← AskMachinist · Guides · Calculator · 🔎 Search
How to Cut a Woodruff Keyseat
Guide · Updated October 28, 2026
Woodruff keys are half-moon keys that sit in a curved seat in a shaft. The cutter matches the key, so the job is mostly centering the cutter on the shaft and plunging to the right depth.
Check the key fit
Measure seat width with gage blocks or pin gages. The free Gage Block Calculator builds the stack.
Step by step
- Read the key number: The last two digits are the nominal diameter in eighths of an inch; the leading digits are width in thirty-seconds.
- Choose the cutter: Use the cutter with the same number.
- Hold the shaft: V-blocks, a vise or collet block, axis parallel to the table.
- Center the cutter: Touch each side of the shaft, split the difference, and offset half the cutter width.
- Touch off: Touch the top of the shaft.
- Plunge: Feed down slowly at low RPM with coolant to the seat depth from the key table.
- Check fit: Try the key; it should tap in with light hand pressure.
Worked example
A #606 key is 3/16 × 3/4 in (width 6/32 in, diameter 6/8 in). From the ANSI B17.2 keyseat table for #606: shaft keyseat width 0.1863–0.1880 in, shaft depth B = 0.2143 in (+0.005/−0.000), key height above the shaft C = 0.0937 in, and keyseat diameter 0.750–0.768 in. B is measured at the side edges of the seat, not at the crown of the shaft. On a 1.000 in shaft, the seat edges sit 0.094 in either side of center, so the shaft surface there is lower than the top by 0.500 − √(0.500² − 0.094²) = 0.500 − 0.4911 = 0.0089 in. If you touch off on the top of the shaft, plunge 0.2143 + 0.0089 = 0.2232 in (up to 0.2282 in at the +0.005 limit). Check: key height ≈ B + C = 0.2143 + 0.0937 = 0.308 in, which matches the #606 key height. At 60 SFM in steel with an HSS cutter, RPM = 3.82 × 60 ÷ 0.750 = 306 RPM.
Why Woodruff keys
The curved seat keeps the key from tipping and is easy to cut with a single plunge, which suits tapered shafts.
Common problems
A loose key means the cutter was worn narrow or the seat walked. Off-center means poor centering. A key sitting too high means not deep enough.
Tips
- Keyways: cut a keyway.
- Broach: use a keyway broach.
- Holding: V-blocks.
FAQ
What does a Woodruff key number mean?
Last two digits: diameter in eighths; first digits: width in thirty-seconds.
Which cutter do I use?
The cutter with the same number as the key.
How deep do I cut?
Use depth B from the ANSI B17.2 keyseat table (0.2143 in for a #606), measured at the seat edges.
Can I use an end mill?
No, use a Woodruff cutter.
How should the key fit?
Snug, tapping in lightly.
Related guides and calculators
- How to Cut a Keyway
- How to Use a Keyway Broach
- How to Use a V-Block with Gage Blocks
- Milling Speeds and Feeds
- How to Cut a T-Slot
- How to Cut a Radius with a Rotary Table
- How to Use a Slotting Attachment on a Mill
- Shaft Key Size Chart
- ISO 286 Hole-Basis Fits Explained (H7/g6, H7/h6, H7/k6, H7/p6)
- All free machinist calculators
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.