Feeler Gauge: Uses, Diagram, Brands and Applications

The tiny tool that protects your expensive tooling

In precision machining, the smallest gaps can cause the biggest headaches—chatter, poor surface finish, tolerance drift, even chipped carbide inserts. A feeler gauge is the pocket-size insurance policy that helps you find and control those gaps. Whether you’re validating insert seating in a turning toolholder, checking fixture contact before a heavy milling pass, or stacking leaves to shim a setup, feeler gauges keep your CNC cutting tools honest so your cycle times and finishes stay on target. Major metrology brands (e.g., Mitutoyo, Starrett, Bahco) publish specifications and ranges for these gauges, and all the big insert makers (Sandvik Coromant, Kennametal, ISCAR, Seco, etc.) align on ISO insert nomenclature—so you can connect measurement to the right insert geometry with confidence.

What is a feeler gauge?

A feeler gauge (also called a thickness gauge) is a set of precisely ground leaves (blades), each marked with its thickness. Leaves are typically stainless or spring steel; most sets let you swing out one or more leaves and “feel” for a target clearance. Many pro sets explicitly state that each leaf is marked and detachable, making stacking, cleaning, or replacement easy. 

Typical ranges (metric)

Common professional sets cover roughly 0.03–1.00 mm, often in fine 0.01 mm steps at the thin end and 0.05 mm steps above 0.10 mm—ideal for machine setups and bearing checks.

Step-by-step: How to use a feeler gauge correctly

1. Clean the surfaces: Wipe mating faces; a tiny chip will lie. (Insert makers emphasize clean seats for a reason.) 

2. Pick a starting leaf: Begin a bit thinner than you expect. (E.g., 0.05 mm when checking jaw contact.) 

3. Feel for “just-drag”: A correct leaf slides with light, uniform resistance. No drag = too thin; won’t enter = too thick.

4. Stack leaves if needed: Combine leaves to match the gap. Add their stamped values (e.g., 0.02 + 0.03 = 0.05 mm). 

5. Record & act: If you detect a gap under an insert seat, stop and clean, replace a damaged shim, or re-torque as per toolholder spec. 

Tip: Keep leaves lightly oiled and re-fan them to dry. Replace any nicked leaf; it won’t read true.