Shaving Rivet Heads 1

[Sparweb]

Has anyone tried shaving the head of a flush rivet to prevent a knife-edge condition in the sheet? We would normally use a NAS1097 rivet in 0.032" sheet, but since this is the third time rivets are being re-driven into these holes, it's time to use oversize rivets. That calls for a NAS1241 rivet, but its head would require such a big countersink (0.035" head) that they would be in a knife-edge. Countersinking only about 0.26" into the sheet, and shaving the heads down once they're driven has been suggested.

Any thoughts? Is this accepted practice? What would that do to the joint allowables? Thanks.


STF

 

[koopas]

Good day Sparweb,

Yes, it is standard practice. Take out the old rivet, redrill to accomodate the OS rivet while leaving the countersink intact. Shoot the new OS rivet and shave flush.

Niu's "Airframe Sizing and Stress Analysis" book, Chapter 16, describes the procedure.

My guess is that your joint allowable would actually be slightly increased for that particular OS rivet since the countersink is smaller.

Hope that helps.
Alex

 

[jetmaker]

SparWeb, koopas,

With regards to shaving rivets and joint strength... you should not expect any better performance from an oversized shaved rivet than the per drawing fastener. If you are limiting the csh depth in a thin sheet, failure mode is likely by pull-through, and since your head contact area is the same as the per drawing fastener, it makes sense not to assume that you will have any better strength then the per drawing fastener. In thick, single lap joints, you have to worry about shearing the fastener head off. Again.. this shear area is no different than the per drawing fastener.

The real problem here is that most joint strength tables do not tell you the mode of failure, and so being able to make a reasonable guess at the potential strength increase is very difficult. For that reason, when I analyze a shaved head, I will rarely assume the rivet has any greater capability than the per drawing fastener.

jetmaker

 

[koopas]

Jetmaker,

Wouldn't you think that even though your head contact area does not change, the increase in shank diameter translates into an increased bearing and shear strength allowable?

Even though the resistance of the rivet head prying off due to fastener bending doesn't differ due to identical head contact area, I'd imagine that you would experience a slight overall strength gain with the OS rivet.

What do you think?
Alex

 

[aviat]

Even the manufacture's microshave rivets. FYI the following site gives information on rivets repairs. They limit the number of rivets that may be replaced.

http://www.flash.net/~gila/rivet_spec/rivet_a.htm

However, with your material being only 0.032 you should not be using a counter sink because of edges will be too thin. Anything under 0.040 and under should be dimpled. Both the sheet and the stringer etc.would have to be dimpled.

There is another means to do this if you have access from the back and that is to use a universal head inside and use the formed head to fill the counter sink. This is called back riveting.

http://www.aircraft-tool.com/ecatfiles/ECat2002A.pdf

(under riviting) sell tools for both dimpling in situ or back riviting.

Cheers

 

[koopas]

aviat,

Pertaining to your last point on back riveting, do you need special tools for that?

Back riveting to me sounds like the NACA style of installing rivets. You shoot a protruding head from the inside and the driven head is bucked into a 82 deg. countersink.

Someone correct me?

Alex

 

[jetmaker]

hey koopas.

I would agree with you in that a higher strength would be realized if you were in a thick sheet, where the fastener is critical in shear. However, here is what you must consider. There are many more modes of failure than simple fastener shear or failure by bearing. Heads pop off the shank, the csk shears about the shank, the csk pulls through the sheet, and a few others which are combination modes. If you are on the joint strength charts, and are not shear critical, you have no idea what the failure mode is. Therefore, modifying the head in any fashion MAY result in a change in strength. You most certainly will not reach the values of the oversized fastener, which are based on the full head. On the other hand, you WILL most likely exceed those of the per drawing fastener. Therefore, to be safe, stick with maximum capability of the per drawing fastener when you have to limit csk depths.

This is a good rule, and you will likely never get in trouble using it. If your rework is so critical that you need to use the full fastener values... then I suggest adding a row, or spotting in some additional fasteners elsewhere to be safe. These are just my beliefs... but I think they are worth sharing.

Hope this helps.

jetmaker

 

[Sparweb]

Thanks for the links, and fortunately, I'm not in a situation where I have to back-rivet or wheedle every last pound from the joint for a positive MS.

The situation I'm actually in is putting a doubler over a spot that has universal head rivets installed. Once they're out, the holes just have to be picked up and the head of the c-sunk rivet goes into the doubler. The holes in the parent skin dictate that an oversize fastener be used, hence the shaving so that an 0.032" doubler can be put on.

Reading thru Mil-R-47196... Then I compare the head size of standard sized rivets, the head size of a reduced-head rivet (eg. NAS1097), and the shaving limits in the spec... There's a relation there.

Thanks!


STF