Thread Tolerance
Thread tolerance is the allowable variation in a screw thread's critical geometry, primarily pitch diameter, with associated major/minor diameter, flank form, and lead. It ensures mating parts assemble with intended fit, load transfer, and retention. In CNC machining, it is specified by thread classes like ISO 6H/6g or ANSI/ASME 2B/2A.
On the shop floor, thread tolerance decides whether a part actually assembles, not just whether it looks threaded. A machinist may cut an internal M8 x 1.25 thread that seems perfect, but if pitch diameter drifts because the tap is nearing end of life, the Go/No-Go gauge can bind or the mating bolt feels loose. That is why the drawing must carry the full designation, such as 6H internal and 6g external, along with thread depth and gauging requirements. Practical CNC methods include cut tapping, form tapping, and thread milling. Form taps displace material, which changes allowable tolerances and hole sizing. Blind holes demand proper chip evacuation and depth allowance. In production, tool wear undermines the tightest class quickly, so offsets must be checked against functional gauges. The goal is a fit that supports load transfer and retention without over-controlling features that drive up cycle time and rejection rates.
What dimension is most important in thread tolerance?
Pitch diameter, because it governs how male and female threads actually mate and carry load.
Are major and minor diameters enough to control thread fit?
No. They matter for strength and clearance, but pitch diameter is the primary functional control for thread fit.
How should a CNC drawing specify a thread?
Include nominal size, pitch or TPI, class of fit, thread depth, and any special acceptance criteria such as gauging or plating allowance.