Calculate bore milling parameters: feed rate adjustments, ramp angle per revolution, and helical path geometry for interpolated holes.
Calculate ↓For programming helical interpolation toolpaths on machining centers.
A shop was interpolating a 25 mm through-bore in 304 stainless using a 12 mm 4-flute end mill at 1,200 mm/min feed. The tool chattered on every bore, producing a 15 μm surface finish. The CAM defaulted to 0.7 mm Z per rev. By reducing Z per rev to 0.35 mm and increasing spindle speed to 5,500 RPM (calculated from effective cutting diameter), the finish improved to 2.5 μm Ra and tool life doubled. The cycle time increased by 12% — an acceptable trade-off for eliminating secondary reaming.
In helical interpolation, the cutting speed varies across the tool diameter. The outer edge of the tool (closest to the bore wall) cuts at the programmed Vc. The inner edge (closest to the bore center) cuts at a lower speed proportional to the ratio of the inner radius to the outer radius. This means the chip load is not uniform across the cutting edge — the inner edge produces thinner chips, which increases the specific cutting force locally.
For helical paths on a Mazak or Okuma, reduce the feed rate by 10-20% compared to linear feed at the same chip load to account for this uneven chip distribution. The feed rate adjustment is most significant when the bore diameter is less than 2× the cutter diameter. For bores over 3× cutter diameter, the effect is minor and can be ignored.
What is helical interpolation? A G02/G03 circular interpolation move with simultaneous Z-axis movement, creating a helical toolpath. Used for bore milling, thread milling, and spiral entry.
How do you calculate feed rate for helical milling? Feed at the cutter = RPM × flutes × fz. The Z-axis feed = feed × tan(ramp angle). Use the calculator above for exact values.
What is a safe Z per revolution for helical interpolation? 0.2-0.8 mm per revolution depending on material and tool diameter. Lower values for stainless and titanium; higher values for aluminum. The maximum Z per rev should never exceed the tool's corner radius.
When should I use helical interpolation instead of drilling? For bores over 20 mm diameter, helical milling with a standard end mill is more versatile than using a dedicated drill or boring head. The tool can machine multiple bore sizes, and the process is not affected by coolant pressure or chip packing.