Tapping

Dimensional Accuracy Criteria

• Thread Size (Major Diameter): Must conform to nominal specifications (e.g., M8 x 1.25 or 3/8-16 UNC).

• Tolerance Class Fit: Verified via Go/No-Go Thread Plug Gauges. The "Go" gauge must enter freely; the "No-Go" gauge must not enter more than 1.5 to 2 turns (depending on the specific standard).

• Thread Depth: Measured as the full usable thread length, excluding the tapered lead-in (chamfer) from the tap.

• Pilot Hole Integrity: Pre-tap diameter must ensure correct thread engagement (typically 60%–75%) to balance strength against the risk of tool breakage.

Thread Integrity and Form Criteria

• • Pitch & Lead: Must remain uniform and accurate to the specification to prevent cross-threading or binding.

  • Alignment (Perpendicularity): The thread axis must be perpendicular to the mating surface to ensure the bolt head seats flat and applies uniform clamping force.

  • Visual Quality: Threads must be complete and well-formed. No evidence of stripped threads, jagged crests, or "double-threading" (caused by a tap re-entering incorrectly).

• • Cleanliness: Mandatory removal of all chips, swarf, and cutting oil. 

  • Edge Finish: The entrance should ideally have a chamfer (typically 90° to 120° included angle) to assist in fastener starting and to prevent a raised burr from lifting the mating part.