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.

02-10-2008 12:46 PM  #1
Steve_Shannon Will fly beer for rockets   Joined: Aug 2006 Posts: 1653	Re: DIY: Building a bulkplate and centering ring cut-out tool Very nice article John. Do you ever have a problem with those 1/8" bits deflecting?

02-10-2008 01:08 PM  #2
jrcoxx Certified Level Three   Joined: Nov 2006 Posts: 166	Re: DIY: Building a bulkplate and centering ring cut-out tool Deflecting?? No, I try to minimize the bit extened length outside the chuck, this also reduces vibration. I have also used Rotozip "Tile" cutting bits for cutting G10. It works really well. John

02-10-2008 01:34 PM  #3
UncleVanya   Joined: Sep 2007 Posts: 1425	Re: DIY: Building a bulkplate and centering ring cut-out tool Would the higher torque XPR Dremmel be up to this task?

02-10-2008 02:06 PM  #4
denverdoc Rana sapiens   Joined: Aug 2007 Posts: 1732	Re: DIY: Building a bulkplate and centering ring cut-out tool I have been meaning to try my roto-zip, even bought the circle jig attachment. Thanks for the idea about using tile cutter for G10.

02-10-2008 02:28 PM  #5
Steve_Shannon Will fly beer for rockets   Joined: Aug 2006 Posts: 1653	Re: DIY: Building a bulkplate and centering ring cut-out tool Quote: Would the higher torque XPR Dremmel be up to this task? I have the Dremel version of the RotoZip with the plunge router base. The Dremel with the plunge router base leaves too much bit sticking out between the collet and the router base, so it deflects easily. It would be plenty torquey enough otherwise. You might not be able to feed the bit into the wood as fast as you can with a trim router like John used. I also have a couple other full size routers. They allow the collet to be lowered to the same plane as the base, so they work better.

02-10-2008 03:09 PM  #6
jrcoxx Certified Level Three   Joined: Nov 2006 Posts: 166	Re: DIY: Building a bulkplate and centering ring cut-out tool I would be very cautious using smaller tools such as Dremel unless you are cutting very thin material. Even with a slow feed rate the bits will over heat. You can make straight cuts by securing a guide fence (I screwed a straight piece of 1x4) to the plywood sheet. It will not be as clean as using a table saw, but if you do not have a table saw available, it is much better than using a jig saw or hand saws. John

02-12-2008 06:36 PM  #7
mbrocketry New Member   Joined: Aug 2006 Posts: 1	Re: DIY: Building a bulkplate and centering ring cut-out tool I have been using this method for about ten years and still use it to this day only I use a router, router table and a wood jig. I cannot stress how you have to be careful using this setup or similar setups to make center rings. Make sure you wear safety glasses and NO gloves and no loose clothing while using this setup as noted by the author. Lets just put it this way I have experience with this setup and how dangerous it can be as I have got my finger caught in a 3/8 flute cut router bit spinning at 25,000 RPM cutting a bulkhead for about a tenth of a second. Matter of fact I was pulling my finger away before it even it the bit and it still looked like the end of my finger was blown off. I had to have finger surgery to place the nailbed that was missing so my finger nail would grow back. I was lucky it did not hit bone and my wife was home to take me to the hospital. Graphic pics please do not look if you have a weak constitution: http://mbrocketry.com/imag...tures/finger/finger1.jpg http://mbrocketry.com/imag...tures/finger/finger2.jpg http://mbrocketry.com/imag...tures/finger/finger3.jpg This is what can happen So PLEASE, PLEASE PLEASE be careful. John S. if you plan on cutting G10, be careful of the bit burning up, breaking off and heading towards your eye. G10 can heat and fatigue metal fast.

02-12-2008 06:54 PM  #8
Steve_Shannon Will fly beer for rockets   Joined: Aug 2006 Posts: 1653	Re: DIY: Building a bulkplate and centering ring cut-out tool That last is a very good point. In a very short time a bit can wiggle loose. Wood Magazine had an article by a guy whose spiral bit wiggled loose and darted upward in penetrated his forehead. A full face shield would not be a bad idea when using spiral bits that pull the material toward the machine. I've had templates yanked from my hands also when using a pattern following bit and a router. I keep a very loose hold. I would rather lose the workpiece than my finger.

02-12-2008 06:56 PM  #9
denverdoc Rana sapiens   Joined: Aug 2007 Posts: 1732	Re: DIY: Building a bulkplate and centering ring cut-out tool MB, Thanks for the tip, after getting bonked in the face a couple of times from parts like a section of 3 inch PT that seemed well chucked in my lathe, I'm investing in a good full face shield. I have been unable to cut G10 with a circle cutter on my drill press with any kind of accuracy, so I thought using a spiral saw might help matters. I have a decent router I bought on ebay and have meaning to make a table for it--somehow haven't gotten around to it. Usually I use at most 1/8" G10 and more often 1/16 as facing for CR's, using light ply in a sandwich arrangement. 2 of those and a 1/4" BBply core in between makes for some strong rings IMHO.

02-12-2008 07:10 PM  #10
Rokitman Certified Level One   Joined: Oct 2006 Posts: 11	Re: DIY: Building a bulkplate and centering ring cut-out tool I have noticed if you put bees wax on the threads of your router it helps by keeping the bit tight. This sounds like a good way to make rings but I will make them the way im making them now. Much faster and mass produce with my cnc machine. Hard to beleave I use to make them with Fly cutter for all them years. Patrick www.cnc-rings-fins.com

02-12-2008 07:20 PM  #11
denverdoc Rana sapiens   Joined: Aug 2007 Posts: 1732	Re: DIY: Building a bulkplate and centering ring cut-out tool Patrick, Just the gentleman I've been looking for--I need some custom CR's that accomodate telescoping aluminum tube staging rods, and actually will fit more than one way. I here you'ze da man for this.

04-28-2008 11:10 AM  #12
randym New Member   Joined: Aug 2006 Posts: 2	Re: DIY: Building a bulkplate and centering ring cut-out tool While I love to do DYI type of projects, I opted buy a Jasper Jig to use with my plunge router. I basically works the same way, but your hands are no where near the bit while it's cutting. It's a little more expensive way to go, but it keeps me from injuring myself and having to hear my wife tell the story over and over again!