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Canadian
Homebuilt
Aviation

Rib Routing

There's a very quick and easy method of cutting out ribs and other such parts from aluminum sheet. It uses an electric router with a laminate trimming bit. I've heard that this method has been used at DeHavilland Aircraft to mass produce parts for their aircraft. Gary Wolf has demonstrated this technique at RAA seminars and other aviation events in southern Ontario.

Fig. 1 - Gary Wolf demonstrating the technique

The router follows a wooden form, cutting the aluminum sheet flush with the edge. Each rib is individually cut - although you might be tempted to stack up a few sheets and cut them all at once this turns out to not be a good idea. The cutter tends to get loaded with aluminum and the sheets get welded together. The story goes that DeHavilland used to bond the edges of laminated aluminum sheet in just such a way.

The method is so quick, however, that it doesn't really take that much longer to cut the ribs out individually.

Any woodworking router should work. Bits, on the other hand, can make the difference between success and failure. One thing you may find surprising is that the cost of the bit seems to have little to do with how well it cuts sheet aluminum. Gary has had the best results with an inexpensive Mastercraft cutter that he purchased from Canadian Tire (see Fig. 2). Other more expensive types that he has tried did not work as well. The best advice would be to try different makes until you find one that works well for you. Another factor that may affect results is the type of aluminum you are using. Gary used 6061-T6 for the demonstration.

Fig. 2 - Mastercraft Laminate Bit

As many of us know, woodworking power tools can be used on aluminum provided that the cutting speed can be slowed down. This is often done on bandsaws, for example - pulley systems can be used to reduce the speed while maintaining torque. This doesn't apply to routers, though. Using an electronic speed control you can reduce the speed of the router but you also reduce the torque, and you will find that you don't have enough power to cut the aluminum.

You'll need to develop a feel for cutting the sheet while the router runs at its normal speed. Too slow and you'll have a mess. Too fast and you have a mess. Best to practice first. Fortunately it doesn't take long to get the correct feel of how quickly to move the router so that the chips clear the cutter properly.

Here's how it's done:

	You'll need to cut out two pairs of templates from plywood or MDF. Each pair will sandwich the aluminum sheet in between to hold it in position . One pair is cut to the trim line (the size of the rib blank before it is formed into shape) and includes the cutouts for the lightening holes. The sandwich is held together by pins at both ends. This keeps the parts from moving while the trimming and forming is done. The two pieces making up the cutting template are exactly the same. It's worthwhile taking the time to make up good ones - every little detail will be transferred to the rib when you run the router around it.
	To locate the aluminum blank correctly you can place the top template over it and match drill holes for the pins - best to drill through to a piece of scrap plywood. Then pin the sandwich together. 1/4 inch bolts work fine as pins - make sure that the holes in the top template are countersunk so that the bolt heads don't protrude above the surface, otherwise they will catch the router baseplate. Next, drill holes in the lightening hole locations using a spade bit to give the router bit a place to start.
	In this picture we can see all the holes drilled and everything ready for routing with the laminate trimmer bit. Gary has a handy fixture that he made up for holding the sandwich and catching (most of) the aluminum chips as the router cuts its way through the sheet. Even so, this is probably not something you should do on the dining room table. Make sure that your eyes are protected with safety-rated glasses.
	Now the fun part. Run the router around the template, including the lightening holes. Note the direction of travel - Gary is moving the router from his right to his left on this part of the rib, and will continue in a counter-clockwise circle right around the whole template. This keeps the cutting edge of the bit advancing into new material and also keeps the cutter tight against the template. When you cut the lightening holes you will need to move in a clockwise direction. This is because they are an inside cut, while the perimeter of the rib is an outside cut. The part closest to the camera has already been trimmed. You can see the bit cuts the aluminum sheet flush with the template.
	Garys' pretty quick - he's already got this rib blank cut out and is fitting it to the second pair of templates. These are smaller than the ones he used for cutting because the sheet will be formed over the edges to make the flanges. Again, pins are used to hold everything in the correct position. To see what happens next, check out Rib Forming.