Every shop needs to have an ultimate arbiter of straight and flat. In the above picture a ruler is being used to verify the dimensions on a CAD drawing printed out from an office printer to a precision 1/100 inch. But in guitarmaking there are many situations where much greater precision in the form of straightness really matters. One example is preparing the two halves of a spruce guitar soundboard to be. The edges of the two halves need to be straight to a couple of thousandths of an inch, or else when they are joined and sanded out a glue seam appears.  Seeing this on a finished guitar is a dead give-away that the maker didn’t have “straight” figured out, either they couldn’t see it or couldn’t make it or didn’t care, or all three, indicating a serious hole in their world-view. Another task requiring great straightness is verifying the flatness of a guitar neck blank’s surface prior to gluing on the fingerboard. Here you stand the ruler on the surface and look for light leaks between them, first along one edge, then along the other, and finally across the diagonal. A warped surface can look perfectly straight and flat down both edges, but the diagonal view gives the game away.

Laying a real precision ruler against a part’s edge and inspecting how the ruler and it make contact along their full length tells a story, but the story is of an unusual kind, where fewer words makes a better story. Any deviations from straight add unwanted details to the story – high here, low there, straight from here to here, then low again, etc. The perfect story you work for and want to hear is just one word – straight, and the best and most trusted story-teller in the Ridgewing shop is a precision ruler from L.S. Starrett, an old-line Yankee precision tool manufacturing company located in Athol, Massachusetts. 

L.S. Starrett Tool was founded by one Leroy Starrett in 1881.  An unstoppable technical entrepreneur a/k/a “Yankee inventor”, what today we would call a “Maker”, Starrett’s simple personal ambition was to make an honest living manufacturing and selling some new product that people would find useful and valuable. His first product was the sliding precision square, which was a significant improvement over the standard fixed squares of the day, like carpenters use today.

In creating and manufacturing the sliding square, the problem Starrett faced was that the parts had to have perfectly flat, straight and square surfaces in the short raceway where the ruler slid back and forth and was locked by a thumbscrew. Also required were perfectly straight and parallel edges on the ruler, so when the body and ruler were tightened together anywhere along the ruler’s length, everything was perfectly straight and square. At the time, reliably creating steel straight edges of the required length and precision was beyond the capability of existing machining technology. There had simply never before been the need for such perfect straightness as Starrett called for. To make a long story short, Starrett figured out how to make edges of the required precision, and after creating the sliding square went on to leverage his unmatched straight edge-maklng technology to build up a successful tool manufacturing business that today employs 1600 people in Athol and around the world. And it all came about by figuring out how to get the edge straightness story down to one word – straight, or maybe three words – really, really straight. In engineering terms, this is about 5 microns, 10 wavelengths of light, over the length of the ruler. That is straighter than a person can see or feel, so when you ask one of these rulers and it says “straight”, you can close up shop and go home.

The Ridgewing shop’s ruler is Starrett Model No. C416R.  This is a 36 inch ruler, one-eighth of an inch thick of tool steel with a frosted chrome finish. On one side there are marked 32nds and 64ths of an inch, and the flip side, which is my favorite, is marked with 50ths and 100ths of an inch. This slightly blurred image shows the 1/50” marked edge, while first image shows the 1/100” marked edge. The coolest part is that these marks are all inscribed down to the tiniest detail, so you can feel them with a pointed tool, allowing you to reliably measure and mark a work piece to an accuracy of 1/100”.

Unless you have super-vision, all those tiny hundredths of an inch marks are pretty much a blur, particularly to these old eyes. So to create a particular measured point on a work surface the ruler is stood on edge, and the tip of a fine pointed awl tool is slid carefully across the surface of the ruler until it “clicks” into the inscribed inch’s groove nearest the desired measurement. The point is then carefully slid down the inch groove until it reaches the height of the 1/10” lines. It is then slid over until it “clicks” into the nearest 1/10” line to the measurement, and then is carefully slid lower until it finds itself lost in the forest of hundredths lines. So how to find the right hundredth line?  By ear! From its position on the known 1/10” line, the awl tip is dragged lightly across the hundredths grooves, where each one it slips across gives an audible “click”, and you simply count the “clicks” until you get to the hundredth you want. Then you slide the awl point down that 1/100” groove until it reaches the work surface, and you give it a push, making a barely visible pin-prick. Move the ruler away, and push that point down into the pin prick again to widen it to visibility.  The point of a larger awl can then find that widened pin prick, and with one good tap of a hammer the pin-prick will expand enough to capture the tip of a drill bit.