Feed rate is the speed at which a cutting tool advances through the workpiece, usually given in inches or millimeters per minute. In milling it is calculated as spindle speed × number of teeth × chip load per tooth. It is different from spindle speed, which is how fast the tool rotates, in revolutions per minute.
In short: spindle speed is how fast the tool spins; feed rate is how fast it moves through the material.
Feed Rate vs Spindle Speed vs Cutting Speed
Four related numbers are set for every cut, and they are easy to mix up.
| Term | What it measures | Units | How it is set |
|---|---|---|---|
| Cutting (surface) speed | How fast the cutting edge passes the material | ft/min (SFM) or m/min | From tables for the tool and work material |
| Spindle speed | How fast the tool (or, on a lathe, the part) rotates | rpm | Calculated from cutting speed and diameter |
| Chip load (feed per tooth) | Thickness of material each cutting edge removes | in/tooth or mm/tooth | From tables for the tool, material and cut |
| Feed rate | How fast the tool advances through the part | in/min (IPM) or mm/min | Spindle speed × teeth × chip load |
On a lathe or drill, feed is usually given per revolution instead (in/rev or mm/rev), and the feed rate is spindle speed × feed per revolution.
How Do You Calculate Feed Rate?
First find the spindle speed from the recommended cutting speed V and the tool diameter D:
- Imperial: rpm = 12 × V (ft/min) ÷ (π × D in inches), often rounded to V × 4 ÷ D
- Metric: rpm = 1,000 × V (m/min) ÷ (π × D in mm)
Then multiply by the number of teeth and the chip load:
- Feed rate = rpm × number of teeth × chip load
Worked example: a 1 in (25.4 mm), four-flute high-speed steel end mill cutting aluminum at about 250 ft/min (76 m/min) turns at about 950 rpm. With a chip load of 0.008 in (0.2 mm) per tooth, the feed rate is 950 × 4 × 0.008 ≈ 30 in/min, about 760 mm/min. A university lab guide that uses this example cuts both values to 60% when coolant is applied by hand.
What Happens If the Feed Rate Is Wrong?
- Too high: each tooth takes too thick a chip. Cutting forces rise, the tool and part can deflect, finish and accuracy suffer, and edges can chip or the tool can break.
- Too low: the job takes longer than it needs to. A US Army machining manual notes that more mistakes are made by overspeeding and underfeeding than by underspeeding and overfeeding.
- Roughing vs finishing: roughing cuts typically use a lower speed and heavier feed to remove metal quickly; finishing uses finer feeds and higher speeds for a better surface.
- Chatter: if the cut vibrates, a standard first fix is to lower the speed and raise the feed.
Use lower feeds for small or fragile cutters, deep slots, hard materials and flexible setups, and higher feeds for rigid setups, coarse-tooth cutters and easy-to-machine materials.
What Designers Can Control
Feed rates are chosen by the machinist or CAM programmer, not the designer, but the design sets the limits. Easy-to-machine alloys such as aluminum 6061 and brass run far faster than stainless steel or titanium. Generous internal radii allow larger, stiffer tools and heavier feeds. And only specify fine surface finishes where they matter, because a finer finish means slower finishing passes.
At NICE Rapid, CNC programs are generated in CAM software from your design file after an engineering review of material, tolerance and finish, and standard as-machined finishes are Ra 0.8–3.2 μm. See CNC machining for materials and tolerances.
Frequently Asked Questions
What is feed rate in machining?
Feed rate is the speed at which the cutting tool moves through the material, usually in inches or millimeters per minute. On lathes and drills it is often given per revolution instead, as inches or millimeters of travel per spindle turn.
What is the difference between feed rate and spindle speed?
Spindle speed is how fast the tool or part rotates, in revolutions per minute. Feed rate is how fast the tool advances through the part, in distance per minute. Spindle speed comes from the cutting speed and diameter; feed rate is then built from it.
How do you calculate feed rate for milling?
Multiply spindle speed by the number of teeth on the cutter and the chip load per tooth. For example, 950 rpm × 4 teeth × 0.008 in per tooth gives a feed rate of about 30 in/min (760 mm/min).
Related Services at NICE Rapid
- CNC Machining — 3-, 4- and 5-axis milling and turning in-house, standard tolerance ±0.1 mm and precision to ±0.01 mm.
- Rapid Prototyping — machined prototypes in 3–5 days, with an engineering review included in every quote.
- Request a Quote — send a STEP file, material and finish requirements for a quote within 24 hours.
Sources
- University of Florida, EML2322L Design and Manufacturing Laboratory, "Drilling and Milling Speeds and Feeds" — rpm = 12 × V ÷ (π × D); milling feed f = N × ft × m; drilling feed = N × feed per rev; worked example (1 in four-flute HSS end mill in aluminum, 250 sfm, 950 rpm, 0.008 in/tooth, 30 in/min; scale to 60% with manual oiling); lower/higher feed and speed guidance; running too fast causes accelerated tool wear or failure — web.mae.ufl.edu
- US Army TC 9-524, Fundamentals of Machine Tools, Chapter 8: Milling Operations (hosted by Carnegie Mellon University) — RPM = CS × 4 ÷ D; feed in inches or millimeters per minute = chip per tooth × number of teeth × rpm; coarse feeds reduce finish and accuracy; roughing at low speed and heavy feed, finer feeds and higher speeds for finishing; "More mistakes are made on overspeeding and underfeeding than on underspeeding and overfeeding"; reduce speed and increase feed to cure chatter — uhv.cheme.cmu.edu
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