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How to Use a Center Gage (Fishtail)
Guide · Updated October 28, 2026
A center gage, or fishtail, has 60° notches for grinding and checking threading tools, and a straight edge to set the tool square to the work. It also checks 60° lathe centers.
Check the major diameter
Mic the major diameter before threading. The free Gage Block Calculator builds the stack.
Step by step
- Grind the tool: Grind the 60° point; check it in the gage notch against light.
- Set tool height: Tool on center.
- Square the tool: Hold the gage against the work and bring the tool into the notch; no light on either side.
- Set the compound: Set it at 29.5° (or 30°) for flank infeed.
- Cut and check: Cut the thread.
- Check centers: Use the 60° notch to check a center point.
- Measure: Check with a thread gage or wires.
Worked example: 1/2-13 UNC
P = 1 ÷ 13 = 0.0769 in. The usual single-point depth for an external 60° thread is 17/24 H = 0.61343P (H = 0.866025P) = 0.0472 in. With the compound at 29.5°, compound travel ≈ 0.0472 ÷ cos 29.5° = 0.0542 in. Finish to size with wires or a ring gage rather than the dial alone.
Why square matters
A tool skewed even a degree or two leans the thread form, so one flank is steeper.
Reading the scale
Many gages have scales in 1/14, 1/20, 1/24 and 1/32 in for checking pitch on the work.
Common problems
Leaning thread: tool not squared. Wrong angle: tool not ground to 60°. Torn flanks: dull tool.
Tips
- Threads: threads on a lathe.
- Gages: thread gages.
- Height: tool height.
Worked example: checking a 60° tool against the gage
A center gage (fishtail) has 60° notches for Unified and metric threads and usually scales in 1/14, 1/20, 1/24 and 1/32 in for checking pitch. Grind the threading tool until it fits the 60° notch with no visible light at either flank. Then set it square: hold the gage against the true part diameter and bring the tool into the side notch so both flanks touch evenly.
A tool tipped 1° off square cuts a flank angle error you can see as a lean in the thread form. For 8 TPI the pitch is 0.125 in; check it against the gage scales or with a thread pitch gage before taking finishing passes.
Fuller worked example: 1/2-13 UNC depth and compound feed
Grind the tool to the 60° center gage notch and square it to the work with the gage against the part. For 1/2-13 UNC, P = 1 ÷ 13 = 0.0769 in and the external thread depth is about 0.6134 × P = 0.0472 in.
With the compound set at 29.5°, the compound feed to reach that depth is 0.0472 ÷ cos 29.5° = 0.0472 ÷ 0.8704 ≈ 0.0542 in. Finish by checking with a thread ring gage, thread wires, or the mating nut rather than the dial alone.
FAQ
What is a center gage?
A 60° gage for threading tools and centers.
How do I square the tool?
Against the work with the gage edge.
What compound angle?
29.5° or 30°.
How deep is a UN external thread?
The usual single-point depth is 0.61343 × pitch.
Can it check centers?
Yes, 60° ones.
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