Limit gauges are made from hardenable steel, typically a high carbon or alloy tool steel, hardened and then stabilised and ground. The hardness matters because the gauge is loaded in contact with brass, aluminium or steel caps thousands of times, and any plastic deformation at the contact points changes the dimension the gauge is supposed to embody.
Hardness alone is not enough. A gauge that is hardened but not properly tempered will continue to change size for months as retained austenite transforms. Good practice is a double temper and a stress relief after rough grinding, followed by a stabilising period before final grinding. That sequence is invisible in the finished part, which is why two gauges of the same nominal hardness can age very differently.
Hard chrome plating is common on gauges that see abrasive caps or corrosive atmospheres. Plating adds a layer that can chip at edges, and a chipped edge on a go member will scratch parts. Where plating is used, the sheet dimensions apply to the plated part, and re-plating after wear is not acceptable because it changes the base dimension.
Ask for the material grade, the hardness range, the heat treatment sequence, and the dimensional report against the gauge sheet. A supplier who can only provide a hardness figure is describing half of the requirement.
