Homemade oversize chainring sprockets.
The accompanying image files are layouts of drill patterns for fabricating round chainwheels of various tooth counts.
The images have 90, 110 and 130mm PCD circles inscribed and lines for the common 5-hole pattern.
There are three sprocket patterns per file on the same center, to save on number of files!
The files are JFIF (.jpg) monochrome at 96dpi.
To use them, first print out the file somehow. Straight out of Netscape will do but you may have to join pages together. Check that the 90mm circle really is 90mm. Use spray adhesive to glue it onto a sheet of 2mm or 2.5mm (3/32") aluminum. The thicker size is better for strength but you will have to thin out the teeth more, unless the sprocket is for non-derailer (1/8") chain. It's good to glue a sheet of blank paper on the other side of the metal to protect it from scratching.
Center punch the holes for the teeth on the dots, and the mounting holes on the PCD of your crank. The common ones (90, 110, 130) are drawn on the patterns but you can lay others out with a compass.
The holes should be drilled first with a small (1/8") drill to ensure they stay on center. Then drill with an 8mm (5/16") drill. Cut through the outside of all the holes with a jigsaw leaving as much "meat" as possible on the teeth. Cut out the inside area in the pretty pattern of your choice, leaving enough metal for strength around the mounting holes. I use a half-ellipse between mounting holes coming as close as 20mm (about 7/8") to the roots of the teeth.
Then the slow part: filing off all the teeth! They need to be thinned somewhat too, especially if you are using 2.5mm sheet. Have a piece of NEW chain handy to check. It should roll on and off the teeth easily without sticking, in the absence of lubrication. With practice it goes quite fast, and you don't need to worry too much about taking too much metal off, within reason. It takes a couple of hours to make a 60T sprocket and it's good exercise :-)
If you put the sprocket onto the crank and turn it with the pedal you can thin the teeth with a file held in the other hand, and put points on them (the poor man's lathe). Other ideas, which I have found to work quite well, are to do a Hyperglide by thinning the teeth unevenly, or to remove every 4th or 5th tooth on all but the smallest sprocket. This helps shifting, especially if your tooth range is large. I have 44-56-68T on my trike with a 12-32 rear cluster.
