IS 1369 : Part 2 : 1993/ISO 4755 : 1983 Fasteners - Thread runout and undercuts Part 2 Dimensions for screw thread undercuts for external ISO metric threads

National Foreword

This Indian Standard (Part 2) which is identical with ISO 4755 :1983 `Fasteners - Thread undercuts for external metric ISOthreads, issued by the International Organization forStandardization (ISO) was adopted by the Bureau of Indian Standardson the recommendations of the Bolts, Nuts and Fastener AccessoriesSectional Committee (LM 14) and approval of the Light MechanicalEngineering Division Council.

An undercut of reasonable width allows threads to be cut on acomponent up to end without the end face of the screwing toolfouling the component. Design requirements are usually for areasonable degree of control on the diameter at the root or crestof the undercut to avoid weakening the section at this point, andfor sufficient radius at the end of the undercut to avoid stressconcentration. The production requirement is for an adequate widthof undercut to allow the use of dies and taps with a reasonablethroat angle or taper lead.

It is well known that too narrow undercuts or the demand forperfect or full threads up to the shoulder or to the bottom ofblind holes add substantially to manufacturing costs bynecessitating the use of special tps and dies. This may require asecond screwing operation, or resort to process, such as threadmilling or grinding. It is suggested that where recommendations forany of the standard forms of undercut appear at first sight to beinapplicable, an endeavour shall be made to accommodate them byalterations in design, for example, by using through tapped holes.A counterbore or undercut on the first few threads of an internalthread may readily be drilled when tapping a hole and is muchsimpler to produce than equivalent undercut on the mating externalthread.

References

IS 1369 : Part 1 Dimensions for screw threadrun-outs for external ISOmetric threads (Adoption of ISO 3508 : 1976)

IS 1369 : Part 2Dimensions for screw thread undercuts for external ISOmetric threads (Adoption of ISO 4755 : 1983)

IS 1369 : Part 3 Dimensions for thread run-outs andundercuts forinternal threads (blind tapped holes) (underPreparation)