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tec #1051 12/17/13 11:31 AM
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I do not have /4C in front of me but here is the information. Hopefully this will help. AMS specifications are in inches, feet ..etc. So any conversion to metric are approxment. Does this corraspond with /4C? If so, then it is the same as 2430 and all the 2431/ specifications for steel media.

1" X 1" = 1 sq in or 25.4mm X 25.4mm = 645 sq mm
1/2" X 1/2" = .25 sq in or 12.7mm X 12.7mm = 151 sq mm
1/4" X 1/4" = .065 sq in. or 6.32mm X 6.32mm = 40 sq mm

Whether this method is bias or not, the question that needs to be asked is; does it control the process either manufacturing or peening to produce a desired result? I believe this test method has been in place for decades and has been proven. But, that does not mean that a proposal to the committee for a better method should not be considered. Please feel free to make that proposal and if not already become a member of the committee.

I hope this helps.

tec #1052 12/17/13 12:06 PM
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sorry

Inch mm Area mm
1 25.4 645.16
0.5 12.7 161.29
0.25 6.35 40.3225
0.0625 1.588 2.521744

tec #1053 12/17/13 01:01 PM
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I presented a model that allows an estimate of particle numbers to be made using simple arithmetic. For the model spheres were assumed, not irregular particles. For identical spheres 225 is correct for square packing. Hence we have a ball-park figure of 2475 particles for 11 fields. For actual AWS 17 particles the number will obviously be different - as you have pointed out. It will not, however, be way out. With a maximum of 2 unacceptable particles being allowed that will correspond to 'less than one in a thousand'.

tec #1054 12/17/13 02:01 PM
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We should find some interns with spare time to do actual count of particles in a sample field.
Several years ago David Francis of Metal Improvement Company made model estimates that seemed to support the number of particles for each media size maintaining a (fairly) constant percentage.
I'll search for that document (in my spare time)

tec #1056 12/17/13 07:04 PM
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For the three largest sizes the estimated number of particles to be inspected lies between 217 and 312. For all smaller sizes the number varies (with size) from 1852 to 2945. These numbers are based on my own Excel-based model.
The biggest problem with AMS 2431 shape analysis is that it is based on subjective visual inspection - different people will give different answers for the same samples. It would be very useful if a set of reference images could be produced. These only require a digital photograph to be produced for each of the field being viewed. These images could be displayed on any computer screen to be 'visually inspected'. Assessments by different individuals could then be compared. This could lead to a future insistence on computer-based image analysis!

tec #1057 12/18/13 08:12 AM
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You are right visual inspection is very open to different people opinion of correct shape but this is random uncertainty which is common in all testing and inspection.
The main problem I can see is the range of media as you identified can potential be massive, my boss found similar results to you Socrates.
I believe that the range is far to big to have a numerical figure depicting the total number of unacceptable particles. It should be a percentage that is based upon the screened sample currently being inspected.

tec #1058 12/18/13 06:36 PM
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We have to be practical! A percentage would require that more than two thousand individual shot particles would have to be counted (except for the rarely-used huge particles). That would be very tedious and time-consuming. With the present specification we only have to identify marginal and unacceptable particles. This could easily be converted to a percentage to the nearest significant figure i.e <0.1% but would not add anything useful.

tec #1059 12/19/13 08:34 AM
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Why do you need to know how many marginal and unacceptable particles there are in the sample, If you don’t know how many particles make up that sample, the results don’t mean anything?

As you said there is a huge range as to the potential number of particles in the sample, that is true!
Unless you have number as how many particles are in the sample it doesn't mean anything.
All you know is that the sample you had has 2 failed particles. Does that mean the stock is no good because it had 2 fails in an indeterminable number?

I wouldn't suggest counting them it’s not practical but a percentage of failures based on weight could be useful.
We don’t know how many particles are in each container but we know the weight fairly accurately.

We may even know the hopper weight for instance but as the amount in the hopper it’s impossible!!!
If there is an acceptable amount of fails based on weight the experiment has more meaning.

tec #1060 12/19/13 10:13 AM
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The results do mean something - quantitatively! Consider, for example, that we decide to set a limit of 0.1% of unacceptable particles in a defined batch. 2 unacceptable particles would then be the most that could be tolerated in a batch of 2000 (2 being 0.1% of 2000). We now have to ensure that the sample size examined contains at least 2000 particles. As shown previously there will be at least 2000 particles in the total number of fields specified for each normal particle size.

tec #1061 12/19/13 10:47 AM
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So you would have to go to some extent to count them?

Because 2 fails is based on 0.1% of 2000 particles you need to first ensure there are at least 2000 particle to screen.

But if it was calculated against weight you could have any number of particles you would only have to do some math to calculate a percentage of fails by weight a sample of 2000 particles of CWS62 has a mass of approximately 43.6g. It would be approximation of-course and not as accurate as counting but a lot easier and the results could be used a lot more effectively.

Percentage to weight is already used to calculate size failures.

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