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Speeds & Feeds Calculator - RPM, Feed Rate & Chip Load for Milling and Drilling

Calculate spindle RPM and table feed rate based on cutter diameter, material, and number of flutes

Free speeds and feeds calculator for milling, drilling, and reaming operations. Enter your cutter diameter, number of flutes, workpiece material, and cutting tool material to get the correct spindle RPM and table feed rate in IPM. Getting speeds and feeds right is the difference between a smooth cut with good tool life and a broken endmill buried in your workpiece. Too slow and you rub instead of cut, work-hardening the material and killing the tool with heat. Too fast and you overload the flutes, snap the cutter, or chatter badly enough to scrap the part. This calculator uses industry-standard surface footage (SFM) recommendations for common material and tooling combinations: HSS and carbide cutters in mild steel, stainless, aluminum, cast iron, brass, and plastics. It computes RPM from SFM and cutter diameter, then multiplies by your chip-per-tooth target and flute count to give you the table feed rate. Results include the recommended chip load range so you can fine-tune based on rigidity, depth of cut, and machine capability. For shops running manual mills and drill presses without CNC, the calculator rounds RPM to the nearest available belt speed and adjusts feed accordingly.

Pro Tip: On a manual mill, your biggest enemy is chatter, and the fix is almost never to slow down. Chatter usually means the chip load per tooth is too light - the cutter is rubbing instead of cutting, which excites vibration. Increase the feed rate first. If that doesn't help, try a cutter with fewer flutes (bigger chip per tooth at the same feed) or reduce the tool stickout. Slowing the spindle only helps if you're above the SFM limit for the material and generating excess heat.

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Speeds & Feeds Calculator
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How It Works

  1. Select Tool and Material

    Choose your cutting tool material (HSS, cobalt HSS, or carbide) and workpiece material (mild steel, 4140, stainless 304/316, 6061 aluminum, cast iron, brass, or plastic). Each combination has a recommended SFM range.

  2. Enter Cutter Diameter

    Input the cutter diameter in inches or millimeters. For endmills this is the nominal diameter. For drills, use the drill size. The calculator converts SFM to RPM using the standard formula: RPM = (SFM × 3.82) / diameter in inches.

  3. Specify Number of Flutes

    Enter the flute count on your cutter. Two-flute endmills are standard for slotting and aluminum; four-flute for general steel work. More flutes allow higher feed rates at the same chip load but require more spindle power and rigidity.

  4. Adjust Chip Load

    The calculator provides a recommended chip-per-tooth value. Adjust within the range based on your setup: lighter chip load for long stickout, thin walls, or light machines; heavier for rigid setups and roughing.

  5. Review RPM and Feed Rate

    Get the calculated RPM, table feed in IPM, and chip load per tooth. If your machine can't reach the calculated RPM, reduce to the nearest available speed and let the calculator recalculate the feed to maintain proper chip load.

Built For

  • Manual mill operators looking up correct RPM for a specific endmill and material combination
  • CNC programmers verifying feeds and speeds before posting a new program
  • Shop supervisors standardizing speeds and feeds across multiple operators
  • Students and apprentice machinists learning proper cutting parameters
  • Job shops quoting machining time based on accurate feed rate calculations

Frequently Asked Questions

RPM = (SFM × 3.82) / cutter diameter in inches. SFM (surface feet per minute) is a material constant that depends on the workpiece and tool material. For example, mild steel with an HSS cutter runs at about 80-100 SFM. A 1/2-inch endmill at 90 SFM gives (90 × 3.82) / 0.5 = 688 RPM. Carbide cutters run 2-4 times faster than HSS in the same material.
Feed rate in IPM = RPM × number of flutes × chip load per tooth. For mild steel with a 4-flute carbide endmill, a typical chip load is 0.003-0.005 inches per tooth. At 2,500 RPM with 4 flutes and 0.004 chip load: 2,500 × 4 × 0.004 = 40 IPM. Start conservative and increase until you get a good chip and finish. If you hear chatter, increase feed before decreasing speed.
Chip load is the thickness of material each flute removes per revolution, measured in inches per tooth (IPT). It matters because too light a chip load causes rubbing, heat buildup, and premature tool wear. Too heavy a chip load overloads the cutter and can break it. Every tool and material combination has an ideal chip load range. Staying in that range gives you the best surface finish, longest tool life, and fastest material removal.
Two-flute endmills have larger flute valleys for chip evacuation, making them better for slotting, plunging, and aluminum where chips are large and gummy. Four-flute endmills are stiffer, produce a better finish, and allow higher feed rates in steel and cast iron where chips are small. For general work in steel on a rigid machine, four-flute is the default. For aluminum, pockets, and slots, go two-flute.
The most common causes are: (1) Feed rate too low, causing rubbing and heat that softens the cutter. (2) Excessive tool stickout creating deflection and chatter. (3) Climb milling on a machine with backlash, which grabs the cutter and pulls it into the work. (4) Recutting chips in a slot or pocket instead of clearing them with air or coolant. (5) Wrong cutter for the material - running uncoated HSS at carbide speeds will fail instantly.
Disclaimer: This calculator provides starting-point recommendations based on industry-standard SFM and chip load values. Actual optimal parameters depend on machine rigidity, tool condition, coolant, depth of cut, and workholding. Always start conservative and adjust based on observed cutting performance. Follow all shop safety procedures when operating milling machines.

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