How do I calculate CNC feed rate?

CNC feed rate is calculated using the formula: Feed Rate = Spindle Speed (RPM) × Number of Flutes × Chip Load. The chip load varies by material - for example, softwood typically uses 0.011 inches per tooth, while hardwood uses 0.009 inches per tooth, and aluminum uses 0.003 inches per tooth.

What is a safe plunge rate for CNC?

A safe plunge rate is typically 25-40% of your feed rate, depending on the bit type. Upcut bits can plunge at 40% (best chip clearing), straight bits at 35%, compression bits at 30%, and downcut bits at 25% (chips don't clear well when plunging).

What stepover should I use for CNC roughing vs finishing?

For roughing passes, use a stepover of about 45% of your bit diameter for efficient material removal. For finishing passes, use about 15% stepover (10% for ball nose bits) for a smoother surface finish.

What is the difference between upcut and downcut bits?

Upcut bits pull chips upward, clearing material efficiently but may cause tear-out on the top surface. Downcut bits push chips down, leaving a cleaner top edge but requiring shallower passes. Compression bits combine both - upcut on the bottom and downcut on top - ideal for plywood and laminates.

What is chip load and why does it matter?

Chip load is the amount of material each cutting edge (flute) removes per revolution, measured in inches per tooth. Correct chip load prevents tool breakage (too aggressive) and heat buildup (too conservative). It varies by material: softwood ~0.011", hardwood ~0.009", MDF ~0.013", aluminum ~0.003" for a 1/4" bit.

What materials does this CNC calculator support?

This calculator supports 6 materials: Softwood (pine, cedar), Hardwood (oak, maple), MDF, Plywood, Aluminum, and Acrylic. Each material has calibrated chip load ranges based on GDP Tooling guidelines, with values that scale by bit diameter from 1/8" to 1/2"+.

How do I know if my CNC settings are safe?

The calculator includes a Tool Risk Level meter that scores your settings from Safe to Danger based on multiple factors: feed rate vs guidelines, pass depth relative to bit diameter, stepover percentage, chip load range, plunge rate, and aggressiveness level. Keep settings in the green/safe zone when starting out.

How long will a CNC pocket operation take?

Machining time depends on pocket size, material depth, feed rate, stepover, and pass depth. Enter your pocket width and height in the calculator to see an estimated machining time. The formula is: Time = (Pocket Width × Height × Depth) ÷ Material Removal Rate (MRR).

CNC Feeds and Speeds Calculator

Tool & Material

Material

Aggressiveness

Safe Max 1.0×

Bit Type

Number of Flutes

Bit Diameter

Up-Cut Length

V-Bit Angle (degrees)

Taper Angle (degrees)

Cutting Parameters

Spindle Speed (RPM)

Material Depth

Pocket Size (W × H)

Number of Passes

Cut Type

Using Coolant?

Calculated Results

Tool Risk Level Safe

Safe Caution Danger

Settings are within safe limits.

Est. Machining Time --

Pocket Volume ÷ MRR

Program These

Feed Rate ✎

Plunge Rate ✎

Suggested RPM

Pass Depth ✎

Number of Passes

Stepover ✎

Diagnostics

Chip Load ✎

Surface Speed (SFM)

Material Removal Rate

Compare Roughing vs Finishing

Roughing

Feed Rate--

Stepover--

Pass Depth--

MRR--

Finishing

Feed Rate--

Stepover--

Pass Depth--

MRR--

Summary

Tips

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