Making hatches come out perfectly can be tricky. Generally, the best technique is to cut parts oversized, then sand or file them to fit. This is time consuming and still doesn't give perfect results, so when I had to cut three hatches for my Comanche 3, I decided to make a router jig.
Then, when I needed another staging hatch for my Aerobee, it seemed worthwhile to write up this how-to article. If you're going to be making multiple hatches for the same sized airframe, it definitely pays to make a jig.
A typical airframe hatch has three parts: a hole in the airframe, a piece inside the airframe that the cover mounts to, and the cover itself.
This picture of the Comanche-3 aft ends show these three pieces well. All three parts were cut out with the jigs described here, making them perfect in apperance and fit.
The first thing to do is determine the size of your hatch, which will be based on the electronics you want to contain. In my hatches, I was using G-Wiz LCX units, so they are narrow and tall. This drawing shows the size of the cover and the mount, as well as the position of the G-Wiz LCX.
Note that the router bits we'll be using for these options are "pattern bits". Pattern bits are flush trim router bit with the guide bearing above the cutters, instead of the more usual location at the end (below the cutters).
The most commonly available cutter diameter for pattern bits (like flush trim bits) is ½", but smaller sizes are available. In the photo above, I've mounted an Amana #47224 bit with a ¼" cutter diameter to get smaller corner radii when cutting out the inside of the mount. The tubing cutout and cover are routed with a more standard ½" pattern bit. Note that you should pick a bit with a long enough cutter length to cut through the tube as it moves away from the center.
The cutout jig is a box with open sides. The top and bottom have holes designed to guide a pattern bit. The top is for the cover mount, and the bottom is for the airframe cutout. The ends have a hole sized for the airframe tubing to fit into snugly.
To use the box jig, just mount the tube firmly through the ends. Here you can see me using a bar clamp in front to hold the tube and jig to the work table, and two spring clamps in back to hold the tube to the jig. If the tube is tightly held, you won't have any shifting or chattering during routing.
To cut the hole (either side), just place the router on the top, plunge through the tube, and run the bit around so that the bearing (or rub collar) rides against the cutout in the jig top.
Remove the top, clean up the edges with a little sandpaper, and your cut is done. In the picture above, I've just cut out the cover mount from a scrap of airframe tube. If you need more than one mount, turn the tube segment around and cut another one.
Once the mount inside cutout is made, cut the mount from the section of tube. This is most easily done with a band saw, but any method works since these cuts need not be precise. Above right, you can see me demonstrating how this mount will be installed inside the airframe tube just forward of the centering ring (this is part of my Aerobee sustainer).
Cutting the hole in the airframe is more of the same; just using the other side of the box jig.
Above you can see the airframe tube mounted in the jig and clamped down. If the tube doesn't fit tightly through the holes in the ends of the jig, use some tape to make it secure. This time, I used a ½" pattern bit for larger radius corners.
Now that we have the mounting plate cut out, and the hole in the airframe cut, we proceed to the hatch cover, which uses a different jig.
The cover jig is a simple piece of plywood, cut so that its corners are the same radius as the pattern bit used for the airframe cutout (½" diameter here). The oversized tubing scrap is placed on the block, screwed down with the mounting screws for the electronics, and then routed to shape using a pattern bit in a router table.
Make a drawing of the hatch cover, locating the holes for the flight computer and the mounting holes for the cover itself. For example, here's my drawing for the G-Wiz LCX based hatch.
The cover jig should be cut to match this drawing, with at least two holes to mount the cover during routing. Use T-nuts or threaded inserts so that you can screw the cover down to the jig block. Since I decided to use #4-40 T-nuts, I recessed the bottom a bit so the block would still lie flat.
To make a hatch cover, start by cutting a rectangle of tubing a bit larger than the final size. (I used more of same piece of tube that I cut the mount from earlier.) Then lay out and drill the holes for mounting the electronics unit on it using your drawing.
Mount the tube scrap to the jig using screws and stand-offs or other shims to compensate for the tubing curvature. I also added two strips of double-stick tape along the sides where the tube hits the block (since the G-Wiz mounts with only two screws).
Now, with the cover material firmly mounted to the jig, use a pattern bit mounted in a router table to trim the cover to the same size as the pattern block. I wear heavy leather gloves and am very careful with my hands so near the router bit. Also make sure to observe the direction the bit spins and avoid climb cutting.
After pattern routing, the cover will exactly match the block, and the angle of cut of the edges will also be perpendicular to the tangent of the tube at the center line, just like the airframe cutout. (The routing is always done along a plane tangent to the tube.)
Unscrew the tube and clean up the edges with sandpaper and you have a perfect cover. Drill a hole for pressure equalization in the cover and it's ready for mounting. Note that the mounting screw holes will be drilled later.
Installing the Hatch
To install the hatch, the mount plate is simply bonded in place to the inside of the airframe, lining up the mount with the airframe cutout. The cover remains loose, but holes are drilled through the cover and mount plate for retaining screws.
The first step is to test-fit the parts. The cover may need a little sanding to fit properly.
Once you're happy with the fit of the cover, set it aside and epoxy the mount plate into the airframe. Be sure to use sandpaper to rough up the periphery of the mount plate and the inside of the airframe tube to create a good bond.
After the mount plate bond is cured, the next step is to drill holes and install nuts for the cover retention screws. I chose to use #4 screws here, but #2s would also be fine. Tape the cover in place and use your drawing to locate the four retainer screws. Drill the screw holes through the cover and the tabs in the mount plate.
If you're using fiberglass tube PEM nuts are an option, but for cardboard or phenolic you want T-nuts (or possibly weld nuts). For the Comanche 3, I used T-nuts directly into the phenolic airframe, but they weren't secure enough, so for the Aerobee, I used small pieces of 1/16" plywood as backers.
Finally, mount your electronics unit and test that everything goes together precisely and securely.