Sunday, May 7, 2017

Template tempest

Rule number one: When you buy a four foot open array radar, it ain't four feet. It's something else and it's always longer. In sum, it didn't fit where it was supposed to go. This led to some interesting gyrations to find a solution. A number of problems had to be solved. The first was how to do some careful measuring of curved in space and time radar arch surfaces that were not perfectly symmetrical as you would expect of any hand made product. And who would be surprised to find out nothing on a boat is square and true? The second issue was measuring the swing of the radar array, and the third problem was the design of a cantilevered ledge to mount the radar on. But in the end the lesson here is almost always a way to solve a problem. Oops did I just hedge my statement a bit?

So how did this all happen in the first place? The answer is it wasn't all that hard. Two antique radars were being replaced with two new ones and had been purchased along with a ton of other electronics as part of a major upgrade. The new 6" Fantom radar fit where the old one was, but the 4' Fantom array missed by a few wee inches.

So to do the measuring we need some tools. The radar's template was accurate, but it didn't include the center point of the array's rotation. With the radar sitting square and straight on its template I used an ancient device that the pyramid builders used called a "Plumb Bob". Don't ask. I don't know who Bob was or why they named it after him.

The array end is slightly curved but squarish. I used the plumb bob to plot the curve of the leading edge of array as it turned. I marked the curve in about five degree increments using one of the several compass on my phone.

Now for the arch surfaces. I needed to know where they started and stopped and where the flat portions were located. I'm going to through bolt a platform that is going to be cantilevered off the bottom side of the arch. I also want the bolts to be as far apart as possible and still be on the flat surfaces. A square told me where the ends of the curvaceous surfaces were and a straight edge let me define the start and stopping points of the flat surfaces.

There is an access hatch under the arch that also figures into the big picture so I need to know where that is. When a surveyor does a survey they have to have a starting point. In other words you have to say "This is here"and everything else is referenced from "This is here".

The opening of the access hatch is measured and a longitudinal center line is established.

All of this is being done on the upper deck of the vessel. The material of choice is poster board taped to together. If you blow the pic up you can see all of my cryptic notes. How far do the cables stick out the back of the radar? Where does the arch start and stop? Where is the center line? What distance away does the radar have to be cantilevered out to be to let the array clear the structure as it turns? Lastly despite the fact that I measured carefully I needed to have some safety margin built in. I only have one shot at this on the owner's dime. If it was screwed up, the error would cost me my dimes.

The final frontier of this exercise is the space ship Enterprise shaped template of the platform. It will through bolt on each side of the access hatch. Everything clears and I have some modest safety margins built in. The moment load is about 125 lbs and I'm comfortable the arch will hold it. It is going to be milled out of 1/2" aluminum and I'm removing some material under the radar pedestal to get rid of some weight. The machine shop drilled the mounting holes and put a drain hole in the depression. I snagged it and took it out for coating. It's cleaned, gets an epoxy primer and then was powder coated in white.

The final result looks really nice. It has a factory appearance and cleared by 2" the structure behind it. The two plates plates (wrapped in plastic) job is to spread the bolt loading on the arch over a larger area. Close enough for gummit work. Tedious to do? Yes, but worth it and a satisfying project.

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