Home Forums General Discussion Specifications J444 grit size methods min-min or min-max

Mr. Champaigne,
Please allow me the liberty of introducing myself via this email as I would appreciate your help with some education on classifying grit size.
I am a corrosion engineer working at Kennedy Space Center and currently have some oversight on a depainting and coating project.
I have attached a spreadsheet <> that hopefully you can follow so that you can straighten me out.
Please feel free to call me for discussion as I think I need it.
Sincerely and thank you,
Joe

Joseph J. Curran
ASRC Aerospace: Engineer (Work Control)
mail stop: ASRC-09
Kennedy Space Center, Fl 32899
(321) 867-7558 work (321) 794-3078 cell
http://www.shotpeener.com/ubb/images_misc/grit_size.xls

---Response from Bob Gillespie---
Jack:

Joe Curran has a good point. Table 1 in SAE J444 is for Cast Shot sizing
and uses the min-min designation. It also indicates in the footnote of
Table 1 that the sieves used "Corresponds to ISO Recommendations", so there
appears to be an effort to agree with ISO. Table 2 is for Cast Grit sizing
with the same ISO footnote. However, it does not indicate either min-min
or max-min for the cumulative % values in Table 2. One could assume that
since Table 1 uses min-min values, Table 2 would also. According to Joe
Curran, this would put SAE J444 in conflict with ISO. If the max-min
values are less confusing and also used by ISO, I would think J444 should
adapt the max-min values as well, and clearly state them in both Tables 1
and 2. I do not have a copy of ISO 6106, so I cannot be certain of the
values in it.

Table 1 uses a mixture of min-min and max-min requirements. Table 2 does not specify – except that it implies 'max-min'. The Irvinamasteel web site uses a mixture of min-min (for G10-24) and max-min (for G16 upwards) as J444 specification limits for cast grit! It follows that there is a need for clarification. Table 2 should be 'corrected' to state, explicitly, that max-min values are required.

Correction :Table 1 uses a mixture of min-min and max-min requirements. Table 2 does not specify – except that it implies 'max-min'. The Irvinamasteel web site uses a mixture of min-min (for G10-14) and max-min (for G16 upwards) as J444 specification limits for cast grit! It follows that there is a need for clarification. Table 2 should be 'corrected' to state, explicitly, that max-min values are required.

Hi,
I am posting an email I sent to Jack addressing Bob's reply. I could not use this forum earlier as I was not a member then. So that being said here is another message for thought.

The following is an attempt to clear up some items are written.

1) Actually table 1 uses both min-min and max-min and labels the screens as such.
2) Table 2 in the heading states "Test..........with Maximum and Minimum Cumulative Percentages .....on corresponding test sieves.
So there is a definite denotation to use max-min.
This is where the confusion lies as I do not think that industry uses max-min as stated.
3) The assumption that table 1 uses min-min may be consider incorrect in a few regards. First, table 1 also states max-min in the heading and has max listed through-out the table itself; Second, just because one table uses a format does not necessarily mean that the following table should use the same format; Thirdly, any Standards or Test Methods, be they SAE, ASTM, etc. should be so clear as to leave no room for assumptions or ambiguities.

Please accept my apologies for being anal about this, but clarity is certainly is very important here at Kennedy Space Center and I hope that it is important enough throughout the industry to have complete clarification.
Recently, someone told me that industry has been using min-min for 30 years and therefore it has to be right. Because I have been down this road before (many times) it is hard for me to accept such a statement. Usage does not necessarily make it correct and have found cases where people have been doing something incorrectly for 20-30 years due to such statements.

Again my apologies for opening this can of worms, Sincerely, Joe

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

Jack,
Thank you for posting David Kirk's response. After reading Mr. Kirk's post, I believe that J444 is definitely a max-min spec as stated in the header; with the requirement of all material (100%) passing the first defined large sieve, with a maximum % allowed on the second smaller sieve, and with an accumulated minimum % allowed on the third sieve (sieve 2 plus sieve 3).

How does one know that the authorities are in agreement with max-min? And if the authorities are in agreement, should a governing body inform industry that they are applying a min-min spec contrary to J444? Is there a method to do so? Is there a need to do so?
Sincerely,
Joe

We have been manufacturing cast steel abrasives for many years. As a manufacture of those abrasives we have been producing cast steel grit that conforms to many industry standards, including the SAE J-444 grit specification. The proper interpretation of the grit sieve specification is min, min.

Please don’t take my word for this; review the following industry standards. Although they differ in the description of the specification, they all state the following: All pass and some 1% max on the top sieve. The second specification sieve %min and the third specification sieve %min.

MIL-S-851D -- Military Air Force
SFSA 20-66 – Steel Foundries Society of America
SSPC – AB3 -- Society of Protective Coatings
ISO 11124-3 -- International Organization for Standardization

During the next Surface Enhancement meeting I will be proposing that we add the term (min) in Table 2 of SAE J-444 as intended and to bring it in line with the other industry standards for Cast Steel Grit Abrasive.

If anyone has any questions or concerns please feel free to contact me at [email protected]

Regards
Dennis Scharer
Ervin Industries
Technical Services Manager