The Basics of Knurling
Knurling is a finishing process in metalworking where grooves or patterns are formed on the surface of a finished part. This is achieved by a rotating tool that impressions the desired pattern into the metal.
A knurling tool produces a ridged pattern on shafts, gears, tools, pipes and other round-shaped components. All the while, this textured pattern remains consistent in size to form a uniform knurled surface.
There are a couple reasons for knurling. The first reason is aesthetic as knurling is a decorative finishing process that develops a texture on the given workpiece. Another reason for knurling is to create a surface finish that will enhance grip. For example, the handle of a hand tool can be knurled so that the hand does not slip off and a firm grip is maintained. The knurling application can also be used so that serrated edges of workpieces can come together in certain unit assemblies.
Before you knurl
Before you get to knurling, make sure you know the needs of the job you are working on. Different projects will require completely different knurling criteria. Here are some to add to your “before knurling checklist”.
- Diameter before and after: The diameter of your workpiece will change after your knurling process.
- Knurl pitch: This entails TPI (the number of teeth per inch), the circular pitch (the distance between tooth to tooth), and the diametral pitch (the number of teeth per inch of workpiece diameter)
- Concentricity: Ensure that your workpiece is concentric and make sure the knurling wheel is set on the centerline.
- Coolant: Heavy flow of coolant is suggested to ensure the knurl wheels remain both cool and clean.
- Cut or form type: This will determine the tools you must use. See information below.
Knurl cutting vs knurl forming
There are two types of knurling, cutting and forming. Both of which are knurling processes achieved by pressing the knurl pattern into a round workpiece.
Knurl cutting is the process of removing enough material from the workpiece to actually reduce the diameter. This approach is most ideal for medium to large machining projects that require shoulderless diameter* knurling. If the workpiece material is on the harder side, cutting is your best bet. Full-faced wheels are best for cutting operations.
Knurl forming is essentially the same process just not as severe. Material is certainly displaced, however it is more like an impression and is particularly useful for smaller workpieces. Forming is ideal for shoulder diameter* knurling and for centerless workpieces. If a high surface finish is required, knurl forming is the way to go. Beveled edge wheels are best for forming operations.
*Shoulderless (above) vs with shoulder
There are multiple types of knurls with different patterns and shapes. The knurl shape refers to the leading edge of the knurl wheel and the applications it best supports. For example, a beveled knurl wheel has a blunt leading edge therefore requiring higher forces on both the tool and holder. Convex knurls on the other hand have a sharp leading edge to provide a very smooth and precise finish. and refers to the type of shape that the knurl is creating. The standard knurl wheel has a sharp corner on the leading edge which makes the wheel ideal for heavy loading.
Knurling can produce different types of patterns. Some popular knurl patterns are straight, diagonal, and diamond. Check out this diagram that showcases the variety of knurling tools for different applications.
Interested in learning more about the tools you will need to be successful in knurling applications? Check out this video below by YouTuber and machinist Joe Pieczynski.
Selecting your knurling tool
As mentioned above, there are a wide variety of knurling applications. Here are some knurling categories you will have to work through in order to select the correct knurling tool.
- Knurl pattern
- Pitch style
- Type of knurl
- Diameter range
- Type of material (both of wheel and workpiece)
- Quantity of parts being knurled
- Tool center height
- Tool shank size
Many of those specifications will be outlined via the project requirements, however others will be up to you to figure out. For this reason, there are many formulas that will help you to calculate the depth of cut, tracking pitch and cutting parameters in combination with the given material. Data charts and online knurling calculators should be of use.
From clamps, scissors, wheels and machine tool inserts, knurling involves several different components, all of which have dozens of different options in terms of size, material, etc. All of the different options that are offered make it possible to vary the knurl pattern or style produced. Depending on the way in which these variations are selected and applied, hundreds of unique approaches to knurling are possible.
The knurling holder attaches to the lathe and has a cutting head that holds the knurl wheel in place.
This self-centering knurling tool holder features a knurling head that rotates within a 40 degree arc so equal pressure is automatically applied to both wheels. The tool holder measures 5/16″ wide x 3/4″ tall.
This is a multi-head knurling tool that features 6 separate wheels. The knurling head can be rotated and locked with the side bolt, allowing you to use different patterns.
The knurling wheels are what actually make the impression in the workpiece. Perhaps you have decided you want to switch up your knurling pattern. This is made possible as the knurling wheels can be switched in and out.
This female diamond knurl wheel should be used to create male diamond knurls. This wheel is made out of high speed steel for wear resistance and is used to form metal, not cut.
This straight wheel is made of high speed steel and is designed to create a straight knurl pattern. The high speed steel allows this wheel to perform well in general purpose applications for both ferrous and nonferrous materials.
This right-hand diagonal wheel will produce left-hand diagonal patterns. This wheel is made from cobalt which is harder than high speed steel and provides excellent wear resistance.
The information in this article is truly just scratching the surface of all there is to know about knurling operations. From safety precautions to single or double wheel knurling and knurling best practices, you can dive much deeper and continue your knurling research here.
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