Speeds & Feeds Calculator
Calculate optimal spindle speed (RPM) and feed rate for CNC milling. Supports metals, plastics, and wood.
Speeds & Feeds Calculator
Calculate optimal RPM and feed rate for CNC milling
Tool Parameters
Spindle Speed
4,889 RPM
revolutions per minute
Feed Rate
70.4 IPM
inches per minute
0.0048" per tooth
thickness per flute
640 SFM
surface feet/min
3.52 in³/min
volume rate
0.85 sec
cutting time
G-Code Reference
Formulas Used
Understanding Speeds & Feeds
Proper speeds and feeds are critical for tool life, surface finish, and productivity. Running too slow wastes time; running too fast damages tools and workpieces.
Key Parameters
- SFM: Surface speed determined by material and tool type
- RPM: Spindle revolutions - depends on SFM and tool diameter
- Feed Rate: Table movement speed (IPM or mm/min)
- Chip Load: Material thickness per tooth per revolution
What Is Surface Feet Per Minute (SFM)?
Surface feet per minute (SFM) is how fast the cutting edge travels across the workpiece surface, measured in feet per minute. It is set by the material and tool type, then converted to spindle speed for your tool diameter.
SFM = 0.262 × tool diameter (inches) × RPM. Working backwards: RPM = (SFM × 3.82) / tool diameter. The 0.262 and 3.82 constants are shorthand for π/12 and 12/π. This calculator starts from a recommended SFM for your material and operation, then solves for RPM using the exact form RPM = (SFM × 12) / (π × diameter).
Material Guidelines
Metals
- Aluminum: 600-1000 SFM, aggressive chip loads
- Mild Steel: 80-120 SFM, moderate chip loads
- Stainless: 40-80 SFM, maintain cutting pressure
- Titanium: 30-60 SFM, rigid setup required
Non-Metals
- Plastics: 500-800 SFM, sharp tools, high feeds
- Hardwood: 600-1000 SFM, chip evacuation critical
- Softwood: 800-1200 SFM, prevent burning
- Composites: 300-600 SFM, dust collection essential
HSS vs Cobalt vs Carbide: Tool Material and Cutting Speed
Tool material sets how hot the cutting edge can run before it softens, which sets how fast you can cut. HSS (high speed steel) is the baseline. Cobalt HSS (M42) adds heat resistance. Carbide holds its edge at much higher temperatures again, and a coating adds a further speed margin on top of bare carbide. The calculator applies a multiplier on top of the material's baseline SFM for the tool material you select.
| Tool Material | SFM Multiplier | Notes |
|---|---|---|
| HSS | 1x | Baseline. Tough, cheap, forgiving on older machines. |
| Cobalt (M42) | 1.25x | More heat resistant than standard HSS. Good on stainless. |
| Uncoated Carbide | 3x | Holds an edge at high heat. Brittle - needs a rigid setup. |
| Coated Carbide (TiN/TiAlN) | 3.9x | Coating adds further heat and wear resistance over bare carbide. |
Sources: cutting speed ranges from the LittleMachineShop.com cutting speeds reference, and cobalt/coating figures from Griggs Steel's M42 cobalt reference. Sources disagree on the exact carbide ratio (roughly 2x-5.5x depending on material), so the multipliers above use conservative mid-range values, not the high end.
Frequently Asked Questions
What is Surface Feet per Minute (SFM)?
SFM is the speed at which the cutting edge of the tool moves across the workpiece surface. It's determined by the material being cut and the tool type. Aluminum can handle 800+ SFM, while stainless steel may need only 50-100 SFM.
How do I calculate RPM from SFM?
RPM = (SFM × 12) / (π × Tool Diameter). For example, cutting aluminum at 800 SFM with a 0.5" endmill: RPM = (800 × 12) / (3.14159 × 0.5) = 6,112 RPM.
How do I calculate surface feet per minute?
SFM = 0.262 × tool diameter (inches) × RPM. A 0.5" endmill at 6,000 RPM runs at 0.262 × 0.5 × 6,000 = 786 SFM. The 0.262 constant is π/12, which converts tool diameter and spindle revolutions into feet of cutting edge travel per minute.
What is chip load and why does it matter?
Chip load is the thickness of material each flute removes per revolution. Too low causes rubbing and heat; too high risks tool breakage. Typical chip loads range from 0.001" (hardened steel) to 0.010" (softwood).
Should I use climb milling or conventional?
Climb milling (cutter moves with feed direction) generally provides better surface finish and longer tool life. Conventional milling may be better for older machines with backlash or when roughing tough materials.
How do I know if my feeds and speeds are wrong?
Signs of incorrect parameters: excessive chatter/vibration, poor surface finish, discolored chips (too much heat), broken tools, or excessive tool wear. Blue chips indicate overheating - reduce speed or increase feed.
What's the difference between roughing and finishing?
Roughing uses higher chip loads and deeper cuts to remove material quickly. Finishing uses lower chip loads and shallower cuts for better surface finish. Our calculator adjusts SFM and chip load accordingly.
Can I run carbide at HSS speeds?
Yes, but you lose most of carbide's advantage. Carbide holds its edge at much higher temperatures than HSS, so running it at HSS speeds is safe but slow. Going the other way and running HSS at carbide speeds will burn the edge and ruin the tool - always select the correct tool material in the calculator before trusting the RPM.
What SFM should I use for HSS in mild steel?
80-120 SFM is the commonly quoted range for HSS in mild steel (1018). This calculator uses a 100 SFM baseline for mild steel, reduced to 80 when roughing. Cobalt (M42) tools can typically run 25-50% faster, and carbide 2-4x faster in the same material - select your tool material in the calculator to see the adjusted RPM.
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