Knurl Callout: What to Put on the Drawing
What a knurl callout needs on a drawing: pattern, pitch, pre-knurl diameter and extent, plus the extra diameter limits a press-fit knurl needs to work.
A knurl callout is often left as a single word on a drawing, which leaves the machinist to guess the pattern and size.
How Do You Specify a Knurl on a Drawing?
Give the knurl type, the pitch, the diameter before knurling, and the location and length of the knurl. If the knurl is for a press fit, also give the toleranced diameter before knurling and the minimum diameter after knurling.
An r/Machinists user asked for help adding knurl specs to a drawing, and it's a common gap. In a separate r/Machinists drawing critique, one reply said the knurl doesn't really need to be specified, but suggested a note saying the depth and style don't matter. Another said medium knurls usually suit parts that are turned by hand. Either way, the drawing should say something.
What a knurl callout should include
According to GD&T Basics, a grip or decorative knurl needs:
- Knurl type: straight, diagonal (left- or right-hand), or diamond
- Pitch: the distance between knurl elements
- Diameter before knurling
- Location and length, if only part of the surface is knurled
Press-fit knurls need more
Knurls are also used for press fits. For those, GD&T Basics says the drawing must also give the toleranced diameter before knurling and the minimum acceptable diameter after knurling, because that sets the press-fit force.
Standards for a knurl callout
- DIN 82 is the German knurling standard. It names each type with a code, for example RAA for straight grooves parallel to the axis and RBL or RBR for left- or right-hand knurls, as APPORO's DIN 82 summary shows.
- ASME B94.6 covers knurling tools sized by diametral pitch instead of teeth per inch, with recommended blank diameters in multiples of 1/64 or 1/32 in., depending on the pitch.
- Don't mix systems in one callout. Pick one pitch convention and stick with it.
Design tips
- Put the knurl on a plain cylindrical diameter. GD&T Basics notes knurling is typically done on a lathe.
- Say whether the knurl is cosmetic or functional, so the shop knows what matters.
- Chamfer the knurled length, as the drawing critique above also suggested for threads and knurls.
Related: chamfer vs fillet on machined edges, spot face vs counterbore callouts and straightness vs flatness callouts.
At DigiForge
Our CNC service covers 3-, 4- and 5-axis milling and turning, and every quote is reviewed by an engineer. Put the full knurl callout on the drawing, and if your part needs a knurl, ask us when you quote.
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