I made the part to convert the height comparator to a “face” comparator, it is a bit of mild steel welded to O1 tool steel, has a 8″ radius, was hardened and screwed to the bottom of the tool. I’m a bit unsure of and don’t know how to explain the “why” of the radius but I think you’ll get the idea from the videos.
I don’t have a reference square [yet] so I’m using a couple of Mitutoyo squares BS 939, DIN875, Grade B made in England of all places. I’m taking a measurement at 5″, so, if I’m reading the DIN spec right, the angular tolerance is 0.00080″ + blade length/10,000. So, if the squares are in spec, at 5″ there can be no more than 0.00130″ (one thou) offset. Whew.
First video shows the attachment and then compares 5 squares to the Mitutoyos. Assuming the Mits are square (they match), the results for grade B are: Union and PEC blem match, Lufkin and Starrett are borderline and the no name econo square fails. The Mits, PEC and ecno were purchased new, the Union, Lufkin and Starrett are way old, used and the Lufkin looks worse for wear.
In the second video, I revisit the 123 blocks and check for squareness. They suck.