Monday, July 14, 2014

Why I Took Shop Class

So roughly fifty years later I could make a tool to rebuild a 40 year old motorcycle. In the old days there were mechanical drawing classes that all of us boys were required to take in school and one of the things taught was the six hole layout on a circle. Since the chord length between the holes is the same as the radius of the circle, it takes all of about thirty seconds to lay one out. The layout for the clutch tool was four concentric circles and the tab layout. I put the tabs on the same center line as the holes, so the layout was super easy. Even though the clutch shell/basket has twelve slots, I only put six tabs on the tool.

Having taught this layout process over the years, I'd say the average high school graduate of today would have a tough time laying a tool like this out. The smart guys with a little CAD experience could draw it up on the computer easy enough but doing this sort of thing the old fashioned way might just present a little challenge. For one thing they might not know the relationship between the radius and the chord length. There's something different about the computer skills and the traditional ways of doing things. Neither way is right - neither way is wrong. Just different. Doug Stowe at the Wisdom of the Hands is probably the guy who can explain it the best but I know it to be true. In fact he mentions in a recent post the German concept of fingerspitzengefuhl that might help explain what I mean. I know in this day and age you certainly couldn't justify a mechanical drawing class because you might be called upon to make a layout for a six hole bolt circle some day, but I'm dead sure a nine weeks traditional drafting class in middle school would really help everyone on several levels.

Here's a little challenge for you: Draw up the clutch tool. Granted this will be easier if you know what a motorcycle clutch disc looks like but the layout is four concentric circles with six equally spaced tabs around the perimeter. The hole for the socket is 1-1/8" radius, the bolt circle is 2-7/16" radius, the main radius is 3", and the tabs stick out 3/16" so another circle of 3-3/16" radius. The tabs are 3/8" wide. I made it easy by giving you the radii. If I would have given the diameters, that would have stumped a lot of the high school kids because they can't divide fractions by two.

When you get done the drawing should look like this only with a smaller center hole and the bolt circle.

Once you get your piece of paper, compass and a scale, it shouldn't take more than about three minutes to draw up your layout. Regardless of how long it takes you, when you finish it up, you've got the pattern for the clutch tool in case you ever buy a '73 XLCH basket case.

1 comment:

Nova Run said...

I remember when you had be draw up a piece to be milled on Autocad for the Harley you were building. It didn't quite work once all the coordinates were entered into the milling maching, but it did look good in Autocad! Jose Magallanes