CNC Machining Ball Nose Scallop Calculator

A CNC machining ball nose scallop calculator relates the spacing between parallel ball-end toolpaths to the theoretical cusp left on a planar surface. Given effective ball radius and desired height, it finds stepover. In reverse mode it starts with stepover and finds height. An optional finish width converts that spacing into whole intervals and centerline passes.

The geometry is a circle intersected by two equally spaced paths. The midpoint between passes is the highest remaining point, while the path centers define the tangent depth. Because the exact circle equation is used, the result remains valid across the supported range without replacing it with a small-height approximation.

Scallop height is not the same as measured surface roughness. It describes ideal form at one cross-section. Runout, vibration, feed marks, cutter wear, material behavior and measurement filtering can dominate the real texture. Curved surfaces and tilted tools also change effective geometry, so this page stays limited to a planar cross-feed case.

For a target height h below radius R, the half-stepover follows from a right triangle whose hypotenuse is R and vertical side is R minus h. Solving the circle gives full stepover s = 2 times the square root of 2Rh minus h squared.

Reverse mode uses the same circle. Half the entered stepover is the horizontal side, so the vertical distance from the ball center to the intersection is the square root of R squared minus half-stepover squared. Subtracting that distance from R gives cusp height.

For pass planning, finish width divided by requested stepover is rounded upward to whole intervals. Passes equal intervals plus one. The calculator divides the width by that interval count and recomputes cusp height, ensuring the equal covering spacing does not exceed the requested value.

10 mm ball diameter and 0.010 mm cusp: With 5 mm radius, a 0.010 mm target gives about 0.63214 mm stepover. Across 20 mm, 32 intervals and 33 boundary-inclusive passes give 0.625 mm equal spacing, producing a slightly smaller theoretical cusp.

0.5 mm stepover: A 5 mm effective radius with 0.5 mm spacing gives a theoretical planar cusp of about 0.006254 mm. That number does not predict Ra or approve the path on a curved model.

Equivalent inch geometry: A 0.250 in radius and 0.0005 in target height can be entered directly. Exact conversion produces the same physical stepover in metric, while pass count stays the same when finish width is converted consistently.

Confirm whether the entered cutter dimension is diameter or radius and whether the actual contact profile is a full circular ball. Inspect wear and runout. Define finish width between the intended first and last centerlines, rather than assuming it equals the entire part width without considering containment.

Simulate the actual CAM path and inspect transitions, boundary behavior, stock allowance and surface curvature. On steep or curved regions, use CAM or tool-maker methods that account for local contact geometry. Verify the produced surface with the drawing’s specified texture instrument and cutoff settings.

Keep the calculator record with the drawing revision, units, input source and rounding rule. Recheck the result after any change to the tool, stock, setup, work offset, CAM strategy or inspection method. A correct equation can still be applied to the wrong reference feature, so identify the physical planes, axes and dimensions before transferring a number to a setup sheet.

Before machining, review workholding, rigidity, holder projection, runout, tool condition, coolant or lubrication, chip evacuation, machine travel and control behavior where they affect the operation. Prove out through the shop’s approved process and inspect the resulting feature. The calculator documents nominal arithmetic; it cannot observe the machine, material, tool or part.