Test Sieves: Part-III Methods of Examination of Apertures of Test Sieves
1985 Edition

IS 460 Part 3 requires measuring at least 10 apertures in both warp and weft directions at two widely separated positions on the sieve cloth. If discrepancies occur or results conflict with Part 1 Table 2, measurements at three additional locations are taken. The overall average from up to five positions must fall within specified tolerance limits. The number of fields and minimum consecutive apertures measured vary with nominal aperture size, as detailed in Table 1. For apertures exceeding 3.35 mm, a minimum of 20 apertures should be measured; if fewer than or equal to 20 apertures exist, all must be measured. Measurement fields are selected to ensure no two fields are crossed by the same wire, using masks as per Clause 3.1 and Figure 2.

According to IS 460 Part 3, if a sieve contains 20 or fewer apertures, all apertures must be measured. For sieves with more than 20 apertures, a prescribed minimum number of apertures must be measured in one or both directions as specified in Clause 2.2.5. If the minimum required sample size is not available, all apertures have to be inspected. This ensures adherence to the dimensional tolerances X, Y, and Z outlined in Clause 2.1.4.

The standard recommends beginning with a visual general inspection, followed by detailed scrutiny of individual apertures. Oversized apertures—those differing by approximately 10% or more from the average—can be detected visually by skilled inspectors. For coarser sieves, mechanical tools such as engineer's callipers, limit gauges, and segmental callipers are suggested. For finer sieves, optical instruments like microscopes or projectors paired with coordinate measuring tables are recommended to measure aperture sizes in warp and weft directions accurately, ensuring compliance with tolerances X, Y, and Z.

Oversize apertures are identified primarily through visual inspection, as apertures deviating roughly 10% from the average size are noticeable to experienced observers. For coarser sieves, engineer's callipers, limit gauges, and segmental callipers are employed, while finer sieves require optical magnification tools combined with coordinate measuring tables. Apertures exceeding the maximum permissible deviation (tolerance X) are considered unacceptable, leading to sieve rejection. Apertures falling between nominal plus Z and nominal plus X are counted, and if their proportion surpasses limits, the sieve is rejected. Measurements are conducted in both warp and weft directions and across the sieve surface for thorough evaluation.

Sieve rejection criteria include visible weaving defects such as loose wires, creases, wrinkles, or blinded apertures located away from the sieve edges. Additionally, apertures that exceed the maximum permissible deviation (tolerance X) from the nominal size are cause for rejection. For sieves with 20 or fewer apertures, all must be measured; for larger sieves, systematic inspection and measurement of suspect apertures are performed. Any aperture found to be oversized beyond tolerance results in rejection. These criteria ensure sieves maintain dimensional integrity and performance as per IS 460 Part 3.