What is the formula for CNC feed rate?

Feed Rate (IPM or mm/min) = Spindle RPM * Number of Flutes * Chip Load (feed per tooth). For example, 18,000 RPM * 2 flutes * 0.002 inches/tooth = 72 IPM.

What is radial chip thinning and when should you compensate for it?

Radial chip thinning occurs whenever the radial depth of cut (stepover or WOC) is less than 50% of the cutter diameter. Because the cutter engages the material along a shallower arc, the actual chip produced is thinner than the programmed feed per tooth. Increasing feed rate maintains optimal cutting pressure and prevents tool rubbing.

Why do endmills break or melt when cutting acrylic plastic?

Acrylic melts when feed rate is too low relative to RPM (rubbing instead of slicing), generating friction heat. For acrylic, use a polished single 'O-flute' spiral upcut bit, moderate RPM (14,000–16,000), and a healthy chip load (0.003 to 0.005 inches per tooth) to fling chips away before they weld back to the stock.

What is the difference between climb milling and conventional milling?

In climb milling, the cutter rotates in the direction of the feed, starting with the thickest chip and exiting thin. This produces superior surface finish and less tool wear on rigid CNC machines. Conventional milling cuts against the feed direction, which can be safer for machines with significant backlash.

CNC Feeds & Speeds Pro - Speeds, Feeds & Chip Load Calculator

⚙️ Tool & Stock Setup

Workpiece Material

Cutter / Endmill Diameter (in)

Number of Flutes

Target Chip Load (in/tooth)

Spindle Speed (RPM) 18,000 RPM

Depth of Cut (DOC) (in)

Width of Cut / Stepover (in)

⚡ Cutting Prescriptions

38 IPM

Plunge Rate (Ramp In)

1,178 SFM

Surface Speed (ft/min)

1.58 in³/min

Material Removal (MRR)

0.24 HP

Est. Spindle Load

Radial Chip Thinning Adjustment +18% Feed Required

Because stepover (WOC) is under 50% cutter diameter, actual chip thickness is reduced. Increase feed to 149 IPM to maintain tool cutting pressure and prevent rubbing heat.

💡 Machining Tip: For Hardwood with a 1/4" 2-flute upcut bit, 18,000 RPM at 126 IPM delivers crisp chip clearing without burning edges. Use climb milling for cleanest surface finish.

The Physics of CNC Speeds, Feeds & Chip Formation

Machining success on a CNC router or milling machine depends on producing a defined chip rather than dusting or rubbing the material. When a spinning flute engages the stock, it must bite into the material with sufficient thickness (known as Chip Load or Feed per Tooth, IPT) to carry away heat. Running your spindle too fast with too low a feed rate causes friction rubbing, dulling carbide bits in minutes and causing acrylic or aluminum to weld onto the flute.

The Essential Speed and Feed Formulas

        Feed Rate (IPM) = RPM × Flutes × Chip Load (IPT)

        Surface Feet per Minute (SFM) = (RPM × π × Diameter) / 12

        Material Removal Rate (MRR) = DOC × WOC × Feed Rate

Why Radial Chip Thinning Matters for High-Speed Machining (HSM)

When cutting a full slot (100% radial engagement), the maximum chip thickness occurs at the tool centerline and matches programmed feed per tooth. But when performing adaptive clearing or light stepovers (e.g. 10% to 20% cutter diameter), the cutter enters and exits before reaching full thickness. Without increasing feed rate, the tool "burnishes" and rubs against the stock. Compensating for radial chip thinning keeps chip thickness constant, allowing dramatic feed rate increases while dramatically extending tool life.

Frequently Asked Questions

What is the recommended plunge rate for CNC router bits?

Flat endmills are not drill bits and have poor center-cutting geometry. Always set straight plunge rates to 30% to 50% of the horizontal feed rate, or better yet, program a ramping or helical entry in your CAM software (Fusion 360, Vectric VCarve).

How do I choose between 1-flute, 2-flute, and 3-flute endmills?

Single 'O-flute' bits offer maximum chip evacuation space, making them mandatory for plastics (acrylic, HDPE) and high-speed aluminum milling on hobby routers. 2-flute bits are the universal standard for hardwoods, plywood, and MDF. 3-flute and 4-flute bits are best suited for rigid metal milling machines cutting steel or brass.

What are the signs that my feed rate is too low?

Fine powder or dust instead of distinct curly chips, squealing noises, scorched or charred wood edges, and carbide cutters becoming boiling hot to the touch all indicate that feed rate is too low for the current spindle RPM.