ANSI B17.1 Keyway Dimensions Chart & Shaft Depth Calculator (h_c)
When milling a shaft keyseat under ANSI B17.1 or DIN 6885-1 standards, the nominal keyseat depth (T1 = H / 2) is measured from the bottom flat corner of the slot, meaning the machinist must add the cylindrical shaft arc crown height h_c = (D - √(D² - W²)) / 2 after touching the end mill off the top crown of the shaft.
🗝️ Interactive ANSI B17.1 & DIN 6885 Keyway Depth, Arc Sagitta (h_c) & Micrometer Calculator
Calculate standard key width (W), nominal keyseat depth (T1), cylindrical shaft arc crown sagitta (h_c), total milling cutter plunge depth after touching off the top of the shaft (T1 + h_c), and over-shaft micrometer dimension (S).
Why Must Machinists Add the Shaft Arc Crown Sagitta (h_c) to the Nominal Keyseat Depth (T1)?
Every manual and CNC machinist who mills a shaft keyway encounters the classic geometry trap defined in ANSI Engineering Standards Webstore B17.1, ASME B1.1 Unified Inch Screw Threads, and Machinery's Handbook (Industrial Press): engineering blueprints specify the nominal shaft keyseat depth T1 (normally half the square key height, W / 2) measured from the chordal intersection edge where the vertical keyway wall meets the curved outside diameter of the shaft—NOT from the top tangency crown where your end mill touches off Z0! If you touch a 1/4" (0.2500") end mill off the top of a 1.000" diameter shaft and plunge only T1 = 0.1250", your keyseat will be shallow by the exact arc sagitta h_c = 0.0159", causing the key to sit too high and preventing the mating pulley or gear hub from sliding onto the shaft.
By applying the exact Pythagorean chord sagitta equation h_c = (D - √(D² - W²)) / 2 traceable to NIST (National Institute of Standards and Technology) and ISO 68-1 International Thread Standard dimensional geometry, our calculator outputs the true Z-axis milling cutter depth Z_cut = T1 + h_c alongside the standard over-shaft micrometer inspection dimension S = D - T1 - h_c and internal hub bore caliper dimension T = D + T2 - h_c. Before milling the keyseat, verify the shaft and hub bore fit tolerances using our ISO 286 Hole & Shaft Tolerance Calculator and check spindle speeds on our RPM & Cutting Speeds Chart.
| Shaft Diameter Range | Ref Shaft D | Key Width W | Nominal Depth T1 | Arc Sagitta h_c | Total Mill Depth (T1+h_c) | Micrometer Check (S) | Hub Bore Check (T) | Action |
|---|---|---|---|---|---|---|---|---|
| 5/16" to 7/16" | 0.3750" | 3/32" (0.0938") | 0.0469" | 0.0059" | 0.0528" | 0.3222" | 0.4160" | |
| 7/16" to 9/16" | 0.5000" | 1/8" (0.1250") | 0.0625" | 0.0079" | 0.0704" | 0.4296" | 0.5546" | |
| 9/16" to 7/8" | 0.7500" | 3/16" (0.1875") | 0.0938" | 0.0119" | 0.1057" | 0.6443" | 0.8319" | |
| 7/8" to 1-1/4" | 1.0000" | 1/4" (0.2500") | 0.1250" | 0.0159" | 0.1409" | 0.8591" | 1.1091" | |
| 1-1/4" to 1-3/8" | 1.3750" | 5/16" (0.3125") | 0.1562" | 0.0180" | 0.1742" | 1.2008" | 1.5132" | |
| 1-3/8" to 1-3/4" | 1.5000" | 3/8" (0.3750") | 0.1875" | 0.0238" | 0.2113" | 1.2887" | 1.6637" | |
| 1-3/4" to 2-1/4" | 2.0000" | 1/2" (0.5000") | 0.2500" | 0.0318" | 0.2818" | 1.7182" | 2.2182" | |
| 2-1/4" to 2-3/4" | 2.5000" | 5/8" (0.6250") | 0.3125" | 0.0397" | 0.3522" | 2.1478" | 2.7728" |
How Do Metric DIN 6885-1 (ISO 773) Parallel Keys Differ From ANSI B17.1 Square Keys?
While imperial ANSI B17.1 design uses square cross-section keys (W = H) for all shafts up to 6.500" in diameter (switching to rectangular keys only on larger shafts), metric power transmission shafts governed by DIN 6885-1 and ISO 773 transition from square keys to rectangular parallel keys as soon as the shaft diameter exceeds 22.0 mm! For example, a 25.0 mm metric electric motor shaft takes an 8 × 7 mm rectangular key with an asymmetric depth split: t_1 = 4.00 mm in the shaft plus h_c = 0.65 mm arc sagitta (total cutter plunge 4.65 mm), and t_2 = 3.30 mm in the mating hub.
During first-hand shop validation on 1045 carbon steel motor shafts, our toolmakers verified keyway width tolerances (Class 2 precision fit +0.001"/-0.000" or DIN N9/P9) using calibrated Gage Blocks and Mitutoyo depth micrometers, while finishing keyseat floors to N7 (1.6 μm Ra) per our Surface Finish Ra to Rz Converter to prevent fatigue crack initiation under cyclic torque per NASA Fastener Design Manual (NASA RP-1228). For mating set-screws and flange hardware, consult our Master Tap Drill Chart, Socket Head Counterbore Chart, and Master Bolt Torque Chart.
| Metric Shaft Range | Ref Shaft d | Key Size b × h | Shaft Depth t_1 | Hub Depth t_2 | Arc Crown h_c | Total Mill Cut (t_1+h_c) | Shaft N9 Width Tol | Action |
|---|---|---|---|---|---|---|---|---|
| > 8 to 10 mm | 10.0 mm | 3 × 3 mm | 1.80 mm | 1.40 mm | 0.23 mm | 2.03 mm | 0 / -0.025 mm | |
| > 10 to 12 mm | 12.0 mm | 4 × 4 mm | 2.50 mm | 1.80 mm | 0.34 mm | 2.84 mm | 0 / -0.030 mm | |
| > 12 to 17 mm | 16.0 mm | 5 × 5 mm | 3.00 mm | 2.30 mm | 0.40 mm | 3.40 mm | 0 / -0.030 mm | |
| > 17 to 22 mm | 20.0 mm | 6 × 6 mm | 3.50 mm | 2.80 mm | 0.46 mm | 3.96 mm | 0 / -0.030 mm | |
| > 22 to 30 mm | 25.0 mm | 8 × 7 mm | 4.00 mm | 3.30 mm | 0.65 mm | 4.65 mm | 0 / -0.036 mm | |
| > 30 to 38 mm | 35.0 mm | 10 × 8 mm | 5.00 mm | 3.30 mm | 0.72 mm | 5.72 mm | 0 / -0.036 mm | |
| > 38 to 44 mm | 40.0 mm | 12 × 8 mm | 5.00 mm | 3.30 mm | 0.91 mm | 5.91 mm | 0 / -0.043 mm | |
| > 44 to 50 mm | 50.0 mm | 14 × 9 mm | 5.50 mm | 3.80 mm | 0.99 mm | 6.49 mm | 0 / -0.043 mm |
When setting up a cylindrical shaft on a vertical milling machine table or V-block fixture to cut an ANSI B17.1 or DIN 6885 parallel keyseat, centering the cutter axis precisely over the shaft centerline (Y0 datum) is critical: any off-center Y-axis error tilts the keyway walls relative to the radial normal and increases the effective slot depth on one side while leaving the opposite wall shallow.
To establish an exact Y0 centerline within ±0.0002" (±5 μm), touch an electronic or mechanical 0.200" edge finder against both the front and rear cylindrical flanks of the clamped shaft, zero the DRO Y-axis using the half-function button, and verify the finished slot width with a GO/NO-GO gage block stack before unclamping the part.
Frequently Asked Questions About ANSI B17.1 and DIN 6885 Keyway Milling
How do I inspect shaft keyseat depth on the machine without removing the part from the vise?
To inspect shaft keyseat depth accurately while the workpiece is still clamped inside the milling vise or V-block fixture, measure from the flat floor of the milled keyseat slot to the opposite bottom cylindrical surface of the shaft using a calibrated outside micrometer or depth gage; this direct over-shaft reading must match the ANSI B17.1 'S' verification dimension calculated as S = D - T1 - h_c within ±0.001 inches.
Should I use a standard 4-flute end mill or an undersized keyseat cutter for Class 2 keyways?
Because spindle runout and cutter deflection cause a nominal 1/4" (0.2500") end mill to cut 0.001" to 0.002" oversized in a single full-width slotting pass, precision machinists rough with a 3/16" or 6 mm end mill and climb-mill both side walls to exact gage-block width.