Question(s) for sheet metal gurus

[Pbruce]

I am putting my inspection hatches on. I just completed the last one (the big one by the aileron bellcrank) and I see that when the screws are in around the perimeter, the cover has a “puffed out” appearance. I put a straight edge on it and it bulges out about 5/16” in the center. The other smaller hatches all seem to be flat or slightly concave, by about 1/16”. I can live with them.

I’m not sure what the culprit is. I’m using the nutplates from Avipro (k1000 I think). I match-drilled the holes in the inspection cover frame to the cover itself, then dimpled both for 6-32 100-degree countersunk screws. I noticed that the dimpling caused a lot of distortion in both the ring and the cover, which took some effort to flatten out (mostly). Then I dimpled the #40 nutplate mounting holes in the frame, flattened it out again, then drilled out the screw holes to15/64 in the frame so there is clearance around the nutplate screw hole. Then I flush-riveted the nutplates on. I match-drilled the cover frame to the skin and dimpled for flush rivets. The frame riveted down nicely under the skin. I do see that my screw holes in the cover don’t exactly match with the nutplates anymore. Some are off by 1/4 of a hole (maybe 1/64 to 1/100 of an inch). I tried to widen those out a bit, then screwed the cover on. It goes on but as I said, it’s buckled into a puffed shape.

I want to get this cover to lie reasonably flat, by cutting and or putting stiffeners in if I must. I’m not sure how to proceed. I also want to improve my technique to allow for the distortion caused by the dimpling, or eliminate any error in mounting nutplates, or whatever else is causing the holes to line up initially but not once riveting nutplates on and dimpling is completed. I’m using a nutplate as a jig, with a pan head screw tightly clamping the nutplate onto the frame through a #30 hole in the frame.

I have some .032 material which I could use to make another cover with, or I could use this one for the other wing(not yet started) and use the other blank cover for this hole. However, I’m doubtful I could get the holes to line up perfectly now that the ring is solidly riveted to the skin. Is it possible to accurately mark through from the back of the nutplate onto the cover and then drill?
Any advice is appreciated. Thanks.

[BravoGolf]

If you choose to make a new cover you could use a hole finder. ACS calls it a strap duplicator

https://www.aircraftspruce.com/catal...Duplicator.php

put the pin in the hole and use the bushing as a drill guide.

[N942VT]

You're not alone on this, aluminum moves a lot when lots of dimples are pressed into that small an area. Expansion of the rivets will make a weak piece crawl a little too.
Install the backing ring as you did and then match drill the cover to the ring and install the nut plates. Don't over dimple, just press enough to do the job, or even a little less.

[rodsmith]

Lay the cover over the wing opening with it pressed flat, is it now a little too large? I'm wondering if trimming the edges a bit would fix the problem.

[Pbruce]

If you choose to make a new cover you could use a hole finder. ACS calls it a strap duplicator

https://www.aircraftspruce.com/catal...Duplicator.php

put the pin in the hole and use the bushing as a drill guide.

i May have to do this. Will it fit snugly into the nutpate though? because if it’s loose, then I will have an accuracy issue again.

[jaredyates]

You can always make a strap duplicator and size the pin as you want. Either swell a rivet, or jb weld something that fits.

[Pbruce]

Lay the cover over the wing opening with it pressed flat, is it now a little too large? I'm wondering if trimming the edges a bit would fix the problem.

No, In fact it’s a bit small on three sides. I will double check to make sure the last edge is clear.

[Pbruce]

You're not alone on this, aluminum moves a lot when lots of dimples are pressed into that small an area. Expansion of the rivets will make a weak piece crawl a little too.
Install the backing ring as you did and then match drill the cover to the ring and install the nut plates. Don't over dimple, just press enough to do the job, or even a little less.

I struggled with this issue on the other small covers. First, because I had the wrong screw head (82 degrees angle on the head). So I ordered screws with 100 degree heads, but they still stuck out proud. I didn’t realize that a 1/8” dimple does not accept a 6-32 screw head. I borrowed the dimple die for 6-32 screw and they now fit flush when I tighten them right down.

If I don’t dimple completely, it seems to leave my cover all wavey near the hole. I will try that In earnest though, since it might straighten out when the screw is tightened, and in any case might look better than the bulge. Thanks for that suggestion, and for the moral support.

[davzLSA]

After dimpling the holes on the access panel you might try rolling the edges of the access panel ever so slightly. I found that it made for a better fit of the access panel.

[svyolo]

I am glad you asked this because I have never found or got an answer I liked. If you under-dimple you get a huge amount of distortion. I haven't seen the definition of "over-dimpling" but I can understand what it is. I think you would squeeze/crush the sheet and expand the metal.

Does one tool do a better job than another? Squeezer, manual or pneumatice, DRDT?

Under-dimpling definitely doesn't cut it.

[davzLSA]

Hi Pbruce, I built a pair of RV4 wings. The inspection panels I made turned out good and flat. Here is what I did if you are interested. To start with I cut the hole in the wing and smoothed and de-burred the hole, Next I laid the access panel its self over the hole and traced the opening onto access panel, Next I put in the reinforcement ring and flush riveted it to the wing skin. Then I carefully cut the access panel along the traced line but made it a little large, then I would sand the edges of the access panel until it finally fit in the opening perfectly. Next I drilled #40 holes at 1 inch intervals around the perimeter of the access panel, then I placed the access panel in the opening and used the pre drilled holes in the access panel to drill the access panel to the reinforcement ring, Only then did I open up the holes to full size and de-burred and dimple the holes. Then I installed all the nut plates on the reinforcement ring. I hope this helps.

[rodsmith]

I am glad you asked this because I have never found or got an answer I liked. If you under-dimple you get a huge amount of distortion. I haven't seen the definition of "over-dimpling" but I can understand what it is. I think you would squeeze/crush the sheet and expand the metal.

Does one tool do a better job than another? Squeezer, manual or pneumatice, DRDT?

Under-dimpling definitely doesn't cut it.

Not sure how you overdimple. For screws I use the correct dimple die, then use the screw itself with the pneumatic squeezer to further dimple the hole. Ends up with nice flush screws and I have never had unacceptable distortion in the access panels, tank covers or anything else.

[N942VT]

Smash it down so hard you distort the metal around the die, that's how you over dimple.

[svyolo]

That is my question. Define "so hard". I would love a good answer to this, and I appreciate any and all attempts at a response. Most of mine have been done with a pneumatice squeezer at 80-90 psi.

[Archer39J]

The instructions I was using for pneumatics, and what I've done with what looks like good success so far, is to set the dimple dyes just so that they touch with no material in them. That seems to be giving me good dimples that doesn't affect the overall dimensions of the part, or at least that I've noticed yet. The problem, it seems, is when you have a negative net interference between the dimple dies (you back one of the dies up even more when open), then there's too much force when you try and squeeze a piece of aluminum between them.

[rodsmith]

That is how I set my pneumatic squeezer and set it at 90psi. I see no evidence of distortion in the aluminum, just nice looking dimples.

[Pbruce]

Not sure how you overdimple. For screws I use the correct dimple die, then use the screw itself with the pneumatic squeezer to further dimple the hole. Ends up with nice flush screws and I have never had unacceptable distortion in the access panels, tank covers or anything else.

How do you dimple using the screw? I’m using a pneumatic squeezer. With a male die in one jaw and a female one in the other. Can you explain how to do it using a screw please? I need to go back and re-dimple the fuel tank cover nutplates because I mistakenly used a 1/8” dimple die in the squeezer. It needs to be a bigger dimple for the 6-32 screw. Thanks. Might have to drill out the nutplates. Hope not.

[Bcone1381]

There is a lot of information in the above 12 posts. Don't discount Jared comment in post #5. You could fabricate that hole duplicator (kind of long) with a #6 screw, and attach the screw into your nut plate for a tight accurate fit.

I'm thinking Rod might insert a very short machine screw into the dimpled hole, then re-dimples the hole by replacing only the male half of the #6 dimple die with a flush die used for riveting countersunk rivets.

I think when I fabricated my frames and covers, I first clecoed them both together and then dimpled them both together. I think my dimples were less than fully developed, so I re-dimpled them all a second time to drive the calibrated dent home. I don't own a pneumatic squeezer; the simple hand squeezer is all I have and its not possible to over squeeze any thing. After dimpling, my frames looked like a potato chip, but installed inside the skin with no problem and the covers went on nicely.

I also used a dimple die from Aircraft Tool Supply called a Spring Back Dimple Die. https://www.aircraft-tool.com/shop/detail.aspx?id=SB6

As a last resort, removing everything and doing it over is something I do a lot. That is what I call the educational part of homebuilding

[rodsmith]

A #6 screw shank fits in the hole where the die is inserted in the squeezer. I use a flat set on the other side. I believe this technique was detailed in the quick build wing manual specifically for the tank covers.

[Mark Goldberg]

I suspect that "over dimpling" happens with a pneumatic squeezer quite easily. I have always used a normal squeezer where you can control very easily how "hard" you squeeze. Mark

[Pbruce]

Hi Pbruce, I built a pair of RV4 wings. The inspection panels I made turned out good and flat. Here is what I did if you are interested. To start with I cut the hole in the wing and smoothed and de-burred the hole, Next I laid the access panel its self over the hole and traced the opening onto access panel, Next I put in the reinforcement ring and flush riveted it to the wing skin. Then I carefully cut the access panel along the traced line but made it a little large, then I would sand the edges of the access panel until it finally fit in the opening perfectly. Next I drilled #40 holes at 1 inch intervals around the perimeter of the access panel, then I placed the access panel in the opening and used the pre drilled holes in the access panel to drill the access panel to the reinforcement ring, Only then did I open up the holes to full size and de-burred and dimple the holes. Then I installed all the nut plates on the reinforcement ring. I hope this helps.

You know, I think I did the first one like this, and it was the best. In quit doing it that way because I thought it would be more accurate to fit the panel in the ring then mount the whole assembly in the wing. Doing it your way allows the final hole placement allows any distortion from riveting the frame to the skin to be accommodated. Maybe it makes a difference.