Monday, November 3, 2008

Tap Troubles

I am making another chandelier consisting of two pieces of fabricated steel that are joined together by four machine screws into blind holes . Since the screw holes had to line up I clamped the two fabbed pieces together, drilled the four holes through both the first piece of metal into the second piece of metal. Then I tapped the blind holes, drilled a slightly large hole in the first piece of metal, and screwed them together. All the holes were aligned just the way I had in mind.

Then I decided I was unhappy with the way the two pieces of metal were lined up. I've commented in an earlier post that one can get away with a lot in art, but less so with design. These pieces of steel were definitely design. Why did I notice the misalignment now and not before I did the fabrication? The truth is that I did notice but I was so focused on getting the piece done I just ignored the flaw thinking it was not that bad.

A half hour of cutting apart, grinding, and welding and I was again ready to join the two pieces. Two holes lined up, two did not. No problem, the welder was right at hand, so I welded the two holes shut, ground them flat, and remarked the new holes in their slightly different location.

In the back of my mind I remembered reading somewhere that weld material was a lot harder than mild steel. The first clue about this memory came when I drilled the new holes and noticed it took longer. The next clue was when I broke a tap in the first hole. Maybe the tap was worn somehow as it was part of a lot of old taps I had purchased at a closed machine shop? No, the final clue was when I broke a second tap in the second hole.

A definition of insanity I read once was doing the same thing over and over again and expecting a different result. I do this all the time on computers. Type something, it does not work. Maybe I miskeyed it, so I type it again. But I only do it twice. My typing is not that bad, and two taps in a row could not be bad either.

I am learning machining by experience. This was a pretty cheap lesson with a price of two taps and an hour or so of rework. Learning how to do it right the first time is a harder lesson to learn.