Saw-teeth, tap, reamer, and milling-cutter teeth, may be drawn, and usually should be drawn, down until a file will barely catch them; then they will do excellent work. Many inexperienced temperers are apt to complain if such tools can be filed at all when drawn to the proper color, forgetful or ignorant of the fact that a file should always contain about twice as much carbon as a tap or reamer, and that if both are drawn to the same color the file must necessarily be the harder. Such men often destroy much good work by trying to get the tools too hard. If a tap-tooth be left file hard, it will be pretty certain to snip off when put to work.
TEMPER COLORS.
When a clean piece of iron or steel, hardened or unhardened, is exposed to heat in the air, it will assume different colors as the heat increases. First will be noticed a light, delicate straw color; then in order a deep straw, light brown; darker brown; brown shaded with purple, known as pigeon-wing; as the brown dies out a light bluish cast; light brilliant blue; dark blue; black.
When black, the temper is gone. It is well established that these colors are due to thin films of oxide that are formed as the heat progresses.
These colors are very beautiful, and as useful as they are beautiful, furnishing an unvarying guide to the condition of hardened steel.
The drawing of hardened steel to any of these colors is tempering.
So we have the different tempers:
| Light straw | For | lathe-tools, files, etc. |
| Straw | “ | “ “ “ “ |
| Light brown | “ | taps, reamers, drills, etc. |
| Darker brown | “ | “““ “ |
| Pigeon-wing | “ | axes, hatchets, and some drills |
| Light blue | “ | springs |
| Dark blue | “ | some springs; but seldom used |
This is the unfortunate second use of the word temper, which must be borne in mind if confusion is to be avoided in consulting with steel-makers and steel-workers. The meanings may be tabulated thus:
| Temper. | Steel-maker’s Meaning. | Steel-worker’s Meaning. |
|---|---|---|
| Very high | 150 carbon + | light straw |
| High | 100 to 120 C | straw |
| Medium | 70 to 80 C | brown to pigeon-wing |
| Mild | 40 to 60 C | light blue |
| Low | 20 to 30 C | dark blue |
| Soft or dead-soft | under 20 C | black |