Pinewood Derby Stories and Photos from Maximum Velocity
PINEWOOD DERBY SHOP TALK Making Weight Pockets
Mounting weights on pinewood derby cars can be done in many ways. The easiest way is to screw plates on the bottom of the car. The next simplest is to drill holes into the side, back or bottom of the car, and then insert lead or tungsten into the holes.
Another way to weight a car is with pockets in the bottom of the car. To create a very low-profile car and to focus the weight in one area, tungsten cubes are oftentimes used. These cubes are tightly packed and glued into pockets in the bottom of the car.
Although weight pockets can be made with more sophisticated machinery such as routers and mortising machines, they can also be made with a hand drill and a chisel. Today, I will share some tips to take the pain out of making pockets by hand.
Before we start drilling, let's cover some preliminary steps:
1. To make clean pockets, you must use a Brad Point or Forstner drill bit. These bits create relatively flat-bottomed holes with straight sides, which greatly reduce the amount of chiseling which will be required, and minimizes the risk of damaging the car.
2. Always make the pockets on the raw block. It is much easier to clamp, drill, and chisel into a block than it is to work on a shaped car. Also, if damage is done, then you can easily start again.
3. Select a drill bit that is properly sized for the work you are doing. If you are a making pocket to hold one row of 1/4 inch cubes, then a 1/4 inch drill bit is perfect. If you are making a pocket for multiple rows of cubes, then use a larger bit (3/8 or 7/16 inch).
4. After marking the perimeter of the pocket, clamp the block firmly to a work bench, or place it in a vise. Don't try drilling into a block that is not firmly locked in place.
Now, let's move on to the actual drilling. First we need to establish the depth of the pocket. With a hand drill, the easiest way is to wrap a piece of masking tape around the bit at the pocket depth. Then when drilling, you can stop when the masking tape reaches the wood.
Figure 1 - Masking Tape on Drill Bit (marking 1/4 inch of depth)
Start by drilling one hole at each corner of the pocket, making sure to stop at the masking tape. Then continue drilling holes around the perimeter of the pocket, and the interior of the pocket. The goal is to remove as much wood as possible with the drill bit.
Figure 2 - Drilling Holes
Now let's look at chisels. Similar to selecting a drill bit, select a wood chisel that is appropriately sized. For a 1/4 inch wide pocket, use a 1/4 inch chisel. For larger pockets, a 1/2 inch chisel will work fine. Next, make sure the chisel is very sharp - a dull chisel will lead to a damaged car. Finally, be very careful with a chisel. NEVER, EVER pull a chisel towards you, or push a chisel towards your hand.(1) Treat the chisel like a knife and handle it accordingly.
Now with the block still clamped in place, hold the chisel vertically and press it down along the perimeter of the pocket - generally a hammer is not needed. Work around the pocket, squaring out the corners. Then, clean out any remaining wood in the interior of the pocket. Two tricks:
1. When squaring corners, square along the grain of the wood first, then square across the grain, and, 2. Cut a small amount of material at a time - taking too much material can result in a damaged block.
Figure 3 - Chiseling
Test fit the weight into the pocket. If it doesn't quite fit, then expand the pocket slightly with the chisel.
Figure 4 - Testing the Pocket
The bottom of the pocket will have tiny holes from the center tip of the drill bit. If the car will be thicker than the depth of these holes, then they can be left alone. But if the car will be just a little thicker than the depth of the pocket, from the bottom of the pocket fill the holes with some wood filler. Allow the filler to dry before slicing the car.
Now that the weight has been accommodated, you can shape and finish your car. Good luck with the race!
(1) Since Maximum Velocity began over 10 years ago, we have only experienced two significant accidents in our shop. One of the accidents occurred during a workshop for our local race. A father broke a chisel rule (Never push a chisel towards your hand) resulting in a decent sized gash on the palm of his hand.