A Little Update

Since it's a slow news day in the space news I've been following, I thought I'd post an update to some work on my tower fix I last talked about in early April.  A few of you left comments about the bare U-bolts on the pipe not being secure enough.  I got the idea that the heart of the issue is the contact area of the bolt on the 4" schedule 40 pipe is simply a very thin line – infinitely thin in principle – and the fix would be spreading that load over a larger contact area.  Reinforcing the pipe is another option and remains available.

Over the course of the next few weeks after that post, I struggled to envision some sort of thing that could go in between them, eventually settling on cutting a sliver of a cylinder.  I looked at U-bolts that came with things to spread the load, and eventually thought of a chunk of aluminum that would go between the U-bolt and the pipe, about a quarter inch thick and at least a half inch wide.  The place where that U-bolt went would be milled out the width of the bolt – half inch.  Ideally, the stainless U-bolt would tighten down into the aluminum, creating (crushing) a groove the bolt sits in and really spreading out the contact. 

I don’t think that my CAD drawing is any easier to understand than this image of the part after machining the two big channels and as it’s ready to come off the mill.  

The load spreading portion of the part is the semicircular arch that starts just in from both the bottom left and right corners of the piece.  You can see it’s very thin at the bottom and much thicker in the top middle (about 3/10 inch).  Looking down you can see a large aluminum (almost) semicircle; that’s leftover (scrap) and the bottom of the arch surrounds and presses onto the schedule 40 pipe (4-1/2 inch actual diameter).  There’s another arch cutaway above the one the U-bolt presses onto.  That one is the one the U-bolt sits in.  The clamping force extends over that arch that clamps onto the pipe.  

You can see that there’s an opening in the top of the part there, the top of the U-bolt sticks out there, and on both sides of the channel you can see what are 1/2” holes drilled through, top to bottom.  That’s because my original idea of how to mill the part was to stand it up in the milling vise and mill it left to right, raising and lowering the cutter to cut out the shape of the channel and the half inch holes removed a lot of what had to go.  I gave up on that idea after a lot of simulations and attempts to get tool paths that would do that.  After that I milled it in the position you see it, lower the cutter into the work on the right end of the semicircle, cut the semicircle counterclockwise, lift the cutter out of the work and move (at high speed) back to where it started and repeat the cut a little deeper.

It took longer to figure out how to machine it than to decide what it would look like or how it would work.  Along the way, the name morphed from the U-Bolt Load Spreader to the U-Bolt Thingy then to just plain Thingy or the UBT.  

This is a photo of the Thingy in place, with the U-bolt tightened down (but not torqued to spec.)

For scale, the U-bolt itself is half-inch diameter and the channel it’s sitting is just a bit deeper than that (about .050 in.)  The “wall” on the right is the channel stock used for the arms.  The other channel stock is on the left side, well out of picture.  The really big round thing is the 4 inch pipe.

Looked at from 90 degrees to the right, the channel and UBT look like this:

The slot in the top of the channel and the bolt sticking beyond the UBT are very obvious; so are the half inch holes that were drilled (the second UBT doesn't have those holes).  The holes look like they're a perfect place to attract mud dauber wasps, so I filled those with caulk.  

When I started to work on the tower to install these, it became obvious that the tower had been driven from vertical by some wind episode we had.  Part of the installation then became to temporarily clamp the tower to the old house bracket to pull it vertical, like my temporary fix described in that April update, then to torque down the bolts before taking the clamp off. We haven't had particularly intense winds in the two weeks since this was added, but the tower has remained vertical.