How do you calculate the maximum depth of cut from spindle power?

Divide the power at the cut (spindle power x drive efficiency) by the power each inch of depth needs (unit power x radial depth x feed). With 7.5 hp at 80%, a unit power of 0.3, 0.25 in radial and 96 in/min: 6 / 7.2 = 0.833 in, removing 20 in³/min at 100% spindle load, the Depth of Cut calculator's own example.

The power it takes to remove one cubic inch of a material per minute, in hp per in³/min. It depends on the material and its hardness and is given by tooling suppliers and machining handbooks. The workbook holds no unit power data; the example values are invented, so use your supplier's.

Why does the usable depth stop at the flute length?

Because the cutter cannot cut deeper than its flutes, however much power is left. In the example the 0.5 in end mill on VMC-1 could drive 1.667 in in aluminium but has 1 in of flute, so 1 in is usable at 60% of the spindle.

What does the spindle load at the programmed depth tell me?

The share of the spindle's power the depth in your program needs: programmed depth x cutting power per inch / efficiency / spindle power. Above your limit the operation is Near capacity; above 100% it is Over capacity and will stall or slow the spindle. In the example the rough core at 0.6 in on the 7.5 hp machine needs 135%.

Can I use metric units?

The workbook works in inches and horsepower, like the Depth of Cut calculator. Convert before typing: 1 kW = 1.341 hp, 25.4 mm = 1 in, and unit power in kW per cm³/min x 21.98 gives hp per in³/min.

Does it check chip load or tool deflection?

No. It checks power and flute length only. Use the CNC feed rate calculator for chip load and chip thinning, and your tooling supplier's data for torque, rigidity and deflection limits.

Production and Throughput

CNC Depth of Cut and Spindle Power Excel Template

Power-limited axial depth = spindle power × drive efficiency ÷ (unit power × radial depth × feed rate), and the usable depth is the smaller of that and the flute length. Give each milling operation a row and see its usable depth, removal rate, and the spindle load your programmed depth asks for.

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 depth of cut template is a working Excel workbook for CNC programmers, machinists and manufacturing engineers. Each milling operation is one row with its machine, material, radial depth, feed, flute length and programmed axial depth; the workbook gives the deepest pass the spindle can drive, the removal rate at that depth and the spindle load the program asks for. It ships with eight operations of a fictional mould shop on three machines, so every tile and chart is filled in when you open it.

Every inch of axial depth needs unit power x radial depth x feed. A 0.75 in end mill roughing 4140 at 0.375 in radial and 30 in/min needs 13.5 hp per inch of depth: on the 15 hp VMC-1 at 80% efficiency that allows 0.889 in, but the same operation moved to the 7.5 hp VMC-2 allows only 0.444 in, and the programmed 0.6 in asks for 135% of its spindle.

Four of the eight operations are limited by spindle power rather than flute length, and one is Over capacity. 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: machine, material, tool, radial depth, feed rate, flute length and the axial depth the program uses, with a Check column
Per operation: spindle power, drive efficiency and unit power looked up from Setup, cutting power per inch of depth, power-limited and usable axial depth, which limit binds, removal rate, and spindle load at the usable and the programmed depth with a status in words
Setup lists of machines (spindle power and drive efficiency) and work materials (unit power from your tooling supplier), and your spindle load limit
Dashboard with four tiles (operations over the spindle, highest load at programmed depth, power-limited operations, operations), programmed against usable depth, spindle load against your limit, and the operation table
Method sheet with every formula, and a Verify sheet that re-checks the formulas against an independent reference and the Depth of Cut 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 before moving a job to a smaller machine, when a program stalls the spindle or chatters, or to plan roughing depths that use the machine's power without overloading it.

How to use it

On Setup, type the shop and job and your spindle load limit.
List your machines with their spindle power and drive efficiency, and your work materials with the unit power your tooling supplier gives.
On Operations, give each milling operation a row: machine, material, tool, radial depth, feed rate, flute length and the axial depth the program uses.
Fix anything the Check column names.
Open the Dashboard: operations that ask more than the spindle can give, the usable depth of every operation and its spindle load.

Frequently Asked Questions

How do you calculate the maximum depth of cut from spindle power?: Divide the power at the cut (spindle power x drive efficiency) by the power each inch of depth needs (unit power x radial depth x feed). With 7.5 hp at 80%, a unit power of 0.3, 0.25 in radial and 96 in/min: 6 / 7.2 = 0.833 in, removing 20 in³/min at 100% spindle load, the Depth of Cut calculator's own example.
What is unit power?: The power it takes to remove one cubic inch of a material per minute, in hp per in³/min. It depends on the material and its hardness and is given by tooling suppliers and machining handbooks. The workbook holds no unit power data; the example values are invented, so use your supplier's.
Why does the usable depth stop at the flute length?: Because the cutter cannot cut deeper than its flutes, however much power is left. In the example the 0.5 in end mill on VMC-1 could drive 1.667 in in aluminium but has 1 in of flute, so 1 in is usable at 60% of the spindle.
What does the spindle load at the programmed depth tell me?: The share of the spindle's power the depth in your program needs: programmed depth x cutting power per inch / efficiency / spindle power. Above your limit the operation is Near capacity; above 100% it is Over capacity and will stall or slow the spindle. In the example the rough core at 0.6 in on the 7.5 hp machine needs 135%.
Can I use metric units?: The workbook works in inches and horsepower, like the Depth of Cut calculator. Convert before typing: 1 kW = 1.341 hp, 25.4 mm = 1 in, and unit power in kW per cm³/min x 21.98 gives hp per in³/min.
Does it check chip load or tool deflection?: No. It checks power and flute length only. Use the CNC feed rate calculator for chip load and chip thinning, and your tooling supplier's data for torque, rigidity and deflection limits.