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Thank you for your comment! You are correct; real straight edges are 3D, and that fact is important. That is exactly why many people advocate flipping the straightedge about a vertical axis -- so they can keep the same side of the straight edge against the plane. This can allow one to avoid the 3D issue, but as I've shown, that method is sensitive only to the "even" portion of the straightness error. Actually I do have more to say about this, and I hope that you are interested enough to stick around to read it. I haven't thought about this in several years though, so I need some time to put a more comprehensive answer together. I will do that.

Are you there? I have quite a bit further to say, including some practical suggestions for moving forward, but I'm not going to bother writing them down if no one is reading.

Thank you for your interest!

Thanks for your comment, Mel. I really appreciate your interest. I looked again, and you are correct. The scale is coated with some sort of mottled gray material, and the marks are burned into that layer to a small depth that can be seen clearly with a 20X loupe. I can't feel the engraving with my fingernail, but it is definitely there. Now the question is, how robust is that mottled gray coating? I didn't want to do any sort of really destructive testing, but I did rub a small portion of the scale with a pink rubber eraser, a blue scotchbrite pad, and a green scotchbrite pad in turn. With the green pad, I could start to see scratches appearing in the coating, but the marking I was rubbing over remained clearly visible. Overall, that's good enough for me!

It has the forged steel head. I don't think it is hardened, unlike the scale, which is marked "HARDENED". Of course both the steel heads and the cast iron heads are ground. Do you think there is an intended accuracy difference between Starrett forged steel heads and cast iron heads?

Yes, but if were only that, we'd simply see a different result when one edge of the scale is against the reference points inside the head as opposed to when the other edge of the scale is against it. That is, we'd get two configurations square (if any of them were) with the other two configurations non-square. I believe that what is happening is that one edge of the scale has a curve at one end. If only there were time to do it, I could carefully characterize the straightness and parallelism of the two sides of the scale and relate that data to the squareness test results. I have all the necessary equipment, but not the time.

It's definitely a crapshoot. I tried to buy a Chappell framing square, supposedly guaranteed accurate to within 0.003 inches, but they were discontinued before I got there. Then I tried to buy a Johnson CS 12 (stainless steel) but they also disappeared before I could buy. (No information about how accurate they were; they may be available again.) Then, just to have something I could use for a couple of new projects, I bought a cheap Johnson CS 7 (white on black aluminum) from Lowe's. The damn thing turned out to be almost perfect! It was so good that I decided to put it away and buy an Empire white on blue aluminum for everyday use. That one is pretty good too!

@@practicalmetrology8599 well your video is quite interesting because I use a Johnson speed square (or is it rafter square?) among others. The 90 deg measured using the base of the square stood up on a flat surface is out because the extrusion was not quite flat (had a convex bow to it). I had to hand dress it to get it somewhere near but it had been throwing me off for weeks because I was measuring things with one side of the base and then checking with the other and thinking my work was off. I just can't understand why it is so hard for someone somewhere to manufacture an accurate square. I can understand obviously why a rafter square might have a loose tolerance after all it's not intended for fine measurement but even the engineers squares I have come across (many marketed as DIN rated etc are terrible and if you drop them then forget it). I hapen to think 0.001 over 4" is quite alot. It makes a difference if you're using a square to square up a shooting board for example. Anyway rant over sorry bout that.

Absolutely no need to apologize. I agree with you. I was not really upset with the 16 inch Empire combination square that started this whole thing for me in 2018 (the oldest video). I just thought it was important that other people be made aware of the issue. That problem did cause me to screw up a couple of parts, but the satisfaction of figuring out what was going wrong made up for that. But when I bought the little 6 inch Starrett and found that it was 0.010 out of square, I went ballistic. I'm still upset about that; as I said (paraphrasing) in my Amazon review, I am personally offended that Starrett's current management would throw away a 140 year tradition of quality. However, overall I have come to accept the situation. It is possible to purchase a cheap square that is accurate. But you have to test everything, and keep testing if you drop things. And dropping things is a bigger issue every day as I get older, so I keep my square testing setup handy.

Thanks for commenting! Cool that you know the founder. I would not pay full price for them, but their e-mail flash sales change the value proposition quite a bit (for me) on some of the tools. I haven't pulled the trigger on any others so far, though.

Thank you for your comment. I appreciate your interest.

Thank you for your comment. "Looking for trouble" might be the thing that I am best at. While we disagree about this particular issue, I do have to admit that as my eyes get older, I have less and less use for 1/64 graduations.

Thank you for commenting; I really appreciate it. I did check the edges of the 6 inch rule for straightness and parallelism. It is excellent. The longer video I had in mind would have covered that and more. What I said in my Amazon review of the 6 inch about it was: "Finally, in fairness, I have to say that the scale itself of the 6 inch Starrett is excellent. Its edges are straight within +/- 0.00015 inches and parallel within 0.0006 inches. So maybe the scale is still made in the US, at least."

Thank you for your interest in this information and for your question! What I have done is to mark the square with the preferred configuration(s), so that when I use it to determine squareness, I get an accurate result. My initial reason for changing the configuration of the square was to get the preferred measurement scale in position for the measurement that I wanted to make. I realize now that doing that has a cost, and I now usually don't bother changing the configuration from the one that gives me the most accurate square.

Thank you and others for the recent comments. I am glad that at least a few people are finding something of value here.

Very glad to hear that!

Glad I could help!

Once again, thank you for watching and commenting!

Wow, thanks for the comment! Would have been perfectly OK to be critical -- I just hoping that someone would say something! I know this video could have been a lot better -- but I also found out that more effort would not have been justified by the level of interest.