A few posts back, I checked some of my squares, one was way out and some others were borderline. That bugs me so I’m going to fix [some of] them; the out of whack machinist square because I’m too cheap huck it.
First things first however – if I’m going to change things, I need references. What have I got? Well, I have a surface plate¹. But, it is a cheap import, so I’d like to verify it (as much as I can). How? Well, I have a Starrett rule (a bunch of them in fact). So, I have several things I believe are flat so if they all agree, that is good enough for me.
I’ll check with light² (since it is way more accurate than anything I have access to but it is go/no-go only) using the handy little pinkie sized red LED flashlight that Suburban threw into my order. It does look like my plate has a concave top but it appears to be slight so it could be dust or dirt on either the rule or plate (the plate was cleaned but I only wiped the rule). Now, I have a flat [enough].
Next, a reference “standard” for square. I have three Mitutoyo squares that are maybe good enough. I just bought some Suburban Tool import 123 blocks³ that I will use to check the the first three inches of the square and use the surface plate to verify that the blade is straight. If so, I know that square is square (but only on the outside). (The photo isn’t very good as there was more light than there appears to be, that bright line isn’t the edge of the block, it is the gap). If you look at the big image, at the bottom, you’ll see the black where they touch perfectly). (The numbers on the block are the readings from the comparator (eg .2 thou from something) as I checked the block).
Now I have a 7″ reference and can proceed to squaring the squares. Note that when you are working at these tolerances, cleanliness is next to godliness. Not shown in the images is the sheet of paper on the plate, I wipe the edges of the square on it to clean it. The paper is very slightly abrasive and will get rid of gunk.
First up, the machinist square. I used a DMT 250 grit diamond plate to flatten the stock and take some meat off the blade. Just like sharpening a plane iron. I don’t trust sandpaper on a plate, I’ve had bad luck with drubbing the ends. Lucky me, the inside is square so I don’t have to figure that out. I mostly worked the blade, but also cleaned up the stock, which, as you can see, was slightly convex [across the width]. I got pretty close to matching the Mit, the light bends as it exits the gap so that gap looks a lot bigger than it is. I probably should have measured the width of both [the blade and stock] to see, and keep them or bring into, parallel. It is important to remember when you are lapping, you need to be abrading the entire edge while also abrading the high side faster than the rest of the edge (that’s the idea anyway, there are several ways to get there depending how far you need to go, I use more pressure on the high side). That way the edge stays straight. But it isn’t quick, especially as these types of squares are typically hardened.
I was going to correct the two old combo squares but I compared them to the Mit and decided they were just fine for what I use them for. So the test victim is a double square I got from Rockler long ago and really dislike (right, I dearly love the crusty Lufkin on the left). Most “adjustable” squares are manufactured to be user true-able. This is a photo of the stock, you can see the little pip (above the + that marks the side that is high). There are two lands cast into the stock that the blade rests on. By filing one or the other down, you can tilt the blade. Before you do so however, make sure the stock and blade are parallel and clean. It is pretty tedious; take apart the square, file a tiny bit (a thou is a lot here), reassemble, check. I think this one took five to seven rounds to get it dead nuts.
¹I can’t find the cert for this plate but I do have three other certs. The cert that came with the smaller plate that I got about the same time, from the same place (Enco) says the plate measured within ±0.00001″ per inch. It even has a serial number. But I can’t find a number on the plate itself. Hmmm. Anyway, assuming the worst and the big plate matches that, that would be 0.00036 (3.6 tenths) over 18″, which is getting close to half a thou.
²This was really hard to photograph; I had to be level with the plate with the lights very dim, there isn’t much light getting under the rule.
³ Suburban Tools is a well respected name in measuring instruments and they own a even more respected brand (Taft-Peirce). They say they these blocks are “GLOBALLY SOURCED” and they inspect them. I believe that because they are wrapped in “Made in USA” paper. The specs are square/size to ±0.0002″ and flat/parallel within 0.0002″, ie better than I can [reliably] measure. As far as I can tell, they meet spec (I’ve only checked squareness so far) but they aren’t perfect. Still, quite nice for $25 (get them through Amazon, everywhere Suburban else sells them, they have a huge shipping charge, eg on ebay, the shipping was over $20).