Stepover Ratio
What is the formula for stepover distance?
Stepover distance = tool diameter × stepover percentage. For example, a 10 mm end mill with a 50% stepover results in a 5 mm lateral offset between passes.
Why does stepover ratio strongly affect surface finish?
The spacing between passes determines the height of residual scallops left on the surface. Smaller stepover reduces scallop height and improves finish, but requires more passes and longer cycle time.
What is a typical starting stepover ratio for general CNC work?
A common starting point is 10–20% of tool diameter for general work. For roughing, increase toward 40–60%; for cosmetic or high-precision finishing, decrease to 5–10%.
Stepover ratio is the lateral offset between adjacent toolpaths, expressed as a fraction or percentage of tool diameter. In CNC milling, it equals the radial depth of cut (RDOC). This ratio determines how much the cutter overlaps each pass, directly influencing surface finish, cycle time, and material removal rate.
On the shop floor, stepover ratio governs practical toolpath spacing. A 50% ratio on a 10 mm end mill offsets passes by 5 mm. For roughing, ratios of 40–60% remove material quickly but leave rough scallops. Finishing passes typically use 5–10% to achieve smooth surfaces and minimize post-machining work. In ball-nose finishing, stepover directly controls scallop height on contoured surfaces; reducing it improves finish but exponentially increases machining time. The ratio must be coordinated with tool deflection, cutter stickout, and stepdown—aggressive radial engagement can worsen deflection even with correct programmed geometry. In nested-based manufacturing like raised panels or MDF finishing, low stepover ratios hide raster marks and reduce sanding. For rough profiling or pocket clearing in cabinet parts, higher ratios prioritize throughput. Using the same ratio across different cutter geometries leads to poor results because a ball nose, flat end mill, and V-bit respond differently to the same lateral offset.
Visible scallop lines: Too large a stepover on a finish pass leaves uncut material between toolpaths, causing poor surface gloss and out-of-spec cosmetic finishes that require extra manual sanding.
Wasted cycle time: Running an excessively small stepover in production roughing consumes machine time with negligible improvement in finish, raising costs without proportional benefit.
Inconsistent results: Applying the same stepover ratio across different cutter geometries overloads some tools or fails to produce the intended finish, since ball noses, flat end mills, and V-bits behave differently.