The Magnificent Drill Cart

Winslow and I meet every couple of weeks and try to build something from parts to finished in a couple of hours. As it happens, he likes cars and from the first session wanted to know when we were going to build one. One day as we worked our way through some of the basic skills of constructing with blocks and plates, we talked through the important parts of a car.

According to Winslow, you need three important things in a car; some wheels, a body, and a motor. “What about brakes and a steering wheel?” I asked. “Oh yeah,” he said, “those are good too.”

Advertising Works
I bought some drills from Harbor Freight for the first Tinkering session and since then I get an e-mail every week or so. Because I’m kind of window shopping for an arc-welder, I haven’t unsubscribed from their service… Then this 18v drill pops up, and I remember Winslow’s list. A motor?

Is an 18v drill enough to push a 9 year old around? I start poking around with some torque calculations, but I don’t get far since there are few details about the drill on the site. I do some experiments with the drills purchased earlier and determine that I can’t stop the chuck on my smaller drill with my hand. One blister older, I decide that the bigger drill will be sufficient.

You Call That A Body?
I didn’t bring a circular saw, so Winslow and I decide to go with the simple rectalinear lines of the hunk of scrap wood I had brought. We prop it up on blocks to get a look at it, and start talking through how the pieces could go together. It looks like the plywood will be just big enough. We can tell already that we aren’t going to win any beauty pagents with this vehicle.

You Need Wheels If You Wanna Roll
The nice thing about this design is that it doesn’t require a lot of precision. The wheels are all freewheeling, so we won’t be able to drive them from the axle (axle drive requires a differential which is a pretty tricky bit of construction). After some more discussion we settle on a simple friction-drive mechanism since it will be the easiest to build and debug.

Here, we see Winslow testing the fit of the wheel and axle into the receiving hole in the drilled block. His black t-shirt of invicibility is protecting him from the bolt head as he presses the axle through the hole using his chest.

Engine Number One
After a couple of attempts to lock the first drill in place using blocks, we decide to fabricate (ahem) a mounting bracket using a short length of plumber’s tape. It turns out that an ergonomically shaped drill is not really the most convienent shape to attach to a board.

I’ve brought 3/4″ sanding wheels to help the drill make good contact with the wheel. Note the use of a block at the butt-end of the drill. This block ensures that the sanding wheel in the chuck is aligned with tread of the tire.

A Tail-dragger
I don’t have a good picture of it, but we’ve put a 3″ swivel gate-wheel on the tail of the cart. The friction drive would probably work better as rear-wheel drive (if the examples in the Microcar Museum are to be believed), but Winslow is going to have to steer this by driving the wheels independently and doing it backwards seems like a bit of a stretch.

Test Drive!
Looks like we are ready to roll. Note the conspicuous lack of a helmet on Winslow. One thing I’ve learned in working with 9 year-olds – if you want them to wear a helmet, you have to glue it to their head.

Notice the steering posture that Winslow is adopting here. To get a good grip on the triggers he has to turn his hands out almost 90 degrees. Probably not the best ergonomics for long-haul carting (doesn’t look like we’ll be entering the Paris-Dakar Rally with this beast).

Video Highlights
So, what’s it like to drive? Winslow tests it’s limits in a grueling test-drive and nearly runs over his friend Clark.

Not to be out-done, Clark decides to take it downhill through the slalom course.

Some Lessons Learned
1. As you can see in the video, the king-post on the gate-wheel is a dangerous appendage. Both of the brave test-drivers managed to avoid getting scraped, but there were some serious near-misses.

2. It turned out that the drills had a tendency to work themselves loose despite tightening the metal straps twice. The drivers developed a technique that ameliorated the problem (they pulled the handles of the drills towards the center), but it added to the fatigue of driving and most of the rides ended with some sanded rubber coming off the wheels as the tension was released.

3. 18v drills will last for a good 45 minutes of intermittant driving and course building.

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