Technical Foundation
What Is a Radius Gage?
A radius gage checks a part's fillet, corner or groove radius by direct comparison — a leaf with a precision-ground concave or convex edge of a known radius is held against the part feature, and light gap visible between the gage edge and the part surface indicates a mismatch, while a flush, gap-free fit confirms the radius is within the gage's tolerance. This go/no-go comparison method is faster than optical or CMM measurement for shop-floor inspection, though it's less precise and doesn't produce a numeric measurement of the actual radius, only pass/fail against the gage's stated value.
Sets bundle a range of radius leaves, commonly from about 1/32 in to 1/2 in in fractional sets or 1mm to 15mm in metric sets, each leaf marked with both a convex (for checking concave part features like fillets) and concave (for checking convex part features like rounded corners) profile. Leaves are typically hardened stainless steel for wear resistance and dimensional stability, and are checked periodically against a certified reference or replaced when wear is suspected, since a worn gage edge no longer represents its stamped radius accurately.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Radius Range | Fractional: ~1/32 in to 1/2 in; Metric: ~1mm to 15mm typical set range | Set must span the range of radii expected on the parts being inspected |
| Gage Style | Multi-leaf folding set vs. single fixed-blade gage | Multi-leaf sets cover a range in one compact tool; single blades suit dedicated repetitive checks |
| Profile per Leaf | Each leaf ground with both convex and concave edges | One leaf checks both inside (concave) and outside (convex) part radii of that size |
| Material | Hardened stainless steel, standard for wear resistance | Maintains edge profile accuracy over repeated shop-floor use |
| Tolerance | Manufacturer-stated accuracy on the ground radius | Determines suitability for tight-tolerance inspection vs. general shop checks |
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Applications
Where Radius Gages Get Used
Radius gages are specified anywhere a fillet, corner or groove radius needs a quick shop-floor comparison check against a drawing tolerance.
Machine Shop Inspection
Radius gages verify fillet and corner radii on machined parts against drawing callouts during in-process and final inspection.
Die & Mold Making
Toolmakers check radius features on dies, punches and mold cavities where a specific fillet radius affects part formability.
Sheet Metal Fabrication
Radius gages verify bend radii on formed sheet metal parts match the specified minimum bend radius for the material.
Weld Inspection
Fillet weld gages check weld leg size and profile against welding procedure specification requirements.
Quality Control & Receiving Inspection
Radius gages provide a fast go/no-go check on incoming or in-process parts without CMM programming time.
FAQ
Radius Gages Questions, Answered
How does a radius gage check a part's radius?
The gage leaf's precision-ground edge is held against the part feature; if the radii match, the edge sits flush against the part with no visible light gap. A visible gap on either side of the contact point indicates the part radius doesn't match the gage — this is a comparison check, not a numeric measurement.
What's the difference between the concave and convex edges on a radius gage leaf?
Each leaf is ground with two edges of the same nominal radius — one concave (curving inward) and one convex (curving outward). The convex edge checks a concave part feature like an internal fillet, and the concave edge checks a convex part feature like a rounded external corner.
Should I buy a fractional or metric radius gage set?
Match the set to the drawing standard used on the parts being inspected — fractional sets (in 1/32 in or similar increments) suit imperial-dimensioned drawings, while metric sets (in mm increments) suit metric-dimensioned parts. Some sets are offered in dual fractional/metric marking.
How accurate is a radius gage compared to a CMM measurement?
A radius gage is a comparison (go/no-go) tool, not a precision measuring instrument — it confirms whether a radius is close enough to the gage's stated value for shop-floor purposes, but doesn't report an exact numeric radius the way a CMM or optical comparator would. Use a radius gage for fast in-process checks and a CMM for certified dimensional reports.
Do radius gages need periodic calibration?
Yes, in quality systems requiring traceable measurement equipment, gages should be periodically checked against a certified reference standard or replaced when wear is suspected, since repeated contact with parts can gradually wear the gage edge and shift it away from its stamped radius value.
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