As you’ve probably read elsewhere, most of the speed comes from the reduction of friction, which is all where the wheels rub against the axles. Do reduce this, you want to spend lots of time polishing them. Here’s what I found can be done to get a mirror-like finish.
Ideally use a drill press (like shown in my previous post) to hold your axle nail. If you don’t have a drill press, a corded or cordless drill in a mount or vise of some sort will also work (make sure it’s secure though).
Optionally mill away the middle section where the wheel would touch, leaving only the inner- and outermost areas. This reduces the surface area the wheel will touch the axle. I did this with a file mounted in a vise on my drill press.
Use a file and grind away any burrs that are around the nail head. Also bevel the inside of the nail head if allowed. For grinding I use a set of jewelry files.
Start with a coarse sand paper and work your way down to finer grits. I used 400, 800, and 1200-grit. Cut strips of sandpaper about 1/4″ x 3″. While the axle is rotating at medium-high speed, pull the sand paper back and forth over the axle. Do this over the areas where the wheel will touch the axle, including the inside of the nail head. It might help to have a small piece of wood to push the sandpaper against the nail head.
1200-grit produces a pretty reasonable result, but you won’t quite achieve a final mirror finish yet. To get to the final finish, I decided to use compressed graphite sticks. Again while the nail is spinning, press hard (but not too hard to bend the nail) with the graphite stick to polish and lubricate. Parts of the graphite stick will break off, and this is OK. After a few minutes of this you should have a very shiny and pre-lubricated surface for your wheel to run on.
As you can see, there are still some minor scratches on the polished part. Next time I might add an additional higher-grit polish before the graphite.
Of course you will want to add additional graphite powder to your assembled wheel, but with a polished axle you will have greatly reduced the friction produced.
I have quite a few tools, but I don’t yet have a lathe. In this case, I needed to mill grooves in the axles for my son’s Cub Scouts Pinewood Derby car.
There are plenty of pages out there that talk about using a hand drill to hold the axle and then polish it up, but to reduce friction even more ideally the wheel only makes contact at a few small points on the axle. Plus if you mill a groove, technically you have somewhere to store more graphite, which could possibly help improve lubrication over many runs (assuming it doesn’t just all fall out).
In my case, I was going for essentially one small point on the inner and outer edge of the wheel. This way the wheel shouldn’t wobble any more than when the entire axle is present.
Obviously a drill press would work better than a hand drill, since it is more stationary. For the cutting tools I had a set of jeweler’s files that work well for fine grinding like this. As for holding the tool I initially started by resting my hands on the drill press table and tried manually grinding down the area I wanted. I couldn’t get a very consistent result though, given occasionally the tool would get pulled more, causing it to graze the area where the wheel would make contact, ruining the smoothness of this area.
I then decided to make a tool holder of sorts by using a vise and a few blocks of wood (in this case oak) to hold the file in place. Then I could simply raise and lower the table to the correct height, then slide the vise along the table to grind the axle at the location I wanted without worrying about it running away from me.
It takes a bit of patience to adjust the table to the correct height, but with some practice I was able to finish the last axle in less than 10 minutes.
The head is also ground back with a triangle file to add a groove at the head as well as an angle to reduce the surface area touching the wheel. This I could do reliably enough by hand that I didn’t make another fixture for it.