Feed in machining is the distance a cutting tool advances into or along a workpiece during the cutting process. It controls how much material each tooth or cutting edge removes per pass, and it directly affects surface quality, tool life, and how long a job takes. Along with cutting speed and depth of cut, feed is one of the three fundamental parameters you set for any machining operation.
Feed vs. Cutting Speed
These two terms get confused constantly, so it helps to separate them up front. Cutting speed is the velocity at which the cutting tool moves across the workpiece surface, measured in surface feet per minute (SFM) or meters per minute. It relates to the rotational motion of the tool or workpiece. Feed rate, on the other hand, is the linear motion of the tool along the workpiece. Cutting speed determines how fast the tool edge meets the material; feed determines how quickly the tool advances through it.
Think of it this way: cutting speed is about rotation, feed is about travel. A lathe spinning a steel bar at high RPM has a high cutting speed. The rate at which the tool slides along the bar’s length is the feed. Both need to be set correctly together, because changing one without adjusting the other throws off the balance of heat, force, and chip formation.
How Feed Is Measured
Feed is expressed differently depending on the operation, which is one reason the concept can feel slippery at first.
- Feed per revolution (mm/rev or in/rev): Used in turning and drilling. This is the distance the tool moves each time the spindle completes one full rotation. On a CNC lathe, typical feed rates range from about 0.001 in/rev for steels up to 0.005 in/rev for plastics.
- Feed per tooth (mm/tooth or in/tooth): Used in milling. Since a milling cutter has multiple teeth, each tooth takes its own bite. Feed per tooth is the distance the workpiece advances between one tooth engaging and the next tooth engaging.
- Linear feed rate (mm/min or in/min): The overall speed at which the tool moves through the workpiece. This is the value you typically program into a CNC machine, and it’s calculated from the other values.
Calculating Feed Rate
The math is straightforward once you know which operation you’re running.
For turning and drilling, linear feed rate equals spindle speed (in RPM) multiplied by feed per revolution. If your drill spins at 2,000 RPM and you’re feeding at 0.004 in/rev, your linear feed rate is 8 inches per minute.
For milling, you add one more variable: the number of teeth on the cutter. Linear feed rate equals RPM multiplied by feed per tooth multiplied by the number of teeth. So a 4-flute end mill running at 10,000 RPM with a feed per tooth of 0.002 inches gives you a feed rate of 80 inches per minute. This formula matters because it reveals that a cutter with more flutes, at the same RPM and chip load, moves through material faster.
How Feed Differs Across Operations
In turning, the workpiece rotates and the cutting tool moves along its length. Feed is simply how far the tool travels per revolution of the part. It’s a single, clean number.
Milling is more complex because the cutter itself rotates and has multiple cutting edges. Each tooth scoops out a small chip of material, and the size of that chip (the “chip load”) is determined by the feed per tooth. Getting the chip load right is a balancing act. Too thin a chip generates excessive heat because the tool rubs rather than cuts, which shortens tool life and can cause breakage. Too thick a chip overloads the tool with torque, leading to poor edge quality, broken cutters, or the part shifting in the vise.
Drilling follows the turning convention, using feed per revolution. The key concern in drilling is chip evacuation: feeding too aggressively can pack chips into the flutes and snap the drill bit.
Why Feed Rate Matters for Surface Finish
Feed is the single biggest lever you have for controlling surface roughness. Higher feed rates leave more visible tool marks on the workpiece because each pass of the cutting edge is spaced farther apart, creating taller peaks and deeper valleys on the surface. Research on CNC-milled surfaces confirms that surface roughness increases significantly as feed rate goes up, particularly on angled and curved surfaces.
For finishing passes where appearance or tight tolerances matter, machinists reduce the feed rate substantially compared to roughing passes. A roughing pass prioritizes material removal speed; a finishing pass prioritizes surface quality. It’s common to use a feed rate two to four times lower on a finishing pass than on a roughing pass of the same part.
Feed Rate and Tool Life
Cutting speed has traditionally been considered the dominant factor in tool wear, but feed plays a significant and sometimes underappreciated role. Higher feed rates increase the mechanical load on each cutting edge, generating more force and heat at the tool tip. Lower feed rates reduce force but, counterintuitively, can increase heat per unit of material removed because the tool spends more time rubbing against the workpiece without taking a productive cut.
The ideal feed sits in a zone where each tooth removes a chip thick enough to carry heat away from the cut, but not so thick that it overloads the tool. This is why chip load recommendations exist for every combination of tool material, workpiece material, and cutter geometry. Staying within the recommended chip load range is one of the most reliable ways to get predictable tool life.
Setting Feed in CNC Programs
On CNC machines, feed is programmed using an F-code paired with a G-code that tells the controller how to interpret it. The two most common modes are G94 and G95.
G94 is the default on most machines. It sets feed in millimeters per minute (or inches per minute), meaning the tool moves at a fixed linear speed regardless of spindle RPM. This is the standard mode for milling.
G95 sets feed in millimeters per revolution. The controller ties the tool’s advance to the spindle rotation, so if the spindle slows down, the feed slows proportionally. This mode is standard for turning and is also useful in drilling, where maintaining a consistent chip load per revolution matters more than maintaining a constant linear speed.
A typical CNC program line might look like: G94 F1500 X150, which tells the machine to move to position X150 at a feed rate of 1,500 mm/min. Switching to G95 in the same program changes the meaning of the F-value entirely, so mixing up these modes is a common source of crashes for new programmers.
Choosing the Right Feed Rate
Feed rate selection starts with the material you’re cutting and the tool you’re using. Tool manufacturers publish chip load recommendations for their cutters in specific materials. You look up the recommended feed per tooth (for milling) or feed per revolution (for turning), then calculate the linear feed rate from there using spindle speed and number of flutes.
Softer materials like aluminum and plastics tolerate much higher feed rates than hard materials like steel or titanium. A plastic part on a CNC lathe might run at five times the feed rate of a steel part with the same tool. Beyond material, the rigidity of your setup matters: a long, thin workpiece or a tool with a lot of stick-out will chatter at feed rates that would be perfectly fine on a rigid setup.
When in doubt, start conservative and increase. Listen to the cut. A smooth, consistent sound usually means the feed is in a good range. A squealing or chattering sound often means you need to adjust feed, speed, or both. Experienced machinists treat published recommendations as starting points and fine-tune based on what the machine, tool, and material are telling them in real time.