CNC & Precision Machining

Chip Load (Feed per Tooth, fz)

Chip load, or feed per tooth, is the distance a milling cutter advances during the engagement of each cutting edge. It sets the chip thickness each tooth removes and is the basis for calculating milling feed rates.

For a cutter with z effective teeth turning at n rev/min, the table feed is vf = fz z n, so the feed per tooth fz is the programmed feed divided by teeth and speed. Tool suppliers publish recommended chip loads by tool diameter, geometry and workpiece material. The actual maximum chip thickness also depends on radial engagement and the cutter's entering angle.

When radial engagement is less than half the cutter diameter, the maximum chip thickness becomes smaller than the feed per tooth, an effect called radial chip thinning. Light radial engagement strategies therefore raise the feed per tooth to restore the intended chip thickness. Similarly, cutters with an entering angle below 90 degrees, such as 45-degree face mills, produce thinner chips for the same feed per tooth.

Too small a chip load makes the edge rub instead of cut, generating heat, work hardening and rapid wear, particularly in stainless steels and heat-resistant alloys. Too large a chip load chips or breaks the edge and can overload the spindle. Starting values from supplier data are refined by observing chip shape and colour, tool wear, spindle load and surface finish.

Key points

• Feed per tooth fz = vf / (z x n)
• Light radial engagement thins the chip, so fz is often increased
• Entering angles below 90 degrees also reduce chip thickness
• Too small a chip load rubs; too large a chip load chips the edge

Related terms

• Feed RateCNC & Precision Machining
• Spindle Speed (RPM)CNC & Precision Machining
• Depth of Cut (ap and ae)CNC & Precision Machining
• Trochoidal and Constant-Engagement MillingCNC & Precision Machining
• Tool WearCNC & Precision Machining

Terms that refer to Chip Load (Feed per Tooth, fz)

• Climb Milling and Conventional MillingCNC & Precision Machining