Learn to Build A Winner (Cont'd)

Woodshop 101 (Continued)

3. Wheel Preparation

The concerns in wheel preparation are to assure that:

The wheels are nearly equal in size,
All contact surfaces are smooth and slick so that the wheel rolls smoothly,
The tread cross section is parallel to the hub so that the wheel rolls in a straight line,
The distance from the hub to the tread is exactly the same all around the wheel so that the Car doesn't bob up and down as it rolls, and
The remaining friction has the least possible braking action (torque).

When we are done with each wheel, we want its cross section to look much like this picture. (Remember that a "cross section" view is what we would see if we sliced the wheel through its center and looked at the cut side.)

[Fig 11. Wheel cross-section]

There are three approaches with which I am familiar. All involve a process called "turning". "Turning" is rotating the work piece (in this case, the wheel) about its axis and applying a cutter so as to produce radial symmetry. The cutter is slow- moving, and usually moves perpendicular to or parallel to the axis of rotation (center).

Method 1.

This method is described in the paper insert in the Grand Prix Pinewood Derby Kit. It requires an electric drill, a straight nail, some fine sandpaper, and a flat-sided wood block. The nail should be a close fit to the inside of the wheels hub. (You may need to file the shoulders of the nail's point to allow the nail to enter the hubs without cracking them.)

The nail is passed through the hubs of all four wheels and into the chuck of an electric drill. With the drill turned on and held very steady, gently hold the sandpaper against the tread surfaces with the wood block.

[Fig 12. Wheel turning]

Some possible problems:

Problem 1. If the pressure is too hard, the wheels may spin on the nail. This can cause heating of the hub and result in distortion, such as an oversized, non-cylindrical or non-parallel hole. If this happens, start over with new wheels.
Problem 2. If the pressure is too hard or is held too long, the tread surface of the wheels can become too hot. If this happens, then the tread surface may bubble and distort. This is very hard to correct.
Problem 3. If the pressure is unequal along the wheels, then the wheels at one end of the nail may be smaller than the wheels at the other end. The treads will be conical rather than cylindrical. If this happens then the car will have difficulty running straight, since each wheel wants to run in a circle!
Problem 4. If the sandpaper is backed by something soft (compressible) instead of a wood block, the edges of the tread will be cut more than the center. This can also happen if a compressible abrasive, such as steel wool, is used. The result is wheels with a rounded profile. Most rules disallow tread profiles which do not fully contact the track.

Method 2.

Each wheel is held in a mandrel, such as the PineCarTM P357, and the mandrel is "chucked" into a drill press. Alignment guides (reference and stop blocks) are clamped onto the table of the drill press. The wheel is passed down across a cutter, which can be sandpaper on a wood block or a bit of sharp-edged metal clamped to a wood block. The alignment guide is set to limit how close the cutter can get to the wheel so that each wheel is the same size. Gradually move the cutter closer to the limit block. With each step, pass the wheel across the cutter.

After the last cut is made, use a small piece of cloth with a bit of jewelers rouge on it to polish the wheel tread. Hold the cloth gently against the turning wheel for a few seconds.

[Fig 13. Wheel turning]

Some possible problems:

Problem 1. Overheating. The plastic of the wheel will pucker if friction with the cutter causes too much heat to accumulate in the wheel. If sandpaper is used, avoid prolonged contact and too much pressure between sandpaper and wheel. If a metal cutter is used, avoid cutting too deeply on each pass and avoid the cutter idling at one spot on the wheel. If the surface puckers, start over with a new wheel. (Extra wheels may be purchased separately, but make sure they are "legal".)
Problem 2. Conical tread profile. If the cutter slips as the wheel passes, the tread will not be parallel to the axis of the wheel. To avoid this, hold the cutter firmly against the table and stop as the wheel passes by the cutter.
Problem 3. Hub damage. If the wheel slips on the mandrel while cutting, a number of problems may arise. Heating may distort the hub or pucker the interior surface. The main way to avoid this is to start with the wheel snugly gripped by the mandrel, and then make shallow cuts, only a few hundredths of an inch each time. If sandpaper is used, keep the pressure very light.
Problem 4. Wheel too small. If you remove too much tread, the wheel judge may disqualify the car. (If you can't race, you can't win!) The best rule is to take off just enough from each wheel so that all the wheels have a uniformly flat tread cross section, and all the wheels are the same size. The tradeoff is that reducing the wheel diameter reduces the angular inertia of the wheels, which reduces the energy used to accelerate them. At the same time, it increases the effect of the braking torque from axle and hub friction. (If the rules permitted, the angular inertia reduction could be accomplished by removing mass from the inside of the rim! This would avoid the loss of leverage resulting from the diameter reduction. Most rules prohibit such "tampering" with the wheels, however.)
Problem 5. Runout. The spindle, chuck, and mandrel must operate to cause the wheel to spin exactly on its own axis as the tread is being cut. If this doesn't happen then the distance from the inside of the hub to the outside of the tread will not be uniform around the wheel. The condition is called "runout". As the Car runs it will bob up and down slightly. Gauges to detect this condition are common in machine shop work, but not so common among wood workers. To some extent the condition can be detected by feeling the mandrel as it spins in the drill. If runout seems excessive, try rechucking the mandrel.

Method 3.

Turning on a jeweler's lathe. A jeweler's lathe is a precision machine tool. Each wheel is turned individually. The cutter is fed across the tread parallel to the wheel axis in a series of shallow cuts. If you take this route, work with its owner for details on operation.

After the last cut is made, use a small piece of cloth with a bit of jewelers rouge on it to polish the wheel tread. Hold the cloth gently against the turning wheel for a few seconds.

The possible problems are about the same as with method 2.

Polishing:

Regardless of the method used to dress the wheel tread, gently polish the tread surface. Use jeweler's rouge on a clean cloth. Hold the treated cloth gently against the wheel tread for brief periods as the wheel spins. Be careful! This process can produce enough heat to pucker the tread surface if too much pressure is used or if it is applied for too long.

Hub Shaping:

This step need not be done on a lathe. There are several ways. The objective is to create a narrow band on the inner hub, near the wheel's axis, which will contact the side of the Car. If it were permitted, a short, brass sleeve fixed inside the inner hub could be used to place the contact surface even closer to the axis. Since most rules prohibit this, the next best is to place the contact at the central edge of the hub.

[Fig 14. Hub shaping]

4. Axle Preparation

Goals:

Our goals in this section are to produce axles which allow your Car's wheels to turn with the least possible resistance and which allow the wheels to be aligned to perfection.

Summary:

When you first take the nails from the kit, look at one of them carefully. You will see that the underside of the nail head is rather rough. It is also more-or-less flat across.

Next, look at the shaft near the nail head. You will notice some ridges on one side of the shaft. The last set of nails that I inspected had 3 such ridges on each nail, separated by about 0.04 inches.

Finally, look at the point. It looks like the metal of the nail has been cut four times to produce the point. When that was done, it caused the nail to bulge where each two adjacent cuts meet the shaft.

Shaping:

The underside of the nail head needs to be shaped and smoothed, and the bulges near the point need to be removed. When you are done, it should look like this.

[Fig 15. Axle shaping]

Here is one way you can accomplish the changes:

Detect the bulges by rolling the nail shaft on the edge of a smooth hard surface. You should be able to feel the vibration as the nail shaft rolls over each bulge. You may also hear the slight "chattering" as the nail shaft bobs up and down.

A more precise measurement of the bulges involves using a micrometer caliper. With the caliper closed gently on the shaft of the nail, draw the shaft out until the cut end is under the caliper pad, then turn the nail shaft. Notice whether any bulges interfere with the turning of the nail.

Use a small, fine metal file to gently remove the bulges near the point. It is easiest if the nail can lie in a shallow grove in a piece of wood while you are filing it. Repeat the test above to evaluate progress.

[Fig 16. Axle shaping]

Once the bulges are removed, chuck about 3/8" of the nail in the drill press. Check the nail for any wobble as it spins in the drill. (Look at the shaft of the nail, not at the head.) If there is any wobble, try rechucking the nail. It is important that the nail spins on its own axis.

Next, with the nail spinning, hold the metal file gently against the underside of the nail head. Make sure that the teeth of the file will cut into the nail as it spins. Try not to touch the nail shaft with the file. Tilt the outside of the file down about 5 to 10 degrees and allow the spinning nail to cut its head into a slightly conical shape.

Polishing:

The ridges on the shaft may remain. Decide if the rolling wheel is to press on the smooth portion of the nail shaft (ridges up) or on the ridged portion of the nail shaft (ridges down). In either case, polish the portion of the shaft that the wheel may contact. I prefer "ridges up" since the span of the ridges is too narrow (less than 0.08 inches) to adequately control the wheel orientation.

The last step is to polish the nail shaft and underside of the nail head. Rub some more jeweler's rouge into a strip of cloth and hold the cloth against the spinning nail and nail head for a few seconds. Then hold a clean strip of cloth against the spinning nail to remove any jeweler's rouge that may have been left on the nail.

Avoid polishing the outside of the nail head. You want it to be obvious to the judges that nails from the kit are being used!

Some people like to sprinkle some graphite on a cloth and hold that against the spinning nail for a few seconds. The idea is to try to impregnate the metal surface with graphite.

Remove the nail from the chuck. It is best to handle the nail with a clean cloth to prevent finger oils and perspiration from being left on the nail.

Use a felt-tip pen such as a "Sharpie" to mark the outside head of the nail to indicate where the shaft ridges are located.

Set the nail aside in a clean container.

Note:

Unless the shaft surface is damaged, I prefer to avoid using sandpaper, emory cloth, etc. because these will reduce the diameter of the nail. Reduced diameter of the shaft hurts performance unless the wheel hub inside diameter is similarly reduced!