What Is Key Stock and How Is It Made?

Huyett Marketing Department
03/17/2022
Updated 11/17/2023

Transmitting torque from a shaft to radial components like gears and couplings is a common engineering requirement. Machine keys, which are manufactured from keystock, are among the most versatile, inexpensive, and easy-to-install power transmission hardware for linking shafts to hubs, gears, and other radial components to facilitate this need.

What is Key Stock?

To put it simply, keystock is metal stock longer than 12 inches from which machine keys are cut. However, to qualify as keystock, the material must adhere to strict dimensional standards with precise tolerances. “Tolerance” refers to the amount by which a part is permitted to deviate from its nominal size.

Tolerance requirements are outlined in the ANSI-B17.1 standard, which specifies three different classes of key fit (the fit of a machine key in a keyway).

Class 1 is a clearance fit. The width of the machine key is smaller than the keyway so the shaft and the mating part can move relative to each other.
Class 2 is a transition fit. The width of the key may be slightly larger or slightly smaller than the keyway. Transition fit keystock ensures mating parts are held together while allowing key joints to be disassembled without difficulty.
Class 3 is an interference fit. The key is larger than the keyway so installation requires considerable pressure or a temperature differential to expand or contract the key and keyway/key seat.

Technically, only stock that adheres to Class 2 tolerances is keystock , although the term is often used more broadly in casual reference.

In addition to adhering to dimensional standards, keystock must also be straight with perpendicular sides and sharp corners. These qualities allow keys cut from the keystock to fit tightly within keyways, maximizing surface contact and reducing bearing stresses.

Keystock vs. Bar Stock

Bar stock and keystock are superficially similar. Both are lengths of metal manufactured to nominal dimensions. However, bar stock has lower tolerance standards than keystock. Whereas keystock must conform to ANSI-B17.1 Class 2, bar stock usually conforms to looser tolerances.

Machine keys manufactured from bar stock must fit in the keyway, but there may be considerable variance in the tightness of the fit between different lengths of bar stock. Machine keys manufactured from keystock, in contrast, are either a small amount over (oversize keystock) or under (undersize keystock) the nominal size.

What is Key Stock Metal?

Keystock can be made from many different materials, including carbon steel, stainless steel, copper, brass, aluminum, monel, and nylon. However, the terms keystock metal, bright steel, keystock steel, and key steel usually refer to square or rectangular medium carbon steel keystock which conforms to the standards discussed in the previous sections.

Medium carbon steel is ideal for keystock and machine keys. It is a fairly strong and hard steel, but it is soft enough to be cut with a hacksaw and filed to the desired dimensions.

Read our Keystock Material Selection Guide to learn more about keystock materials, how to identify keystock steels, and whether you should add a finish to keystock.

How is Key Stock Made?

Keystock is cold drawn because the cold drawing process allows for tighter dimensional tolerances, greater cross-sectional accuracy, a superior surface finish, and sharper corners.

During cold drawing, the metal stock is pushed through a die of the desired shape. The die reduces the stock’s cross-sectional size and increases its length. After cold drawing, the keystock conforms closely to the shape of the die, allowing for greater dimensional accuracy and a superior surface finish compared to other techniques such as hot rolling.

The cold drawing process also changes the keystock’s mechanical properties, increasing the metal’s tensile strength and yield strength.

Key Stock Secondary Processes: Polishing and Heat Treatment

Once the keystock has been cold drawn, it can receive several secondary processes to achieve the correct hardness and surface finish. The most common keystock secondary processes are heat treating and tumbling.

Key Stock Tumbling

Tumbling, also known as vibratory finishing, vibrates keystock in the presence of abrasive pellets. The vibration causes friction between the keystock and the pellets, polishing the surface and removing imperfections such as burrs.

Heat Treating Key Stock

Heat treating hardens keystock by heating it to a critical temperature range and then quenching to reduce its temperature rapidly. Heat treating modifies the crystal structure of the metal, allowing manufacturers to control the metal’s hardness or softness, creating keystock with properties calibrated to the specific needs of each component.

For more information on a metal’s hardenability and other important properties, read our Introduction to Mechanical Properties of Metals.

Key Stock Types

Huyett stocks a comprehensive selection of machine keys, keystock, and keystock assortments. Our range includes multiple types of bright steel keystock, as well as a range of specialist stock, including hexagon and round keystock and bar stock.

Undersize Key Stock

Undersize key stock is slightly smaller than its nominal size while still conforming to ANSI-B17.1 Class 2 tolerances. We stock:

Oversize Key Stock

Oversize key stock is slightly larger than its nominal size while still conforming to ANSI-B17.1 Class 2 tolerances. We stock:

Moltrup

Moltrup is a precision-manufactured oversized product that meets ANSI B17.1 Class 2 fit. The product possesses a bright and smooth surface finish and is designed to fit tightly into a keyway. It is most often used in OEM applications where the mating keyways are unworn and precision made. Moltrup is available in medium carbon steel (AISI C1045), alloy steel (AISI 8630), and low carbon steel (AISI C1018). Medium carbon and alloy steels allow for heat treatment, with alloy providing some additional strength characteristics.

MAK-A-KEY

MAK-A-KEY is manufactured to wider oversized tolerances and is made this way to allow for in-field fitting to the keyway. It is generally an aftermarket product used in repairs and maintenance. The product is typically sold in one-foot and three-foot bars that the user can “cut, file, and fit!™” at the worksite. As the mating keyway may often be worn, starting with a slightly wider “stock” allows for a more accurate and efficient fitting process.

MAK-A-KEY is available in carbon steel and stainless steel. Both imperial (inch) and metric sizes are offered and extend to an array of cross sections, up to about 2” square.

Bilateral Key Stock

Bilateral key stock may be over or under the nominal size while still conforming to ANSI-B17.1 Class 2 tolerances. Our range of bilateral key stock includes:

Step Key Stock

Step key stock is a T-shaped stock used when damaged keyways and key seats have been repaired. It is often referred to as repair keystock.

When a keyway becomes damaged or worn, it can be repaired by removing the damaged material and widening the keyway. However, a repair results in the keyway on the shaft and the key seat on the mating part having different widths. Square and rectangular machine keys cannot be used following this type of repair.

Step keystock is shaped to accommodate keyways and key seats of differing dimensions. There are two different types:

Type 1 step key stock is used when the key way on a part attached to the shaft has been widened during a repair.
Type 2 step key stock is used when the repair was made to the shaft's key seat.

We stock Type 1 and Type 2 step key stock in carbon steel and stainless steel.

Hex Key Stock

Hex key stock is a hexagonal (six-sided) metal stock manufactured using the same techniques as standard keystock. Hex keystock is used to make hexagonal keys, but it is also widely used for tool shafts, transmission shafts, axles, drill bits, and tool steel.

Hex keystock is particularly useful when making shafts connected to hubs and gears. Its hexagonal shape allows it to transmit torque to the connecting part without the use of a separate machine key or keyway.

We stock:

Round Key Stock

Round key stock is a cylindrical stock often used to make shafts, pins, rails, and in many other applications where lengths of round stock are required. At G.L. Huyett, we stock thousands of varieties of round keystock and bar stock suitable for a diverse range of applications.

What is a Machine Key?

A machine key (or shaft key) is a simple metal part—often square or rectangular in cross-section—that fits into a slot cut along the length of a power transmission shaft (the key seat). The key protrudes above the surface of the shaft to intersect with a matching slot cut into the inner circumference of a gear or hub (the keyway). When the shaft spins, rotational force is transferred to the key and then to the mating component. Together, the machine key, keyway, and key seat are called a key joint.

The benefits of using machine keys include:

Lower construction costs: Key joints have lower construction costs than other methods of attaching radial parts to shafts.
Quick installation and removal: Key joints have lower construction costs than other methods of attaching radial parts to shafts.
Versatile: Machine keys are appropriate for high torque transmission scenarios.
Expendable: Machine keys can be made of softer metal than the mating shaft and hub. This ensures that the inexpensive key breaks before more expensive components if a machine malfunctions.
Standards compliant: Machine keys and key stock are manufactured in accordance with ISO and ANSI standards.

As with anything, there are pros and cons to using machine keys. Consider the following for your power transmission application.

Shaft keys may not be suitable for joints that experience alternating directional loads or significant vibration.
Cutting a key way can reduce shaft strength.
A key joint can unbalance a shaft and the connecting part.

For many projects, these disadvantages have no impact, but it's important to be aware of them and assess each key joint individually.

Alternative Hub-to-Shaft Connection Methods

Machine keys are one of several ways a shaft can be connected to the hub of a mating part. They are widely used because they are relatively inexpensive and straightforward to manufacture and install. However, they may not be suitable for every application, and it is helpful to understand the machine key alternatives that are available.

Interference fitting, also known as press fitting or friction fitting, relies on precise machining of the mating parts so that they fit together and produce sufficient friction to maintain the joint.
Set screws, also known as grub screws, are threaded fasteners that hold components on a shaft while allowing for easy adjustment.
Splines are ridges machined to interlock with grooves on a mating part to hold it in position and transmit torque. Splines function similarly to machine keys, but matching splines and grooves must be broached or milled into each component, a more complex task than cutting a keyway and key seat. Additionally, while a machine key can easily be replaced if it breaks, broken splines may require the replacement or re-machining of the part.

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