Knurling in CNC Turning: Types, Design Rules, and Common Problems

Knurling in CNC turning creates a textured pattern on a round part. It can make a knob easier to grip, improve the look of a handle, or help a fitted component resist rotation. But a good knurl needs more than the right-looking pattern. The tool, material, and part diameter must work together.

This guide explains how knurling works, how to choose a pattern, and what to check before production. It also covers common defects and practical ways to prevent them. If you are new to the process, our guide to CNC turning explains the basics of machining rotating parts.

What Is Knurling in CNC Turning?

Knurling adds raised or cut ridges to the outside of a rotating workpiece. It is often used on adjustment knobs, tool handles, collars, and hand-operated fasteners. Unlike ordinary turning, the goal is to create a repeatable surface pattern rather than a smooth cylindrical finish.

There are two main methods:

Form knurling: A hardened wheel presses against the part and pushes the metal into a pattern. It creates little or no intentional chip removal. However, it can increase the outside diameter and place a high load on the part.
Cut knurling: A cutting wheel removes material to create the ridges. It can be useful when forming forces are too high or material displacement is a concern. It still needs suitable tooling and chip control.

The right method depends on the material, part shape, machine rigidity, and required finish. Knurling is one of several ways to change a part's surface. For other appearance and texture options, see our surface finishing overview.

Straight vs. Diamond Knurling

Straight knurling produces lines that run along the part's axis. It is useful when the design needs a simple linear grip pattern.

Diamond knurling uses crossing diagonal lines to form small diamond-shaped peaks. This pattern is common on handles and knobs that users turn by hand.

Neither pattern is automatically better. Choose based on how the part will be held, its appearance, and the space available for the tool.

What Should You Specify on the Drawing?

A drawing that says only “knurled finish” leaves too much open to interpretation. Before programming the CNC lathe, define these six items:

Pattern: State whether you need straight, diagonal, or diamond knurling.
Pitch: Specify the spacing of the pattern using the tool manufacturer's system or an agreed standard.
Length and position: Mark where the knurl starts and stops. Leave enough space near shoulders and grooves for the tool to enter and exit.
Finished diameter: Give the acceptable outside diameter after knurling. Form knurling can raise the diameter above the original turned size.
Material and wall thickness: Thin or hollow sections may bend under forming pressure. Hard or brittle materials may need a different tool or method.
Acceptance criteria: State what must be checked, such as pattern coverage, tooth definition, burrs, and finished diameter.

Why does pitch matter? A wheel must form a repeating pattern around the part's circumference. A poor match between wheel pitch and blank diameter can lead to double tracking. As a rough planning check, compare the blank circumference (π × diameter) with the selected pitch. Confirm the final setup with the tool manufacturer's guidance and a trial part; the calculation alone does not guarantee a clean pattern.

How Is Knurling Done on a CNC Lathe?

The exact settings depend on the machine and tool. Still, the setup usually follows the same basic sequence. These checks also apply to other CNC machining operations where tool alignment and workholding affect part quality.

Step 1: Prepare the Workpiece

Turn the area to the planned starting diameter. Check that the surface is concentric and free from damaging burrs. Do not assume that the starting diameter will be the final diameter after form knurling.

Step 2: Install and Align the Knurling Tool

Mount the correct wheel and check its condition. Make sure the tool contacts the intended area evenly. Poor alignment can cause uneven depth or an incomplete pattern.

Step 3: Support the Part

Keep the unsupported length as short as practical. A long or thin workpiece can deflect under pressure. Use appropriate machine support when needed. A scissor-style knurling tool may reduce net side load compared with a single-sided tool.

Step 4: Set Speed, Feed, and Infeed

Start with the tool manufacturer's recommended settings for the material and knurl type. Form knurling needs enough pressure to establish the pattern. Too little engagement can cause shallow or doubled teeth. Too much can damage the part or tool.

Avoid copying one speed or infeed value across different materials. Validate the settings on a first-off part.

Step 5: Form or Cut the Pattern

Engage the wheel as planned, then move along the required knurled length. Keep the feed consistent. Allow room for a clean exit, especially near shoulders.

Step 6: Inspect the First Part

Check the pattern around the full circumference. Look for double tracking, uneven ridges, burrs, and damaged edges. Measure the finished outside diameter at the start, middle, and end of the knurled area.

Record the successful settings. This makes the next production run easier to repeat.

Common Knurling Problems and How to Fix Them

Problem	Likely cause	What to check first
Double or overlapping pattern	Pitch and blank diameter do not register well; poor initial engagement	Check wheel pitch, starting diameter, alignment, and engagement
Shallow or uneven teeth	Low infeed, worn wheel, or weak setup	Inspect tool condition and rigidity before increasing pressure
Finished diameter is too large	Material was displaced during form knurling	Measure the crest diameter and adjust the pre-turn diameter after a trial
Burrs or torn edges	Excessive load, unsuitable material, or poor edge preparation	Review material, wheel condition, and entry/exit details
Pattern becomes deeper at one end	Workpiece deflection or tool misalignment	Check support, blank straightness, and tool contact
Chatter or wheel damage	Loose setup, excessive force, or unsuitable settings	Stop the operation and inspect the tool and holder

Example: A knurled collar looks even, but its outside diameter is too large. First measure the diameter at several points. If the size is consistently high, review the starting diameter and approved infeed. If the diameter changes along the length, check support and alignment before making a size adjustment.

Change one setting at a time when troubleshooting. Otherwise, it becomes difficult to see which adjustment solved the problem.

How Do You Keep the Pattern Consistent in Production?

A successful sample is only the starting point. Repeat orders also need consistent material, tooling, and inspection.

Keep a record of the following:

Material grade and condition
Knurling wheel type and pitch
Starting and finished diameters
Tool position, speed, feed, and infeed
Approved appearance and inspection results
Tool wear or replacement limits

A worn wheel or a change in material can affect the pattern even if the CNC program stays the same. Check the first piece after a setup change, and inspect production parts at an agreed frequency.

Also remember that a knurl's appearance is not the same as its function. A visually uniform pattern may still have the wrong diameter or poor fit. If the part has other texture requirements, assess them separately from the knurl geometry. Our article on CNC surface finish discusses why surface condition matters for functional parts.

Form Knurling or Cut Knurling: Which Should You Choose?

Form knurling is often suitable for ductile materials and well-supported parts. It produces the pattern by moving material, so finished diameter and forming load need close attention.

Cut knurling removes material. It may be worth evaluating for harder materials, delicate parts, or designs where high radial force is a problem. However, it requires the right cutting geometry and chip management.

Do not choose only by appearance. Ask the manufacturer to review the material, wall thickness, part diameter, knurl length, and required quantity. A test on a representative part is the safest way to confirm the result.

What Should You Ask a CNC Turning Supplier?

Before placing an order, make sure the supplier can answer a few practical questions. Can they produce the specified pattern and pitch? How will they control the final diameter? What support will they use for thin parts? How will they inspect the first part and monitor tool wear?

Share a drawing that identifies the knurled area, material, quantity, and important dimensions. Clear information helps the supplier choose a suitable process and reduces avoidable revisions.

Conclusion

Good knurling starts with a clear design and a stable setup. Choose the right pattern, confirm the pitch, and define the finished diameter. Then inspect a trial part before moving into production.

For custom turned parts with knurled features, contact Kaierwo to discuss the drawing, material, and manufacturing requirements.