Rough grinding the easy way: say goodbye to the drudgery of hogging out a curve in your mirror blank.
Such was the situation facing telescope maker David Groski in advance of the annual Delmarva Mirror Making Seminar in Maryland. Dave had to prepare several blanks for attendees and he needed a faster way to rough grind the curves. He decided to machine generate the blanks the way professional optical fabricators do. His approach is simplicity itself and worth considering even if you're only going to make one or two mirrors. "It takes around 30 to 45 minutes to cut an f/4 curve in a 10-inch mirror and about the same amount of time to cut the corresponding curve on the glass tool," Dave notes. "If you were to grind it by hand it would take six hours at least, and most likely much longer."
The equipment requirements are minimal. You need a drill press--a piece of gear found in virtually every home workshop. You also need a diamond coring bit such as the ones available from THK Diamond Tools, which has an online store on eBay. The bits cost around $30 (depending on the size). Lastly you'll need to fix the glass disk to some kind of turntable with a fine tilt adjustment. If the table on your drill press allows for precise tilt adjustments, you only need to mount a simple turntable to it and you're up and running. But more likely you'll need to make what Dave refers to as a "sine table," shown in the photographs here. The tilt of the glass blank is precisely set by adjusting a nut and bolt at the far end of the table.
To generate a concave curve for a mirror, you position the blank under the coring bit so that the edge of the bit just touches the center of the glass blank. For a convex curve, the bit just touches the outer edge of the glass. The bit needs to be at least half the diameter of the glass disk, but it can be larger. For example, a 5-inch-diameter bit will suffice for mirrors up to 10 inches in diameter.
Crucial to the process is calculating the correct amount of tilt to generate a curve of the necessary depth for your mirror and tool set. The sine of tilt angle is the diameter of the coring bit divided by two times the radius of curvature of the mirror, or tool. For example, let's say you're using a 5-inch bit to generate an f/4.5 curve in a 10-inch mirror. The mirror's radius of curvature is 90 inches and requires that the table be tilted 1.59[degrees]--the angle with a sine of 5/(2x90). You can look up the angle in a trig table, or use a scientific calculator's arcsine key.
How do you achieve such a tiny, precise tilt? This is where the sine table comes in. "I set the table to the angle I need by using simple trig and measuring a known distance from the hinge," Dave explains. "Then I set the vertical height by adjusting the set screw until I achieve the correct angle. This gets me close to the radius I want, but I use a spherometer or template to measure the curve after I make the cut and, if needed, adjust the angle and take a second pass."
To generate the curve, set the drill press at its slowest speed (under 100 rpm) and lower the spinning bit until it engages the surface of the glass. As glass is ground away, you continuously, slowly rotate the mirror with the turntable (it may turn on its own), all the while ensuring that the mirror and bit are kept moist to prevent dangerous glass dust from becoming airborne. Grinding continues until the bit has ground the full diameter of the glass and the curve extends from center to edge.
The curves for both the mirror and its matching tool should be generated in succession without changing the angle of the sine table. It's especially important to ensure that the mirror's bevel doesn't wear down and cause edge chipping. Dave finishes off the process by grinding the mirror against the tool with a few wets using #80 grit. "In most cases," he notes, "the curves generated with the drill press will be close enough that some grinding with #80 will quickly get me to the curve I want. And because I need to mate the tool to the mirror, there's no reason to try to get things dead perfect when initially generating the curves by machine."
Readers wanting to know learn more can contact Dave at email@example.com.
Contributing editor Gary Seronik has made numerous mirrors and is about ready to give up his caveman ways. Some of his scopes are featured at his website, www.garyseronik.com.
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|Title Annotation:||Telescope Workshop|
|Publication:||Sky & Telescope|
|Date:||Nov 1, 2011|
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