Cutting Bulkheads with a Router & Templates
Sun, Nov 25 07 @ 04:48 PM Eastern Standard Time
I was introduced to this method of cutting the bulkheads by the gang at the now famous "Plane Day". I really like how clean the bulkheads turn out. The edges are perfectly square, the lines are straight, and the curved edges are gentle. Personally it makes sense to make a part over sized and then bring it down to shape.
Opponents of this method claim that is too messy and you get covered in glass dust. I'll put forth that the mess is no worse than any other wood routing. In fact the dust is probably less than I've seen on some woods. Cutting the parts outside will reduce the amount of dust to cleanup. And sure I'm covered in itching fiberglass dust, but no worse than insulating a house with fiberglass. A good soapy shower will take that away.
Here's a video demonstrating the technique from start to finish. It's 10 minutes long and about 25 megs. Enjoy!
The excuse that it makes a mess is just non-sense. You're building a composite airplane, it's a messing process. You'll spend hours sanding glass, micro, and epoxy, so what's the big deal with a little mess? Detractors say it'll make you itchy. There's an easy cure for that; A HOT SOAPY SHOWER! Done, gone, no itch. Have you ever fiberglassed a house? It's about the same thing. I don't find it all that bad, but that's an individual thing.
Second, lean against any composite part with bare arms and your arms will itch a little. That's just the nature of the business. If you've done any sanding, there will be glass dust around.
Some say to follow the plans. The plan has you cut the foam to shape. Then glass and scissor trim within 1/2 inch of the foam. Then come back several hours later to knife trim. Or even reheat with a heat gun to cut. So this is the third time the shape is cut (First cut foam, second cut scissor trim, third cut knife trim). Compare this to the template process where you don't even have to scissor trim because it's oversized. You cut the templates once, then simply cut the bulkhead by using them as a guide.
Now the more common method is to use the fein. If you're thinking ahead, you don't cut the foam to shape. After cured, use the fein to cut the oversized foam and glass all at once. This isn't that radically different than the router method. I use the fein to cut glass all the time. It does a fein job cutting straight lines if you use a straight edge (a rudimentary template). But it's not great for cutting around corners. So now cut it over sized and then sand it down. That works fine for an outside corner, but cutting the inside corners of the leg openings would be more difficult. So now you’re sanding shapes by hands using multiple methods.
I find having the templates themselves are useful. Most people probably copy the M-Drawings to the foam. I instead copied them to the template. After it was cut, it was supper easy to trace the template onto the Foam. So the layout (copying) process still is only done once. Then after it's glassed I retraced it again onto the glass surface. So there's no lost energy laying out the templates. Cutting the templates is even easier. Good grief it's wood. I used MDF, it’s doesn’t get any easier to cut and sand. Certainly if I’m going to cut and sand something to shape, this is what I want to be working with. I’d loan my templates to you, but they’re in Australia and liable to be laid out upsidedown and backwards!
To this day I still use the templates to look at the original layup. They are handy to have around the shop to use for fitting purposes or simply confirm that it was sized currently. Questions like, gee that hole doesn't look like it's in the right position, let's hold up the template for comparison. Yep that's right, let's overlay that to the M-Drawings. Looks ok-great.
Someone said a slip of the router and you make a mess of your part. That’s same problem can occur if your not carful with the Fein. It comes down to knowing how to use the tool properly and what you’re comfortable using.
I’ve used a router more than I thought I would. So far I have:
-routed the bulkheads using templates.
-Leveled the NACA foam.
-routed the 1/16" depression around the landing brake, F22, & for LG bulkhead
-rounded over bulkheads prior to glassing during ch7
-cut the channel for the Coax going to the Antenna
My ultimate advice to anyone considering the technique is to do what you’re comfortable with. If you have used routers and templates in the past and like the process, then go for it. You’ll be happy with the results. If you don't like the idea, then skip it. It's just a technique. It's doesn't matter if you freehand cut it or use a CNC machine, most of the shapes are never seen. The inside edges of the IP and F-22 are the only ones that’ll be seen.
I'm a carpenter’s son and was taught to build ever part to the best that your skill level will allow regardless if it'll be seen or not. I take pride in ever part. Granted this technique may be higher precision than needed, but early in the build YOU NEED CONFIDENCE. I took my bulkheads to the SAAA (EAA in OZ) meeting and impressed many of the old-timers. That builds confidence.
BTW, study Nat's pictures closely at the end of the chapter. You CAN make the IP out of one piece of foam instead of two. There's a picture showing that he did it different than the plans.
job cutting straight lines if you use a straight edge (a rudimentary template). But it's not great for cutting around corners. So now cut it over sized and then sand it down. That works fine for an outside corner, but cutting the inside corners of the leg openings would be more difficult. So now you’re sanding shapes by hands using multiple methods.