Everything You Need To Know About Knurling

knurl lines and shapes are common on door handles and the grips of hand tools such as hammers and wrenches. But have you ever wondered what is the purpose of these features? Or how a textured pattern is imparted on items?

This article covers everything you need to know about the knurling processes. It will outline the types of knurling patterns and applications of knurling, among other areas. Read on for more information on this process plus a few knurling tips. Remember to ask for a quote and enjoy our manufacturing capabilities.

What is Knurling?

Knurling

Knurling is a manufacturing process used to create a textured pattern on the surface of a cylindrical or flat workpiece, typically made of materials like metal, plastic, or wood. The purpose of knurling is to improve the 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 is characterized by a rotating set of machine tools coming into contact with the object surface and embedding a straight knurling pattern or any other pattern. Knurling wheels are designed to impart their patterned texture on the part.

In form knurling, the axis of the fine, medium, or coarse knurling wheels is parallel to the axis of the workpiece. As the knurl wheels force into the workpiece, they create the desired pattern. In this type of knurling, the pressure for the formation of knurling patterns is not as severe as in knurl cutting.

To get the best results from form knurling, you may need to use beveled edge knurl wheels. These wheels prevent chipping of the edges, thereby making the surface finishing process simple or unnecessary.

The appropriate knurling tool comes with a unique pattern that transfers to the object surface during the knurling operation. It is worth noting that knurling can also refer to the part of the workpiece that has been subjected to the force of the knurling tool. You can achieve a textured surface through either cut knurling or form knurling.

How to measure the size of knurls?

Knurling Size

To achieve the best results on a knurling process, one needs to adhere to best practices for knurling processes. the following are measurements that we recommend one to have when performing a knurl pattern.

Diameter Before Knurling

It refers to the original or initial diameter of an object before the knurling process is performed on its surface. This measurement is taken before any material is displaced or removed due to the knurling process.

Before knurling, the object has a smooth or textured surface, depending on its initial design and manufacturing. The diameter before knurling is an important measurement because it serves as a reference point for the manufacturing process. It helps engineers, designers, and manufacturers ensure that the knurling process does not deviate the object’s dimensions from the intended specifications.

Diameter After Knurling

Diameter after knurling refers to the measurement of the diameter of an object, typically a cylindrical one, after a knurling process has been performed on its surface.

The process of knurling can slightly alter the dimensions of the object, including its diameter. This is because the material on the surface is displaced to create the knurled pattern. Therefore, when measuring the diameter of the object after knurling, you would be measuring across the outermost points of the raised knurled pattern.

Knurling Angle

The knurling angle refers to the angle of the diagonal pattern created by the ridges or grooves during the knurling process. This angle can determine how deep or sharp the knurl patterns turn out to be.

Knurl Diameter

The knurl diameter refers to the outer diameter of the wheel. This diameter typically falls between 6 mm and 32 mm.

Knurl Hole Diameter

This is another parameter on the knurling wheel. It is the diameter of the internal hole of the wheel. The hole goes into the knurl holder during the knurling operation. The holder size and the knurl hole diameter need to match to prevent machining inaccuracies.

Grades of Knurling

There are at least three grades of knurling based on the kind of knurl lines: Course, medium, and fine knurling. Course knurling presents the most notable ridges for the most effective grip. Fine knurling has the smallest ridges so the grip may not be as pronounced as in course and medium knurling.

The circular pitch for the knurling wheel is within the 8 to 80 TPI (Teeth Per Inch) range. For course, medium, and fine grades of knurling, the pitch is 14, 21, and 33TPI respectively.

Types of Knurling Patterns

Knurling Patterns

There are several types of knurling patterns, each with its own distinct design and functional characteristics. Here are some common types of knurling patterns:

Prevents sink marks and warping

  1. Straight/standard knurl

  2. Right-handed knurl

  3. Left-handed knurl

  4. Diamond knurl

  5. Square and beveled knurl

  6. Concave and convex knurl

Straight/Standard Knurl

Straight/Standard Knurl

This is a manufacturing process that is created by a hollow or straight knurling wheel. The knurling holder in this case holds only one knurling wheel. Also, the wheel is typically cylindrical. During the knurling operation, the knurling wheel should rotate leaving a slight clearance in the knurl tool holder and the borehole. The result for straight knurling or standard knurling is a straight knurl pattern on the surface of the workpiece.

Right-handed Knurl

Right-handed Knurl

In the case of right-handed knurl, the setup is such that diagonal lines or knurls emerge sloping to the right. The knurling wheel has teeth whose helix angle is 30 degrees.

Left-handed Knurl

Left-handed Knurl

Since the left-handed knurl is the opposite of the right-handed knurl, the lines or knurls lean to the left. Still, like in the right-hand knurl, this type features a 30-degree helix angle. It is possible to combine the right-handed knurl and left-handed knurl. The two knurl wheels in this instance would produce diamond knurling patterns.

Diamond Knurl

Straight/Standard Knurl

This pattern, as explained above, emerges when the left-hand knurling and right-hand knurling combine. Another way to create this knurl is through a diamond knurling wheel. This wheel is capable of forming diagonal lines that slope to the right and left concurrently.

There are male and female diamond knurl patterns. The male diamond knurl carves the diamond patterns into the workpiece while the female diamond knurl creates protruded diamond patterns.

Square and Beveled Knurl

Beveled means that the angle is not a right angle but a slanted one. In this case, square knurling creates a heavier load on the tooth edge as compared to the beveled knurl. That is why beveled 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 knurl pattern forms when the curvature of the knurling wheel teeth is towards the center of the wheel surface.

When it comes to the convex knurling operations, this curvature is towards the outside because the contour is essentially rounded. As you would expect, moving the convex knurl is simpler because of this rounded shape.

Types of Knurl Tool Holders

Knurling working

As the name suggests, the knurl tool holder is a device that holds the knurl. It connects to the lathe and features a mechanism for holding the knurled wheel intact. There are different types of knurl tool holders, as we discuss next:

Square and Beveled Knurl

The bump style knurl holders are the most common. This type of holder is characterized by a space for one or two knurls. For the single or two knurling wheels, the type of the bump holder is 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 head comes with a setup for two wheels that the machinist can position according to the diameter of the workpiece. As long as you have each wheel perpendicular to the surface of the workpiece, the knurls will be balanced.

This holder is easy to use when it has a self-centering feature. A pair of screws can allow for 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.

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

Applications of Knurling

Knurling has many applications, ranging from mechanical engineering labs in learning institutions to commercial applications in the automotive industry. As for specific uses, the knurling processes are effective in the creation of mechanical pencils, marking control knobs, and many other items.

Here is a list of areas where machine knurling and hand knurling are applicable:

  • Decoration on parts

  • Enhance grip on devices

  • As a repair method for worn-out parts

  • low-precision assembly

  • Easy identification

Conclusion

Screw bosses are an important feature in injection molded parts. They serve as a positioning and fixation aid while also strengthening the part. Careful consideration of the basic design guidelines ensures that your bosses are durable, reliable, and free from any cosmetic flaws.

In need of quality plastic parts equipped with reliable screw bosses? Aria’s injection molding services are here to fulfill your specific needs. Our team of experts will work with you to manufacture an ideal solution for your application. We are equipped to work with a diverse range of materials and we offer options for a variety of surface finishes and complex geometries.

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