CNC machining time calculator

A feature-based estimate adds up the time for each kind of work. Roughing time is the volume removed divided by a removal rate; finishing time is the area finished divided by a finishing rate; holes take a fixed time each plus their depth; every setup adds a load and unload. Harder materials divide both rates by how much slower they cut than 6061 aluminum.

The milling formula, in minutes per part

(part.removed_volume_in3 - part.narrow_volume_in3) / (6 * material.machinability * part.tool_scale ^ 1.5) + part.narrow_volume_in3 / (min(6, 100 * part.narrow_tool_in ^ 3) * material.machinability) + (part.machined_area_in2 - part.thin_wall_area_in2) / (8 * material.machinability * part.tool_scale) + part.thin_wall_area_in2 / (3 * material.machinability) + part.holes_count * 0.3 + part.tapped_holes * 0.4 + part.holes_depth_in / 10 + part.setups * 0.75

tool_scale is 1 for parts up to 10 in across, rising to 2 at 20 in: bigger parts get bigger tools, so they rough and finish faster per cubic inch. Gaps narrower than 3/8 in (narrow_volume_in3) are slotted at the rate of the end mill that fits, 100 × its diameter³ in³/min, and walls thinner than .100 in that stand at least four times their thickness (thin_wall_area_in2) are finished in shallow steps at 3 in²/min. QuotedPart measures both from the STEP model; the calculator above leaves them at zero.

The turning formula

part.removed_volume_in3 / (12 * material.machinability) + part.bore_area_in2 / (10 * material.machinability) + part.area_in2 / (40 * material.machinability) + part.holes_count * 0.3 + part.tapped_holes * 0.4 + part.holes_depth_in / 6 + part.grooves * 0.3 + part.setups * 0.75

These are starting points, not your machines. A rigid 40-taper mill with high-feed tools will beat them; an older machine or a long reach won't. Change the constants to match your own cycle times.

Default removal rates by material

For parts up to 10 in across, from the formulas above.

From run time to price

Multiply the run time by the machine's hourly rate, add setup and programming once per lot, material and outside processing, then markup. Work out the rate with the shop hourly rate calculator, price a whole part with the machining cost estimator, or put it all in the CNC quote template.