Building Techniques

Making Identical Ribs - Mark C. Rzadca
Here is one more way to make large numbers of identical ribs that improves on using a template for producing identical outlines. (See the picture with insets below.) By drilling a 3/32 hole just inside the perimeter of the template, a brass tube can be cemented in place to act as a drill guide/bearing.
Holes drilled (using a pin vise) at each guide location provide identically located slots for wing spars. Small pins in the template stick into the rib’s balsa to maintain the relative position of the template to the piece being prepared. Once the holes are drilled, all that is needed to create slots is to connect the edges of the holes to the perimeter of the rib using a single edge razor or modeling knife. One advantage of the round bottom of the slots is that the spar extension of the top of the rib can be adjusted since the radius provides a little “give”. With this method spars cannot be made square. The width is established by the inner diameter of the guide bushing and the depth of the spar must be at least a little greater than that to account for the thickness of the tube wall thickness that must be inside the outline of the rib. So far I have used 3/32” tubes to produce slots for 1/16” by 3/32” spars for two models and it has worked great.
The only part that may be a little difficult to obtain is the 3/32” drill that will cut cleanly and produce a flat bottom. A flat bottom is not essential but a normal high speed twist drill bit will not cut cleanly through soft balsa. The tungsten carbide bit shown in the picture was purchased as part of a set from Harbor Freight some time ago at a cost of about $10 for 30 bits. These bits were industrial discards. Perhaps an end mill will work but too few cutting surfaces may not cut as cleanly as desired.