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.
Live tool
Razor & Fine-Edge Angle Calculator
Enter spine thickness and blade width to find the honing angle built into a straight razor, per side and inclusive, with guidance against the classic targets.
Run it
Your numbers, live
A straight razor’s edge angle is not chosen at the stones — it is built into the blade: honing flat on the spine, the spine thickness and blade width fix the angle forever. This calculator does the trigonometry: enter both measurements and read the angle per side, with guidance against the classic targets.
Your numbers
The arithmetic
Behind the number
How this is calculated
Honing on the spine forms a triangle: half the spine thickness is the rise, the blade width is the hypotenuse. Angle per side = arcsin(half the spine ÷ width). The classical proportion — spine about one quarter of blade width — lands at the traditional 7–8° per side; see Straight Razor Geometry for why that ratio rules the design stage.
Using it in the shop
Design first: pick the width, multiply by roughly 0.25, and that is your spine before grinding. Check honing reality second: tape on the spine (a common protective habit) adds thickness and raises the angle slightly — add the tape to the spine number to see by how much. For knives honed on their spine (some traditional single-bevel work), the same math applies; for normal knives, angles come from the bevel-angle chart instead.
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.