One-page reference and calculators for spindle speed (RPM), feed rate (IPM), and tapping feed.
Enter chip load, flutes, and RPM to get feed (IPM). Use the advanced SFM checker only if you need to compare surface speed to charts.
Formulas follow standard machining references; always verify on the ShopBot with conservative test cuts first.
General Preset Calculator
Pick a material, tool type, and bit size to get generalized starter values for the ShopBot. We prioritize vendor-provided chip loads and RPM ranges. Always verify with the actual tool manufacturer and run test cuts.
Suggested Starters
Chip Load (in/tooth)—
Flutes—
Diameter (in)—
—
Calculated
Recommended RPM—–—
Feed Range (IPM)—–—
These results will also populate the detailed calculators below (midpoint RPM + suggested chip load).
Cut order, pass depth, and hold-down still matter — run a small test tile first.
Theory & Formulas
Teaching flow: Start from vendor chip load + RPM ranges to compute feed (IPM). Use SFM as a derived/diagnostic metric, not the starting point.
Spindle Speed (RPM)
RPM = (SFM × 3.82) / Dia(in)
3.82 ≈ 12/π (unit conversion from surface feet/min + circumference to diameter in inches).
Feed Rate (IPM)
Feed (IPM) = RPM × Chip Load (in/tooth) × Flutes
Chip Load is the thickness of chip removed per tooth per revolution.
Tapping Feed (IPM)
Feed (IPM) = RPM / TPI
TPI = Threads Per Inch of the tap.
Notes for ShopBot Users
Start conservative; increase feed or lower RPM to avoid burning in wood. Increase RPM or lower feed if you see chipping.
Bit geometry (upcut/downcut/compression) and workholding affect results—test on scraps first.
For pockets → inside profiles → outside profiles: plan cut order for stability.
Feed Rate Calculator
Feed (IPM)—
Feed per Rev (IPR)—
Feed per Rev (IPR) = Chip Load × Flutes
Advanced (Optional): Surface Speed (SFM) Checker
Enter recommended SFM for your tool & material.
Common sizes: 0.125, 0.25, 0.375, 0.5
RPM—
SFM (recalc)—
Use this to understand surface speed or compare to industrial charts. Most router tooling begins with chip load + RPM ranges, not SFM.
Tapping Feed Calculator
Feed (IPM)—
Pitch (in)—
Pitch (in) = 1 / TPI
Sanity Checks & Tips
If RPM looks huge or tiny, re-check diameter and SFM units.
Wood burning? Try higher feed or lower RPM. Chatter/tear-out? Try higher RPM or lower feed; verify hold-down.
Run the ShopBot preview and do small test cuts before full-sheet jobs.
Key Terms & Definitions
SFM (Surface Feet per Minute): The surface speed of the cutting edge as it moves past the material. It’s derived from RPM and bit diameter and helps understand heat and cutting efficiency.
RPM (Revolutions per Minute): The spindle speed—how many times the cutting tool spins in one minute.
IPM (Inches per Minute): Feed rate, or how fast the tool moves through the material in one minute.
IPR (Inches per Revolution): Feed per revolution, the total distance the tool travels per spindle revolution; equals chip load × flutes.
Chip Load: The thickness of material removed by each flute of the bit on every revolution. Determined by feed rate, spindle speed, and number of flutes.
Flutes: The cutting edges or spirals on a bit. Fewer flutes remove larger chips and are common for wood and plastics; more flutes are used for metals or fine finishes.
Toolpath: The programmed route that the CNC tool follows to cut material.
Pass Depth: The maximum vertical depth that the tool cuts per pass, often related to the diameter of the tool (e.g., ≤ 1×D for wood).
Feed Rate: The speed at which the cutting tool moves through the material; measured in IPM.
Plunge Rate: The speed at which the cutting tool moves vertically into the material.
TPI (Threads Per Inch): The number of threads per inch on a tap or screw; used to calculate feed in tapping operations.
Tool Diameter: The width of the cutting end of the bit, used in feed, speed, and SFM calculations.
Toolpath Order: The sequence in which the CNC cuts features—commonly pockets first, then inside profiles, then outside profiles.
Hold-Down: Any method used to secure material to the CNC table during cutting, such as clamps, screws, or vacuum.
Bit Geometry: The shape and style of the bit (e.g., upcut, downcut, compression, ball nose) that determines chip removal and edge quality.