Making the bars and triangles
for the mirror mount
Eighteen point flotation cells are recommended for mirrors in the 18 to 25-inch aperture range, and I decided to go with that. Rather than use the Kriege bar and triangle sizes specified on page 122 of The Dobsonian Telescope, I used a program freely available on the Internet, called PLOP, to calculate triangles and bars sized to support a fast 22-inch mirror more effectively.
The PLOP (plate optimizer) application was not available when the book was written. It’s used by entering criteria for the desired mirror, diameter, thickness of mirror, focal length, diameter of secondary obstruction, number of point in cell, etc. The program calculates support point positions that minimize flexure and provides specific sizes and hole positions for the required bars and triangles. Whether the improved sizing information makes that much difference in an 18-point cell is an interesting question. I understand sizing improvements are far more critical for 9-point than 18 and 27-point cells, which makes sense when you consider the shorter spans between support points on the more complex cells. But as one CloudyNights member observed, why not use an optimized design?
The triangle size recommended by PLOP for my criteria was slightly scalene (none of the sides were exactly the same length) with apices spaced at 5.29 inches, 4.41 inches, and 4.28 inches. The triangle support points for the bar were specified as 6.87 inches apart with the center point of the bar (where the collimation bolt connects) at 3.43 inches. I used another free Internet program, CellParts, to print templates for my components with 0.5 inch spacing around the specified points. The printouts were quite accurate (to within a hundredth of an inch, or so) and the triangles had nice rounded ends.
With the sizes in hand, my next decision was what material to use. Kriege recommends 1/8 inch stainless steel for the triangles and 1/4-inch for the bars so this was the first thing I considered. Stainless is an extremely hard material to cut. I have a well-equipped woodworking shop but do not have a milling machine or much experience with metalworking. I found a local machinist who had done projects for other members of our local astronomy club, the Astronomical Society of Kansas City. After reviewing the part templates, he said he could make them with his CNC machine, but warned me that using a plasma cutter on stainless steel 1/8-inch thick would warp the parts slightly so they wouldn’t be perfectly flat. Not encouraging.
Kriege suggests using aluminum “if you cannot work with steel.” I had already noticed that many ATMers do use aluminum for their mirror cells, apparently with excellent results. I talked with Steve Kennedy about the mirror cells he uses and found that his bar and cell components are aluminum. I figured, “if it’s good enough for Steve (uber perfectionist), it will be fine for me.”
I decided to use a 6061 T6 aluminum alloy, which is commonly available. It is a harder grade of aluminum, but still easy to work. In fact, aluminum is inexpensive, can be readily machined using standard wood working tools, is weather resistant like stainless steel, and plenty strong for the bars and triangles. Because aluminum is not as strong as stainless, it’s a good idea to double the thickness, making 1/4-inch triangles and 1/2-inch bars. This has the added advantage of increasing the allowable space between the back side of the mirror and the collimation bolt retaining nuts on top of the bars.
After working all this out, I concluded that Kriege’s advice to use stainless steel for the bars and triangles is extremely misleading. What’s the point? Aluminum is clearly a better choice.
I purchased a 30-inch length of 5-inch aluminum bar stock with a 0.25 inch thickness from a local metal company (less than $20) for the triangles, and a 36-inch length of 1-inch by 0.5 inch aluminum barstock for the bars (about $12).
Shown above is the 5-inch bar stock with the six triangle templates cut out and fastened onto the stock using spray adhesive. The barstock needed some cleanup when I got it home from the store, so I wiped it down with acetone and gave it a light sanding with a 150 grit sterated sanding disk using a 5-inch random orbit sander. If I had it to do over again, I would use a more agressive spray-on adhesive. 
The one I used was okay, but the template edges tended to curl up slightly on the sanding station which made it harder to see precisely where the finished edge was supposed to be. I covered the bottom side of the stock with blue 3M masking tape on advice from a CloudyNights member who told me it would help keep the bandsaw blade from gumming up with aluminum filings. I also treated the blade with paraffin wax occasionally in use to help it cut cleanly.
I had a 1/4-inch 10 TPI blade mounted on my bandsaw (a Delta 14-inch) and decided to try that out on the aluminum. It cut like a dream. In fact, it was so easy to work with I’m looking forward to my next aluminum project. My first step was to separate the triangles, making rough cuts between the templates leaving enough space to go back and make finish cuts with the individual pieces. (Note the dust mask! Aluminum dust is not something you want in your lungs.) If I had it to do over again, I would leave slightly more space between the triangles. The following image shows a finish cut on one of the triangles. Smaller pieces are easier to manipulate, which is why it’s a good idea to cut the triangles apart first. Also, I was careful to stay outside the template lines (leaving 1/32 to 1/16 inch) so I could clean up the edges on the stationary sander.
Cleaning up the edges of each triangle on the sander took only a minute or two, and the sander table ensured that the edges remained square.
I sprayed WD-40 on the sanding belt (more advice from my CloudyNights friends) to keep it from loading up with aluminum. The result was acceptable, as can be seen in the image below, but I used some 220-grit silicon carbide paper, wet with WD-40, to polish the edges up a bit more. I wasn’t trying to make them perfectly smooth, but it wouldn’t have taken that much more effort to do so. I would have started with slightly coarser paper and and worked through to a slightly finer grit.
Before removing the paper templates, I center punched the CG position (where the triangle is attached to the support bar) and all three apex points. I punched the apex points in case I later decided to mount the flotation pads with machine screws, which is exactly what I decided to do.
It only takes a minute, and even if you decide to stick the pads right on the triangles, the center punch mark won’t get in the way or anything. Drilling the CG hole for each triangle was easy. The picture at the top of this article looks cool, but after a little experimenting I found that parts could be positioned more accurately by inserting the tip of a metal scribe (which has a very fine point) in the press, centering that on the punch mark, clamping the part in place, and then swapping in the drill bit. It is very difficult to position even a small drill bit precisely. The idea to use the metal scribe (shown in images of making the bars below) was one of the key things I discovered while working on this part of the project.
The Bars
The bars proved more difficult to make than the triangles. A lot more! I have a nice collection scrap aluminum barstock in my junk box, now. Fortunately, the stock is inexpensive, so the learning experience didn’t cost a fortune. The problem with the bars, compared with the triangles, is that the bar template had to be precisely aligned with a pre-cut bar — the opposite technique used for the triangles, which were cut to the template after it was attached to the stock. I’ll skip the gory details about what didn’t work and simply describe what did.
I needed one bar to set up my drilling jig — one of my first mistakes was to try drilling the bars without using a jig. For repeatability and accuracy, using a jig is critical. Attempting to measure and mark each hole position allows tiny inaccuracies to creep in no matter how careful you are and even a small misposition spoils the project. For the setup bar I scribed a line down the middle of one bar and attached a template I printed from CellParts and modified by cutting windows to ensure correct template alignment. The bar shown in this picture is 1&1/2 inches wide, but I eventually decided to go with 1-inch stock for the bars.
I’m shown here cutting a bar to length with the miter sled on my table saw. A significant advantage of T6061 aluminum is that it can be worked with regular woodworking tools. (Don’t try this with stainless steel!) Even for aluminum, safety goggles are important. Note the stop clamped on the miter fence on my right. It ensures that each bar is precisely the same length and makes the cutting operation quick and certain.
After setting stop block positions with the scribed bar, I drilled the center hole for the additional bars.
Drilling the end holes is where accuracy is critical. If the center holes are off slightly, the bar itself might be shifted a small amount one way or the other but the support points for each triangle won’t change. If the end holes are not accurately positioned from the center hole, the support geometry of the mirror cell itself is affected. Personally, I don’t believe a small shift one way or the other is going to matter much in an 18 or 27-point design, but if you want the mirror cell support to be as effective as possible, the end hole position on the bars is where accuracy counts the most.
After messing up my first few attempts at making the mirror cell bars, I wasn’t taking any chances on my third try and cut some MDF bar sample pieces to use for setup and testing. I realized the drill bit used for the center hole was an accurate pivot point for the end holes when set in the hole it made in the jig base. To check the setup accuracy, I mounted a bar template on an MDF sample, placed the metal scribe in my drill press, positioned the jig so that the scribe was exactly over one end hole mark, and then swapped the bar sample end for end. I found that the the scribe was several hundredths of an inch off the second end hole mark. That might not sound like a lot, but it was unacceptable from my point of view.
After reviewing the setup and carefully measuring the printed template with a 1/100ths scale, I found the template itself was most of the problem. I guess the accuracy of the printer (a Canon Pixima 1500) was to blame. To resolve the problem I placed one of the aluminun bars with a center scribe mark (but no template) on the jig and made slight indentations on either end with the metal scribe mounted in the drill press (rotating the bar on the center hole pivot). I tested the end to end distance and adjusted the jig position until the end to end distance was as close to 6.87 inches as I could get it. With the jig clamped firmly in place, I drilled all of the end holes.
The finished bars are shown here with drills inserted in the center and end holes, demonstrating the close tolerance of the final parts. It took a lot of work to get to this point!