CNC Tool Terms

Essential CNC Cutting Tool Terms

Understanding cutting-tool terminology helps machinists select tools, interpret setup instructions, diagnose poor finishes, and communicate clearly on the shop floor.

Cutting Tools

• End mill: A rotating cutter used for profiling, slotting, pocketing, contouring, and other milling operations.
• Drill: A tool designed primarily to produce round holes.
• Reamer: A precision tool that enlarges and finishes an existing hole to a controlled diameter.
• Tap: A tool used to cut internal threads.
• Thread mill: A milling cutter that produces threads through helical interpolation.
• Insert: A replaceable cutting element, commonly made from carbide, mounted in a toolholder or cutter body.
• Spot drill: A short, rigid tool used to establish an accurate starting location for a subsequent drilling operation.
• Chamfer mill: A cutter used to bevel edges, deburr features, or create countersinks.

Tool Geometry

• Flute: A groove along a cutting tool that forms cutting edges and provides space for chip evacuation.
• Helix angle: The angle of a tool’s flutes relative to its axis. It influences cutting action, chip movement, and axial forces.
• Rake angle: The angle of the cutting face that affects chip formation, cutting forces, and edge strength.
• Relief angle: Clearance behind the cutting edge that prevents the tool from rubbing against the workpiece.
• Cutting edge: The portion of the tool that shears material from the workpiece.
• Corner radius: A rounded transition at the tool tip that can strengthen the cutting edge and affect the finished profile.
• Cutting diameter: The effective diameter of the portion of the tool that removes material.
• Shank: The non-cutting portion of a tool held by the toolholder.
• Flute length: The axial length over which the flutes extend.
• Overall length: The total length of the cutting tool from tip to end.

Toolholding Terms

• Toolholder: A device that connects a cutting tool to the machine spindle or turret.
• Collet: A slotted sleeve that contracts around a tool shank when tightened.
• End mill holder: A holder that typically secures a tool with a setscrew against a flat on the shank.
• Hydraulic holder: A precision holder that clamps the tool through hydraulic pressure.
• Shrink-fit holder: A holder heated to expand its bore; it contracts around the tool as it cools.
• Pull stud: A threaded retention component used by a machining center’s drawbar to secure a holder in the spindle.
• Runout: The amount by which a rotating tool or holder deviates from its true axis. Excessive runout can shorten tool life and produce inconsistent dimensions.
• Stickout: The distance a tool extends from its holder. Shorter stickout generally improves rigidity.
• Gauge length: The distance from the holder’s reference plane to the tool tip.

Cutting Data and Performance

• Spindle speed: The rotational speed of the spindle, normally expressed in revolutions per minute.
• Surface speed: The speed at which the cutting edge travels across the workpiece surface, commonly expressed as surface feet per minute or meters per minute.
• Feed rate: The rate at which the tool advances relative to the workpiece.
• Feed per tooth: The distance the tool advances for each cutting edge engaged during one revolution.
• Chip load: The theoretical thickness of material removed by each cutting edge during a pass.
• Axial depth of cut: The cutting depth measured parallel to the tool axis.
• Radial depth of cut: The width of tool engagement measured perpendicular to the tool axis.
• Tool life: The useful cutting time or number of parts produced before a tool no longer meets performance requirements.
• Chatter: Self-excited vibration that can cause noise, poor surface finish, dimensional error, and accelerated tool wear.
• Built-up edge: Workpiece material that adheres to the cutting edge and changes its effective geometry.
• Chip evacuation: The removal of chips from the cutting zone through flute geometry, coolant, air, or other methods.

Tool Setup and Measurement

• Tool offset: A stored value used by the CNC control to account for tool length, radius, diameter, or wear.
• Tool length offset: The value that relates the tool tip position to the machine’s reference system.
• Cutter compensation: A control function that shifts the programmed path by the cutter radius, allowing size adjustments without rewriting the programmed geometry.
• Wear offset: A small correction applied to compensate for tool wear or minor dimensional variation.
• Tool presetter: A device used to measure tool length and diameter before or after installation in the machine.
• Touch-off: The process of locating a tool relative to a known surface or reference point.
• Tool change position: A safe machine location where an automatic or manual tool change occurs.

Practical Use

Tool terminology should always be interpreted in the context of the machine, control, tooling system, workpiece material, and manufacturer’s recommendations. Verify holder compatibility, tool dimensions, spindle limits, and cutting parameters before machining.