Usage and Operation Views: 16

Chamfering internal thread hole mouths - methods and timing: Mouth chamfering of threaded holes is common: removes burrs, guides the screw in, prevents edge chipping. Part 1 - Before or after tapping? Recommended: BEFORE tapping. Reasons: 1) pre-tap chamfering removes drilling burrs so the tap enters smoothly - prevents tap drift and chipping; 2) prevents mouth chipping during tapping (cast iron, brittle materials); 3) post-tap chamfering can hit the threads, damage the thread profile: screws run rough or strip; 4) pre-tap chamfer leaves a clean chamfer; post-tap chamfer raises burrs on the thread teeth. Special cases: 1) burrs after tapping - a light chamfer is OK, but cut small (0.1-0.2mm) and be careful; 2) large mouth chamfers (C1+) - MUST be pre-tap; post-tap cannot do it well; 3) stainless and tacky materials - pre-tap chamfering is even more important: prevents tap sticking and chipping. Part 2 - Chamfer size: 1) generally C0.5-C1.5 - enough to deburr without hurting thread strength; 2) M3-M4 - C0.5-C0.8; 3) M5-M6 - C0.8-C1.0; 4) M8-M10 - C1.0-C1.5; 5) M12+ - C1.5-C2.0; 6) critical threads - C0.5-C1.0; too large shortens effective thread engagement; 7) thin-sheet threads - slightly larger OK, prevents mouth deformation. Part 3 - Tool selection: 1) 90-degree countersink - most common for pre-tap mouth chamfering; 2) single-flute - recommended: low force, less risk to hole wall and threads; 3) small diameter - threaded holes are small: heads φ6-φ10; 4) cobalt HSS - for stainless and hard materials; standard or cobalt HSS for carbon steel; 5) piloted - for through holes (not tap holes) piloted spot facers work, but generally not on tap holes (the pilot could scratch the hole). Part 4 - Procedure: 1) drill the tap hole - e.g. M6 φ5, M8 φ6.8; 2) mouth chamfer - C0.5-C1.5, not too deep (widening the top hurts tap entry and thread strength); 3) tap - the chamfer guides the tap in, prevents drift; 4) check - inspect threads and mouth; light burrs can be eased with sandpaper or a file - do NOT cut heavily with a countersink (thread damage). Part 5 - Notes: 1) not too large - large chamfers shorten effective thread length: hurts joint strength, especially thin sheets and small threads; 2) not too deep - generally under 1/4 of the tap-hole diameter: M6 φ5 - depth under 1.2mm; 3) ALWAYS chamfer before tapping - stainless, cast iron, brass especially: no chamfer = chipping, burrs, tap chipping; 4) no heavy post-tap chamfering - thread profile damage: rough entry, stripping, poor sealing; 5) blind holes - chamfer before tapping too, but not too deep (limited effective length); 6) through holes - chamfer both ends: one before tapping, the other lightly after (if burred). Part 6 - Common problems: 1) mouth burrs after tapping - poor pre-tap chamfer or dull tap: ease with sandpaper; chamfer better next time; 2) mouth chipping - brittle materials (cast iron, brass): no or too small pre-tap chamfer; increase it, use sharp taps; 3) screws run rough - post-tap chamfering damaged the threads: run the tap through again or thread-file; 4) tap chipping - no pre-tap chamfer: the tap hits the hard mouth edge; always chamfer first; 5) weak threads - chamfer too large: reduce it. Part 7 - Summary: chamfer internal thread mouths BEFORE tapping, C0.5-C1.5, with a 90-degree single-flute countersink, not too deep or large. No heavy post-tap chamfering. This basic practice improves thread quality, extends tap life and keeps joint strength. Our single-flute and cobalt HSS countersinks suit thread-mouth chamfering - ask us.