Building a wooden tripod on the CNC without using any hardware.
A project by:
Gabriel B. Soares
One of the RF engineers at work approached me a few months ago asking me how easy it would be to build a tripod made entirely out of wood for a new antenna that we were about to purchase. I had previously built a plywood table for the RF Chamber that we have at work using only fitted joints (no metal hardware) in order to support some of the units under test without interfering with the RF measurements, and he wanted something similar to support the antenna. I decided I would do some research and see what designs I could come up with to build the tripod.
I started by doing examining existing tripods online that are made specifically to go in RF chambers and to support these types of devices, and was astonished by the costs. The cheapest tripods were nearing $1,000 and I was convinced that I could build something for 1/5 of the price.
The main requirements for this build were that it needed to be very sturdy and stable (we can't afford to damage an antenna that costs thousands of dollars due to an unstable tripod), the tripod could not interfere with the RF measurements that the antenna would be collecting (therefore avoid using screws, nails, or other metal materials), and the height of the antenna needs to be adjustable. Inspired by the table design, I began sketching on AutoCAD the plans for the tripod.
I began the design phase by taking the .dwg files that I produced for the plywood table build and started analyzing the drawing to see what I was going to be able to edit. I took the design for the legs of the table and extended them / modified the angle for what I thought was appropriate for the tripod. I also added a middle platform to the legs which was going to be to hold the bottom horizontal piece of the tripod. Once the legs were designed, it was fairly simple to design the top and bottom platforms of the tripod which hold everything together. I designed the legs to be 32" tall, which was the minimum height requirement for the antenna in the RF room.
The first image below shows the arrangement of all the components in the tripod such that they all fit in a single 30" x 48" sheet of plywood. NOTE: all units in the drawing are in millimeters.
Threading Center Dowel:
In order to be able to adjust the height of the antenna, I decided to go with a threaded center dowel which can be spun in order to get the platform to move up and down. Since I couldn't use a metal threaded rod, I decided that I was going to make a wooden thread. I did some research online and found that I could just buy a wood threading kit with a center tap to do exactly the type of work that I wanted to do.
I purchased a kit for and a 1.5" dowel and played around with the kit. The first thing that I did was remove and sharpen the blade on my water sharpening stones to make sure that I would get the best results. I also used oil as a lubricant throughout the process to aid the cutting process. With the right tool, threading the dowel and tapping the holes was a straightforward process.
Final Tripod Assembly:
With the tripod designs ready to go and with the center dowel fully threaded, the last piece of the puzzle was to buy a 30" x 48" sheet of plywood and let the CNC machine make all of the cuts. I converted the AutoCAD designs to G-Code, set up the machine, and 40 minutes later I had all the pieces ready for assembly.
The end result came out exactly as I wanted it to, and all it took was some light sanding, a mallet, and some glue to put it all together. With the tripod complete I started to work on making an attachment for the antenna to sit on. The total material cost was under $200, the tripod it very sturdy, and has around 18" of vertical extension.
The last part of the build consisted of making the actual antenna mount. The antenna is a Waveguide Horn antenna and costs almost as much as a new car, so it goes without saying that sturdiness is of the essence... I swung by Woodcraft and picked up a piece of 3" x 3" x 36" hard maple and began sketching the design. The antenna can be positioned in two orientations, so the mount needs to allow for that rotation and I wanted to be able to thread the mount directly into the tripod's central dowel.
I began by taking all the necessary measurements of the antenna and transferring the values into one of my favorite phone apps called My Measures. The mount itself is fairly simple and consists of a round hole for central support of the antenna, a carved out section for the bracket to fit in (and lock the antenna in a particular orientation), and the hole for the dowel thread. I completed the build in a few hours and was happy with the results.