Reference chart
Belt Speed by Task Chart (SFPM)
Set your VFD by the job: working SFPM ranges for hogging, bevel grinding, hardened steel, handles and leather, with percentages for a direct-drive 2x72 setup.
Reference chart
Steel Temper Color Chart
The oxide temper colors of carbon steel from pale straw to blue, with temperatures, what each means for a blade, and grinder-burn diagnosis.
The chart
Read it off the table
Clean carbon steel heated in air grows a thin oxide layer whose color reports its temperature — the oldest thermometer in the smithy, and still the fastest way to read a temper or diagnose a grinding burn. Colors below are for clean, bright steel; scale, oil and alloy shift them.
| Color | Approx. temp | What it means on a blade |
|---|---|---|
| Faint / pale straw | ~390–410°F | The classic low temper — maximum hardness territory for most carbon steels (razors, fine slicers) |
| Straw | ~420–440°F | The everyday knife temper zone — the usual 1-2 point HRC trade for stability |
| Dark straw / bronze | ~450–470°F | Working knives wanting extra forgiveness; many camp blades live here |
| Brown-purple | ~480–500°F | Big-blade and impact territory – hardness clearly traded for toughness |
| Purple | ~510–530°F | Spring and tool tempers; softer than most blade edges should run |
| Blue | ~540–570°F | Springs, spines in differential draws – and the classic grinder-burn verdict on an edge |
| Pale / gray-blue | ~590–640°F | Fully drawn soft; screwdriver country |
Reading a grinder burn
Color at the edge mid-grind is this chart applied in reverse: compare the color’s temperature against your tempering temperature — straw on a 400°F-tempered blade is cosmetic, purple or blue means that zone is now over-tempered soft, and sanding off the color does not restore hardness. Full triage in Burned Edge on the Grinder.
Tempering by color, honestly
Torch-tempering by color is real and traditional (spine draws on differential blades use it deliberately — see the differential guide), but an oven with a verified thermometer beats eyes for whole-blade tempers: color reads the surface’s peak, not the soak that hardness actually wants. Use the chart to diagnose and draw; use the oven to temper. The full process lives in the heat treat guide.
Browse more tools in the Calculators section.
More shop math
The rest of the calculators
Eight tools, all free, all running in the page.
Live tool2×72 Belt Speed Calculator
Find your 2x72's surface speed in SFPM from drive wheel diameter and motor RPM, with optional VFD frequency, and see how hard a belt cuts and how hot it runs.
Live toolGrind Height Calculator
Turn stock thickness, edge thickness and bevel angle into the scribe height for a flat grind, with notes on reading the answer and using it at the machine.
Same bench
The machine behind the math
Speed, angle and pressure only mean something on a grinder that holds its setting. I machine the parts, hand-assemble, track and test Torus grinders in Sterling, Illinois.
Nova 2×72 Aluminum Belt Grinder
Current dual-axis platform with a fiber-laser-cut 6061 chassis and single-knob tracking.
Starts at$1,295
Nebula 2×72 Steel Belt Grinder
Current dual-axis platform with a steel chassis, more mass and less vibration.
Starts at$1,295
Torus 2×72 Classic Aluminum Belt Grinder
Prior-design aluminum chassis, 90-degree tilt and 1.5-inch tooling arms.
Starts at$995
One-man shop
Number not matching your machine?
Send me your wheel size, motor and VFD details so I can compare them with the belt-speed calculation.