Interestingly, I just looked up the type of bit to see if I could find at which point it should be measured. Wikipedia identifies it as the "Irwin" type, "invented in 1884, and the rights sold to Charles Irwin who patented and marketed this pattern the following year."
I have just deciphered the shank markings as
| IRWIN |
MADE IN USA
So I have the archetype product!
The other day while looking for a used auger bit for the can crusher below, I encountered a bit which looked about the size I needed, but age had taken its toll, and the size marking was not evident. I scrounged a bit of emery paper from the proprietors, but couldn't decipher the markings. I returned the item to stock and resumed browsing, as the shop is filled with all kinds of tools you never know you might need and I can't resist looking.
Then I noticed a couple of used quality sets of calipers on display in their cases, and cheekily sneaked one of them over to use to measure the bit I had been looking at. At a measured 17.8mm = 45/64" at the spurs, I reckoned it would suffice to make the hole for the 19mm metal I needed to insert. I bought the bit, which, embarrassingly, I now find upon inspection to have "11" (that is, 11/16") stamped quite clearly on the square taper end, (see Left) rather than on the round shank where I was looking. (Oh well, the proprietor didn't pick that up either!)
The calipers were about $35 with case, and I contemplated buying them as something I have always admired as a nice piece of precision engineering. However, I thought I had previously seen some in another shop down the road, and resolved to compare prices first. That shop turned out to have none, but rather than go back I then resolved to check new prices. Thus I ended up buying new the rather cheaper $6.99 blister-packed set shown above (see Left, top). Considering I am not machining crankshafts to the 0.01mm, I figured they would do me.
Subsequently, during construction of the crusher, and afterwards playing around with magnets, I found myself reaching for the calipers so often I began wondering how I had been doing without them.
I have always found it awkward to measure the OD of round things, and now that capability, plus the ability to measure ID of holes and depth of holes has turned out to be really handy. They are also good for transferring measurements, with the locking screw adding to the usefulness of that function.
A few days later, when I was hunting for oval headed screws, I was in an establishment called Geoff's Emporium, when I happened to glance down the aisle, and there is a bloke looking through the stock of assorted nuts bolts screws washers etc. with notepaper and a set of calipers in hand. I don't think I have ever before in my life seen a customer with calipers in a shop.