Radar Construction #2

By Mike Loewen

Basic Disc

Modified Sander

Ready to Glue
I found a bin full of scrap plexiglass at a local surplus store, and stocked up on 1/8" sections. I used a Rotozip with the circle cutting attachment to cut out 10 discs, 5 each of 6-7/8" and 7-1/8". In order to do the final sizing and sanding, I modified my Craftsman Disc/Belt Sander by drilling and tapping a 6-32 hole in the shaft. I then stacked up each set of 5 discs and bolted them to the shaft, fired up the sander and used a double-cut mill file (for plastics) and progressively finer sandpaper to cut the discs down to size and sand them. Back to the Rotozip again to cut out the center of the discs, to leave a hollow 5-1/2" interior. Then, the stack is ready to be glued with Weld-On #4 (acrylic solvent).

Centering the Stack

Welded!

Adding the Top
To keep the discs centered, I cut two 5-1/2" wooden discs, sized and sanded them and glued them together. It was then easy to slide each disc on the stack, smear it with Weld-On #4 and drop the next disc on. I also ran the needle around the edge of each disc so that capillary action distributed more solvent between the discs. Finally, I cut a 2" hole in the center of the top disc and welded it to the stack.

Center Ring Added
The 4-1/2" ring on top of the stack was made by welding a 1/4" and a 1/8" piece of acrylic together, cutting a disc with the Rotozip, and sizing and sanding the outside. Then the center was cut out with the Rotozip to leave a 1/8" ring, sanded and then welded to the stack. I found that when cutting circles with the Rotozip circle cutting attachment, you should drill a pilot hole for the Rotozip bit to fit into, then insert the center and bit into their respective holes before applying power. 

Large Discs
The two large 12-5/8" discs for the bottom section were cut, sized and sanded together, then a 5-1/2" hole was cut in the top disc to match the stack, and the bottom disc was cut to leave a 3/4" wide ring. Be aware that when spinning the discs together, the friction heat from sanding and grinding tends to fuse the edges together and they're hard to pry apart.

Large Wooden Form
To make the 1-1/4" high center ring for the bottom section, I cut two long strips out of 1/8" acrylic and fashioned a circular wooden form 12-1/8" in diameter. Then I heated the first strip in the oven at 275 degrees for a minute or two, bent it around the form and glued the ends with Weld-On #16. After the first strip had set, I heated and formed the second strip around the first. I then clamped the strips and worked my way around the circle using Weld-On #4 to fuse the strips together. When I reached the end I found that I had a slight gap, so I mixed a slurry of acrylic shavings with Weld-On #4 and used that to fill the gap.

Bottom Center Ring

Clamps

Center Section
I had a little sanding to do after the solvent dried on the center ring (that Weld-On runs freely), but I ended up with a fairly round ring, 1/4" thick with an outside diameter of 12-3/8". Then I attached the top and bottom plates with Weld-On #16, one at a time, and clamped them to dry.

Alignment Decal

Vanes

Vanes Attached
Despite my best efforts at keeping things centered I ended up with some variation, and the vanes for the top of the stack had to be individually fitted. I printed the image on removeable label media and stuck it to the top as an alignment guide. The 8 vanes were cut from 1/4" acrylic, each one traced from an 1/8" aluminum template that I cut. The slots were started with a Dremel tool with a medium size ball cutter, and finished off with a bastard file. I opted for rounded edges on the vanes, as that seems to fit better with most of the screen captures I've seen. Finally, the vanes were attached to the stack with Weld-On #16.

 

Radar Section
Finally, the stack is attached to the lower section with a bead of Weld-On #16, followed by Weld-On #4 around the inner edge. What I ended up with has a lot of room on the inside for motors, electronics, etc

Final Thoughts
I learned a lot during this project, and went down a few false trails in the process. My initial idea for forming the center part of the middle section was to use a wooden form with the center cut out, rather than a round form. I soon discovered that it was nearly impossible to bring the edges together inside the form. I also discovered that the smoked gray acrylic I had originally picked out for the center didn't work well with either Weld-On or Goop. Also, I should have heated the long strips for the center a bit longer, as they ended up with some stress cracks.

The other thing I learned about acrylic is, that 1/8" acrylic does not necessarily measure 1/8". The stuff I found is close, but slightly smaller. The biggest effect this discrepancy had was on the small center ring on top of the stack. Because the 1/4" and 1/8" pieces I welded together didn't quite add up to 3/8", I had to modify the vanes a bit so they wouldn't go all the way up to the top of the ring.

After I had run out of the media I used to cut the discs for the stack, I found some different stuff for the large discs for the lower section. To my dismay, this new stock cut VERY badly with the Rotozip, with large chips all around the edges. I finally found some more acrylic similar to what I had used for the small discs, which cut much smoother.