EUKnurling is a manufacturing process that forms a textured pattern on the surface of a cylindrical or flat part, usually metal, but also plastic or wood, to improve grip, handling, and appearance. You see knurl lines and shapes on door handles and on the grips of hand tools such as hammers and wrenches. The same feature also helps with part identification, low-precision assembly, and some repair work.
For engineers, product designers, OEMs, and procurement managers evaluating part function and manufacturability, this article explains how knurling works, the main types of knurling patterns, the process and tools used to make them, how knurl dimensions and grades are specified, the common types of knurl tool holders, practical knurling tips, and where knurling is applied in real parts.
What is Knurling?
Knurling is a manufacturing process used to create a textured pattern on the surface of a cylindrical or flat workpiece, typically made of metal, plastic, or wood. The purpose of knurling is to improve grip or enhance the appearance of the object, making it easier to handle or manipulate by hand.
The Knurling Process: How it’s Done
The process involves a rotating knurling wheel coming into contact with the workpiece surface and embedding a pattern. It is commonly applied on metals such as steel and brass and is commonly performed on a lathe, milling machine, or CNC machine. Knurling wheels are designed to impart their patterned texture onto the part.
In form knurling, the axis of the fine, medium, or coarse knurling wheel is parallel to the axis of the workpiece. As the knurl wheels are pressed into the material surface with consistent pressure, they create an even pattern. In this type of knurling, the pressure required is not as severe as in cut knurling.
To get the best results from form knurling, you may need to use bevelled edge knurl wheels. These wheels prevent chipping of the edges, making the surface finishing process simple or unnecessary. However, brittle and harder materials are more challenging to form and may require rolled knurling or cut knurling, especially on hardened steel.
The knurling tool comes with a unique pattern that transfers to the workpiece surface during the knurling operation. Knurling can also refer to the textured area of the workpiece after the process is complete. Knurling improves engagement and resistance to pull-out and rotation when a part is assembled into another component, including plastic moulding applications. A textured surface can be achieved through either cut knurling or form knurling.
How to measure the size of knurls?
Achieving precise, clean knurling on CNC lathes requires strict control over workpiece dimensions and knurling tool specifications. Below are the key geometric parameters and measurement standards used in knurling operations.
Diameter Before Knurling
This is the original outer diameter of the workpiece before the knurling process is performed. This measurement is taken before any material is displaced or removed.
The diameter before knurling serves as the reference dimension for the entire process. Because form knurling displaces metal outward, the blank diameter is often turned slightly smaller than the final target diameter to prevent oversizing.
Diameter After Knurling
The diameter after knurling is measured across the outermost points (crests) of the raised knurled pattern.
The relationship between the two measurements is straightforward: the post-knurl diameter approximately equals the pre-knurl diameter plus the pattern depth. In cut knurling, material is removed rather than displaced, so the outer diameter may stay closer to the original or decrease slightly.
Knurling Angle
The knurling angle is the angle of the diagonal pattern created by the ridges or grooves. A standard knurling angle is 30 degrees, though 45 degrees is also used for some diamond patterns. This angle determines how deep or sharp the knurl pattern turns out to be.
Knurl Diameter
The knurl diameter is the outer diameter of the knurling wheel. This diameter typically falls between 6 mm and 32 mm (0.25 in to 1.25 in).
Knurl Hole Diameter
The knurl hole diameter is the internal bore of the knurling wheel. This bore fits onto the pin in the knurl tool holder. The holder pin and knurl hole diameter must match precisely to prevent axial play, chatter, or uneven patterns. In CNC knurling, this alignment is especially important when producing high-precision components.
Grades of Knurling
There are three grades of knurling based on the size of the knurl lines: coarse, medium, and fine.
Coarse knurling has the largest ridges and provides the most effective grip. Fine knurling has the smallest ridges, so the grip is less pronounced than in coarse and medium knurling.
Knurl pitch is specified in TPI (Teeth Per Inch) for imperial tooling or circular pitch in millimetres for metric tooling. The standard TPI ranges for each grade are as follows:
| Knurl Grade | TPI Range | Circular Pitch | Typical Applications |
| Coarse | 14 to 21 TPI | 1.2 mm to 1.8 mm | Heavy-duty grips, structural handles, barb fittings |
| Medium | 30 to 40 TPI | 0.8 mm to 1.0 mm | Thumb screws, control knobs, hand tools |
| Fine | 60 to 80 TPI | 0.3 mm to 0.5 mm | Precision instrument dials, medical devices, decorative texturing |
Types of Knurling Patterns
There are several types of knurling patterns. The chosen knurl type depends on grip, appearance, and application. For example, a diamond pattern is often used when a secure hand grip is needed. Knurled surfaces on inserts for plastic moulding also help prevent sink marks and warping.
The most common types of knurling patterns are:
- Straight/standard knurl
- Right-handed knurl
- Left-handed knurl
- Diamond knurl
- Square and beveled knurl
- Concave and convex knurl
Straight/Standard Knurl
Straight knurling is created by a straight knurling wheel. The knurling holder in this case holds only one knurling wheel, and the wheel is typically cylindrical. During the knurling operation, the knurling wheel should rotate with a slight clearance in the knurl tool holder and the borehole. The result is a straight knurl pattern, also called a linear knurl, on the surface of the workpiece.
For low-volume work, straight knurls can also be made by manual knurling, where a hand tool creates the pattern manually.
Straight knurling is often chosen for delicate materials and decorative purposes when a simpler texture is preferred.
Right-handed Knurl
In a right-handed knurl, diagonal lines emerge sloping to the right. The knurling wheel has teeth with a helix angle of 30 degrees. This diagonal style is often chosen for aesthetic effects and a smoother grip than more aggressive patterns.
Left-handed Knurl
The left-handed knurl is the opposite of the right-handed knurl. The lines slope to the left, and like the right-handed knurl, this type features a 30-degree helix angle. Combining a right-handed knurl wheel and a left-handed knurl wheel produces a diamond knurling pattern.
Diamond Knurl
Diamond knurling is created by combining left-hand and right-hand knurling wheels, or by using a dedicated diamond knurling wheel. It is one of the most common knurling patterns for grip enhancement.
Diamond knurling provides the strongest grip of all standard patterns and supports high torque transmission. It is widely used on tool handles, knobs, and screw heads where secure finger grip is critical.
There are two variations. Male diamond knurl carves the pattern into the surface. Female diamond knurl raises the pattern above the surface. Any sharp edges should be deburred after the operation.
Square and Beveled Knurl
Bevelled means the angle is slanted rather than a right angle. Square knurling creates a heavier load on the tooth edge compared to the bevelled knurl. That is why bevelled knurls tend to move more effortlessly than square knurls.
Concave and Convex Knurl
With special knurling wheels, it is possible to create both concave knurl and convex knurl. The concave With special knurling wheels, it is possible to create both concave and convex knurl patterns. The concave knurl pattern forms when the curvature of the knurling wheel teeth is towards the centre of the wheel surface.
In convex knurling, this curvature is towards the outside because the contour is rounded. Moving the convex knurl is simpler because of this rounded shape.
Types of Knurl Tool Holders
The knurl tool holder is a device that holds the knurling wheel. It connects to the lathe and features a mechanism for holding the wheel in place. There are several types of knurl tool holders.
Bump holders are the most common type. This holder has space for one or two knurling wheels. For single or double knurling wheels, the holder type is called single-die or double-die respectively. The bump holder is fixed to the lathe’s tool post and presses against the rotating workpiece.
Straddle Holder
The straddle holder comes with a setup for two wheels that the machinist can position according to the diameter of the workpiece. As long as each wheel is perpendicular to the surface of the workpiece, the knurls will be balanced.
This holder is easy to use when it has a self-centring feature. A pair of screws allows adjustment to suit different diameters.
Scissor Holder
The other name for a scissor knurl holder is a pinch knurling holder. This knurling tool somehow resembles the straddle knurling tool. It is designed to absorb pressure from the knurling process. As a result, there is less strain on different parts of the lathe including bearings and screws. Keeping both wheels aligned also helps maintain proper alignment and uniform contact on cylindrical parts.
Swivel Holder
To hold multiple knurling wheels, you need this type of knurl holder. It can accommodate as many as six wheels. A major advantage of this holder is that it can help operate course, medium, and fine knurling wheels without having to change the holder. This can significantly reduce the machining time and cost.
Applications of Knurling
A knurled surface may also be chosen where plastic moulding around a metal insert benefits from improved retention.
Knurling has many applications, ranging from mechanical engineering labs to commercial automotive work. It is commonly used on metal parts and can also be applied to nonmetal materials such as wood and leather. Common examples include mechanical pencils, control knobs, and tool handles.
A knurled surface may also be chosen where plastic moulding around a metal insert benefits from improved retention.
Common applications include:
- Decoration on parts
- Improved grip on knobs, screw heads, and hand tools
- Repair of worn-out parts to restore functional diameter
- Low-precision assembly
- Easy identification of parts by touch
Conclusion
Knurling is a versatile process that adds functional texture to cylindrical and flat parts. The choice of knurling pattern, grade, and tool holder depends on the required grip level, material, dimensional tolerances, and production method.
Knurled inserts are also an important feature in injection moulded parts, particularly in screw bosses. The knurled pattern on the insert resists pull-out and rotation within the plastic boss. Careful consideration of knurl pattern, insert dimensions, and boss wall thickness ensures that the bosses are durable, reliable, and free from cosmetic flaws.
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