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How to Start a Tap Straight
Guide · Updated October 28, 2026
Most broken taps and crooked threads start in the first turn. Guide the tap square from the start with a tap guide or a spring center in the drill press or mill, while the part is still in its drilling setup.
Check the tap drill
Use the right tap drill and check the hole. The free Gage Block Calculator builds the stack.
Step by step
- Use the drill setup: Tap in the same setup you drilled in.
- Use a spring center: Put a spring center in the spindle to hold the tap wrench in line.
- Or use a tap guide: A block with a square hole on the work.
- Start gently: Turn a couple of turns.
- Check square: Check with a square in two directions.
- Break chips: Back off as needed.
- Finish: Use a plug or bottoming tap.
Worked example: angle error
A tap 1° off square, 0.500 in deep, puts the thread at the bottom off by 0.5 × tan 1° = 0.0087 in. That is enough to bind a bolt in a mating part.
Lubrication
Use tapping fluid suited to the material.
Spiral point vs hand tap
Spiral point taps push chips forward in through holes; hand taps need chip breaking.
Worked example: checking square
After two turns, stand a machinist square on the work against the tap shank in two directions 90° apart. If there is a 0.010 in gap at the top of a 2 in tall square, the tap is leaning atan(0.010 ÷ 2) = 0.29°. Back out, re-align with a guide and restart; at that lean the thread at 0.5 in deep is off by 0.5 × tan 0.29° = 0.0025 in.
Making a tap guide
Drill and ream a block for the tap shank diameter, square to its base. Hold it on the work over the hole and turn the tap through it.
Hand vs machine
Hand tapping in the drill press with a spring center gives a straight start and the feel of hand tapping.
Common problems
Crooked thread: no guide. Broken tap: forced or chips packed. Torn thread: no lube.
Worked example: how far off is too far?
If a 1/4-20 tap leans 2° when you check it with a square, the tip and a point 0.50 in up the tap are offset by 0.50 × tan 2° = 0.0175 in. That is more than a third of the 0.050 in pitch, enough for a visibly crooked thread and a side load that snaps small taps. At 0.5° the offset is only 0.0044 in. Check two directions 90° apart in the first two turns, while you can still correct.
Tips
- Drill: tap drill chart.
- Mill: tap on a mill.
- Speeds: tapping speeds.
FAQ
How do I start a tap straight?
Use a guide or spring center.
Why tap in the drill setup?
The hole is already aligned.
How far off is 1°?
0.0087 in at 0.5 in deep.
Spiral point or hand?
Spiral point for through holes.
Why do taps break?
Forcing or packed chips.
Related guides and calculators
- Tap Drill Chart Explained
- How to Tap on a Mill
- Tapping Speeds and Feeds
- How to Use a Power Tapper
- Helicoil Thread Insert Basics for Machinists
- How to Use Toe Clamps and Strap Clamps
- How to Set Up a Horizontal Mill
- How to Use a Pipe Tap (NPT)
- How to Use a Tap Wrench
- Taper, Plug and Bottoming Taps: How to Use a Hand Tap Set
- How to Thread Pipe with a Hand Pipe Die (NPT)
- All free machinist calculators
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