ISO INTERNATIONAL STANDARD 7721-2 First edition 1990-09-01 Countersunk flat head screws Part 2: Penetration depth of cross recesses Vis ametauxatete fraisee- Partie2:Profondeur de penetration des empreintes cruciformes Referencenumber ISO7721-2:1990(E) ISO7721-2:1990(E) Foreword Iso(the International Organizationfor Standardization)is a worldwide federation of national standards bodies (isO member bodies).The work technical committees.Eachmemberbody interested in a subjectfor which atechnical committee has been established has the right to be represented on that committee.International organizations,govern- work.Iso collaborates closely with the International Electrotechnical Commission (IEC)onall mattersofelectrotechnical standardization. Draft International Standardsadopted bythetechnical committees are circulated to the member bodies for voting.Publication as an Interna- tionalStandardrequiresapprovalbyatleast75%ofthememberbodies casting a vote. International StandardIsO7721-2waspreparedbyTechnical Committee ISO/TC2,Fasteners. Iso 7721 consists of thefollowing parts,under the general title Countersunk flathead screws: - Part 1:Head configuration and gauging Part2:Penetrationdepthofcrossrecesses AnnexAofthispartofIso7721isforinformationonly ISO1990 permission inwritingfrom thepubllsher. InternationalOrganizationforStandardization CasePostale56·CH-1211Geneve 20·Switzerland Printed in Switzerland ii ISO 7721-2:1990(E) Introduction The penetration depth of cross recesses for countersunk flat head screws has to satisfy two requirements which act in opposite directions foragivenhead dimension. First, there is the requirement for sufficient head strength to attain the proof and breaking loads of the respective property class.A shallow cross recess increases the head strength. On the other hand, the wrenchability of the screw should be satisfactory:this can only be achievedbyasufficientlydeepcrossrecess. Iso 7721-2 was developed in order to find a compromise which, as far as possible, wouldmeetbothrequirements. This part of iso7721 specifies deep cross recesses for countersunk head screws of low strength:a good wrenchability is achieved andthe head strength is still sufficient.This execution will be used in Iso 7046-1 (see annex A). For screws of higher strength,sufficient head strength can only be at- tained by a shaliower penetration depth of the cross recesses. If such screws also require good wrenchability,then,under the conditions of the common head style, a shoulder has to be provided under the head, in addition to the larger penetration depth, in order to guarantee suffi- cient head strength. This compromise, which unfortunately results in different but inter- changeabie types of cross recessed flat countersunk head screws, is at the moment the only way of reaching an agreement at the international level. ili