120 deg. Blind Rivets in 100 deg. Dimples Mar. 11, 2002

=====================================================================

Suggestions/Recommendations

My problems pulling flush blind rivets stirred up the Sonex crowd and I received many good suggestions by the Sonex email list as well as private mail. Here is an extract may it be of some help for all the 'newbies':

 Tooling problem?  Hugh suggested to check the nose piece of the riveter. A too large hole could lead to protruding stem fragments. Steven build a dedicated dimpling press for the job, suggests a 'good' pressure on the dimpling die  nose piece is probably not a problem with my tool, but worth checking. If there is a visible 'halo' around the dimple the pressure should be right.
 Wrong air pressure?  opinions differ here, Dan uses only 30PSI with good results and grinds off the offending stems, Chris pulls 40 PSI and hits the protruding stems back to where they belong, Ed uses 50 PSI pulling the trigger slowly, Pat works at much higher 90 - 120 PSI with good results  seems I'll have to experiment a bit with air pressure
 Bad holes?

 Pat also recommends tight holes drilled with high speed and good lubrication.

Ryan drills #40 pilot holes, dimples them and then reams to #30

 I use a Sioux air drill, cobalt drill bits and WD-40 as lubricant. Contrary to Pat my #30 holes were outside of spec after dimpling. As Ryan suggests, I also dimple now directly the pilot holes with specially ground dies. For me this is the only way to prevent oversized holes.

 

The Experiment (aren't we building 'Experimentals'?)

I already dimpled (once again deviating from the Sonex plans :) a lot of rivet holes in control surfaces and other parts using Avery's spring-back' dimple dies. However not all rivets can be bucketed (particularily not at both sides of the sontrol surfaces). So blind flush rivets have to be installed instead. Unfortunately the blind rivets have an 120 degrees shop head ('real' AD solid flush rivets have a 100 degrees head).

So the 1000$ question is: how will the flush blind rivets occupy the 100 degrees dimples? Before I will spend $$$ for the rivets I posted this question at the Sonex email list.

There builder-buddy Chris Boultinghouse was so kind to offer me a few flush blind rivets for experimenting. After this eleborate introduction, here is what I found out:

 

the good News

The blind rivets are nicely flush, viewed from the outside.

So my aesthetic feelings are not hurt much using these ones. The little holes are a bit disturbing, so I will use the blind rivets wherever possible in ereas which are not directly visible (bottom side of wing and control surfaces, right side of vertical tail).

 

The real question however is: how does the head fill up the dimpled hole? To find out I split one rivet joint in the middle and polished and etched the surface.

As one can easily see, the rivet head fills up the dimpled hole completely. However the stem is pulled too far up and the usual 'barrel' shape of the shop head (bottom part of the rivet) is very faint. A fragment of the broken stem protrudes above the skin surface... And this is where we approach the 'bad news' section...

 

the bad news

One can see here a good installation at the left and a bad one at the right (stem is pulled too far into the rivet). At my first try with a couple of rivets it was not possible to produce consistent results. I used about 60PSI at the pneumatic rivet puller. Sometimes I got the impression that pulling the trigger slowly through produced better results than a quick pull. Almost all rivets where the stem was pulled too deep inside showed a fragmet of the stem protruding above the rivet head.

This problem my be caused by my ineperience in case of pulling blind rivets, but it may also be caused by this special application. I never experienced this problems with protruding head blind rivets.

 

This above mentioned 'barrel' problem needed also a closer look. Here I joined two strips of 6061-T6 and then tried to tear them apart using a big screwdriver. The question was: will the rivets stay or will they be pulled thru the lower sheet.

 

Here is the result. As anticipated the 'barrels' held the rivets nicely at their place. When I tried to peel off the top sheet, the rivet head at the left side stayed at its place, a perfectly round hole was ripped into the upper sheet, exactly the diameter of the rivet head.

 

Somewhere in 1999...

I always have this idea to flush rivet the wing and fuselage

skins - not for any practical reasons, it's just for the

looks (and to irritate the RV guys). So I bought a pair of

the good Avery's springback dimple dies (1/8") some time ago.

 

Using my heavy duty drillpress, I dimpled two strips of

2024-T3 0.025 aluminum. Then I riveted them using AN-426A

rivets (these are the 'soft' ones I use for practicing),

in real word the 'AD' hardened rivets will be used.

 

This picture shows the result

 

 

It looks quite good - but not perfect. Especially the

leftmost rivets are dimpled a bit to 'deep'. I currently

don't know why this is so. There is no play in pressing

the dimples - I think. If I should grind the mating

surfaces a bit to reduce the dimple's depth? How shall I

maintain the proper alignment?

 

Anyone an Idea? please send me a hint