I've just finished cutting wood spar stock for use in our quarter scale Piper PA-25 150 Pawnee kits. Due to the fact that it's readily available from our local Kansas City Missouri lumber yard, I've chosen to use C grade Douglas fir for the spar stock for all of our larger kits specifying dimensional wood structural members larger than those available from model aircraft supply houses such as Balsa USA, Sig, Midwest etc.
Before attempting any of these operations yourself, be sure to thoroughly read, understand and comply with the operation, safety and maintenance manual of your power tools as they are inherently dangerous and can cause serious injury and even death. Make sure your work area is safe and free from obstructions and trip hazards and that you are wearing all of the recommended safety equipment and that clothing cannot accidentally come in contact with the blade. In addition be sure to keep fingers and arms and hands well clear of the blade.
I generally choose a 1x6x12 and hand-select the piece by first checking for warps, twists, and cupping or crowning since I will have to straight-line one edge and rip the final parts on the table saw. Next I inspect both 5-1/4" faces for defects such as knots or pitch pockets and then examine the 1-1/2" edges by looking down the full length of the plank I looking for straight and tight growth ring spacing, 1/8" or less, and zero runoff of grain from one end of the board to the other. Usually the best boards will meet this criteria and I can usually find one or two boards that will work from the stack.But even if one or two growth rings run off in eight to twelve feet or knots are near one edge, a plank can be acceptable and I can work around these defects. A1x6x12 foot C grade fir board costs about $30.00 at my supplier.
Next I will place my board in the shop on a flat surface (better not to put it on the floor) and let it acclimate further to my shop and to make sure that it doesn't twist or warp over the next few weeks before I am ready to cut parts. This way if there are any stresses in the board due to moisture being trapped in the board from transport or storage before I purchase it, I will show up before I rip the board and get parts that are less than straight or suitable for kitting.
The first step in preparing the board for ripping parts on the table saw is to cross-cut the board to the nominal lengths needed and then straight-line one edge of each to remove the chamfered or rounded edges typical of dimensional construction lumber. I do this using an a thin-kerf carbide-tipped blade and an outfeed table to safely support the end of the heavy plank as it exits the table saw and blade and run the straightest edge against the rip fence set to remove only about 1/8" of material. I keep my blade clean and lubricated with dry-silicone spray to make the cleanest cuts without chatter or burning of the wood.
Next I set up my thin-strip ripping jig which I found in Wood Magazine in a construction article http://www.woodmagazine.com/wood/pdf/delta/da-00139rip.pdf. This handy jig may be made in a few hours using bits of plywood and hardwood scrap from the shop and a 1/4-20 x 1-1/2" flat head machine screw, washer and nut. It allows one to rip thin strips or parts on the left of the blade without the inherent danger of the workpiece being trapped between the blade and fence and being shot back at the operator by the rotation of the saw blade.
Once set up for the size of stock thickness needed, I rip cut as many pieces as needed to make up the number of parts called out by the kit or kits I am supplying. Finally, once all of the first cuts are made, I send the parts back through with the blade lowered for safety to just over the thickness of the pieces to rip each to its final width. This time since I am not cutting the full 1-1/2" thickness of the board I cut them in the usual way by setting up the rip fence to blade distance and run each final part against the fence.
Here are a few links of interest for making your own structural parts for scale model sailplanes:
Wood Magazine Thin-Strip Ripping Jig
Sport Air Article on Selecting Wood for Homebuilt Aircraft
Flight Magazine July 1943 Article on Wood for Aircraft Construction
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