CNC Knurling: How Grips, Press Fits and Textures Are Formed

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  What CNC knurling actually does to the surface
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  How pitch, diameter and face width are chosen
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  Which materials form well and which fight back
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  Speeds, feeds and the mistakes that show up first
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  Where knurling helps and where it hurts
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  Forming roll vs cutting knurl vs milled pattern

Knurling is not a cutting operation in the usual sense. A hardened roll with a patterned face is pushed into the rotating workpiece, and the metal flows into the space between the teeth. The result is a raised diamond, straight, or helical pattern. With a manual lathe the operator feeds by hand and watches the pattern form. On a CNC lathe or mill-turn center, the feed, spindle speed, and depth are all programmed, so the same pattern repeats on part one and part ten thousand.

Because the material is displaced rather than removed, the outer diameter grows during knurling. A blank turned to Ø20.00 mm may finish at 20.2 mm, depending on the pattern and how deep the roll sinks. That growth is the reason press-fit and thread dimensions sit outside the knurled band, not inside it. Grip is the usual goal, but the pattern also adds a light interference surface that holds a bushing or insert in place without adhesive.

Two families of tooling cover almost every job. Forming rolls cold-work the surface and leave a rounded crest. Cutting knurls have teeth that shear the pattern into the part and leave a sharper edge with a small chip load. The second option needs less radial force, which matters on thin-wall tubes and on long shafts held between centers.

The pattern itself comes in three common geometries. Straight knurls run parallel to the axis and resist axial slipping. Diamond knurls cross at an angle and give the best finger grip in both directions. Helical knurls sit between the two and are often used when the part must also turn or thread into a mating piece.

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  Material grows, it does not shrinkPlan the pre-knurl diameter so the finished OD lands in tolerance.
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  Forming vs cuttingForming is quieter and stronger; cutting suits thin walls and soft alloys.
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  Pattern direction mattersStraight resists pull-off, diamond resists twist, helical does a bit of both.

Pitch is the distance between adjacent teeth, given in teeth per inch (TPI) or in millimeters. Coarse pitches of 8 to 16 TPI make a deep, aggressive grip that suits hand knobs and adjustment collars. Finer pitches of 20 to 33 TPI produce a shallow pattern that suits small-diameter parts and cosmetic surfaces. A rule that has held up in our shop: the pitch should not exceed roughly one third of the shaft diameter, or the pattern starts to look like a single groove rather than a knurl.

Workpiece diameter sets a hard limit on what a forming roll can do. A roll cannot start cleanly on a diameter much below 6 mm without deflecting the part or chattering. Below that, a cutting knurl or a milled pattern is the practical route. Long, slender shafts need a tailstock or a steady rest, because the radial force of a forming roll will bend anything with a length-to-diameter ratio above about 8:1.

Face width controls how many passes the tool makes. Rolls wider than the pattern area allow a single plunge; narrow rolls require a stepping program across the face, which costs cycle time and can leave a faint seam where passes overlap. For a band wider than 20 mm, we usually program a small step-over of 0.5 to 1 mm per pass to keep the pattern even.

Diametral pitch and roll angle have to mesh. If the roll pitch and the programmed feed do not line up, the second pass lands between the teeth of the first and the pattern smears. The fix is a whole-number relationship between roll revolutions and workpiece revolutions, which is why knurl speeds are often written as a fixed ratio rather than a raw rpm.

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  Rough guidePitch ≈ 1/3 of shaft diameter or finer.
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  Minimum forming diameterAbout 6 mm; below that, cut or mill the pattern.
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  Long partsUse a steady rest above roughly 8:1 length-to-diameter.

Ductile metals form the cleanest knurls. Low-carbon steel such as 1018, 1045, and 12L14 flows into the roll teeth with little tearing. Brass C36000 and copper C110 also knurl easily and hold a crisp crest. Aluminum behaves differently: 6061 and 2024 form acceptably at moderate depth, but soft tempers tend to gall and pick up material on the roll, so a light lubricant and a slightly shallower depth help.

Stainless steel sits in the middle. Grades 303 and 304 form a usable pattern, but they work-harden fast. Once the surface hardens under the first pass, a second pass at the same depth can crack the crest. For 316L and 17-4PH we usually cut the pattern instead of forming it, and we keep the depth conservative. Titanium TC4 (Ti-6Al-4V) is the hardest common case; it galls, it work-hardens, and it springs back, so forming rolls rarely give a clean result.

Plastics need their own approach. POM and PA cut cleanly with a sharp tool, and the pattern stays crisp. ABS and PC tend to smear or stress-whiten at the tooth root. PEEK accepts a shallow cut pattern but is expensive enough that a scrapped part hurts, so we cut a test ring first on any new geometry.

Hardened or case-hardened parts cannot be formed at all. If the surface is above roughly 35 HRC, the roll simply skates. The options there are to knurl before heat treatment, to cut the pattern with a carbide tool, or to use a slip-on sleeve that is knurled separately and pinned in place.

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  Easy1018, 1045, 12L14, C36000 brass, C110 copper.
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  Workable with care6061-T6, 303, 304, POM, PA.
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  Cut, do not form316L, 17-4PH, TC4 titanium, hardened steel above 35 HRC.

Knurl speed is lower than turning speed. Forming rolls run at roughly 60 to 120 surface m/min on steel and 150 to 250 m/min on aluminum. Feed per revolution for a forming roll runs 0.3 to 0.8 mm, and cutting knurls run lighter, around 0.1 to 0.3 mm. Plunge depth is usually set so the finished pattern reaches about 0.25 to 0.4 times the pitch. Deeper than that and the crest starts to fold over instead of standing up.

Coolant matters more than most people expect. A forming roll generates heat and pressure at the contact line, and a dry pass on stainless will often tear the crest. Flood coolant or a heavy-duty cutting oil keeps the roll clean and the pattern even. We also stop the roll briefly at the end of the pass to let the pattern settle, rather than pulling straight out, which can leave a torn edge at the run-out.

The three defects we see most are double-tracking, smeared or flattened crests, and a measurable diameter drift. Double-tracking means the roll is not tracking its own groove, usually because the pitch ratio is wrong or the roll is worn. Smeared crests point to too much depth or too little lubrication. Diameter drift is a setup issue: the pre-knurl diameter was not adjusted for the growth the pattern adds.

Inspection is straightforward. We check the pattern visually against a sample, measure the over-pins or over-wires diameter, and verify the crest height with a profile trace when the customer needs a number. On a run of parts, the pre-knurl diameter is the variable we watch most closely, because a 0.02 mm error there shows up directly in the finished OD.

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  Surface speed60-120 m/min on steel, 150-250 m/min on aluminum.
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  Feed per rev0.3-0.8 mm forming, 0.1-0.3 mm cutting.
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  DepthAbout 0.25-0.4 × pitch; deeper folds the crest.

Knurling earns its place when a surface has to transmit torque by hand. Adjustment knobs, thumb screws, collet nuts, and instrument collars all use a diamond pattern so a bare hand can turn them without a tool. The pattern also adds a thin hardened skin on the surface, which raises wear resistance at the contact points. That is a side benefit, not a substitute for a real heat treatment.

Press fits are the second common use. A straight knurl on a shaft gives a controlled interference surface for a bearing, bushing, or plastic housing. The teeth bite into the softer mating part and stop rotation without a key. This works well in die-cast and plastic housings. It works poorly in hardened bores, where the knurl cannot bite and the joint relies on friction alone.

There are real costs. The pattern breaks the surface finish, so a knurled band is not a sealing surface and cannot be used where a gasket or O-ring must seat. Fatigue life drops because every tooth root is a stress concentrator; on a rotating shaft under bending load, that matters. Corrosion resistance also suffers on stainless, since the cold-worked crest is more active than the base metal and can pit first.

Cosmetic knurling is a different game. Some buyers ask for a fine pattern purely as a decorative finish, and there the priority shifts to uniformity across a large batch. That is where CNC control pays off: the same program, the same depth, the same pitch ratio on every part, so a run of 5,000 handles looks like one part repeated rather than 5,000 hand-fed variations.

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  Good fitHand grips, press-fit shafts, plastic and die-cast housings.
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  Poor fitSealing surfaces, fatigue-critical shafts, hardened bores.
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  Watch corrosionCold-worked stainless crests can pit before the base metal.