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When it’s necessary to cut a thread then the hole size needs to be a smaller diameter to allow for the thread to be tapped. The necessary tapping hole size can be easily calculated by deducting the thread pitch from the metric bolt diameter. For example, an M8 bolt with a standard coarse thread pitch of 1.25mm would require a hole of 6.75mm diameter to be tapped. But if the M8 bolt had a fine pitch thread of 1mm then the diameter of the hole required for tapping would be 7mm.
The first number of the bolt class corresponds to the ultimate strength e.g. 400 MPa for classes 4.x, 500 MPa for classes 5.x, 600 MPa for classes 6.x, 800 MPa for classes 8.x, and 1000 MPa for classes 10.x.And BS3643 part 2 provides tabulated diameters and tolerances for coarse and fine pitch threads, based on ISO 965/2. Metric Bolt Sizes The standarized properties of metric bolts are specified in the international standard ISO 898-1:2009 'Mechanical properties of fasteners made of carbon steel and alloy steel - Part 1: Bolts, screws and studs with specified property classes - Coarse thread and fine pitch thread'. They are designated as above also including the pitch of thread in mm e.g. M8 × 1, M14 × 1.5, M27 × 2 etc.
The width of the hexagon nuts across flats s is specified in ISO 898-2 Table A.1 for bolt sizes M5 to M39. Bulgarian, Croatia, Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Romania, Slovakia, Slovenia Metric Threads are measured in Millimetres (mm) units and were developed to simplify the imperial systems. Europe moved to this system but the Americans choose Imperial as their default. A coarse thread has less helical coils per mm and a fine one has more. Coarse is used for heavier loads and fine is used for lighter loads prone to vibrations.Here’s a short list of standard spanner and allen key sizes that apply to the most commonly used metric nuts and bolts. Standard metric spanner and allen key sizes. Size For full table with more Property Classes - rotate the screen! Metric Bolts - Fine Threads - Minimum Ultimate Tensile Loads Thread
The following table provides examples of clearance hole sizes, standard or coarse thread pitches along with fine thread pitches and their corresponding tapping hole sizes. Metric clearance and tapping drill hole sizes. Size M2 is the partial safety factor for the resistance of bolts in accordance with EN1993-1-8 §2.2(2) Table 2.1 and the National Annex. The recommended value in EN1993-1-8 is γ M2 = 1.25. By approximately ignoring the corner rounding for a perfect hexagon the relation of the distance across points s' and the distance across flats s is s' = s / cos(30°) = 1.1547⋅ s.According to EN1993-1-8 Table 3.4 the bearing resistance F b,Rd of the bolt is not affected by the spacing p 1, p 2 and edge distances e 1, e 2 provided that the following limits are observed: e 1≥ 3.0 d 0, e 2≥ 1.5 d 0, p 1≥ 3.75 d 0, p 2≥ 3.0 d 0. The standard coarse pitch thread metric bolt sizes are: M3, M3.5, M4, M5, M6, M7, M8, M10, M12, M14, M16, M18, M20, M22, M24, M27, M30, M33, M36, M39. The nominal gross area A g corresponds to the cross-sectional area of the unthreaded part of the bolt: Nuts and bolts are essential fastenings, critical in the safe assembly of machinery, equipment, furniture and much more. In most circumstances, it’s absolutely vital that the right size of fastening is used. In this brief guide we provide an introduction to standard ISO metric sizes for nuts and bolts which will hopefully help dispel any confusion. BS3643 Parts 1&2
Thread lengths listed are a guide value for bolts under 125mm. For bolts longer than 125mm and shorter than 200mm add additional 6mm of thread. For preloaded bolted connections which are slip-resistant at the Serviceability Limit State or the Ultimate Limit State the corresponding shear load F v,Ed should not exceed the design slip resistance as specified in EN1993-1-8 §3.9 and Table 3.2. Only bolt assemblies of classes 8.8 and 10.9 may be used as preloaded bolts. According to EN1993-1-8 § 3.8(1) for long joints where the distance between the centers of the end fasteners measured in the direction of load transfer is more than 15 d the design shear resistance F v,Rd of all the fasteners should be multiplied by the reduction factor β Lf specified in EN1993-1-8 equation 3.5. The yield strength f yb and the ultimate tensile strength f ub for bolt classes 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, and 10.9 are given in EN1993-1-8 Table 3.1. The design resistance of a group of fasteners may be taken as the sum of the design bearing resistances F b,Rd of the individual fasteners provided that the shear resistance F v,Rd of each individual fastener is greater than or equal to the design bearing resistance F b,Rd. Otherwise the design resistance of a group of fasteners should be taken as the number of fasteners multiplied by the smallest design resistance of any of the individual fasteners as specified in EN1993-1-8 § 3.7(1). For this case elastic linear distribution of internal forces should be used as specified in EN1993-1-8 §3.12.
The second number corresponds to the ratio of yield strength to ultimate strength e.g. 60% for class 4.6 leading to a yield strength of 0.60 × 400 MPa = 240 MPa. The interaction between shear and tension is expressed in EN1993-1-8 Table 3.4 according to the following linear relation:
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