Router Table centering rings and bulkheads

Centering rings and bulkheads are not the most expensive components of rocket building, but the costs can really add up if youre a prolific rocket builder who purchases these items ready-made. If you're like me and find yourself making a variety of unconventional projects, having rings custom made can also be a hassle. So for many of us it would be highly desirable to have a cost-effective and accurate way to make various size rings and bulkheads. Accurate and cost-effective are often mutually exclusive objectives. A well-equipped wood shop will have a number of expensive power tools with specialized features for cutting circular items. If you have access to one of these, go away. This article is for the rest of us.

I have friends who cut the outside diameters of their rings and bulkplates with a benchtop bandsaw and are happy with the results. Ive tried it, but I find this technique extremely frustrating. It requires precision setup of the guides and rollers, which can be difficult to achieve on a hobby tool. It also requires a bit of skill, so you may end up with a lot of shaping and sanding before your done. There are circle-cutting jigs available for professional-grade bandsaws, but if you have one of these you should have stopped reading by now.

If you have access to a drill press, you can make circular cuts with more precision using an adjustable circle cutter. Also called a trepanning cutter, this tool has one or two adjustable cutting arms that lock down to cut to accurate, repeatable circles up to 6 inches in diameter. A hole saw is a circular cutting tool commonly used in the installation of electrical conduit, plumbing, hoses, etc. It not only cuts holes, it also cuts a nice circular disc -- but even though they are available in a variety of sizes they rarely provide the exact fit you need. Smaller center holes can be cut with the tools described above or with more conventional and less expensive spade bits or Forstner bits, but again with these your size range is limited.

I own all of the above, but I dont often use them for cutting rings. I use a router and a simple jig to cut perfect rings of almost any size. A router is a portable housed motor that drives a shaft and collet, and can be purchased for less than half the cost of a benchtop bandsaw. A router table is a handy accessory to make this setup work, and thats what I use -- but before I got a router table I just used a piece of reinforced 3/4-inch particleboard as a router table and clamped it to my workbench.

Mount the face of the router to the underside of the horizontal surface you will use as your table, beneath a hole made for the router bit and collet to extend through. Cut a piece of 1/4-inch Masonite the approximate size of the table and put a slot starting at the position where the router bit protrudes upward. This is the work surface and adjustable guide. Lay this on top of the table with the bit through the slot. Use a router bit that is long enough to extend past the guide surface and at least halfway through the workpiece. A straight double-flute router bit, 1/4-inch diameter is the best for the purpose. These are available in lengths up to 3 inches with a 1-inch long cutting tip.

Calculate the exact radius of the disc you want to cut. Measure this distance from the near edge of the router bit extended past the end of the end of the slot, and mark this point. This marks the placement of your axis point. Drill and countersink a 1/4-inch hole on the underside of the Masonite and install 1/4-inch x 2-inch flat head machine screw from underneath to use as a pivot. Remove the prongs from a T-nut and use it to secure the spindle in place. Double-check your measurements and secure the adjustable work surface to the table with a couple of clamps on the corners.

Be extremely cautious with your fingers on this next part:

Select a piece of scrap material slightly larger than the disc you intend to cut. Drill a 5/16-inch hole in the center and install it on the pivot. Turn on the router. Carefully press the piece down onto the router bit. Rotate the piece around on the pivot until youve rotated it 360°. Shut off the router, let it stop spinning, then remove the workpiece. If the bit did not cut all the way through, flip the piece over and repeat the previous steps. Check the size of the finished disc against your tube and adjust accordingly by shifting the work surface. If desired the work surface can be locked in place on the table with flat head screws. Switch from scrap material to your good stock when you get the size right and try it for real.

If you're making a bulkhead, your work is done. If you're making a ring, then cutting the center hole with the router jig is similar to the steps just described. The only difference comes when you measure the position of the pivot -- this time you measure from the far edge of the router bit.

1. This method can also be employed to cut rings out of plywood, Plexiglas, Styrofoam, G10, and more, provided you use a suitable bit.
2. This may seem like a no-brainer, but remember that you need to cut the disc first and the center hole second.
3. If the outside diameter is too large, shift the work surface toward the bit and take another pass on the router.
4. When the hole in the ring is too small to fit over the tube, a sanding drum in a drill or drill press makes quick work of enlarging the diameter.
5. If the fit is a little loose, use fiberglass drywall tape around the inner component. It will hold the loose ring or tube in place and epoxy will flow right through it, firmly bonding the two items. Fiberglass drywall tape is used to cover wall joints prior to the application of joint compound. It looks a little like gauze and has adhesive on one side.
6. I havent tried it, but this idea could probably be adapted for use with a laminate trimmer or rotary tool, which would be an even cheaper alternative.