Nutplates and inspection panels

[Archer39J]

It is important for us all to understand where and why of any particular fastener system before it is used. I don't know anyone recommending tapping all nutplates. In a few applications it makes sense, but the installer needs to understand the where and why aspect before moving forward. The discussion on Vans was about seat pans and a specific area of the seat pans. Some took that to mean a recommendation for all nutplates. I don't believe that was the intent.

http://www.vansairforce.com/community/showthread.php?t=152553&page=3

Lol, I actually hadn't even read the above thread, but the last post is one. I've seen various people advocate it occasionally.

Apologies, but there is no application where this is acceptable, in my opinion (and that of a major commercial aircraft manufacturer) of course.

If you're having issues snapping screws, use some boelube like they sugges on that thread. Or upgrade to some NAS514 screws which are a harder steel.

[svyolo]

This is the first I heard of running a tap through the nut plate thing. At first I thought it was a horrible idea. But if you just did it enough to clean up the "entry" threads, I would think it wouldn't have cut into the extra material needed to friction lock the fastener.

[BravoGolf]

http://www.vansairforce.com/communit...=152553&page=3

Lol, I actually hadn't even read the above thread, but the last post is one. I've seen various people advocate it occasionally.

Apologies, but there is no application where this is acceptable, in my opinion (and that of a major commercial aircraft manufacturer) of course.

If you're having issues snapping screws, use some boelube like they sugges on that thread. Or upgrade to some NAS514 screws which are a harder steel.

Boy I missed that thread altogether. I was thinking about a different thread. The topic shows up from time to time. I guess there is always someone. I'll stay with your advice.

[PaulSA]

You can also countersink the aluminium even though it is less than the thickness that you normally need for a #6. Install the nutplate first. If you then use a #40 cutter, the guide pin keeps it centred in the nutplate so that you don't end up with a raggedy hole. For #8 nutplates, you can use a #30 cutter.

[fairchild]

FYI ----- I was told by a long time machinist and aerospace engineer that there is one special lube for stainless steel when tapping it or drilling to prevent
it from galling----- this lube might be applied to the SS screws so they wont tend to stick in the crimped nutplates. It is called "anchor lube".
I would switch to the green (cad?) plated screws before I tapped out my nutplates.

[marcusofcotton]

... Or upgrade to some NAS514 screws which are a harder steel.

This is great advice, I wish more people would realize the significance of this! That and keep extras on hand and whenever one is even slightly questionable, replace it. Even do a group change out occasionally as the sacrificial plating gives way. It's not a major expense!

[svyolo]

I have spent a bunch of time the last couple of weeks looking up different techniques with sheet metal. The nut plate dimpling is still having me scratching my head. I can find no specific guidance on it anywhere. It seems like sometimes you have to choose the "least bad" method.

I will use dimpled nut plates where I can or where I think they are necessary. For others I have no issue with having a nut plate sitting on top of 3 dimples. No primary structure.

But when the substrate is too thick to dimple, and not thick enough to countersink (without enlarging the hole and creating a knife edge).

An example is .025 or .032 skin, over an .040-.060 substrate (boot cowl) with a #6 or #8 nut plate. The skin dimple won't sit in countersink without over-countersinking. This is supposedly bad.

Or countersink the skin and substrate together. This creates a knife edge on the skin. This is supposedly bad.

Any idea which one is the least bad? I am leaning toward dimpling the skin, and over-countersinking the substrate, using Paullsa's method of using the nut plate as a countersink guide with a #30 or #40 guide.

But countersinking the skin and substrate together would be a whole lot easier.

[jaredyates]

Option 1. Overdo the substrate, dimple the skin. Same answer as earlier with regard to the meat vs bread of the sandwich. I used countersunk nutplates on the boot cowl like these: https://rover.ebay.com/rover/0/0/0?m...2F252633317845

[svyolo]

Thanks Jared, I will give it a shot. I don't think PaulLSA's technique will work for the CS-ed nut plate. I will have to figure out something for a guide for the CS pilot to ride in to keep from massively chattering and trashing the hole.

Another thing I might try is CSing as much as I can, and then try to dimple the CS a little more. Anyone tried that?

[PaulSA]

I've just been working on these. Here's a few I prepared earlier ..... Done as per my earlier post - seems to work OK!!

IMG_0520.JPG

[svyolo]

Paul, were those K1000 nut plates, or the CS version?

[PaulSA]

K-1000. If you use the K1100 then you dimple rather than countersink. There's also the argument about whether to dimple the nutplate attach holes. I do. There's a lot of them and it's a bit of a PITA but everything sits much better.

This is a sandwich of 2 x 032 and you can see that it has only gone about half way through the second layer - so 032 + 025 should work fine. One reason you don't want to knife edge is that the CS cutter guide pin starts to wander. But by fitting the nutplate first and using the smaller pin in the nutplate, you avoid this.

Good luck getting CS #6 nutplates - although I believe they are just coming back into stock around the retailers......

[PaulSA]

There seems to be a bit of confusion about the possible methods so if I might be allowed to summarize ...... As Svyolo alludes to, there are basically 3 ways to deal with fitting nutplates where a countersink or dimple is required in the skin:

1. If the skin and substrate are both thin enough, you can dimple both and use K1100 dimpled nutplates. If the substrate is thick enough, you can C/S the nutplate attach holes. If it is thinner then you need to dimple. In some cases, (032) you have a choice. If you dimple the attach holes, I would recommend dimpling the corresponding nutplate holes. This is the method I have used on my inspection plates.

2. If the substrate is too thick or is already assembled and can't be dimpled then you need to countersink. If the substrate is thick enough then you can use the appropriate cutter. If it is too thin, then use the method I described by fitting the nutplate first and then C/S using a #40 cutter (#6 hole) or #30 cutter (#8) hole. In this case, the nutplate attach holes need to be C/S. Dimple the skin. This is the method I have used for my fuel tank covers

3. If you need to use a C/S screw but the substrate is too thin or is difficult to C/S or can't be dimpled (steel tabs for instance) then you can fit the nutplates and then C/S the skin and substrate together. In this case, you will knife edge the skin. The problem here is that you end up with very little bearing area around the hole edge for the screw so I would recommend using tinnerman washers under the screw. Nutplate attach holes may be dimpled or C/S as appropriate. This is the method I have used on some of the flooring, boot cowl and wing tip attach.

Hope this clarifies .......

Of course, some people are happy to use K1000s under dimples but that just offends my sensibilities .......

[davzLSA]

I agree with PaulSA, if you are going to dimple the hole use a dimple die to dimple the hole and use the K1100 plate nut but if it is thick enough use the countersink and a K1000 plate nut.

[Bcone1381]

In many of my nut plate installations NAS1097 rivets are used to secure the nut plate. NAS 1097 rivets are a reduced head rivet used for shear applications, the are designed for holding nut plates. The small head of these special solid rivets significantly reduces the depth of the countersinking and will not knife edge a .025 thick piece of aluminum. Using NAS1097 rivets to hold a nut plate eliminates the need for dimpling nut plate rivet holes. The 3/16" NAS1097 rivets are available at Vans Aircraft or Wicks.

Last week I installed my fuel tank Nut Plates like Paul described, countersinking the screw hole in the substrate after the Nut Plate was installed using the #40 countersink. I educated myself, practiced and set up using scrap until I was confident. Using an electric drill and going slow did not result in any chatter issues. I am very satisfied how things turned out.

I followed Eric Newtons instructions to countersink just deep enough to allow the screw head to sit flush in the hole. The countersink did not contact the nut plate but it got real close. I edge rolled the fuel tank cove then dimpled it. The dimple sits down into the countersink nice, and a feeler gage showed I have .002" clearance between the cover and the substrate. The rolled edge of the cover seals up that .002 clearance. I'm happy with the results.

I suppose I could have countersunk the substrate a bit deeper. But I think an over countersunk substrate would result in a bit of slop between the dimple and the receiving countersunk hole. Mine is tight.

[PaulSA]

This is fine if the application is non-structural. In the case of the tank covers, the rivets that hold the nutplates are also structurally holding the skin to the ribs and spars. In this case, standard rivets should be used.

[svyolo]

One reference I read said that when you have a dimpled skin sitting in a CS substrate, the dimple shouldn't sit down completely in the CS. He said he would "soften" the upper edge of the CS, but then the skin still had a bit of clearance. The screw or rivet would pull the two together, snugly.

I read not to CS through the skin down into the substrate, but using a tinnerman washer sounds like this might be a good option as well.

Thanks for taking the time to respond to my questions. It is a big help for me, and anybody else searching at a later date. I was pretty surprised how difficult this info was to find, and how much conflicting information is out there.

[Bcone1381]

Paul;

Thank you for your comment on post #27. I had not thought about this and was concerned. I called Bob Barrows ran it buy him before drilling them out.

He said that the Tank Cover installation is designed so that all the load and stress is carried by the machine screws. With the screws tightened onto the tank cover it sandwiches everything together to carry the forces. He approved the NAS1097 rivets and said their purpose is only to hold the nut plates in position.

[PaulSA]

Fair enough 👍👍