How To Build a Pinewood Derby Car/Axles
Pinewood derby axles are one inch long, 0.087 inch diameter zinc plated steel nails. The minimal axle treatment is the removal of the burrs and crimp marks with a file, followed by sanding and polishing. When it is allowed by the rules, the axle shaft can be cut with grooved and the head tapered. The grooves reduce the contact area between the axle and wheel hub bore and tapering reduces the contact area between the axle head and the outer hub.
The steps to axle preparation are
- File to remove burr and crimp marks
- Bevel (if desired)
- Notch (if desred)
- FIle, sand and polish
Straightening the axle is best accomplished using an axle press or with a small hammer and anvil such as on a bench vise. In either case, the worst of the burr and crimp material near the axle head should be filed off by hand or with the axle chucked into a rotary tool or hand drill.
With an anvil, hold the axle flat against the face with the nail head over the edge. Rotate the axle until you can see a slight gap between the face and axle then gently tap near the head. Continue rotating the axle and gently tap around the axle. Check the axle straightness by chucking it into the drill and observing the runout as it rotates. If you are running a raised wheel car, select the axle with the most runout for the raised wheel position.
The axle press is an anvil split in two with a hole for the axle and an indentation for the axle head. A hammer is used to tap the head into the indentation and forge the angle in the head. The press has a flat side and an indented side. To straighten the axle, first make a mark on the nail head. Insert the axle into the flat side of the press with the mark at the 12 o'clock position. Place the axle press on its side and strike it several times with a hammer, then rotate to the 4 o'clock position. Hammer, rotate to the 8 o'clock position, then hammer again.
Beveling and tapering
Axles can be beveled or tapered with a file, axle press, or lathe. To taper with a file, clamp the axle in the chuck of a drill or drill press. Hold the file at an angle near the axle head. It is helpful to clamp the drill in a vise to hold the axle steady. A drill press is highly useful here, but excellent results can be obtained with hand held tools.
To bevel the axle with the axle press, place the press on a flat surface with the indentation facing upward and insert the axle with the notch at the 12 o'clock. Hammer lightly on the axle head. Repeat the process at the 4 and 8 o'clock positions. If you do not want a beveled axle head, use the opposite flat side of the axle press to straighten the axle head.
If you are fortunate to have both access to a lathe and the appropriate skills, bevels, grooves and any number of axle and wheel modifications can be performed. Since such use is rare in car construction, details of such processes will be omitted.
Notches and grooves
Axles can be notched with grooved to reduce the contact area between the axle and the wheel. This won't directly reduce the friction because the downward force is distributed over a smaller area (see the Physics chapter). However, the groove can also serve as a lubricant reservoir and make it easier to get a good coating of lubricant on the axle.
You can use a metal file to make a 1/4" groove that is 3/16" from the axle head. This process requires a lathe, drill press or rotary tool bench mount and a jig that will hold the file perpendicular to the axle shaft at the proper point. Remove a small amount (about 0.01") with the file and polish the axle using the method below. If you choose to purchase a lathed axle (check your pack rules to see whether this is allowed), you should polish it as well, starting with the finer grit sandpaper.
Polishing should be done last, after straightening, beveling and notching. Put the axle in the chuck of a rotary tool or drill with about 1/2" of the axle showing. A rotary tool wrapped in a shop towel and gently clamped into a bench vise makes an excellent axle polishing lathe.
Start with a file to gently remove any burr material that you missed in the above steps then move on to sandpaper.
Use wet/dry sandpaper starting with CAMI 400 grit (30 micron = 30 μm = 0.03 mm). Cut the sandpaper into 1/4" strips that are several inches long. Dip the paper in water and sand the exposed portion of the axle for approximately 30 seconds with the axle rotating at high speed. Don't forget to polish under the axle head. Folding the sandpaper in half can help here. Proceed to 600, 1200, and 1500 grit sandpapers. Cushioned abrasives are available down to 3 micron grit. Diamond paste polishes are available from 45 μm down to 0.1 μm.
After sanding to about 3 - 5 micron grit with sandpaper, use a shop rag with a small amount of metal polish to get a shiny finish.
Take the axle out of the chuck and rub away any remaining grit or polish.
Official BSA axles are 0.087 inch diameter. A slightly oversize axle can reduce wheel wobble.