Defining knurl on a technical drawing is essential for controlling the grip, roll, and insertion behavior of cylindrical features. Engineers use standardized symbols and notes to specify the exact geometry, direction, and finish of the knurl pattern.
This article explains how to represent knurl on engineering drawings using clear, industry-aligned practices. The guidance supports consistent communication between designers, machinists, and inspectors.
| Knurl Type | Symbol on Drawing | Typical Use | Key Specification Notes |
|---|---|---|---|
| Straight Standard Knurl | Diagonal lines or pattern with leader | Grips, dash handles | Specify pitch, depth, and start point |
| Diamond Standard Knurl | Angled cross-hatch pattern | Tool handles, medical devices | Angle orientation and surface finish |
| Helical or Spiral Knurl | Helical lines with arrow | Sealing grooves, drive features | Direction, pitch, and depth control |
| Fine Braid or Serration Knurl | Fine zigzag or braided pattern | Thin-walled assemblies, connectors | Controlled tooth form and stress limits |
Standard Knurl Symbol and Basic Callout
On a drawing, a knurl is typically shown with a revolved or aligned sectional view and a pattern line. The leader line attaches to the cylindrical surface and may carry a note such as "Standard Knurl per ASME Y14.35M." Use enough section views to clearly show tooth form and engagement length.
Specify whether the knurl is rolled or cut, as this affects tolerance and stock allowance. Include a note for stock removal direction, blank size, and any undercut requirements that prevent interference during forming.
Defining Dimensions, Pitch, and Diameter
Tooth Geometry and Major Diameter
Define the major diameter of the knurl based on the finished outside dimension. Note the preferred blank diameter, typically 0.025 to 0.075 mm smaller than the major diameter for rolled knurls.
Pattern Pitch and Axial Spacing
Specify the axial pitch, the distance between corresponding points on adjacent teeth. Standard pitches range from 0.4 to 6 mm, and tighter tolerances are required for small, fine-pitch knurls.
Radial Depth and Form Control
State the radial depth, measured from the cylinder surface to the bottom of the groove. Provide a maximum material condition note if consistent engagement is critical for function or tooling alignment.
Surface Finish, Tolerances, and Notes
Surface Requirements and Roughness
Indicate the surface roughness for the flanks of the teeth and for the valley surfaces separately. Rolled knurls often call for a smoother valley finish to reduce stress concentration, while cut knurls may show machine marks.
Geometric Tolerances and Runout
Apply runout or circularity controls when knurl alignment affects concentric features. Include a pattern tolerance if indexing is important, and clarify whether the knurl is freehand or referenced to datums.
Material and Process Notes
Specify the material condition, such as normalized, annealed, or pre-hardened, since forming pressure depends on hardness. Add a note about lubrication, tooling type, and whether the knurl is formed in a single pass or multiple passes.
Representation in Sections and Exploded Views
Use aligned sectional views to show the full tooth form without distortion. In assembly drawings, an exploded view can illustrate proper engagement depth and interference-free assembly with mating parts.
Avoid crowding dimension lines near the knurl pattern; instead, place general notes and surface requirements outside the pattern region. Maintain consistent arrow directions and leader paths for clarity across multiple detail and assembly drawings.
Key Takeaways for Clear Knurl Definition on Drawings
- Select the correct knurl type and match the symbol to the intended function, such as grip, drive, or sealing.
- Specify major diameter, blank size, pitch, and radial depth with preferred plus and minus tolerances.
- Include surface roughness for valleys and flanks, plus material condition and forming notes.
- Use sectional views and aligned leaders to prevent crowding and improve readability.
- Apply datum references and pattern tolerances when indexing, concentricity, or assembly behavior is critical.
FAQ
Reader questions
How should I dimension a fine-pitch standard knurl to avoid crowding?
Use a leader line from the major diameter to a nearby extension line and place the pitch dimension outside the pattern. Provide a general note for overall length and tooth count instead of repeated individual dimensions.
What is the proper way to specify a rolled versus a cut knurl?
State the manufacturing process explicitly, for example "Rolled Standard Knurl, 1.5 mm pitch, per finish callout," and include a note about undercuts and stock hardness to control forming behavior.
Can I apply a profile tolerance to the entire knurl pattern?
Yes, a profile tolerance can be applied at a controlled reference, but it must be balanced with material condition and forming limits to avoid impractical inspection requirements.
How do I reference a knurl to datums when indexing is critical?
Define datum references for axis and radial alignment, and add a pattern tolerance callout that controls angular spacing and runout relative to the established datum system.