A few months back, I constructed a small center finder out of scraps to mark centers on some round stock for further turning of round tenons. No big deal. However, last week the surveying firm that I work for accepted a project with a local power and water purveyor to set up a monitoring project on an 8’ diameter penstock that feeds a hydro-electric generator at the bottom of said penstock. The monitoring project was to determine, over time, any displacement in the horizontal and vertical axis of the penstock due to geologic displacement (erosion, slide creep, earthquakes). Our concern was how to delineate the top dead center (TDC) of a 8’ diameter pipe within 1/100 of a foot along the circumference to determine the centerline of each section of pipe. It occurred to me that my little center finder would fit the bill if the legs were longer, the bisecting tang was movable for varying diameters of a cylinder, and a builders level was attached to the tang to find plumb.
Using some 42” x 1 1/2’ x 3/8” survey lath, I half lapped and glued the square legs using a drafting triangle to check for square.
Using the miter saw, I cut two (2) 45 degree cuts on the ends of two additional lath which I just glued to the top of the square legs with a gap to receive the bisecting lath. to tighten the gap along the sliding tang, I just tapped the off leg with a hammer with the clamps set lightly.
Again, I checked the bisecting 45 degree angle with my drafting triangle and tightened up the clamps. To mount the builders level, I fell back on an old reliable and temporary fix, duct tape.
It worked adequately. I set up over the pipe, adjusted the tang for depth, plumbed it up and marked the pipe. To check the accuracy, I flipped the center finder 180 deg. re-plumbed and made another mark. The accuracy was a bit less than desired (perhaps due to imprecise machining and/or accuracy reading the plumb vial on the level) but measuring midway between the two center marks provided repeatable accuracy of the 1/100’th of a foot criterion.
I’m sure that there is a tool sold out there somewhere to find the TDC of large diameter pipes, but it is satisfying to come up with something that does the same job out of low cost material and came out of a small hobbyist shop.
Thanks for look’in in.
-- Greg, No. Cal. - "Gaudete in Domino Semper"