How do you calculate CNC step-over?

Step-over = cutter diameter x radial engagement. A 0.5 in end mill at 40% engagement steps 0.2 in (5.08 mm) per pass, so 2 in of radial stock takes 2 / 0.2 = 10 passes, the Step Over calculator's own example. When the stock is not a whole number of step-overs, round the passes up: 1 in at 0.15 in takes 7 passes, the last one 0.1 in wide.

How do you calculate scallop height for a ball-nose cutter?

Scallop height = R - sqrt(R^2 - (step-over / 2)^2), where R is the ball radius. A 0.5 in ball nose (R = 0.25 in) at a 0.06 in step-over leaves 0.0018 in (1.81 thou) on a flat surface. Halving the step-over cuts the scallop to about a quarter and doubles the passes.

What step-over meets a scallop limit?

Step-over = 2 x sqrt(R^2 - (R - h)^2) for a scallop height h. In the example the 0.125 in blend ball needs 0.0087 in instead of 0.01 in to meet its 0.15 thou (0.00015 in) limit: 139 passes instead of 120, about one more minute at 45 in/min.

Can I work in millimetres?

Yes. Choose mm on Setup and type every diameter, stock, pass length and feed in mm; scallop heights are then in µm. Step-over is always shown in both units with the exact 25.4 mm per inch, so a 5 mm step-over also reads 0.1969 in.

Is the cutting time the full cycle time?

No. It counts feed moves only: passes x pass length / feed. Add rapids, tool changes, accelerations and dwells from your CAM simulation or a timed first part for the full cycle; the Cycle Time Tracker template records it.

Does the workbook check chip load or spindle power?

No. Engagement and feed come from your tooling supplier and CAM program. Use the CNC feed rate and depth of cut calculators for chip thinning and spindle power.

Production and Throughput

CNC Step-Over and Passes Excel Template

Step-over = cutter diameter × radial engagement, and passes = radial stock ÷ step-over, rounded up. Give each milling operation a row and see its step-over in inches and mm, its passes, final pass and cutting time, and for ball-nose finishing the scallop height against your limit.

Free, and a plain .xlsx with no macros to enable. A CSV with live formulas and a printed how-to guide come with it.

Overview

This CNC step-over template is a working Excel pass planner for CNC programmers, machinists and estimators. Each milling operation is one row with its cutter, radial engagement, radial stock, pass length and feed; the workbook works out the step-over, the passes across the stock, the width of the final pass, the cutting time and, for ball-nose cutters, the scallop height. It ships with eight operations on a fictional pump housing, so every tile and chart is filled in when you open it.

In the example the part takes 577 passes and 41.5 minutes of cutting at the programmed feeds. The sealing-face finish, a 0.25 in ball nose at 4% engagement, is 0.01 in per pass: 300 passes and 20.0 minutes, almost half the total, for a scallop of 0.10 thou (0.0001 in). The blend pass, a 0.125 in ball nose at 8%, also steps 0.01 in but leaves a 0.20 thou scallop, above its 0.15 thou limit, so the Dashboard flags it.

Step-over and passes agree with the Step Over calculator for every operation. The download is a plain .xlsx with no macros. A CSV with live formulas is also included for other tools; the dashboard, charts, dropdowns and self-check are in the .xlsx.

What this template includes

Operations table, one row per milling operation: cutter type and diameter, radial engagement, radial stock, pass length, feed rate and an optional scallop height limit in thou or µm, with a Check column
Per operation: step-over in your unit and in both inches and millimetres, passes required, final pass width, cutting time, and for ball-nose cutters the scallop (cusp) height with a status in words
Inch or millimetre entry chosen on Setup, with the exact 25.4 conversion
Dashboard with four tiles (cutting time per part, passes, operations over their scallop limit, longest operation), charts of cutting time and passes by operation, and the operation table
Method sheet with every formula, and a Verify sheet that re-checks the formulas against an independent reference and the Step Over calculator (Self-check: PASS)
CSV with live formulas for the Operations table; the dashboard, charts, dropdowns and self-check are in the .xlsx

Suggested use case

Use it when you program or quote a milled part, to see where the cutting time goes, or to pick a ball-nose step-over that meets the finish without wasting hours on passes you do not need.

How to use it

On Setup, type the shop and part, and choose inch or mm for every dimension.
On Operations, give each milling operation a row: cutter type and diameter, radial engagement, radial stock, pass length and programmed feed.
For ball-nose finishing rows, type the scallop height limit your finish needs.
Fix anything the Check column names.
Open the Dashboard: cutting time and passes by operation, and any finishing pass whose scallop is above its limit.

Frequently Asked Questions

How do you calculate CNC step-over?: Step-over = cutter diameter x radial engagement. A 0.5 in end mill at 40% engagement steps 0.2 in (5.08 mm) per pass, so 2 in of radial stock takes 2 / 0.2 = 10 passes, the Step Over calculator's own example. When the stock is not a whole number of step-overs, round the passes up: 1 in at 0.15 in takes 7 passes, the last one 0.1 in wide.
How do you calculate scallop height for a ball-nose cutter?: Scallop height = R - sqrt(R^2 - (step-over / 2)^2), where R is the ball radius. A 0.5 in ball nose (R = 0.25 in) at a 0.06 in step-over leaves 0.0018 in (1.81 thou) on a flat surface. Halving the step-over cuts the scallop to about a quarter and doubles the passes.
What step-over meets a scallop limit?: Step-over = 2 x sqrt(R^2 - (R - h)^2) for a scallop height h. In the example the 0.125 in blend ball needs 0.0087 in instead of 0.01 in to meet its 0.15 thou (0.00015 in) limit: 139 passes instead of 120, about one more minute at 45 in/min.
Can I work in millimetres?: Yes. Choose mm on Setup and type every diameter, stock, pass length and feed in mm; scallop heights are then in µm. Step-over is always shown in both units with the exact 25.4 mm per inch, so a 5 mm step-over also reads 0.1969 in.
Is the cutting time the full cycle time?: No. It counts feed moves only: passes x pass length / feed. Add rapids, tool changes, accelerations and dwells from your CAM simulation or a timed first part for the full cycle; the Cycle Time Tracker template records it.
Does the workbook check chip load or spindle power?: No. Engagement and feed come from your tooling supplier and CAM program. Use the CNC feed rate and depth of cut calculators for chip thinning and spindle power.