Basics of CNC rotary engraving

CNC rotary engraving is a subtractive process. A cutting tool removes material from the surface of a part while the part itself turns on a rotary axis. On a flat engraving job the tool moves in X and Y across a stationary plate. Here the fourth axis becomes the primary positioning system, so any feature you program can be wrapped around a cylinder instead of laid flat on it.

The rotary axis indexes to a programmed angle, then the tool feeds along the part. A 10 mm long line of text on a Ø50 mm shaft is not a straight line in machine coordinates. It is an arc, and the CAM software unwraps it before the toolpath is generated. If the unwrapping is wrong, letter spacing drifts apart at the edges of the arc.

That single difference explains most of the practical rules in this article. Depth control, fixturing and feature spacing all follow from the fact that the part surface is not a fixed plane.

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  Fourth axisWork rotates, tool stays on a fixed radial path.
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  Wrapped toolpathFlat artwork is converted to cylindrical coordinates.
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  Constant surface speedFeed is adjusted as diameter changes.

It starts with vector data. Suppliers usually send .step, .iges or .dxf files, and we check the engraving layer for closed contours, line weight and text size. If the artwork was drawn for a flat screen and then wrapped onto a cylinder, lettering can compress at the sides. We will flatten the design, scale the text, and confirm the character height before cutting.

Next comes workholding. A cylindrical part goes into a collet, a three-jaw chuck or a custom soft jaw. Irregular parts sit in a machined cradle that matches the outer profile. The part must run true, because a 0.05 mm runout on a Ø40 mm shaft shifts the engraved depth by the same amount across one rotation. We indicate the part before the first cut.

A test pass on a scrap piece of the same material sets the depth. For most metals we aim for 0.05–0.2 mm of cut depth, which gives readable contrast without weakening thin walls. The tool then follows the wrapped path, and the part indexes to any additional angular positions. A final inspection checks depth, character height and spacing.

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  File prepClose contours, size text, unwrap for the cylinder.
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  SetupIndicate runout under 0.05 mm before cutting.
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  First cutTest depth on scrap of the same material.
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  InspectionDepth, height and spacing checked before shipment.

Aluminium is the easy case. 6061 and 7075 cut cleanly with a sharp single-flute cutter, and the engraved groove stays crisp after anodizing. Clear anodize leaves a slightly darker cut line, which is usually enough contrast for scale marks. Hardcoat anodize hides the mark more, so we often deepen the cut or switch to laser marking.

Stainless steel behaves differently. Grades 303 and 304 work with carbide cutters and slower feeds, but 316 and 17-4PH work-harden at the cut. Light passes and a sharp tool matter more than speed. On titanium, especially Ti-6Al-4V, the same work-hardening applies, and the engraved area can hold heat. We keep depth shallow and use coolant.

Plastics are more forgiving on force but less forgiving on burrs. POM and ABS leave a clean edge, while PC and PMMA can chip or craze around the cut. Engraving on PMMA is possible but a laser usually gives a better optical result. Soft materials like PP and HDPE tend to smear, so we increase the cut depth and reduce spindle speed.

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  Best contrastAluminium, brass, anodized finishes.
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  Watch work-hardening316 stainless, 17-4PH, titanium.
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  Prone to burrsPC, PMMA, soft polyolefins.

Rotary engraving works across a wide diameter range. We have cut features on parts as small as Ø3 mm, such as surgical tool shafts, and on industrial rollers up to Ø500 mm. The practical limit is not the machine envelope alone. It is whether the part can be held true and indexed without deflection.

Positional accuracy on a wrapped feature depends on the rotary table and the setup. With an indicated part and a rigid fixture we hold ±0.005 mm on diameter features and maintain character spacing across a full rotation. Longer parts are more sensitive. A Ø20 mm shaft 300 mm long can flex slightly under cutting load, which shows up as a depth variation on the far end.

When the feature must run the full length of a long part, we sometimes engrave in multiple indexes and blend the passes. When the part is short and the artwork is dense, the rotary axis can index once and the tool does all the work. Both approaches are normal. The choice depends on length-to-diameter ratio and how tight the spacing tolerance is.

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  Small endØ3 mm shafts, surgical and instrument parts.
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  Large endØ500 mm rollers on a Ø400 mm rotary table.
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  Tolerance±0.005 mm on diameter with a rigid setup.

The common thread is traceability. A part that must carry a serial number, a date code, a torque value or a scale mark is a candidate. Shafts, fittings, valve bodies, instrument housings and motor cans all fall into this group. The mark is cut into the metal, so it survives cleaning, handling and most coating steps.

Aerospace and medical work often adds a second requirement: the mark must not create a stress riser. We control depth and corner radius on the engraved contour for those parts, and we can move the marking to a non-critical surface if the drawing allows. Medical device work also tends to require documented inspection, which fits our 100% inspection before shipment.

Automotive and EV parts usually need high volume and repeatable placement. Once the fixture and program are proven, the same angular position repeats across a run from one prototype to 10,000+ parts. There is no minimum order quantity on our side.

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  TraceabilitySerials, date codes, lot numbers on curved surfaces.
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  Functional marksScale marks, torque values, alignment lines.
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  RepeatabilitySame angular position across a production run.