Dimple with blue plastic not removed...

Miles Bowen and David Buntin got my attention in another thread by suggesting dimpling with the blue plastic not removed was not a good idea. I dimpled with the blue material not removed after seeing some scratches between holes on the HS. With the VS there are no scratches, just what is desired with a plan to polish the airframe.

Well, this morning while working on the rudder and its stiffeners I decided to do a test with and without the material removed. Each stiffener has 11 dimples which were measured after dimpling. (The micro meter is mechanical but checked out OK by confirming the thickness of the material at .025.)

Here are the results of measuring the dimple depth of the 11 holes in each of the two pieces.

Dimple with blue stuff removed,
.052,.055,.053,.054,.052,.055,.054,.055,.053,.054,.054 (average .0537)

Dimple with blue stuff not removed,
.052,.054,.051,.050,.051,.053,.053,.053,.050,.052,.052 (average .0527)

There appears to be a slight difference, .001 less depth with the blue stuff not removed. As far as the rivet resting in the dimple, I can not tell a difference comparing the HS (no plastic) with the VS (dimple with plastic).

Not all the set rivets are perfect but that probably has more to with my solo riveting that the quality of the dimple.

I thank Miles and David for expressing their concern but my prime concern here is a neat surface and to that end I will continue dimpling with the blue stuff in place.

PS The date and time of the camera are not set correctly, the image was taken this morning. I was too tired to do it last night.



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Dave,

Like you, I dimple through the blue plastic and sometimes not. Like you, I never could discern any real world difference in the comparative result. What you do find by dimpling through the plastic, the chances of casual scratching are greatly reduced.

BTW, the other day when you said you were going to start the VS, I was going to post that you should be able to finish it in less than a day but refrained from commenting because I did not wish to come across as bragging. It was gratifying to me you were able to assemble it in 10.4 hours. Like I said earlier, that empennage kit slams together so fast, you should be able to complete it in 3 weeks and it sounds like you are on track to do just that.

I usually dimple with the plastic on the dimple side but removed from the back side. The plastic protects the part from the male die and is not needed on the back. Worse, the back side die will often cut little 'washers' around the dimples which are left behind when pulling the bulk of the plastic off. They can be a bit tedious to remove individually and easy to forget about if the plastic is not blue. You don't want to leave them on and rivet parts together. If you prime after dimpling, the etch will lift any plastic left behind - but that's not when you want to discover it.

Dimpling with the Blue on

It was my understanding that the issue in dimpling with the blue on was that when the hole was drilled to final size small metal fragments could be trapped under the blue plastic. During dimpling these metal fragments could be the source of future cracking around the rivets...so shouldn't the plastic be removed in at least the area of the dimple to avoid this possibiltiy? Just asken...

Berchmans said:

It was my understanding that the issue in dimpling with the blue on was that when the hole was drilled to final size small metal fragments could be trapped under the blue plastic. During dimpling these metal fragments could be the source of future cracking around the rivets...so shouldn't the plastic be removed in at least the area of the dimple to avoid this possibiltiy? Just asken...

Click to expand...

The inside plastic material can be removed before drilling and dimpling and must be removed before prime. Its the outside that needs protection and that's what this is about.

It is not that big a deal, the important thing is to keep moving forward, get it done, and fly the airplane sooner than later.

Agreed..

Berchmans said:

It was my understanding that the issue in dimpling with the blue on was that when the hole was drilled to final size small metal fragments could be trapped under the blue plastic. During dimpling these metal fragments could be the source of future cracking around the rivets...so shouldn't the plastic be removed in at least the area of the dimple to avoid this possibiltiy? Just asken...

Click to expand...

What he said. I agree. It means you ahve to be careful when dimpling not to scratch the male die down the skins, but it ensures you don't have shavings or shallowness in the dimple. My $0.02..

Nice

Nice research David (with the micrometer). I didn't know what the actual difference was when I compared the two, but as previously mentioned in my other post, it wasn't gross. I would have expected more of a difference. My plastic covering was clear on both sides (before the blue came around), so I wonder if that was a little thicker than the blue plastic?

As Burke mentioned, I could see where shards of alum. could stick to the plastic and cause a problem with the dimpling. I'm guessing that's the reason Vans recommends to dimple with the plastic off, though I'm not for sure? Dave

Good data point, David. As long as the rivets don't protrude out of the dimple you're good to go. I did have trouble adequately deburring (or at least checking for burrs) with the plastic still in place.

Just out of curiosity, are you using an impact-type C-frame tool or a DRDT-2 to do your dimpling? I'm guessing the impact dimpler would be less prone to underdimpling with the plastic on. When I set my DRDT-2 per the instructions (lower the handle to the stop, then adjust the ram down until the dies contact), I could still feel the edge of the rivet head after dimpling 0.032 skins (without the vinyl). About a half turn or so more and the rivets fit much better. You CAN under-dimple with the DRDT.

Rick,
I seem to recall on your post about building (and even flying) with the blue vinyl still in place, you said you removed a small circle of vinyl from around each rivet hole. Was this something you did in the beginning, then found to be not necessary?

My thoughts.. ;-)

Just dimple.. Keep dimpling. Then dimple some more. Then 2-3 years later keep dimpling..

After that start working on electrical and firewall forward.

Then bang your head on the wall.

Berchmans said:

....small metal fragments could be trapped under the blue plastic....

Click to expand...

I found that the gulf between "could" and "would" was so great that I chose to leave the blue plastic in place. In over 6 years of operational service with the -6A, I have yet to observe any downside to that original decision.

Rick,
I seem to recall on your post about building (and even flying) with the blue vinyl still in place, you said you removed a small circle of vinyl from around each rivet hole....

Click to expand...

That is true. Unlike the -6A, to facilitate my goal of having the first RV to fly with its blue vinyl intact, I did in fact remove a small circle of plastic from around each rivet location on the exterior airframe skins of the -8 prior to dimpling. That still left one heck of a lot of other rivet locations that had to be dimpled and in those cases, chose to leave the plastic intact until permanent assembly required its removal.

longranger said:

Just out of curiosity, are you using an impact-type C-frame tool or a DRDT-2 to do your dimpling? I'm guessing the impact dimpler would be less prone to underdimpling with the plastic on. When I set my DRDT-2 per the instructions (lower the handle to the stop, then adjust the ram down until the dies contact), I could still feel the edge of the rivet head after dimpling 0.032 skins (without the vinyl). About a half turn or so more and the rivets fit much better. You CAN under-dimple with the DRDT.

Click to expand...

Using the DRDT-2. It is adjusted so the dimple dies come together and then the handle moves over center for the final compression. Seems to be doing a fairly consistent job.

I switched the dies and have the male on top as you suggested. It works much better, thanks for that tip.

(primer curing right now, should be back riveting later this afternoon)