Accurate Wood Cutting for Aircraft ConstructionSteve Sadler, La Salle, Manitoba, Canada
T-bevel sliding square
Burning an inch.
Figure 7: Cut-line technique.
Reprinted from the January 2009 issue of The Winnsock, the monthly newsletter of the Winnipeg-area chapter of Canada’s Recreational Aircraft Association.
One thing you quickly find when building a wooden airplane is that there are very few square corners. And, while “close enough” may be okay for building a doghouse, you want something better than that in your airplane. Here are a few tips to get accurate joints the first time and (almost) every time.
You will want a cutoff saw (also known as a chop saw) that can swivel left and right and pivot up to 45 degrees from vertical. The saw doesn’t need to be expensive, but it does need to be accurate. Check it with a combination square for 90 and 45 degrees. The second and most important feature is that the saw needs to have good bearings. Grab the saw blade and wiggle it back and forth. If the arbor moves back and forth when you wiggle it, good cuts are impossible. Fortunately most new saws these days have reasonably good bearings. With very little force you can bend the saw blade slightly and that’s okay, but the spindle at the centre of the blade should not have any excessive play.
Since you will be making cuts at many angles, check that you can set and rigidly lock the crosscut angle wherever you want. Most saws have detent settings at 90 degrees (typically shown as 0 on the scale), 45 degrees, and possibly some others such as 22.5 and 60 degrees. The trick is setting the angle at something close to a detent setting. On some saws you cannot set a 43-degree angle, for example. The saw will jump into the 45-degree detent setting and there’s nothing you can do about it. If you are buying a saw, check it out before you buy. Set the angle to 43 degrees and clamp the setting, then grab the handle and wiggle it back and forth. See if the setting on the scale is at 43 degrees, or if it has slipped into the detent setting at 45 degrees.
You will need a crosscut saw blade that will make a smooth cut. I use a 60-tooth fine crosscutting carbide blade, but others may do a good job, too. Sharpness matters.
For measuring and transferring angles there is no substitute for a T-bevel sliding square. This can transfer any angle from paper to wood or from wood to your saw. Good ones can cost as low as $30.
Additionally, you will need a combination square, a support or table extension to hold the ends of long pieces, sharp pencils, good lighting, and a good tape measure.
I have found that a quality wall-mounted pencil sharpener is well worth the small cost. Keep a box of sharp pencils handy. When one gets dull, use a new one. Then at the end of the day, sharpen them all and put them away for next time.
You can’t make accurate cuts if you can’t see. If the light in your work area isn’t ideal (and whose is?), get a couple of student desk lamps with the long pivot arms. Mount one at your saw and one where you are measuring your project. They can clamp on to most surfaces, so you can move them where you need them.
Finally, where precision is required, I like to start a project with a new tape measure. Tape measures do get inaccurate with age, mainly at the measuring hook. The hooks start out square, but the first time you drop the tape on the floor, it probably won’t be square anymore. This means a slightly different measurement depending on the thickness of the work piece and whether you are measuring to an inside or outside location. Also, the sliding slot on the hook wears slightly over time, so you will get inaccurate measurements as the tape measure ages.
Check your tape regularly. Many people don’t like to use tape measures at all and prefer instead a steel rule, or if they use a tape, they “burn an inch.”
Measuring and Setup
Once the layout lines are drawn on the work surface, transfer the measurements to the top of the work piece, in this case a longeron, with a sharp pencil and a combination square as shown in Figure 2.
Use the sliding bevel to measure the angle of the line laid out on the work surface. Once it’s set, you need to make sure you don’t bump it on anything. If it moves, you won’t be cutting the right angle. Take the sliding bevel over to your saw and set the angle as shown in Figure 3.
Astute readers will notice something is wrong here. I have an angle smaller than 45 degrees to cut, but my saw only goes over to 45 degrees! What now? The solution is relatively simple. Add 90 degrees to the cut angle. You do this by taking an accurately sawed piece of wood and using it as an additional fence; a 2-by-6 works well for this. This should put your angle into a range that the saw can handle.
To make an accurate angle setting, look for a thin sliver of saw base to show at the edge of the bevel blade (if your miter saw has one). See Figure 4. Move your head back and forth as you look at the edge of the blade. If the angle is just right, the sliver of base will appear equally at the front and back. If you see a triangular sliver of base appear at the back (or front), then the angle isn’t set quite right. Figure 4 shows a setup with the saw bevel angle exactly matching the sliding bevel. Note the thin sliver of saw base casting appearing to the right of the bevel blade.
When making cuts with the temporary 2-by-6 fence in place, you’ll need three hands. Awkward work with tools that can easily remove parts of your body is usually a bad idea, so clamp the piece to be cut to the 2-by-6 as shown in Figure 5. Make sure the clamp won’t interfere with the blade or the guard.
For the second cut, both the angle and length are important. Setting the angle is the same as for the first cut. Figure 6 shows the technique for measuring length.
You can’t accurately use the hook at the end of the tape measure for this. I like to use the 10-inch mark on the tape as shown (similar to burning an inch); just make sure you subtract 10 inches from the total length when you cut! When measuring for length, it’s a lot easier if you measure to the pointy end of the first angle cut. This will allow you to hook the end of the tape measure over the first bevel when measuring length.
To get accurate length cuts, marking technique is important. As mentioned earlier, a sharp pencil is important. Use the T-bevel to lay out the line on the wood and put a small “x” on the cut side. It is disappointing to cut on the wrong side of the line and find that your piece of wood is one saw width too short. Figure 7 shows an example of a good cut line.
Cutting: Lower the saw blade to the cut line and check for accurate placement. Move the wood back and forth until you get a perfect alignment with the edge of the blade. When everything looks good, lift the saw blade a bit and start the motor. Wait for the saw to get up to speed and then slowly lower the blade through the wood. If you rush any of this, the saw will tear the wood rather than make a clean cut.
That’s it. Repeat as necessary. With some care, every joint will come out like the ones in Figure 8.