reduce edge chipping (I should have provided support under the workpiece as well but was too lazy). The main thing I wanted to show is the wood block attached to the rip fence. The workpiece is butted up against this block to gauge the correct cut length, and then it slides off the block as you hold it against the miter gauge and push it through the blade. This prevents the newly-cut piece from getting crooked and binding against the blade, which could present a safety hazard as well as mucking up the end of the part.