Friday, February 3, 2017

Shim Stock Is Where You Find It

Here's 0.0043".

I needed to shim the 0.750" bore of a 4-step v-belt pulley, so it would fit more-or-less concentrically on a 0.741" diameter wood lathe spindle. My meagre assortment of brass shim stock didn't yield up quite what was needed, so I went for a rummage in my scrap aluminum bin, and came up with the item pictured above.

It miked out at 0.0043" -- close enough. I cut a suitable rectangle of beer can material and tucked it into the pulley's bore. My shimmed pulley and spindle went together nicely, and the pulley now runs as close to true as it's ever going to get.

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Addendum -- SUNDAY, FEBRUARY 5, 2017

One is put in mind of the following passage from Robert M. Pirsig's "Zen and the Art of Motorcycle Maintenance":

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His handlebars had started slipping. Not badly, he said, just a little when you shoved hard on them. I warned him not to use his adjustable wrench on the tightening nuts. It was likely to damage the chrome and start small rust spots. He agreed to use my metric sockets and box-ends.

When he brought his motorcycle over I got my wrenches out but then noticed that no amount of tightening would stop the slippage, because the ends of the collars were pinched shut.

"You’re going to have to shim those out," I said.

"What’s shim?"

"It’s a thin, flat strip of metal. You just slip it around the handlebar under the collar there and it will open up the collar to where you can tighten it again. You use shims like that to make adjustments in all kinds of machines."

"Oh," he said. He was getting interested. "Good. Where do you buy them?"

"I’ve got some right here," I said gleefully, holding up a can of beer in my hand.

He didn’t understand for a moment. Then he said, "What, the can?"

"Sure," I said, "best shim stock in the world."

I thought this was pretty clever myself. Save him a trip to God knows where to get shim stock. Save him time. Save him money.

But to my surprise he didn’t see the cleverness of this at all. In fact he got noticeably haughty about the whole thing. Pretty soon he was dodging and filling with all kinds of excuses and, before I realized what his real attitude was, we had decided not to fix the handlebars after all.

As far as I know those handlebars are still loose. And I believe now that he was actually offended at the time. I had had the nerve to propose repair of his new eighteen-hundred dollar BMW, the pride of a half-century of German mechanical finesse, with a piece of old beer can!

Ach, du lieber!

Since then we have had very few conversations about motorcycle maintenance. None, now that I think of it.

You push it any further and suddenly you are angry, without knowing why.

I should say, to explain this, that beer-can aluminum is soft and sticky, as metals go. Perfect for the application. Aluminum doesn’t oxidize in wet weather...or, more precisely, it always has a thin layer of oxide that prevents any further oxidation. Also perfect.

In other words, any true German mechanic, with a half-century of mechanical finesse behind him, would have concluded that this particular solution to this particular technical problem was perfect.

For a while I thought what I should have done was sneak over to the workbench, cut a shim from the beer can, remove the printing and then come back and tell him we were in luck, it was the last one I had, specially imported from Germany. That would have done it. A special shim from the private stock of Baron Alfred Krupp, who had to sell it at a great sacrifice. Then he would have gone gaga over it.

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