Re: Scored a Nice 16" Parabolic Reflector. HID or Short Arc? Test Beamshots w/ HID.
Ok, here's that frame again.
While I used a TIG welder and a lathe to make this, it could be done with much simpler tools. A $50 MAPP gas torch, a drill press, and a file would go a long way. A Dremel, belt sander, or die grinder would be nice.
I didn't 'design' this, in that it was not dimensioned and laid out as a drawing, or anything like that. I visualized what I wanted, built the first part, checked the fit, and did that again and again until it was done.
First I made the mounting ring (#1). I did not make this from scratch. I started with a steel ring intended to be a pipe joint, which I got from the local scrap yard for $2. I just looked for something that had an ID as close to what I needed as possible, but smaller. I ended up having to remove about 0.100" radius material from the ID. I used a lathe, but a grinder would work fine. It doesn't even need to be precise, really. It gets clamped under a threaded ring on the reflector assembly that is quite wide. Hole needs to be big enough, but sloppy is OK.
Once this ring was installed on the reflector, I needed to start shaping the frame.
I started by setting my glass cover on a blanket, flat, on my bench. I put (3) 1/2" nuts on the glass cover to set my air gap, and then set the reflector on these, face down. I centered the reflector by putting my index fingers on opposite sides of the inside of the glass cover's edge band, and feeling it. This works surprisingly well, and is plenty close for the task. Easily within 1/16". This put the reflector just where I wanted it in final configuration, and made the ring (#1) the foundation of the frame layout. Everything else starts there.
I built off this, directly. I hardly measured anything. I would hold a piece of metal up to the reflector, put a mark on it, bend it, and go compare it again. If it fit, I would tape it in place, with a clothespin or chunk of cardboard under it as a spacer. Coat hanger wire or pipe cleaners would be good to use to develop these shapes if you haven't done this before, so you won't have to bend and unbend the steel tubing to get it right. Then use the hanger wire as a template when you bend the steel.
I simply bent the tubing in a vice, with a pipe stuck over the end to give me leverage on the larger ones (3/8 OD tube). For the smaller ones (1/4 OD solid), I would bend by hand and then bang with a hammer to get a sharper bend. I needed two of each part. I'd make the first, then use it as a template to make the second.
I would leave each piece about 1/4" longer than I thought it needed to be! That let me adjust by trimming to make up for the inevitable errors and asymmetry that would show up when I brazed it, since I did not have a sturdy fixture. First timers might want to leave 1/2". Welds and braze joints warp when they cool. It also gave me a little room to grind an angle on the end of one piece, to match the side of the one it would be brazed onto.
I made the main braces (#2, brazed to the ring) first.
I bent the tube sections to rough shape, two identical pieces. I set the ring on a flat surface, set the 2 bent rods along side it, and brazed only the tips of the tubes together. Then I tweaked the shape to make it sit flat with equal heights on both ends, and brazed it onto the ring.
Now, welding would be tricky here. The tubing is thin, the ring is thick. Tubing is Chrome Moly Steel, ring is stainless. It is possible to weld the two parts together, but not good practice, and the tube would tend to melt away before the ring was hot enough.
Brazing does not care so much about different materials and thicknesses. You don't have to get things as hot, and you don't have to melt the parts you are attaching. You can attach combinations of mild steel, brass, stainless, copper, nickel, cast iron, titanium, and more.
With a TIG welder, I use Silicon Bronze rods. This material is available for MIG welders, too, but I have not used it that way. Oxy Acetlyne brazing is an option, if you have that gear.
The easiest brazing I have ever done is Silver brazing. It is strong, but the melting temperature is within the range of a MAPP Gas torch. A regular propane torch will not work well, get one for MAPP gas, or dual fuel. A decent Bernz-o-matic is only about $60. Chinese ones are 1/4 that much. You will need a small roll of Silver Braze wire from your local welding store, and some flux. And some kind of decent eye protection with tint. Not necessarily welding goggles, it is not arc-weld bright. But a little too bright for naked eyes. I have done it with sunglasses, not recommended though.
I next made 2x of (#3) and brazed these in place. I did one side first, only on one end, the top! I can't final braze at the bottom with the lens in place, because the heat would wreck it. More on that in a minute.
I held a tube in place, eyeballed the angle I would need on the end, reshaped it on the grinder, and repeated until it made good contact at one edge, and was within 1/16 or so everyplace else. That is close enough for this application, the braze will fill it. Strong enough for what it is.
Finally, I did the shorter legs in the back. Not numbered in the pic above.
This left me with a spider shaped frame that would stand flat on the bench, but each leg was a slightly different length and angle. Bend, grind, recheck until all are sitting close to the bench.
For the attachment tabs (#4) I made a cardboard part first for fit, then hammered a scrap of stainless in the vise to match. 3 total.
Here is where my technique would need to be modified a bit to adapt to using a MAPP gas torch. With TIG, I can pump a bunch of heat in, fast! To put it in appropriate language here, it is a 20 kW Argon arc lamp with no enclosure, and an arc length of about 4 MM. :sick2:
This let me quickly put a drop of braze on a joint to 'tack' it, wait a few seconds, then blast compressed air onto the part before it could melt the rubber gasket in the cover lens. Once they were all tacked like this, I could pull the lens out, and finish the brazing. (fast cooling a braze joint will weaken it! But these are only tacks, and once final brazed, will be fine.
With a MAPP torch, you would need to figure out how to hold the attachment tabs in place without using the lens, during brazing. All else is the same. If the MAPP torch can't heat a big part quite enough, you can put it on a barbeque grille to get a head start. Or invite a friend and use two - simple propane for assist, even.
Oh yeah, one last thing. Don't use stainless unless you have to! It is a pain to drill and cut. Mild steel is much nicer to work with. I only used stainless for the ring, because that is what I found in the scrap pile with suitable dimensions.
