Here's a quick post in praise of 1-2-3 blocks. They come in pairs, they're quite affordable, and I think you should at least know about them. Most woodworkers haven't heard of them, most woodworkers don't have them. They come from the machine shop, but why should those guys have all the cool precision stuff to themselves? Other items that woodworkers have lifted from the machine shop include the combination square, the engineer's square, the dial indicator (for setting up table saws and planers), the dial caliper (for measuring thickness) and the precision straightedge (used both to check machine tables and the straightness of wood workpieces). This is another machine shop item we should be thinking about borrowing.
What can you do with them?
Machine setup & testing. I first bought mine because I thought the cutterhead of my planer was out of parallel to its bed. I wanted a very precise way to measure that parallelism, and to adjust it if needed. Turned out I was right, it was way out of whack, and by adjusting the left end down until both ends rested on the 3” blocks precisely, I got it back into parallel. They also came in handy to adjust the drum on my old Performax drum sander, something I had to do (or at least check) every year or so.
They're also great little squares, that stand up on their own without your needing to hold them. Set one on its end while you check to see whether your drill press or router table is set up truly vertical.
Standoffs. Once in a while you need to elevate a workpiece above a surface so your router or drill won't damage your workbench top or drill press table. Since they're steel, they won't flex under pressure, and since they're exactly the same thickness, they will hold whatever you put on them parallel to the surface they're on.
Work supports. This is similar to "standoffs", but check out this setup I used this weekend. I was chamfering chair legs with a handplane, and my end vise wasn't holding the entire 36” long blank steady. I simply had too much leverage at the outside end of the cut. So I put one of my handy bench risers on end, only to find it's just a bit short. 1-2-3 blocks to the rescue! 2” was too much, 1” was just right. I could put all the weight I wanted into my plane stroke.
Measurement. Want to see what a 1 in 12 rise looks like? (or any other slope?) Check this out: Ruler and 1-2-3 blocks. You can also measure a taper if you have a caliper to rest on the 1” face and then the 2” face - - think about it a minute and it will come to you.
Another measurement: the chair legs I'm making are 1-3/8” thick, tapering to 1” on each end. I used the 1” edge as a rule to lay out the center 1” of the stick by eye:
Tablesaw: sometimes your miter gauge fence can't handle the length of cutoff you want to make. You're tempted to use the rip fence as a length stop, but it's not safe to use the miter gauge and rip fence simultaneously: it's not going to take long at all for you to experience an amazing kickback. Don't do it! So set the rip fence 3” longer than the finished cutoff you want, put the 3” block against the rip fence, and register the workpiece against that at the beginning of each cut! You can certainly do this with a piece of wood, but with a 1-2-3 block you KNOW it's exactly 3”, and even better, it's heavy enough that it stays put until you want it to move. And the way my scrap bin works, as soon as I need a 3” wide scrap, all the 3” pieces migrate to the bottom of the bin.
1-2-3 blocks are cheap, like less than $20, unless you buy one of the fancy brand names. Here's a vendor I like because of the niche they're taking advantage of. Little Machine Shop 1-2-3 Blocks I'm not a customer, but they've been around a while and their website is full of good information on working metal.
ps) there are also 2-4-6 blocks, but they cost a lot more. And I wonder what machinists in metric countries us? 25-50-75 (mm) blocks?