Zero Timing 1

[Asanga]

Okay I asked this question as part of an answer for the "Mental Challenge For Aero Engineers" Thread. But Wil Taylor who started that thread felt this was a topic of its own and therefore I am asking this as a separate question.

The question is:
If the fastener holes that are deburred are inspected with the use of HFEC (High Frequency Eddy Current) prior to installation into the aircraft, could those qualify as being zero timed?? Also a second question is how effective is the zero timing procedure as a safety precaution against crack growth? Anybody's comments on that would be great.
Cheers,
Asanga

P.S: I found out about zero timing while looking through an older (1980's) Boeing Structres Repair Document (D6-81987) so that is the extent of my knowledge of the process.
Asanga

 

[9scott]

I do not think deburring and BHFEC the hole alone constitutes zero timing. The hole would have to be reamed 2 times the BHFEC notch standard used after it was BHFEC. I.E. if a 0.020" notch standard was used, the hole would have to be reamed an additional 0.040" after inspecting. The purpose of this is that the actual probe can only detect cracks equal to or greater than the notch standard used. Thus if you BHFEC the hole with a 0.020" notch standard, there still might be a 0.019" crack in the hole.

As for the purpose of zero timing, it removes all the fatigued material from around the hole, thus you end up with a ‘new hole’. Zero timing the hole doesn’t really act as a safety precaution against crack growth, for that you might want to also look into cold working the hole. By cold working the hole, a mandrill larger than the hole is pulled through the hole putting the material around the hole into compression.

 

[WKTaylor]

Asanga..

After I finish with my "mental challenge question" in another thread I'll throw in my 2-cents worth here... Regards, Wil Taylor

 

[KSshopper]

I agree with 9scott's comment with and added comment: Setting up on an .020 notch doesn't necessarily mean you'll find a .020 flaw. There is a "probability of detection" factor that needs to be incorporated. For example, depending on the specific inspection (i.e. access, diameter, materials)an inspector who sets up on an .030 notch can find a .050 initial flaw size with a 90/95 confidence level.

 

[9scott]

I just read my response again and realized that I told you how to perform an insurance cut v.s. zero time the hole. To zero time you just need to oversize the hole either 1/64" or 1/32" (I am at home and cannot remember which) after BHFEC. If you are working on Boeing Commercial Aircraft check chapter 51 of the Service Repair Manual, under the investigation and damage clean up, or the fastener installation section. (ch 51-10-02 & ch 51-40-? For 757) They give a good explanation on how to zero time (at least in the 757 srm)

For the insurance cut, the 0.040" ream is performed after crack removal to ensure the hole is clean & that any fatigued material has been removed.