From David Kirk:
GRIT SPECIFICATIONS
INTRODUCTION
J444 2005 contains tables of specifications for cast shot and for cast grit. Table 1, for cast shot, specifies "Test Sieve Opening Size and Designation With Maximum and Minimum Cumulative Percentages Allowed on Corresponding Test Sieves". This is included, verbatim, in Table 2 for cast grit. Whereas maximum and minimum values are explicit in Table 1 they are only implicit in Table 2. This is clearly an omission and should be corrected.
User requirements for grit (used for blast cleaning) are quite different from those for shot (used for shot peening). Effective blast cleaning can normally be carried out with deliberate mixing of different grades of grade. With such a combination mixture the larger particles carry out the major work of removing the contaminants whilst the smaller particles, much greater in number, carry out the lighter removal and provide coverage. Shot peening requires that there is a much narrower range of sizes in the mix – in order to ensure uniform coverage. These different user requirements underpin the need for different specifications for grit and shot.
Grit cost per kilogram increases as the size decreases. That is primarily because more work has to done on the particles to break them down to smaller sizes. It is therefore in a manufacturer's interests to bias particle size to the top end of a given specification range. On the other hand it is in the user's interests for it to be towards the bottom end of the specification range – allowing a cheaper admixture of larger and smaller grades.
Specifications are based on a combination of two parameters – sieve opening size and measured weights of accumulated particles. Both of these parameters are objective – sieves are made to close tolerances and weighing is easily calibrated and inexpensive. The major difference in particle specifications is the use of so-called "max-min" and "min-min" requirements. These are discussed in the following sections in terms of their respective significance for both users and suppliers of grit.
MAX-MIN SPECIFICATION
With max-min specifications we see three defined restrictions. The first is that all particles must pass through a defined, largest-opening, sieve. Secondly, there is a maximum weight percentage that can be allowed on a second, smaller, sieve. The third limit is that the weight percentage accumulated on the second and a third, smallest-opening, sieve must exceed a stated value.
Fig.1 is a schematic representation of a max-min specification example – for SAE J444 G25. All particles are required to pass through a No.16 (1.18mm) sieve. A maximum of 70 weight % is allowed on a subsequent No.25 (0.710mm) sieve) – 'Upper Specification Limit'. Finally the minimum of the combined weights on Nos.25 and 40 (0.425mm) sieves must be a minimum of 80% - 'Lower Specification Limit'. The required usages of the three sieve sizes automatically divides the range of grit sizes into three 'bins' – which we can arbitrarily call "fines", "small" and "large". The lower specification limit, by subtraction, requires that there be a maximum of 20% of 'fines'. Since the upper specification limit sets a maximum of 70% for the 'large' particles we have an indirect requirement that we have at least 10% of particles that can be classed as 'small'.
In order to satisfy this G25 specification, a manufacturer has to 'fraction' grit from a particular production process. On the one hand a process may aim at producing a single size grade of grit. On the other hand the process may produce an output that aims to produce several size grades of grit. For single-size output we need a much
Fig.1 Schematic representation of SAE J444 G25 cast grit size specification using
MAX-MIN interpretation of limits.
narrow 'normal distribution' of size. Multi-size grit output requires a broad 'normal distribution' of sizes that are subsequently 'fractioned'. A third alternative is 'progressive' where a given batch of as-cast particles is broken down in stages to produce a corresponding range of sizes.
It would be simple to construct a laboratory sample of G25 shot that contained, to the nearest gram, exactly 20% of fines, 10% of small and 70% of large particles. That would not, however, reflect actual commercial grit production. A commercial sample taken from a normal distribution might contain, for example, 5% of fines, 35% of small and 60% of large particles. It makes sense to keep the percentage of fines low - because they can form part of more-expensive smaller-sized grit batches. The percentage of large particles must be significantly below the 70% maximum – to avoid breaking the specification limit when using a production process that has necessary variability.
MIN-MIN SPECIFICATION
With min-min specification we again have three defined restrictions. The first is that all particles must pass through a defined, largest-opening, sieve. Secondly, there is a minimum weight percentage that can be allowed on a second, smaller, sieve. The third limit is that the weight percentage accumulated on the second and a third, smallest-opening, sieve must exceed a stated value.
Fig.2 is a schematic representation of a min-min specification example – again based on SAE J444 for G25. All particles are required to pass through a No.16 (1.18mm) sieve. A minimum of 70 weight % is allowed on a subsequent No.25 (0.710mm) sieve) – 'Upper Specification Limit'. Finally the minimum of the combined weights on Nos.25 and 40 (0.425mm) sieves must be a minimum of 80% - 'Lower Specification Limit'. The required usages of the three sieve sizes again automatically divides the range of grit sizes into three 'bins' – which we can arbitrarily call "fines", "small" and "large". The lower specification limit, by subtraction, requires that there be a maximum of 20% of 'fines'. Since the upper specification limit sets a minimum of 70% for the 'large' particles we no longer have an indirect requirement of at least 10% of particles can be classed as 'small'.
Fig.2 Schematic representation of SAE J444 G25 cast grit size specification using
MIN-MIN interpretation of limits.
With a MIN-MIN interpretation of the specification limits manufacturers would be required to ensure a substantially larger fraction of 'large' particles' (than with a MIN-MIN interpretation). That would incur substantial additional production costs and would offer no perceived advantages to the user.
CONCLUSIONS
It may be concluded that there are significant differences between using "max-min" as compared with using "min-min" interpretation of SAE J444 specification limits for grit sizes. Hence it is reasonable to propose that the potential ambiguity in the specification be removed – in favour of "max-min".
D. Kirk
January 18th, 2007.
also available as download:
http://www.shotpeener.com/ubb/images_misc/grit.doc
