One of the phrases that I've come across in recent months with regard to layout tools is 'Accuracy has to start somewhere.' Typically, this phrase is used in conjunction with a review to justify a new try square or bevel gauge or some such, as a reliable reference standard. I'm in agreement with the phrase, and I'm using it in a tool review, but not the way I've seen it used.
I think accuracy has to start somewhere, but I think it has to start with an understanding of accuracy, and degrees of accuracy.
This is a picture of two lines, drawn with a sliding bevel gauge.
Please note that a) the lines diverge, and b) the divergence isn't really readily apparent for the first couple of inches. THIS is where accuracy begins: with the understanding that minor and minute errors aren't apparent until magnified or multiplied. And you may not detect them until they can affect a bigger picture. (If this was a picture frame, or something with big miters, and your angles are slightly off, your miters won't close. Period. Yes, you can use putty or wood filler, but the joint will lose strength.) It's easier to detect a minute discrepancy if it's projected out far enough. This is why you need to draw LONG lines to set your bevel gauge to. It's why it helps to have a bevel gauge with a long blade, AND a long beam: You want to make sure that the angle is true, even when projected out.
I've seen some stores offer little 3" setup blocks milled out of aluminum from companies like Incra or Woodpeckers. I've also seen firsthand that even inside the 3", these little doo-dads weren't actually square. If you can't make a block square enough within 3", that's beyond egregious. If you're shopping at the store, ask to borrow a Starrett combination square to check any other squares or setup blocks, and see for yourself.
These are my Shinwa bevel gauges, and my Starett gauge:
I bought the Starett years ago. I like it.
I like the Shinwa gauges better.
I like the 9" long beams, the 8" long blades, and the fact that there's room to write on them, to keep track of multiple angles. (Sharpie marker ink wipes clean with denatured alcohol)
I also like that the handle has a hole, not a slot. The slot in the Starrett can lead to errors, like so:
I've exaggerated the issue for the sake of illustration, but a little bit of beam protrusion can interfere with the ability to set up the blade on a table saw accurately.
Lastly, I like the screwdriver slot in the nut on the Shinwa. I don't torque down on it too hard, but the blade locks very rigidly in place. I usually set to finger-tight while I fine-tune the setting, and tighten afterwards, to lock it up.
Quick tip, for an even tighter lock-up, pulled from an old book on drafting: Old-school draftsmen would heat up their dividers and melt beeswax onto the pivot point. Unlike paraffin, beeswax is a little sticky. Melted into the milled steel surface, beeswax will add just a bit more 'sticktion,' which is the static friction that must be overcome before an object moves. In motion, beeswax glides beautifully. But it will also help hold a setting a little bit better. I haven't needed it on the bevel gauges that I have, but for those of you who have been fighting with the gauge that you have, it might help... And, it'll help prevent rust.