Cross boring on the lathe (and the absolutely frightening potential for cumulative error)
As I’ve mentioned in previous posts, my friend Les and I have been getting together one day a week, during the seemingly unending winter, to work on projects. The original thought was that we’d help one another finish up any number of uncompleted projects that reside in both of our shops. But it’s so much easier to go off on little “flights of fancy” and start even more projects. There are just so darned many “fun things” that we could do and the old stuff can be a little boring…
Well we decided that making some little post and rung high chairs would be good fun. You know the kind, little ladder backs with turned posts, rush seats:
I mean, these are easy. People’ve been making them for centuries. You turn or shave the posts. You “eyeball” and bore for the stretchers. “Knock” it together. Throw a little paint on it. Weave the seat. Voila! Instant heirloom!
Okay…So Les and I just can’t seem to take the “path of least resistance”. We’ve got to “up the anti”. So we decide we’ll make multiples of this little baby. We’ll give ’em to the grandchildren. Doll collectors would probably want one (who knows, maybe two). Anyone serious about colonial stuff just wouldn’t be able to resist. To do this we decide that we’re going to have to employ some production techniques. You know, knock ’em out fast…and accurate. We decided that we’d bore the stretcher holes while the posts were still on the lathe. Makes sense, right?
We’ve both cross bored posts and columns on the lathe. You determine the height of the spur center, build a little two point block that you clamp to the ways, then bore away. Easy, breezy! But this time, rather than build a (or find the existing) boring “block” we decided to use a new drilling jig that Les had picked up. The device is one built by a first class lathe manufacturer. It’s mounted on a frame not unlike those used for certain “steadies”, the type built with urethane skate wheels. The unit has a slide with depth control stops. First class, all the way. So we mounted the drill motor into the single point collar, designed for that purpose. I had the responsibility of aligning the bit. I ran the slide in towards the spur center and adjusted the height accordingly. Everything perfect. At least so I thought. In fact, I had failed to notice that the bit was not running parallel to the ways (or, in other words, at a right angle to the center of the lathe). The slide of the device was at 90° to the lathe bed. But the single point collar had turned, ever so slightly, and changed the tangential relationship between the bit and the workpiece. Without going into a lot of illustrations, suffice it to say that as the actual boring axis was not at 90°, the stretcher hole was off. But we hadn’t really noticed the error at this point, so we bored both posts (ignorance is bliss). We put the back slats into the posts, then the first stretcher and… Sweet angel of mercy, what went wrong? Well it took about 1/100 of a second for us to realize that we missed the boring alignment, but what we were reminded of was that when boring pieces that will be placed in “mirror” opposition of one another – the error doubles! In “tech speak”, this is cumulative error!!!
Most carpenters and cabinetmakers will tell you that (usually) one error cancels out another. And, many times this is true. But not so when you’re cross boring on the lathe. So it might be a good idea that in some future post, we explore the construction of a very simple, extremely reliable two-point drilling fixture. In the meantime, most of our younger friends should take away this lesson – even the old dogs can screw up! So, if our paths cross, you’re permitted a little back slapping humor, at our expense. But, don’t overindulge, or from now on we won’t share our “unfortunate mishaps” with you.