Thread Pitch

Thread pitch is the axial distance between corresponding points on adjacent threads of a screw, bolt, tapped hole, or thread-milled feature. Metric drawings state pitch in millimetres; imperial practice commonly uses threads per inch (TPI). In shop terms, pitch determines whether male and female threads actually assemble, and the parts will not screw together properly if the internal and external pitches do not match.

On a CNC machining shop floor, thread pitch drives the spindle-to-axis sync used for tapping, thread milling, and lathe threading, so the tool advances exactly one pitch per revolution for single-start threads. A programmer pulls the thread standard from the drawing, verifies major diameter and pitch, then chooses the tap drill or interpolation diameter. A common metric pilot-hole estimate is tap diameter minus pitch: M10 × 1.5 gets an 8.5 mm hole. For inspection, machinists verify pitch with a thread pitch gauge or by measuring the spacing between adjacent thread peaks; imperial threads are checked by counting TPI over one inch and metric threads by measuring in millimetres. In production, a pitch error can make a thread look clean but fail to mate with the fastener, causing assembly rejection or scrap.

• ⚠️Wrong pitch called out from the print: When the program uses 20 TPI but the part requires 18 TPI, the thread form can cut cleanly, yet the fastener cross-threads or seizes because flank spacing is wrong.
• ⚠️Pitch and lead mixed up on multi-start threads: If lead is treated as pitch, the helix angle is wrong because pitch is smaller; the thread may look right but will not function as intended.
• ⚠️Oversized or undersized tap drill: A hole drilled too small raises tap torque and can break the tap, while too large a hole leaves shallow thread engagement and weak pullout strength.