This an extension of a Wired Magazine article entitled "Metal" in the April 2011 issue, which explains how you can make your own spiderless chainring if you have access to some basic CNC equipment. This story is in a broader package of do-it-yourself stories separated by materials. You can check out the original article in print at your local newsstand or online shortly.
1. The first step is sourcing your material. There are many online stores that sell smaller quantities. But always shop local first if you can. Sometimes you can score find remnants—leftovers from larger pieces that have been cut for bigger projects—for a discounted price. I use 7075 aluminum, which is fairly expensive and hard to find, however it is the strongest. It’s big in the aerospace industry.
2. Cut the material to size. I have my local metal supplier shear the material into 6"x48" strips, as it's easier to handle than a full sheet, which is 12'x4' and very hard to work with in a smaller shop. The materials end up as a 6"x6" square after this operation. The thickness varies depending on the crankset it will be used for, but it is generally about 3/16" to 1/4".
3. Next you're going to want to remove the burs that the band saw leaves on the edges of the squares. This is needed so the part will sit perfectly flat during the rest of the operations. I use a belt sander for this.
4. Now you're going to have to design your ring. You can do this several ways. I use Mastercam to draw and program the parts. If you don't have Mastercam money, there are several free, open-source CAD/CAM programs online that are probably suitable for this. After the ring is designed, you're going to need to write the programs to make your machines run. Mastercam has a built-in utility for doing this. Your CAM software most likely will, as well.
5. With that done, it's time to load a deburred square into a CNC mill for the first machining process. This is where I cut the splines that are used for fitting the ring onto the crank arm, and also where I cut some of the lightening pockets to remove weight. I made a fixture to hold the material so I can easily load and locate the parts, for efficiency and quality reasons. But you can get by with a vice for this step.
6. You'll want to remove the burs again, which can be done with sandpaper, buffing pads, a bur knife, or other abrasives. Remember, the material needs to sit flat in the following steps to produce a quality ring.
7. Now it’s time to machine the square into a circle, and machine the overall thickness of the teeth, along with the taper on the tips of the teeth that help guide the chain on. I use a CNC lathe for this, however you can use a CNC mill if you don't have one. The taper can be done using an endmill with a taper on the end, which can be ordered from most tool suppliers. For this step, use the spline that you machined in Step 5 for locating purposes. You can machine an arbor on a lathe that fits in the spline and make a fixture plate that the arbor fits into as well. Run a bolt through the arbor to clamp it all together, and you’ve got yourself a fixture. It's absolutely critical that the fits between the arbor and spline, and between the arbor and fixtures are held to a tight tolerance; otherwise, your chainring will have a wobble to it. I typically want the fit within .001" or less. The overall thickness of the ring usually needs to be machined in this step as well, which can either be done on a mill or lathe. It would be nice to start with this thickness of material, but the spline thickness is generally defined by the crankset, so you usually end up with a slightly thicker hub in the center.
8. Again, we need to make sure there are no burs for the next step. If you used a lathe, chances are they'll be pretty minimal. If you used a mill, and your design permits, you can save this for after the next step, as long as you don't remove the part from the fixture.
9. It's now time to machine the teeth, which is done on the CNC mill. You can use the same arbor-type fixture to locate the ring for this step. I do a rough and finishing pass to ensure that the tooth pitch is spot on. If there's any deviation, you can experience what is called chainsuck, which is when the chain gets stuck on the ring and starts to follow it back up until it binds. A poorly machined ring will also be noisy and cause premature chain wear.
10. I also machine some more lightening pockets in this step, on parts of the ring that weren’t accessible in Step 5. But, again, that will be dependent on your design.
11. Lastly, it's time to do some final deburring and finishing. You can use a similar process to step 6 for the debur work. I use fine abrasive pads for this, as it removes no material other than the burs. Too much material removed from the teeth in this process will void all the hard work you put in during the previous step to maintain quality. As for the finishing, you can do any number of things from a brushed finish to a mirror polish to an anodized finish, which is what I do. Remember, if you do decide to anodize your rings, the finish before anodizing is going to represent the finish after anodizing. So if you want it to be shiny, you'll need to put in some elbow grease before you send it out.
In conclusion, there is definitely a bit of work involved for a one-off ring. However, riding your own masterpiece can be pretty rewarding. If you take your time, you can create something much better than what's currently on the market, or even make a spiderless ring for a crankset that is currently without an offering.
