How to Make a Nipkow Disc
The Nipkow (pronounced Nip-kov) disc is the most popular mechanical scanner and is also the easiest to make. The NBTVA often has readymade 32-line discs for sale in different sizes for sale to club members, but if you would like to build one yourself or want to create one in a custom format, here is a good place to start.
There are many different ways to make a Nipkow disc, and the results obtained are largely determined by the amount of effort put into the process. Good quality discs require high precision, and the maximum acceptable aperture deviation is generally considered to be one quarter of a picture element. (I hesitate to use the term "pixel" here, because although the signal driving the monitor nowadays is usually stored in a digital format, this display device is not digital and does not have discrete pixels. Each dot along a line simply blends into the next with no definite boundary.) With that said, respectable results can be achieved with the method that I used prior to having access to a laser cutter, described below.
Step 1: Make/Obtain a Pattern
The first step is to obtain or generate a pattern. If you are making a Club disc or a Baird disc, then there are several different patterns already available to choose from. Here is a PDF pattern for a 32-line NBTV disc. This one is the same but has markers for sync holes on the outside of the disc. However, if you are in need of a custom disc, there are a few programs available to generate your own. Some of them can be found on this forum post, but are no longer under active development.
Viddisc5 is a simple program written by Sam Hunt that makes disc patterns with or without sync holes. When generated, the image is placed on the computers' clipboard. You simply paste it into the image processing software of your choice. One drawback of this software is that the images it creates are relatively small, less than 1000 pixels on a side, so it only works well on discs smaller than 8 inches or so. But it's great for making indexing wheels for mirror screws or small slotted discs for syncing.
Another is club member Gary Millard's program DXFNipkow. As the name suggests, it generates simple DXFs for CNC or laser cutters, but can also be printed. This software is considered experimental, so test your design out on paper first before having it professionally cut. It also has the ability to make interlaced and non-interlaced multiple spiral discs. One thing to note here is that the holes are not actually holes, but points that should be replaced with the aperture shape of your choice, and sized using the aperture step feature.
If you still own an older PowerPC Mac, Sergei Ludanov KD6CJI has written a very useful program called Disk Designer but unfortunately no newer version is available. It outputs PDF files that are plenty large enough for big discs. This program was used to make the templates on this site. It can also create multiple spiral (non-interlaced) patterns. I have never personally used this program to generate a pattern since I don't own a Mac, but I have seen/used the results. However I have noticed an issue with certain aspect ratios being rounded to the nearest half-integer.
Step 2: Printing
Once you have your pattern, the next step is printing. For discs 8.5 inches or smaller, you can use your home printer. Otherwise you will need to take it to a local copy and print shop. Many print shops have large format printers for building plans that are suitable for this purpose. Don't forget to subtract at least a half inch to the size print of your pattern, otherwise the first hole will be right on the outer edge of your blank. If you have the option, use thicker paper. It is less likely to stretch or crinkle over time. Also, avoid letting the prints sit for weeks or months for that same reason.
Step 3: Layout
I made my discs out of .020" (or .019") aluminum, which is available in large sheets at a local home improvement store. This gives good rigidity while still being easy enough to cut by hand with heavy duty scissors. I don't recommend simply taping the pattern to the aluminum. The aluminum is too flexible and you will lose your pattern after you cut out the circumference. Instead, use a spray-on adhesive. You can get a can at any craft store or office supply store. If you have the option, get the repositionable kind over the permanent kind; it's easier to remove your pattern after the holes have been drilled.
Cut out a square or rough circle big enough to fit your pattern, then, after spraying, lay the pattern lightly on top of it. Use your hands to flatten it out starting from the center and working your way outwards. Use the edge of a credit card to ensure a good stick, in the same way you would apply vinyl lettering. There should be no creases, wrinkles or bubbles. Don't use excessive force which will distort the pattern.
An experimental disc that was partially drilled, then promptly forgotten in a corner for several years. The pattern has started to wrinkle and peel away from the aluminum.
If your disc pattern doesn't have sync holes included, you can draw them yourself. Just take a ruler and draw a straight line from the center of the disc to the outer hole. (Hint: the sync holes don't have to be exactly aligned radially around the circumference with the outer holes. As long as the offset is constant, the motor controller won't care.) Use a compass to draw the circle that the sync holes will rest on. For larger discs you may have to make something to accomplish this, such as a long stick of wood with a nail on one side and a pen on the other. It may be helpful to position the sync holes closer to the center of the disc and away from the edge of the light box to ensure they don't interfere with viewing. The location is arbitrary, so long as they don't get in the way of the light box. Alternatively, they could take the form of radial slots cut out of the disc's perimeter. But be careful with this method, because when spun the disc will in effect become a circular saw!
Step 4: Drilling
Tools needed:
Micrometer or digital caliper
Drill press
Micro drill bits
Pin vice
All of these items (other than the drill press) are relatively inexpensive and can be found online. After the pattern has been secured, the next step is drilling. In order to make accurate holes, it is first necessary to create a divot to guide the bit. Take a thumb tack and press it into the exact center of the holes on the pattern, including the sync holes and center hole. There is no need to force it all the way through the metal, it's purpose is only to catch the bit and prevent it from "walking" around your disc before it digs in. Take your time with this part, because it is difficult to fix once the divot has been placed. I have found that holding the tack lightly on the hole then looking at it from different angles before pushing it in helps a lot with correct placement.
Following that is choosing the correct drill size. Here's where the micrometer comes in. Measure the distance between the outer edge of the outermost scanning hole to the inner edge of the innermost one and divide that by the number of lines the disc has. The result will be your aperture size. However, you must choose a drill bit at least one size (but preferably two sizes) smaller than your result indicates. It's easy to make a hole bigger by chasing it out with a larger bit. It's not so easy to make that hole smaller again. The aperture size and bit size will not match exactly, so choose the closest one.
Use a drill press to drill out your apertures; a hand drill will likely break the fragile bits. Don't forget to drill out the center and mounting holes if present. The sync holes should be much larger than the apertures to make alignment of the optical fork less critical and to ensure enough light makes it through. There is no hard and fast rule for the size of the sync holes, nor for their shape. The club's motor controller is "edge sensitive", so it doesn't care about the length of the light/dark periods, only the rate at which it changes. You can even elongate the holes towards the disc center if you want more freedom for framing by sliding/moving the optical fork.
One other thing to mention regarding the sync holes. The 32-line Club standard uses a "missing frame pulse" method to indicate the start of a new frame. In my experience, the motor controller works fine with or without a hole blocked out on the disc. Even if you do plan on masking one of the holes, it's best to drill them all out anyway. It's easy enough to cover one hole afterwards at the appropriate point than to have to go back and re-drill the one you missed because you chose the wrong one.
Take your newly-drilled disc and mount it onto the motor hub or some sort of shaft so it can spin freely without flying off. Now, spin the disc as fast as you can by hand and look through the apertures at a bright, diffused light source. It's best to look at the disc face-on and not at an angle. Also, be careful not to touch the edge of the disc while it is spinning because it will be sharp, even if you sand it down. You should see some curved lines of light with black lines in between. If the black lines in between are all the same thickness then that is a good sign. Next, enlarge the scanning holes by one drill bit size and spin the disc again. Note the reduction in the thickness of the black lines. Continue increasing the hole size until the lines nearly disappear. You now have a complete Nipkow disc ready for your first receiver.
Always remember that it's nearly impossible to make a perfect disc by hand, unless you build special tools or jigs for the purpose (which some NBTV forum members have done) and it may take several tries to get the best one.