Thursday, September 26, 2013

Back in Hardware Mode.

So, I stayed up late last night looking though my local Home Hardware superstore's on-line catalog, as I sometimes do, and had one of my ideas. So I went there today, bought some stuff, and before the sun went down I managed to make this.

It's a telescope mount, built from galvanized iron pipe, brass fittings, threaded rod, and some roller-skate bearings. The action is so smooth, I can start it spinning and come back a minute later.

Australians will recognize this as another example of Hills-Hoist based technology.


You can just see one of the bearings on the lower swivel. I've seen a lot of iron pipe used in the construction of amateur 'scope stands and mounts (and even the optical tube assembly) but I've taken it a step further and used matching brass fittings to create bearing blocks that accept standard ABEC 608s.

The bearings can be quickly removed, and disassembled, as I did just then. Those brass parts cost $4 each new, the skate bearings cost me about a buck each back in the day (and have been sitting on a shelf for years) and I had the nuts and 8mm threaded rod lying around from previous projects. 

The real secret (and most expensive item) was the special "self-centering step drill" bit that I used to ream out the ends of the fittings, to create a 22mm 'seat' for the bearing to sit within. I chucked this bit into my drill press, and manually turned the bit, (as if I were tapping a hole) instead of powering up the press.

I have a lathe, but really didn't want to use it for this. Partly this was to find a method that works for people who don't have access to a small machine shop, and partly because I don't like them because they're so insanely dangerous. "Experiment" and "Lathe" are two words that do not go together well.

Each only took a minute or two to carefully ream out to a depth of a couple of millimeters, even by hand. Brass is a lovely material to work... you can feel the chips coming off. (Brass was cheaper and more accurately machined than the equivalent galvanised steel, which was clearly die-cast, and had terrible dimensional accuracy. Don't substitute iron or steel, or use cast parts, you will regret it.)

Actually, the 22mm drill step left the fit a little too snug. The bearings would go in, but getting them out again took a small impact hammer. So I put a small grinding bit in my Dremel, and 'dusted' off a few more thou (just enough to smooth away the drill tooling marks, really) and now the bearings drop in and fall out perfectly. A file, or some sandpaper and time would have worked too.

Total cost for the two bearing assemblies? $20 in parts, and $25 in tooling.

Given that I've see people using four 'pillow block' bearings that cost $15 each on eBay (if you're lucky) I consider this a win. Especially since mine are a fraction of the size, inline, and require no extra mounting hardware to attach to the pipe.

I not only have a new telescope mount, I have a technique for making them at will.

Also, those facing inline threads will come in very useful when I start attaching motors and gearing and torqe plates. I have some designs already, but obtaining Arduinos and servo motors will take a little time. (Assuming I don't reallocate my CNC steppers.)


  1. In case someone actually tries building one of these, I should mention some things about the mechanical design: The two 'central' bolts around the closest 'facing' pair of 608 bearings should be as tight as possible, since the tension across the gap between the bearings is where the entire load of your telescope is going to go. The very central nut is optional - it's only there because I didn't have an appropriate washer. (Technically, it makes the 'bolt' twice as big in this critical zone) It's under compression, and all that force is being borne by the center sleeves of the 608 bearings as well as the outer nuts.

    The tighter this coupling is, the more torsion forces the bearing can take - up to the point where the bolt snaps from the tension, or the bearing inner sleeves are crushed and jam the nut faces against the outer race. We are 'pre-tensioning' these parts of the metal, so they react less to otherwise large outside forces, but that also means we're using up some safety margin.

    The outer two bearings (the ones shown removed) and their associated nuts should be as _loose_ as possible. Nylock nuts would be perfect here. If you tighten these nuts too much, you're applying a large static thrust force to _both_ bearings in the block, and they will quickly fail because that's their weakest mode. The only job of those far 608 bearings is to center the threaded rod's end - to act as a steady hand on a long lever. Not to take any loads. That said, a small amount of axial tension is needed to keep the load bearing in it's seat under the forces it's going to experience.

    The most likely failure mode is for the middle 608 bearings to laterally 'tear out' of their seat, since we've thinned the brass wall at that point. But, that's easily solved by threading an outer fitting onto the end of the bearing block - something I expect to do anyway when attaching motors.

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