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Glass Bead Fracture Count
I am told that how to count broken glass beads has become an audit issue. I’m not surprised – I have been privately wondering for a few years whether the count is to be (a) the number of original particles fractured or (b) the number of pieces resulting from the fracture.
Most specs have copied the ~10% broken limit for cast shot – same numbers for equivalent sizes, excluding only the smallest beads – and do not address the issue above. Only AMS 2430 implies (a) by saying “10% of beads are broken”.
My opinion is that glass bead fracture counting should be deleted from all specs – and not just because of (a) vs (b). When a cast shot particle fractures we worry about a sharp edged particle of at least ½ the original mass causing surface damage. If a glass bead fractures the myriad of sharp particles have little mass with which to do damage.
At Jack's request this will be posted to the EI on line forum.
Written by Pete Bailey
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Reply from Pete Bailey
I agree that excessive broken beads will reduce the effective intensity but believe that the size screening can handle that.
Response from Dr. Cammett
I have no great intelligence on just what to propose as a broken particle limit, but feel that we certainly cannot go without some control in our specifications. True that small broken glass particles may not cause much damage, but a lot of them may muck things up with respect to coverage issues. I have seen some bad looking glass media in my audit travels and have written nonconformance’s thereby. In fairness to current discussion, I should point out that there was no screening classification being done either. As a practical matter, the latter, if present, should eliminate a lot of broken particles.
Written by John Cammett
Response from Walter Beach
You may have opened Pandora’s box with this one.
First, sizes below AGB-18 are next to impossible to count, be it whole glass or broken particles, unless you have specialized, expensive equipment.
Second, I think there is also some confusion as to what AMS 2430 is stating.
Section 3.1.2.3 States: "Glass Beads and Ceramic shot shall be inspected in accordance with 4.3.3 to insure that not more than 10% (by number) of the beads or shot are broken".
Table 2A states the maximum number of unacceptable particles is 5% then a number is listed in the table for each size showing the maximum number of particles.
My question is why does it state 10% in one place and 5% in another, or am I reading it wrong?
A better approach to this would be to require dumping the machine and replace the entire charge after 2 hours of blast time for machines that do not have built-in classifiers. If you are in midstream peening parts you must run test strips prior to dumping to confirm the intensity. After reloading the machine you would again need to run test strips.
Written by Walter Beach
Response from Gary Burns
Walter is right. Further discussion appears necessary about this issue.
For consideration (from someone within GE who deals the most with dry glass bead):
In general we ignore the very small pieces as well. ( to a point.) Obviously, if you have
a very large quantity of small brokens you would have an issue. We also look at
Almen strip tests trending down, or if the patina of the parts goes to a dull flat color vs. a reflective
shiny color.
We typically run into these problems when the weather changes, i.e humidity changes.
Written by Gary Burns
