CNC Machining Gong Pattern: How a 1 mm R0.5 Ball Tool Cuts It

• 1
  What CNC Machining Gong Pattern Means on the Floor
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  Scallop Height: The Number That Controls the Pattern
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  Speeds, Feeds and Tool Deflection at 1 mm Diameter
• 4
  Why Simultaneous 5-Axis Changes the Result
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  How to Inspect and Specify the Pattern
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  Setting Up a Gong Pattern Cut, Step by Step
• 7
  When to Use a Gong Pattern and When Not To

The name comes from the tool, not the part. CNC machining gong pattern work uses a ball nose cutter whose tip radius is 0.5 mm, so the cutting diameter is 1 mm. In a five-axis program the tool tip sweeps a series of shallow arcs across the surface. The arcs overlap like the rings on a gong, and that overlap is what you see under raking light.

Two numbers define the cut. The first is the tip radius, R0.5. It sets the smallest inside corner the tool can reach and the theoretical scallop height between passes. The second is the stepover, the distance between two adjacent passes. Stepover and tip radius together decide whether the surface reads as a deliberate pattern or as a machining defect.

This is not a decorative process. Engineers specify a gong pattern when a surface needs a controlled texture, when a shallow curved form has to be machined in one setup, or when a mold cavity needs a known, repeatable finish before polishing. The arcs are a byproduct of the geometry, not an add-on.

The process only makes sense on certain parts. Thin walls, deep narrow slots and sharp external corners are the wrong fit. A 1 mm cutter has almost no core stiffness, so it deflects before it breaks. If the drawing calls for a corner radius under 0.5 mm, this tool cannot produce it and the feature has to be redesigned or cut by EDM.

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  ToolBall nose, 0.5 mm tip radius, 1 mm cutting diameter
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  Motion3-axis for flat or gently curved faces, 5-axis for steep walls
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  Typical finishRa 0.8–1.6 μm after a clean finishing pass

Every ball tool leaves a ridge between two passes. The height of that ridge is the scallop height, and it is a pure function of tip radius and stepover. For a 0.5 mm tip radius, a 0.05 mm stepover leaves roughly 0.6 μm of scallop. Push the stepover to 0.1 mm and the scallop climbs to about 2.5 μm. The arithmetic is unforgiving because radius appears in the denominator.

That is why a gong pattern looks different on two nominally identical parts. If the CAM programmer used a 0.08 mm stepover on one job and 0.15 mm on the next, the visual difference is obvious even though both are called finishing passes. Buyers who care about appearance should put a stepover value or a scallop height limit on the drawing, not just an Ra callout.

Scallop height and Ra are related but not interchangeable. Ra averages the profile; scallop height describes the worst ridge. A surface can hit Ra 0.8 μm and still show visible banding if the ridges are regular and aligned. Regularity is what makes a gong pattern read as a pattern rather than as noise.

On curved and sloped faces the effective stepover changes with the surface normal. Where the wall tilts toward the tool, adjacent passes crowd together and the scallop shrinks. Where the wall tilts away, they spread apart. Five-axis tool orientation corrects this by keeping the tool tip normal to the surface, which is the main reason the process is run on simultaneous 5-axis centers rather than 3-axis mills.

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  0.05 mm stepoverAbout 0.6 μm scallop, near-polish appearance
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  0.10 mm stepoverAbout 2.5 μm scallop, visible under raking light
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  0.20 mm stepoverAbout 10 μm scallop, clearly textured

A 1 mm carbide ball tool in aluminium 6061 runs at 12,000–18,000 rpm on a spindle that can hold those speeds without chatter. Feed per tooth lands between 0.01 and 0.03 mm. Depth of cut for finishing stays at 0.05–0.15 mm. These are conservative numbers on purpose. At this diameter the tool shank is the weak link, and a broken 1 mm cutter in a half-finished cavity costs far more than the cycle time you saved.

Deflection, not breakage, is the first failure mode. A 1 mm tool with 20 mm of gauge length bends measurably under normal cutting force. The symptom is a taper on the wall or a pattern that fades toward the bottom of a pocket. The fix is to shorten the gauge length, use a shrink-fit or hydraulic holder, and take the finishing pass in one continuous motion instead of stopping and restarting.

Heat is the second issue. Small tools have little material to carry heat away, so the chip has to take it. If feeds are too low, the tool rubs instead of cutting, the edge rounds over, and the surface turns dull and smeared within one part. Raising feed per tooth usually improves the finish on aluminium, which surprises people used to larger cutters.

Harder materials change the picture. In 17-4PH stainless or Ti-6Al-4V, rpm drops to 6,000–9,000 and feed per tooth to 0.005–0.015 mm. Tool life becomes the limiting factor and the pattern is cut in two or three passes with a fresh edge. In hardened tool steel above 45 HRC, a 1 mm ball tool is usually the wrong choice and the pattern should be cut before heat treatment or finished by EDM.

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  Aluminium 606112,000–18,000 rpm, 0.01–0.03 mm per tooth
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  Stainless 17-4PH6,000–9,000 rpm, 0.005–0.015 mm per tooth
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  Titanium Ti-6Al-4V6,000–9,000 rpm, light radial engagement, flood coolant

On a 3-axis mill the tool axis is fixed. The tip of a ball tool therefore meets a sloped surface at an angle, the contact point moves off center, and the effective cutting speed at the tip drops toward zero. The result is a pattern that is wider on steep walls and narrower on flats, plus a band of poor finish where the tool rubs.

Simultaneous 5-axis machining keeps the tool axis tilted so the tip contacts the surface near its center and the surface speed stays even. On a curved form this produces a consistent arc width across the whole face. It also lets the programmer lean the tool into the cut direction to avoid the zero-speed point entirely, which is the single biggest finish improvement on this type of work.

The trade-off is programming time. A 5-axis finishing path on a small curved surface can take several hours to generate and verify, especially when collision checking against the holder is included. For a one-off part with a simple flat face, a 3-axis path is cheaper and just as good. The 5-axis advantage only shows up on steep walls, deep pockets and compound curves.

Tool holders matter as much as the machine. A 1 mm ball tool in a long slim holder will chatter on a 5-axis path no matter how good the motion is. We keep the gauge length as short as the geometry allows and prefer shrink-fit holders for this diameter range, where runout has a direct effect on whether the pattern looks even.

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  3-axisFlat faces, gentle curves, simple one-off parts
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  Simultaneous 5-axisSteep walls, compound curves, molds, repeated parts

A gong pattern is judged in two ways: dimensionally and visually. Dimensionally, you check the corner radii and the wall profile against the drawing. Visually, you check whether the arcs are even and free of steps, chatter marks and dwell lines. Both checks are needed, because a part can measure in tolerance and still look wrong under a low-angle light.

The most useful specification is a scallop height limit, written as a maximum in micrometres. A stepover value also works, but it forces the shop to use your number even when a different stepover would give the same result faster. An Ra callout alone is the weakest option because it averages the profile and hides regular banding.

For appearance-critical surfaces, add a note about the light direction used for inspection, or supply a physical sample as the finish reference. This removes the argument about whether a surface is acceptable. Ambiguity on texture is one of the most common reasons a finishing job gets rejected at incoming inspection.

We inspect finished surfaces before shipment. Reports are available on request, and a first article can be checked against a supplied reference sample. If the pattern has to match an existing part, send that part as well as the drawing so the scallop direction and spacing can be compared directly.

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  Best calloutMaximum scallop height in μm
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  Acceptable calloutStepover value plus Ra range
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  Weakest calloutRa alone, with no texture reference