turning tool

A turning tool is a single-point cutting implement that removes material from the outside of a rotating workpiece on a lathe. Unlike a boring bar, which cuts inside a hole or bore, a turning tool shapes external diameters, faces, shoulders, and tapers by pressing a hardened tip against spinning metal, plastic, or composite stock. The tool sits in a holder mounted to the lathe's toolpost and advances toward the workpiece at a controlled rate while the spindle rotates at a fixed speed.

The cutting point itself is either brazed carbide (a tungsten carbide tip silver-soldered to a steel shank) or an indexable insert (a replaceable chip of carbide or ceramic clamped into a standardized pocket). Brazed tools cost less upfront but require regrinding when dull; inserts are swapped in seconds when worn. The tool's nose radius, typically 0.015 to 0.125 inches, controls surface finish and deflection. Rake angle (the slope of the cutting face) affects shearing action and chip formation. Flank angle must clear the finished surface to prevent rubbing and heat buildup.

Common variants include straight turning tools for facing and cylindrical work, offset tools for shoulders and steps, and threading tools with a pointed 60-degree nose for cut ting screw threads. Finishing passes demand sharper, smaller-radius tools run at high speed and light depth to achieve surface roughness below 63 microinches. Roughing tools have larger nose radii and heavier builds to absorb vibration and take deeper cuts of 0.100 to 0.200 inches per pass.

Material and failure modes

Turning tools fail by flank wear (gradual dulling), crater wear (localized pitting on the rake face from diffusion), chipping (sudden fracture of the cutting edge), and thermal cracks from rapid cooling or interrupted cuts. High-speed steel tools, still common for manual machines, tolerate interrupted cuts better than carbide but require lower speeds (typically 50 to 200 surface feet per minute) and produce more heat. Carbide tools run at 500 to 2000 SFM and generate finer chips but shatter if subject to shock loading or heavy radial forces.

The turning tool's geometry and material grade must match the workpiece material and operation. Aluminum demands large positive rake angles and sharp edges to prevent built-up edge and chatter; steel tolerates smaller rake angles and tougher tool materials. Stainless steel work-hardens aggressively and requires careful speed control to prevent work-hardening zones that multiply tool wear. Cast iron produces brittle chips and wears tools rapidly, so cutters must be rigid and coated carbide is often preferred.