One of the most useful tools you’ll use daily as a machinist is a 6″ scale (also called a “rule”, but not “ruler”).
Scales come in different lengths, graduations and finishes. And you can buy a rigid or flexible one to suite your needs.
If your just starting out consider getting a 6″ 5R or 16R flexible scale. I find black scales with white lettering the easiest to read. SPI makes an EZ-View that is inexpensive, accurate, and easy to read.
What’s the “5R” or “16R” mean?
3R: 10ths and 50ths (front), 64ths and 32nds (back)
4R: 8ths and 16ths (front), 64ths and 32nds (back)
5R: 10ths and 100ths (front), 64ths and 32nds (back)
9R: 16ths and 32nds (front), 64ths (back)
16R: 50ths and 100ths (front), 64ths and 32nds (back)
E/M: 32nds and 64ths (front), 1 mm and 0.5 mm (back)
D/M: 10ths and 100ths (front), 1 mm and 0.5 mm (back)
M: 1 mm and 0.5mm (both sides)
So as you can see, the 5R and 16R scales are the only English scales with 100ths, which (although hard to read) is useful to have on a scale. Of course, if you’ll be working with metric measurements, you’ll want to go with an E/M, D/M, or M scale (English/Metric, Decimal/Metric, or Metric).
Where To Buy:
Check ebay.com first if you’re looking for a high-end brand like Starrett. Otherwise try one of the tool suppliers, like MSC / J&L Metalworking.
One final note on scales, it’s handy to have a “hook rule” in your toolbox. They are a bit harder to find and you won’t use them as often, but when you need one, you need one. The picture shows the 3 scales I use (the one on the left is a flexible SPI, the one on the right is rigid SPI, both are EZ-View), and a Lufkin hook rule in the center. Notice how the hook rule is very thin with a small hook on the end. Also, you can see from the picture how much easier it is to read a black scale vs a silver one (although the Lufkin is very old, so that has something to do with it too).
That’s just about everything you need to know about scales. If anyone has anything to add, feel free to post a comment.