Friday, April 26, 2013

Need A Suggestion on a Mechanical Problem

The truss connector blocks are flat on the base, because they are normally attached to a flat surface.  In this case, I am attaching to a 20" OD round tube.  That is close to flat, but not quite.  Where the blocks screw to a surface, there is going to be about 1/16" gap on either side.  I need some spacer in there that will prevent rocking, and is about 1.25" long and perhaps .375" wide.  This is a bit small to be cutting from steel or aluminum (at least with what I can find sitting around on a weekend).  Any suggestions for something that I might find in a hardware store that is stiff enough that it won't bend much, but can be cut with tin snips?

UPDATE: A conversation with my wife about the subject suggests a solution to try tomorrow: take some of the pieces of aluminum that are too thick (.125") and sand them to a wedge on the sander.  This will produce wedges that can be milled to the required dimensions.  Once to size, I can tape them to the back of the blocks, then drill them through the existing holes in the blocks.

UPDATE 2: Sanding aluminum to a wedge is surprisingly slow.  Does anyone have an idea of some common thin wedge shaped aluminum or steel that I might find in a hardware store?  Are there shims that are wedge shaped?

This might do the trick: they are plastic wedges intended for construction, and self-adhering.  What the various dimensions are describing isn't terribly clear, but I suspect that I might be able to run these through the band saw to cut them into the wedges that I need, and then mill them to the precise dimensions that I need.  Then use the adhesive to stick them on the bottom of the blocks before drilling through the existing hole in the block through the wedge.

UPDATE 3: One reader suggested the wedges used for repairing axe handles.  These usually come as a tiny piece of steel (for exerting force) and wood.  I was hoping to use metal or plastic to reduce the problem of rot.

One possibility is to use a piece of aluminum C-channel; cut down the legs to 1/16" of an inch, and that provides something that prevents wobble, and provides a flat base.  I tried to mill a C-channel from an existing 1/8" sheet of aluminum, and while it looked great, it was very slow.  It probably makes more sense to start with a piece of aluminum C-channel, cut it to the needed length (1.25"), cut the legs down to 1/8" inch on the band saw, then mill them to 1/16" inch on the vertical mill with a fly cutter.  The problem is that I don't have any 3" wide C-channel lying around, so I will probably have to wait until Monday to get it, which means waiting on this rebuild until next weekend.  I wish someone made acetal C-channel-- lighter, and it doesn't need to be spectacularly stiff or strong.

UPDATE 4: Final solution, I think.  Using x2 + y2 = r2, I compute that for a 20.5" OD tube, the drop over a distance of 1.45" in the x-direction means a .103" drop in the y-direction. What I will look for is some1/10" or 1/8" round rod, and I will then epoxy 1.25" lengths of this on to the tube where the two sides of where the block will go.  This will give a flat surface upon which it will mate.

5 comments:

djmooretx said...

The wedges used to afix axe heads to handles.

Kirk Parker said...

Why not coat the two sides with mold release, loosely assemble it, and inject some epoxy into the gap? Instant (well, fairly instant) spacers of the exact size and shape needed.

Once it's hardened, disassemble, clean off the mold release, sand the exposed ends to your required smoothness (epoxy sands very easily) and reassemble to the regular tightness.

takirks said...

You're in Idaho. Go to the nearest chainsaw shop and look at the selection of plastic and aluminum felling wedges. Something ought to suit your needs.

Clayton said...

Kirk: that is a very good idea. I think I have a simpler idea; if that doesn't work, I'll try yours. What is mold release, and where do I buy it?

Kirk Parker said...

Many different types of mold release; if you google the phrase you'll get overwhelmed by the alternatives. :-(

My experience is with PVA liquid and also with using wax paper as an alternative non-stick surface (this is for epoxy and also for marine gelcoat repairs.)

Here's a brief discussion from West System (one of the leading suppliers of epoxy for marine uses) talking about using hair spray and auto wax, so for your purposes you might not have to buy anything exotic:

http://www.westsystem.com/ss/mold-release-pva-and-hairspray/

West System also has a very well-written set of documents on the use of epoxy, that show concepts and techniques in a way that's universally applicable (i.e. not just a cookbook for using their products):

http://www.westsystem.com/ss/how-to-use/