Knurling is that pattern you see on gripping surfaces that give you extra friction.
Knurling is a manufacturing process in which a pattern of lines is physically added onto a workpiece's internal or external surface by rolling wheels with V-shaped teeth. These patterns are typically added to cylindrical workpieces using a lathe.
This article provides detailed information about knurling such as knurling methods, tools, applications, and terminologies.
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Knurling Explained: What is it?
Knurling is a machining operation used to make straight, diagonal, or diamond-like patterns on round or cylindrical parts.
The following are the different types of knurling based on where the knurl pattern is machined on a workpiece.
|Types based on the zone of knurl||Details|
|Full Knurl||Knurled fully across the surface of a part|
|Band Knurl||Only knurled across a certain area of a part|
|Internal Knurl||Knurled on the internal diameter of a part|
|Tapper Knurl||Knurled on the conical end of a part|
|Milling/Face Knurl||Knurled on a plane surface of a part|
A knurl is called a full knurl when the knurl pattern is imprinted fully across the surface of the workpiece.
A knurl is called a band knurl when the knurl is only rolled over a specific area of the workpiece.
In internal knurling, the internal diameter of a workpiece is knurled.
When the knurling operation is performed on the conical end of a round workpiece, it is called a tapper knurl.
Milling/face knurls are made on the plane surface of a workpiece. These types of knurls are generally made using a milling machine.
Pitch (circular) is the distance between each line on a knurl pattern. For an accurate pitch measurement, you must measure it from the tip of the knurl teeth.
Diameter before knurling
It is the original diameter of the stock workpiece before the knurling operation.
Diameter after knurling
It's the diameter of the knurl on the part after the knurling operation. Based on the knurl manufacturing technique, it can be more or less than the original diameter.
An angle of a knurl is the angle between the teeth of a knurling wheel.
The hole diameter of a knurling wheel represents the diameter of its internal hole.
You have to get a wheel with a hole diameter that matches your holder. Otherwise, the knurling wheel won't be a perfect fit, affecting machining accuracy.
Knurl diameter represents the outer diameter of a knurling wheel. It is always larger than the hole diameter.
Face width represents the width of the area where the knurl tooth appears on a knurling wheel. It is the surface that comes in contact with the workpiece.
As the face width of the wheel increases, it requires more pressure (in-feed) to knurl the workpiece.
Grades of Knurling
Fine, medium, and coarse are the three major grades of knurling. Among them, coarse knurling has the deepest ridges and a larger surface area providing better friction.
In terms of TPI (Teeth Per Inch), fine knurling has a larger value of around 33TPI, medium knurling is around 21TPI, and coarse knurling is about 14TPI.
This means the Diametrical Pitch (DP) or the number of teeth per inch of a fine-grade knurling wheel will be larger than a coarse-grade wheel.
Types of Knurl
Knurling wheels are available in the following configurations for ID (internal diameter) and OD (outer diameter) knurling operations.
|Types of Knurl||Details|
|Straight/Standard Knurl||Produces a straight knurling pattern|
|Right-handed Knurl||Produces diagonal knurls sloped towards the right|
|Left-handed Knurl||Produces diagonal knurls sloped towards the left|
|Diamond Knurl||Produces diagonal knurls sloped towards the left and right|
|Square Knurl||Square knurls can only make a band knurl|
|Beveled Knurl||Beveled knurls can produce a full knurl|
|Concave Knurl||Produces protruded bands on a specific area|
|Convex Knurl||Can transverse and knurl throughout the surface of a stock|
A straight knurling wheel produces a straight knurling pattern on the workpiece. The series of tooths on a straight knurl wheel is not bent toward any angle.
A right-handed knurl produces diagonal knurls sloped towards the right. The teeth on these knurls are generally at a 30-degree helix angle.
A left-handed knurl is opposite to a right-handed knurl. It produces diagonal knurls sloped towards the left.
The teeth on these knurls also are generally kept at a 30-degree helix angle.
You can use both a left-handed and a right-handed knurl together in a two-wheel holder to produce diamond patterns.
You can also make a diamond pattern using a single diamond knurling wheel. It produces diagonal knurls sloped towards the left and right.
A female diamond knurl produces protruded diamond patterns, and a male diamond knurl produces diamond patterns carved into the surface.
Square and Beveled Knurl
Square profile knurls are best at performing single-location knurling operations, whereas bevel edge knurls are best when it comes to traversing knurling operations.
This is because, in square profile knurls, there will be a heavy load on the tooth edge when it's moved left or right, but the angled tooth of bevel edge knurls takes some load off the tooth edge, allowing it to move across the surface easily.
Concave and Convex Knurl
The teeth on a concave knurl are curved into the center of its face. It is used to feed one area of the workpiece at a time, axially.
For long traverse knurling operations, a convex knurl is the best bet. It has a round profile that allows it to move around the surface of a stock smoothly.
Based on The Wheel Material
Knurl wheels made out of different materials are available. Among them, carbide knurl wheels are the best option for working on high-speed knurling jobs.
Other materials include HSS (High-Speed Steel), powdered HSS, cobalt, etc.
Types of Knurl Tool Holders
|Bump Holder||Can house one or two wheels|
|Straddle Holder||Holds two wheels in line with the center of the workpiece|
|Scissor Holder||Can adjust knurl distance and is set slightly before the center|
|Swivel Holder||Can hold up to six knurling wheels|
Bump-style knurl holders are available in single and double-die configurations. Single-die setup can house one knurling wheel, while a double-die setup can house two wheels.
They are also available in a modular setup. It has a swappable tool head, which allows it to have a single or double-die setup without changing the holder.
A straddle holder head can hold two knurl wheels. You can move these wheels up or down to work with stocks of various diameters.
On a straddle holder, you must set the knurl wheels in line with the center of the workpiece.
When knurling with two wheels simultaneously, the knurl lines can overlap if the wheels are not equally pressing onto the workpiece.
Also, set each wheel perpendicular to the workpiece to avoid making knurls heavier on one side.
A scissor holder looks like a straddle holder and can hold two knurl wheels. You can adjust the knurl distance by turning a nut on the holder handle.
When using a scissor holder, do not set the wheel at the stock center like in straddle holders.
It needs to be slightly before the center of the part to put a good pressing force on it.
A swivel knurling tool hander can hold up to six knurling wheels on its head. You can set up multiple wheels on the head and easily change the wheel by turning the head.
For example, you can set it up with a set of fine, medium, and coarse knurling wheels to perform multiple patterns in less time, as you don't have to spend a lot of time on tool changing.
Cut and form knurling are the two machining techniques used for making knurls.
In the cut knurling process, the surface material from a round stock is removed to make the knurl.
But in the form knurling process, the surface material is slightly displaced by the pressure exerted by the teeth of a knurling wheel.
Unlike cut knurling, the surface material is not removed in form knurling. So the knurls made using the forming knurling technique will be slightly larger than the original stock diameter.
Form knurling jobs require a higher in-feed than cut knurling jobs. Because of this immense pressure exerted by the tool, the cutting speed of form knurling is lower than cut knurling.
If you want to perform knurling jobs on small parts and soft materials, go with form knurling. It delivers a good surface finish.
Cut knurling is best when working with large parts or hard materials.
Manual vs Machine Knurling
Knurling operations can be performed using hand tools or machine tools. Working with hand knurlers is difficult, but it gives good results for small jobs.
In comparison, machine tools let you quickly make knurled parts. If you want to do highly repeatable jobs, it's best to work with automated machines like a CNC lathe.
Applications of Knurling
Knurls are used for functional and decorative applications on materials like aluminum, brass, steel, plastic, etc.
In functional areas, knurls increase the surface friction of parts to provide a better grip.
For example, it is used to make grips for tools, pistons, screw heads, handles, footpegs, turnable control knobs, pens, etc.
Because of the aesthetic appeal of the knurl pattern, knurled parts are also used for decorating parts.
Knurling is a highly rewarding machining technique, but it's hard to master.
If you are a beginner, start with one-wheel knurling jobs and work your way up to two-wheel jobs.
Follow these tips to get a better knurling pattern on your workpiece.
- Set the wheel threads deep into the stock surface
- Work with concentric stocks
- Use beveled or convex knurl wheels for form knurling jobs
- Use square knurl wheels for cut knurling jobs
- Apply coolant while knurling
Frequently Asked Questions
Can you knurl wood?
Yes, you can knurl wood, specifically hardwood. But knurling wood is more challenging than knurling metals. Also, knurls on wood won't last long, as they quickly fade out when exposed to physical contact.
What speed should you knurl round stocks?
You can knurl round stocks at the speed at which you would otherwise turn them on a lathe. Manufacturers recommend a maximum speed of 50 m/min for knurling operations. In general, you should machine harder materials at a lower speed.
Can you knurl without a lathe?
Yes, you can knurl without a lathe. The knurling process can also be performed manually using hand knurlers. These tools can also make straight and angled cuts.