C-clamp

How it works

The frame is the C: a rigid arc with a threaded boss at one tip and a fixed anvil at the other. Turning the screw advances the pad toward the anvil and compresses whatever lies between them. The frame carries the reaction, so the force is limited by the strength of the frame and the screw.

The Navy’s 1971 manual describes it as a steel frame threaded to receive an operating screw with a swivel head, made for light, medium and heavy service in a variety of sizes. The swivel head or shoe adapts to angular faces: Record’s 1967 catalog states that every size of its G cramps is fitted with a swivel shoe that adapts itself to angular faces.

A quick-release arrangement existed early. Silver and Fawcett’s 1874 patent describes a clamp frame with one arm passing over and one under the work, a screw working in a spring-loaded plug with half threads, so that pressing the plug inward disengages the threads and the screw drops down of its own weight.

Anatomy

Frame: The C-shaped body; Record makes its Junior G cramp frame of drop-forged steel, its ribbed cramp frame of malleable iron, and its No. 121 heavy-duty cramp frame of drop-forged steel heat treated to close limits.
Fixed jaw (anvil): The flat face at the end of the lower arm.
Screw: Steel; Record’s ribbed cramp has a machine-cut Acme thread and its drop-forged cramp a unified-form thread.
Swivel shoe (head): A pad free to turn on a ball end of the screw so that it seats on angled faces; the 1971 Navy manual describes replacing a damaged head.
Handle: A cross-handle or drop handle; Record fits a wing nut to ribbed cramps up to 8 inches and a drop handle to the two larger sizes.

Variants

Light-duty G cramp: Record’s Junior G cramp, for home craftsmen and boat builders, with a drop handle and swivel shoe.
Ribbed malleable-iron G cramp: Record’s standard cramp for light woodworking, with a ribbed frame and Acme-threaded screw.
Drop-forged heavy-duty G cramp: Record’s No. 121, designed for heavy duty, with a drop-forged steel frame and a stove-enameled red finish.
Quick-adjust C-clamp: Gelinski’s 1938 patent describes a C-frame tool with a quick-adjustment stem and spring, in which the screw can be tilted into a half-nut.
Locking C-clamp: Petersen’s 1958 advertisement lists an 11-inch locking C-clamp that is adjusted to the right opening before use and then locks to the work.
Edging clamp: Record lists an edging clamp among its G cramps that grips edging strips on straight or curved work.

Specifications

Opening (capacity) · in: Sold by the maximum opening. The Navy text advises a clamp with an opening about 1 1/2 to 2 inches wider than the material.
Depth of throat (reach) · in: Record’s catalog lists depth of throat for each size; no standard size table has been sourced.
Thread form: Acme on Record’s ribbed cramps; unified on its drop-forged cramps.

Made to

ASTM A47
ASTM A48

Safety and care

Safety

Do not use wrenches or bars to tighten clamps, according to the Army and Navy training text.
When closing the jaw of a vise or clamp, avoid getting any part of the hands or body between the jaws or between one jaw and the work (1971 Navy manual).

Care

Clean the threads and swivel with a rag and lubricate with a light coat of oil (Navy training text).
Keep clamps on a rack, on pins or in a tool box rather than in a pile, so that they are not damaged (Cincinnati Tool).
Threads must be clean and free from rust and the swivel head clean, smooth and free of grit; for long storage apply a rust-preventive compound (1971 Navy manual, Navy training text).
To replace a damaged swivel head, pry open the crimped portion, remove the head from the ball end of the screw, fit a new head and crimp it (1971 Navy manual).

Tips

Choose an opening one and a half to two inches wider than the work
The Navy training text advises selecting a clamp with an opening about 1 1/2 to 2 inches wider than the material, and opening the clamp as wide as it will go before applying it.1
Protect glass and polished surfaces
The same text says that glass and highly polished surfaces must be protected with brass shims or wooden blocks.1
Keep the pressure line at right angles to the joint
Cincinnati Tool’s booklet gives this as the basic rule of clamping glued joints, and for curved joints advises gluing or clamping a batten or block to the stock so that the line of pressure is square to the joint.1
Alternate clamps above and below a glued panel
For edge-to-edge joints the booklet says to alternate the clamps, one above and the next below, to prevent buckling, draw them snug from the center, level the boards with a mallet and block, and then tighten fully.1
The swivel pad seats itself on angled faces
Silver and Fawcett’s 1874 patent describes jaws that adjust themselves to the surfaces they meet, and Record’s catalog says its swivel shoe adapts itself to angular faces.1 2
Match the clamp to the job by opening, reach, strength and screw length
Cincinnati Tool lists the questions to ask: the opening required, the depth required, the strength and weight, whether a full-length screw is needed or a shorter screw would avoid a projecting end, and the type of handle.1
A locking C-clamp is set to the opening first
Petersen’s 1958 advertisement describes its locking C-clamp as adjusting to the right opening before use and then locking to the work with a strong grip, so that it can be placed with one hand while the other holds the work.1

History

Earliest documented

1874

Salem, Ohio, United States

US Patent 150,900, dated 12 May 1874, to Albert R. Silver and Edward W. Fawcett of Salem, Ohio, describes a clamp for fastening material to a work-bench, with a frame having one arm to pass over and another to pass under the work, a screw, universally jointed heads and a spring plug nut for quick release. It is the earliest dated C-frame screw clamp read for this entry. Cincinnati Tool’s booklet reports that wooden screw clamps had been used in Europe for centuries and an iron bar clamp without a screw by French cabinetmakers in the 17th century.1 2

The C-clamp is the metal descendant of the screw clamp. Cincinnati Tool’s history says wooden screw clamps have been used in Europe for centuries and that the clamp as we know it began with the bar clamp of French cabinetmakers in the seventeenth century, made of iron without a screw because screws were still expensive; the work was held between the slide and the bent end of the bar and tightened with hammer blows. The booklet says the all-metal bar clamp was first used in Europe about 1876. This is a company history without citations, and no earlier C-frame clamp has been read here.

The first dated C-type frame in the record is the 1874 patent of Silver and Fawcett. It already has features that last: a frame with an arm above and an arm below the work, a swivel head that adapts itself to the surface so that it can be used for any purpose where a clamp may be necessary, and a quick-release nut. Record’s 1967 catalog still advertises a swivel shoe that adapts itself to angular faces, and the Navy’s manual describes the screw with a swivel head as the standard form.

The frame material separates the grades. Record’s 1967 catalog makes light clamps of drop-forged steel for home craftsmen, ribbed malleable iron for light woodworking, and heat-treated drop-forged steel for heavy duty; the Navy describes light, medium and heavy service. The same catalog gives the thread: an Acme thread on the ribbed cramp and a unified form on the drop-forged one. Gelinski’s 1938 patent and Petersen’s 1958 locking C-clamp show the two later attempts to speed adjustment, one with a spring stem and half-nut and one with a toggle lock.

Cincinnati Tool says the bar clamp is the basic clamp for most cabinet work, with C clamps, quick clamps and hand screws used to supplement it. The Army’s 1941 manual makes the metalworker’s use plain: clamps hold work outside a vise or where no vise is available, generally for light work. O*NET’s Tools Used table lists C clamps for 5 of the 11 occupations that this archive maps to the tool, and C clamps or hand clamps for all 11.

Timeline

17th century
French cabinetmakers use an iron bar clamp with a sliding jaw but no screw, tightened by hammer blows, according to Cincinnati Tool’s history.1
1874
Silver and Fawcett of Salem, Ohio, patent a bench clamp with a swivel ball-jointed head and a spring nut that releases the screw quickly (US 150,900).1
1876
The all-metal bar clamp is first used in Europe, according to Cincinnati Tool.1
1879
The Cincinnati Tool Company is organized by Edward Hollister Hargrave, according to its own history; the booklet lists C clamps among the clamps that supplement bar clamps in cabinet work.1
1909–1930
C. and J. Hampton of Sheffield produces G clamps among its engineering and woodworking vices, T-bar sash cramps and floor cramps.1
1920s–1930s
The Made-in-Chicago Museum says the metal Pony and Jorgensen “C” clamps of the Adjustable Clamp Company were introduced in these decades and attributes their patent to Harry V. Holman; the patent has not been located.1
1938
Paul Gelinski of Chicago patents a C-frame hand tool with a spring-pressed quick-adjustment stem and a half-nut (US 2,133,892).1
1941
The U.S. Army’s TM 10-590 describes clamps as used for holding work outside a vise and generally for light work.1
1958
Petersen advertises an 11-inch Vise-Grip C-clamp with ice-tong jaws.1
1967
Record’s catalog lists Junior, ribbed, drop-forged and heavy-duty G cramps, an edging clamp, sash and T-bar cramps.1
1971
The Navy’s Rate Training Manual describes the C-clamp as a steel frame with a screw and swivel head for light, medium and heavy service.1

Gaps in the record (6)

The first C-shaped metal screw clamp and its maker have not been identified; the 1874 patent is the earliest dated C-frame clamp read here and may not be the first.
Cincinnati Tool’s history of clamps is a company booklet; its dates for the French bar clamp, the 1876 all-metal bar clamp and the 1879 founding are not independently confirmed.
The attribution of the Pony and Jorgensen C clamps to Harry V. Holman rests on a museum blog; the patent has not been located.
The Smithsonian’s C-clamp records could not be read (site blocked).
No Roman, medieval or Renaissance C-shaped clamp has been sourced; “C clamp” and “G clamp” naming has not been traced.
The standards slugs are unverified guesses; no standard for C-clamps has been read. No load rating, independent test or failure data has been sourced, so buying is null.