Rocketry Planet

I typically prefer to scratch build rockets and all of the components that I can. Fortunately I have a fairly well stocked garage and many power tools.

Prior to owning a 6 x 18 metal lathe, I built a jig to be able to cut bulkplates and centering rings as needed. It is essentially a router table that uses a laminate trimmer as the router.

I own a router table and chose not to use it for two reasons: 1) I did not want to fill it with holes that I use for the center pin. 2) My router table has a large insert that would not allow me to make a bulkplate smaller than 3".

I used a Ryobi laminate trimmer for my bulkplate/centering ring tool.

While I use a laminate trimmer, you could certainly use most routers or probably adapt a RotoZip type tool. I do not believe that a Dremel type tool has sufficient power to do this safely.

Speaking of safety, my laminate trimmer runs at about 28,000 RPM and has a considerable about of torque. The bit(s) that I use are not protected with a guard and will cut you without any consideration of your ability to protect yourself. In other words, if you choose to use something like this, it is at your own risk. Always be aware of where the bit positioned and keep your hands, fingers or any other body parts, away. Do not wear loose fitting clothing that could become tangled in the bit and always wear appropriate eye protection.

My version of the bulkplate/center ring cutter was created with following:

• Some type of a router, laminate trimmer or a spiral cutter (like RotoZip)
• RotoZip bits
• 1/4" to 1/8" bit adapter
• 2' x 2' square of 3/4" plywood
• Mounting hardware
• 1/8" music wire (centering pin)

As I mentioned, I used my Ryobi laminate trim router. The key to being able to use this tool is a 1/4" to 1/8" bit adapter. This will allow you to use the smaller spiral cutters normal used in the RotoZip tool.  

I removed the base plate of the router and purchased some longer screws to use when mounting the router to the plywood. The Ryobi is made from plastic, so be sure to use the correct size screws to avoid damaging the internal threads of the router base. (Ryobi is assembled with metric parts.)

The grey base is used to mark the mounting holes on the plywood.

Then drill the holes to the match your mounting screws size.

TIP: I used a hand drill vs. a drill press, and it is difficult to make perfectly centered and straight holes.  I normally drill the holes one size over to allow room for the screws to line up when mounting the router.

Additionally, the holes are counter-sunk. I used a 3/8" Forstner bit. Make the cut deep enough for a washer and the head of the screw.

The router base is then mounted to the bottom of the board and the router attached. I installed a 1/8" drill bit to create a hole in the center and made a plunge-cut with the bit. Afterwards, I used a 1" Forsner bit to create space for the router chuck.

The headspace is not required, but helps to reduce vibration by not having an excessively long exposure of the spiral bit.

Replace the drill bit a spiral cut bit, install the router into the base and adjust the depth to sufficiently cut the material thickness that you are using. Mount the plywood base securely to a work bench or table top.

Now, to determine the centering pin position, you will need to measure the airframe or coupler that you are making the bulkplate for.

For this example, I am making an internal coupler bulkplate for a 3.9" coupler tube.

The inside measurement is 3.74", divide this by 2 and it will determine the "center" position of the pin from the "inside edge" of the cutting tool (1.87").

The centering pin hole is measured from the inside edge of the spiral bit.

Mark and drill the centering pin hole to match the size of your centering pin. I use 1/8" music wire. This hole does not need to completely penetrate the plywood, but should be deep enough to secure the pin in place (about 5/8").

Here you can see the centering pin installed to the right of the spiral bit.

When you are ready to start using your bulkplate cutter, STOP AND THINK ABOUT THIS BEFORE TURNING ON THE ROUTER: I suggest that you try making a few practice, or dry cuts. Pay attention to where the bit is and where your fingers are located. If you do not want to practice without the motor on, then get some clean towels and hand the phone to your wife. Tell her she should call 911 if she hears you screaming.

The cut is made by inserting the centering pin into the material and turn on the router and plunge the material thru the spiral bit. Rotate the material to counter-clockwise direction.

When the cut is complete, turn the router "OFF", then remove the bulkplate.

Centering rings are cut the same way as a bulkplate: Make the outside cut first, then position the centering pin for the internal cut. The only difference is the centering ping is measured form the outside of the bit to the center.

I cannot stress enough about power tools and safety. Any tool bit capable of making a 4" diameter bulkplate in 1/2" plywood is capable of removing your hand. Pay attention and practice.  

For John's contribution of this article, he will receive a free one year subscription to LAUNCH Magazine. This sponsorship is made possible by our friends at MM Publishing, Inc., the producers of LAUNCH Magazine. Want your own free subscription? Read the program details page for complete information.