Wednesday, November 28, 2007

Making Big Circles

Making Big Circles

I need to make some big circles--20" and 17.5" in diameter, out of 1/4" aluminum plate. But that's too big for a lathe that I have. It turns out that there are several ways to do this, both using a router, and using a table saw. Here's one of the better examples of how do this with a table saw (probably the best choice for me in the size that I am going to need).

I've been experimenting with what was laying around the garage, and I have some confidence that I can do this, perhaps even pretty well, and most importantly, while leaving all my fingers firmly attached to my hands. Three tricks to this:

1. The board that holds the workpiece needs to be big enough that you can clamp it very securely to the table. You won't be able to hold the board tightly enough when cutting anything as tough as Delrin or aluminum.

2. The thickness of the board that holds the workpiece, plus the thickness of the workpiece, needs to be less than the height of the blade on your table saw. In this case, I have a somewhat smaller than normal carbide blade in the table saw, so I may need to get the standard diameter blade to get the height up a bit.

3. The example above uses 1/4" holes in the board into which the bolt that acts as an axis of rotation. I've decided that for my purposes, I'm going to use a tapped 3/8"-16 hole in the board (which will be aluminum or steel, depending on what I can find most available). The axis hole in the workpiece is a through hole so that it can turn freely, but using a threaded bolt to hold the workpiece in position to the board prevents the torque of the blade from ripping the workpiece free and sending it flying. You have to make sure that you don't use too long a bolt, of course, because the board has to clamp to the table.

I would not recommend using this approach to cut very small circles, because then your fingers are dangerously close to the blade. Since I'm planning to cut 17.5" and 20" circles (for the mirror cell), my fingers never have to get closer to the blade than a bit less than half the radius.

I'm looking at making a mirror cell rather than buying it because:

1. There aren't many vendors of 17.5" mirror cells intended to put into a tube. Many of the commercial sources are Dobsonian-targeted (which means a big square frame). Discovery Telescopes apparently makes one, but review comments like this one aren't confidence inspiring, especially since Discovery Telescopes webpage seems to have disappeared.

2. AstroSystems makes a 17.5" mirror cell which is probably quite good, since Company 7 sells it, but the weight of 16.6 pounds seems excessive. I believe that I can fabricate my own for a fraction of the cost, and about half the weight. (And weight reduction is the whole reason that I am rebuilding Big Bertha.)

A reflector mirror cell consists of the following parts:

1. A base plate that attaches to the tube. Typically, there are three screws (sometimes more) that pass through the tube into a flange on the base plate. Both for ventilation, and to reduce the weight, I'll will put in some lightening holes.

2. A mirror plate with a ring in which the mirror sits. At the top of the ring are usually three clips that prevent the mirror from falling forward out of the ring. The mirror plate usually has a series of 9 or 18 points that support the mirror arranged in a pattern that provide air flow around the base of the mirror, to speed up cooling. My experience has been, however, that a heavy ventilated solid mirror plate works well, too, and saves a bit of weight.

3. Three big screws that attach the mirror plate to the base plate, with springs in between the mirror plate and the base plate. Some designs have the screws coming off the mirror plate actually part of the stamping, but I have discovered that tapped holes let you use standard bolts. There are through holes in the base plate, and wing nuts on the screws, which provide a secure method of adjusting the collimation. I have used 1/4"-20 bolts in the past for this for big Big Bertha's current primitive mirror cell, but because of the weight involved, it would be tempting to switch to 3/8"-16 bolts. I think the springs that I am currently using for this will still fit over 3/8" diameter bolts.

UPDATE: It turns out that I can get the circles cut out of 1/4" aluminum for $220--which suddenly makes the cost of buying a mirror cell not seem so extravagant. Hmmm. Maybe using 1/8" steel starts to make more sense. My guess is that even 1/8" aluminum would still be more sufficiently stiff, easier for me to cut into circles, and it would take some more weight off the complete telescope--especially at the tail end, where there's the most deflection caused by weight.

UPDATE 2: A reader suggests that trying to use the technique above should be restricted to small cuts--that it would be safest to trim the square to a rough circle first. I may in fact go to hexagons instead.

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