Dimpling or countersinking

[Luca Milesi]

In one month I will start with the build of my Savannah

For some positions countersinking is required and I’m making some experiments with some techniques.

The first hole is countersunk with a 11 mm drillbit driven by hand

The second hole is dimpled

I also ordered some specials  A4 rivets with countersinked head.

I m curious to see how they work.

Comments and advice are welcome

lucas

[kasper]

While they look nice you will need to buy a special drill bit to do the countersinking - the angle of the cut face has to match the angle of the countersink rivet heads and they are not what you get on any standard drill bit.

 

 

This is not just a looks good thing - the strength of the riveted piece and its longevity in servive require that the angle of the cut face allow the rivet head to closely hold.

 

The dimple looks perfect with just the deburring to go.

 

[Luca Milesi]

Thank you very much. I really appreciate your advices.

[IBob]

There's been lots of discussion around this on other threads.

While countersinking into thin sheet is not ideal, it is what the Savannah manual calls for.

 

1. As I see it, the reason it is acceptable in this case is that the there are very few countersink holes, and they are not 'structural': they are mostly used for holding things like nutplates in place, where the final load will be taken by the nutplate at the rear of the sheet, not by the countersunk rivet.

Consider also that, while dimpling may produce a nice countersink, your nutplate will not then sit flush to the back of the sheet. As it needs to do to be properly tightened.

 

2. However, countersinking with an ordinary drill often does not always produce a tidy result: the bit tends to wander in the hole. As noted by Kasper above, you really need a proper countersink drill, which has a central spigot that holds the drill centered.

 

3. The angle required to match the rivets is 120deg. I have several drill sets that are not 120degree, but my Bosch set is.

 

4. If I was building again, I would cut, not dimple, but I would source the correct countersink tool from AircraftSpruce. They are expensive in sets, but you don't need the set, you just need the ones for the A4 and A5 rivets. Just be sure you specify 120deg.

 

Finally, consider that thousands of Savannahs have been built according to the instructions in the manual, and are flying just fine. That suggests to me that the instructions in the manual may be followed with confidence.

 

There is a huge amount of Savannah info on this site to help you.

All the best, and don't forget to post pics of your progress for us to enjoy!

 

 

 

 

 

 

[old man emu]

1 hour ago, IBob said:

The angle required to match the rivets is 120deg.

According to my parts Bible, the angle between the head of the rivet and the shaft for an MS20426A(D)-(L) rivet is 100 degrees.

 

Here is some information of riveting. Sorry that it is upside down, but it's copied from an American publication.  It can be read correctly in the USA.

 

Rivets.pdf

[IBob]

2 minutes ago, old man emu said:

According to my parts Bible, the angle between the head of the rivet and the shaft for an MS20426A(D)-(L) rivet is 100 degrees.

 

Here is some information of riveting. Sorry that it is upside down, but it's copied from an American publication.  It can be read correctly in the USA.

 

Rivets.pdf 1.76 MB · 1 download

OME the Savannah is put together with pop or blind rivets, not solid rivets. There are some solid rivets (at cockpit sides and main spar) but that is all done at the factory.

[IBob]

 

 

Etc.............

[old man emu]

3 hours ago, IBob said:

the Savannah is put together with pop or blind rivets, not solid rivets

Fair enough!

 

Hope he's got a pneumatic riveter and a good sized compressor. 

[IBob]

The kit comes with a pneumatic rivet gun, OME.

And a belt driven compressor (rather than a direct-drive screamer) is definitely the way to go..........)

[kgwilson]

Hi Lucas,

 

You started a thread on this subject in December with many posts and advice from various members. I assume this is more or less a continuation of the subject.

 

Basic rule of thumb. Aluminium sheet should be dimpled and Aluminium plate countersunk. I made my own dimpling tool and used large 120 deg drills with light pressure to countersink the plate substrate with results as good as those I'd seen when specialist countersink tools were used. It is just easier to stuff it up using my methods. All you need though is a bit of practice before you start on the real thing.

 

[Luca Milesi]

I thank you very much fir your reply.

Yes, I have extra bought a tail kit and now I m making som experiments. The advices  from the members  in relation with  my december thread was very interessant and helpful to me.

I countersinked the hole in the picture with a 11 mm drillbit driven by hand .

Blue Skies to all  from south Switzerland