Free Keyway Broaching & Slotting Calculator
Calculate ANSI B17.1 & DIN 6885 standard keyway dimensions, bore arc chord height compensation, caliper inspection depths, and broach shim passes.
🗝️ Shaft / Bore Geometry
Because a keyway is cut flat into a circular cylinder, measuring depth at the edge vs center differs by the arc chord height (sagitta, s): s = R - √(R² - (W/2)²)
Nominal Radial Depth: 0.125" (3.18 mm)
• Recommended Broach: Style III / B Broach
• Push Passes: 1 pass without shim + 1 pass with standard shim.
Shop Tip: When checking an internal broached keyway with vernier calipers or depth micrometer, you must add the chord height $s$: Caliper Depth $Y = D - s + h_{hub}$. Measuring without $s$ results in an undersized keyway that binds on the drive key!
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ANSI B17.1 & DIN 6885 Standards
Parallel drive keys transmit torque between a rotating shaft and a gear, pulley, or coupling hub. The key width $W$ is standardly sized to approximately one-quarter of shaft diameter ($W approx D / 4$):
Standard Key Sizing Table (ANSI B17.1)
Shaft Dia 1/2" - 9/16": Key 1/8" sq
Shaft Dia 5/8" - 7/8": Key 3/16" sq
Shaft Dia 15/16" - 1-1/4": Key 1/4" sq
Shaft Dia 1-5/16" - 1-3/8":Key 5/16" sq
Shaft Dia 1-7/16" - 1-3/4":Key 3/8" sq
Shaft Dia 1-13/16" - 2-1/4":Key 1/2" sq
The Arc Chord Compensation Formula
Because the top of the broach cut originates at the chord connecting the two edges of the keyway rather than the center tangent of the circle, depth measured across the bore is:
External Shaft: Y = (D + s) - h_shaft
where s = R - √(R² - (W/2)²)
Frequently Asked Questions
Why do HSS push broaches require stepped shims?
Standard rotary arbors and broaches cut to a fixed depth on the first pass (e.g. 0.060"). Inserting a precision steel shim behind the broach pushes the cutting teeth outward by another 0.030" or 0.060" on the second pass, gradually reaching full nominal keyway depth without snapping teeth from excessive chip load.
What is the tolerance class for keyways?
ANSI specifies Class 1 (Commercial loose clearance fit) and Class 2 (Precision interference drive fit). For Class 1, keyway width has a +0.002" / -0.000" tolerance so keys slide into place with thumb pressure.
Can keyways be cut on a CNC milling machine without a broach?
Yes, using an end mill for external shaft keyseats or an internal rotary broaching tool (slotting tool / single-point shaper macro) on a CNC lathe/mill spindle with oriented C-axis spindle lock.