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How to Tap on a Mill
Guide · Updated October 28, 2026
Tapping on the mill keeps the tap square and on position, right after you drill. You can hand-tap with a spring-loaded tap guide in the spindle, or use a tapping head or rigid tapping on CNC.
Check the hole
A thread plug gage tells you if the thread is right. Check hole positions with gage blocks; the free Gage Block Calculator builds the stack.
Step by step
- Drill the tap drill: Spot and drill the tap drill on position; chamfer the hole.
- Do not move the table: Keep the same X/Y so the tap lines up with the hole.
- Set up the guide: Put a spring tap guide in the chuck and the tap in a tap wrench.
- Start the tap: Bring the guide point into the tap center with light spring pressure.
- Tap: Turn the wrench, backing off every turn or two to break chips, with tapping fluid.
- Back out: Reverse the tap fully, blow out chips and check with a plug gage.
Worked example
Tapping 1/4-20 through 0.5 in of 6061: spot drill, drill #7 (0.201 in), chamfer. Keep the position. Put a spring tap guide in the chuck and seat its point in the tap. Hand-turn the 1/4-20 spiral point tap through, then back it out. Check with a 1/4-20 go/no-go plug gage.
Power tapping
Tapping heads reverse automatically and slip if the tap binds; set the depth stop and run slow, around 200–400 RPM for 1/4 in in mild steel. On CNC, rigid tapping needs spindle and feed synced: feed = RPM × pitch.
Broken taps
Taps break from the wrong drill, dull taps, packed chips in blind holes or bottoming out. Use spiral flute taps in blind holes, spiral point in through holes, and measure blind-hole depth carefully.
Tips
- Drills: tap drill chart explained.
- Speeds: tapping speeds and feeds.
- Gaging: thread gage basics.
FAQ
Can I tap with a drill press or mill spindle?
Power tapping needs a tapping head or reversing spindle; otherwise hand tap with a guide.
What tap drill for 1/4-20?
#7, 0.201 in, for about 75% thread.
Spiral point or spiral flute?
Spiral point for through holes, spiral flute for blind holes.
What RPM for tapping on a mill?
Slow, around 200–400 RPM for 1/4 in in steel with a tapping head.
How do I keep a tap straight?
Tap right after drilling without moving the table, using a spring tap guide.
Related guides and calculators
- Tap Drill Chart Explained
- Tapping Speeds and Feeds
- How to Drill on a Mill
- Thread Gage Basics
- How to Center Drill and Spot Drill
- Go/No-Go Gaging Basics
- How to Use a Keyway Broach
- How to Use Parallels in a Vise
- Helicoil Thread Insert Basics for Machinists
- How to Use a Power Tapper
- How to Start a Tap Straight
- How to Use a Tap Wrench
- Taper, Plug and Bottoming Taps: How to Use a Hand Tap Set
- Spiral Point vs Spiral Flute Taps
- How to Use Transfer Screws to Locate Holes
- All free machinist calculators
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