Threading Technology – Clamping Technology 520

207 Roll Form Taps UNC UNF M MF G STI SELF-LOCK Tech. Info v c Product Finder 2.16 Thread hole preparatory diameter for roll form taps The influence of the preparatory diameter If the preparatory diameter is too small the workpiece material is overformed in the thread root and there are excessive process forces. If the preparatory diameter is too large the thread root is not sufficiently formed, the minor diameter is too small. In order to preclude such negative effects, the tolerance of the preparatory diameter is narrowed down from the start. In some cases where the forming characteristics are very extraordinary it may be necessary to go without a standard preparatory diameter entirely, and to find the correct diameter by testing. It is important to know that the preparatory diameter has a decisive influence on the minor diameter of the nut thread, as the following example shows. Every lack of precision, every kind of surface roughness will be mirrored in the finished internal thread and its minor diameter. Cold-formed thread M8-6HX in corrosion- and acid-proof material, e.g. material no. 1.4571 or 1.4401, with different preparatory diameters. Nut height = 2 x d v c = 21 SFM n = 255 rpm Coolant-lubricant: EMUGE thread cutting oil no. 5+ HIGH ALLOY While the observation of the pitch diameter tolerance of the internal thread, e.g. ISO metric thread 6H, offers no problems usually, deviations in the minor diameter of the internal or nut thread must be expected, as demonstrated above. The extended minor diameter tolerances for cold-formed internal threads are specified in DIN 13-50. This standard allows a 7H tolerance for the minor diameter of the nut thread, with a pitch diameter tolerance of 6H. Pitch diameter of cold-forming tap 6HX P Workpiece Profile of cold-forming tap Preparatory diameter min. = 7.41 mm Preparatory diameter max. = 7.48 mm Produced minor diameter of nut thread = 6.70 mm Produced minor diameter of nut thread = 6.85 mm