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America on the Move
Collection Exhibition Themes
Machinist's Caliper
Catalog #: 2002.0075.05, Accession #: 2002.0075
Currently on display
From the Smithsonian Collection
Used during work by a machinist on a lathe, during initial stages of cutting or trimming axles or wheels to a prescribed diameter.
Physical Description
Made of mild steel, with stiff hinge, 31" x 17 3/4"
Date Made:
Dates Used:
1820 - Today
Type used with large lathes
Gift of National Park Service

Part of a small array of hand tools displayed in "America On The Move" - such tools were used in the inspection and repair of steam locomotives. Light repairs on steam locomotives were usually done in roundhouses at the many small locomotive terminals throughout a railroad's system; heavy repairs were done in a large, centralized repair shop serving the whole system (often referred to as the "Back Shop"). Most of these tools date from the early- to the mid-20th century, roughly 1900-1955.

Machinists used calipers to measure the sizes and diameters of parts against a standard size or diameter, or to compare a dimension on one part with the same dimension on another, similar part. A large caliper like this one was used by a machinist when operating a large lathe or vertical boring mill, making very large parts. The lathe might be an immense wheel lathe, one that "trued" the wheel treads of locomotive and railroad-car wheels. Or it might be a lathe for axles, in which the outer ends of each axle were machined for later fitting of wheels by a wheel press.

A fairly crude caliper like this one was used for approximate measurements only, during the "rough" cutting of a diameter down to size. In the final, fine stage of a lathe cut, a much more precise caliper was used.

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