Nut plates overly tight

[Marty_d]

Hi,

 

I've been working on my instrument panel today - now will be a removable 0.040" front held by 8 nut plates.  The nutplates are MS21075L3, held by 2x #40 rivets and look like this: 

 

The trouble is the thread is so tight that the bolts which go in them, which have a Phillips drive, won't actually go all the way through without either stripping the Phillips slots or breaking the mounting rivets.

 

In desperation I finally found a hex bolt with the same thread size and cut some grooves perpendicular to the thread, then drove it in and out of each nutplate to "tap" it enough to take the normal bolt.

 

Shouldn't have to do this though.  I understand that they're made tight to retain the bolt, but they shouldn't be that tight should they?

 

Any advice, or have I just bought crap nutplates and should get another sort?

 

Thanks, Marty

 

 

[Yenn]

They are designed to be tight to restrain the screw. I have had some which seemed excessively tight an put a tap part way through just to ease them up. If you put the tap too far through they will be loose.

 

 

[old man emu]

Crap nut plates? First question, did the nutplates come with traceability documentation? If you got them from a reputable supplier such as Aviall, Edmo Australia, Spruce and Specialty, the paperwork will have a reference number called a GRN or a Tracking Number. If this information was on the invoice for both the nut plates and the screws, then they are aircraft quality.

 

As for the rivets you are using, the correct nutplate rivets for that size nutplate is CCR264SS-3-XX, where XX is grip length.

 

 

[Marty_d]

Yes they're from an aircraft supplier in Aus - one of the standard ones, can't remember which.  They're just REALLY tight.  (And yes, I'm using the right bolts in them!)

 

Just to give an example, after one had broken the rivets holding it, I held the nutplate in the vice until it was actually bending the outer casing (had to to stop it moving), then tried to drive the bolt.  It would jam and strip the phillips slots.

 

Ah well, sounds like I'll just have to tap the buggers.  I'd rather they were like normal nuts (ie easy to thread) and use Loctite to hold them.

 

 

[hihosland]

Sounds to me like trying to force a M5 bolt into a 10-32 nut plate.

 

The threads are very close but not identical

 

 'dems my thoughts

 

 

[Marty_d]

Sounds to me like trying to force a M5 bolt into a 10-32 nut plate.

 

The threads are very close but not identical

 

 'dems my thoughts

 

 

 

I thought that too but checked my emails and found the order.

 

The screws (they call them "screws", I call them bolts) - are AN525-10R7 which Aircraft Spruce tells me are 10-32 thread size.

 

The nutplates are MS21075L3N which are also 10-32.

 

 

[hihosland]

try them with a known m5 bolt,

 

if you get the same result your bolts are probably miss labled

 

I've had a bolt place supply me with the wrong ones

 

 

[Downunder]

try them with a known m5 bolt,

 

if you get the same result your bolts are probably miss labled

 

I've had a bolt place supply me with the wrong ones

 

Yeah, I'm with you on that. Not the first time a "part picker" in some warehouse has miss matched similar parts.....

 

 

[facthunter]

I've had the same.  If you put a tap part through it's making a couple of threads do the whole job.  Are they AN.? Try lubing it and working it a couple of times. Nev

 

 

[440032]

No, they should certainly not be that tight. Something screwy is going on (no pun intended.)

 

I wonder if you have 8-32 nutplates. A 10-32 screw into an 8-32 nutplate would precisely produce the results you are seeing.

 

 

[kgwilson]

Something definitely wrong. Nutplates are as as previously stated designed to be tight with a tapered female thread. Even if you have the right male (usually machine) screw they can be easy to cross thread and seem OK as they get really tight. If this is the case unscrew the male part & inspect the thread very closely. It will be very bright and the start will be distorted. Biff it & get another. When the correct thread is used they tighten to a point and then will fully close down with just firm pressure on the driver. The one you have pictured doesn't look like a quality nutplate to me.

 

 

[onetrack]

Sounds like the Chinese substitutes strike again!

 

 

[old man emu]

Without wanting to insult you, Marty, but what side of the nutplate are you screwing in the screw from? Have a look at this EAA video on how to set a nutplate:  

  and see which side the screw goes into.

 

Also the AN525 screw is a structural screw, which means that there is a section of unthreaded shank between the underside of the head and the start of the thread. In a 10R7 screw that unthreaded length is 1/16". Is it that unthreaded section that is stopping the screw going home? If so you should be using a Truss head, Non Structual screw AN526-1032R7

 

There's no way that Aircraft Spruce and Specialty would have metric hardware, and definitely no Chinese knock-offs. 

 

AN525-10R7 = Washer head, structural screw, Phillips drive,  10-32 (0.19" major diameter - approx 3/16"; 32 TPI; Grip 1/16"; Overall length 7/16"

 

MS21075L3 =  Mini Floater Nutplate; steel; black moly; 10-32 thread size; Rivet Hole Style Not Dimpled; Rivet spacing 0.5"

 

CCR264SS-3-XX = Nutplate rivet; Steel; Flush head 100* head; Nominal rivet diameter 3/32"; Hole drill size #40; XX = Grip length

 

 

[Marty_d]

Attached are photos.  Everything seems kosher and sourced from Superior Air parts at Archerfield.

 

Yes OME I know which end the screw goes in - would be a bit hard to mount something flat the other way around!

 

1/16" is around 1.58mm.  Where they're binding up there's still a good 5 threads visible on the screw, so it's not that.  

 

No, they should certainly not be that tight. Something screwy is going on (no pun intended.)

 

I wonder if you have 8-32 nutplates. A 10-32 screw into an 8-32 nutplate would precisely produce the results you are seeing.

 

10-32 is around 4.8mm diameter where 8-32 is around 4.1.  From laying a ruler across the screw hole in the nutplate it's definitely closer to 5mm than 4mm, so I reckon they're 10-32.

 

I've had the same.  If you put a tap part through it's making a couple of threads do the whole job.  Are they AN.? Try lubing it and working it a couple of times. Nev

 

Lube is a good idea.  I'll try that tomorrow.

 

 

 

 

[old man emu]

For starters, throw away any Metric measuring device you have. AN, MS and NAS hardware is all Imperial measurement. 

 

Looking at the labeling on the packages, I'm happy that these parts are fair dinkum and traceable via the Invoice number.

 

The letter "L" indicates that the lubricant in these nutplates is black moly grease, so they should not need any other lubricant.

 

How many threads are NOT going into the nut plate? 5 threads on a 32 TPI screw is 5/32", or about 1/8". A 10R7 screw has an overall length of 7/16" and a grip length of 1/16". That gives a threaded length of 6/16" (12/32"). You should have 7 threads out of the nutplate, plus 1/16" unthreaded.

 

I suspect that your assembly instructions call for AN525's, but I suspect that AN526 in a shorter length would do.

 

What exactly are these screws securing, and to what?

 

 

[Marty_d]

For starters, throw away any Metric measuring device you have. AN, MS and NAS hardware is all Imperial measurement. 

 

Looking at the labeling on the packages, I'm happy that these parts are fair dinkum and traceable via the Invoice number.

 

The letter "L" indicates that the lubricant in these nutplates is black moly grease, so they should not need any other lubricant.

 

How many threads are NOT going into the nut plate? 5 threads on a 32 TPI screw is 5/32", or about 1/8". A 10R7 screw has an overall length of 7/16" and a grip length of 1/16". That gives a threaded length of 6/16" (12/32"). You should have 7 threads out of the nutplate, plus 1/16" unthreaded.

 

I suspect that your assembly instructions call for AN525's, but I suspect that AN526 in a shorter length would do.

 

What exactly are these screws securing, and to what?

 

Hi OME,

 

For a start, there are no assembly instructions.  This is a scratch build and the plans don't go into the detail of inspection ports etc.

 

In this particular instance I'm using 8 nutplates to attach a 0.040" instrument panel to the standard 0.025" panel (the majority of which has been cut away, leaving a frame for the front panel to attach to).  This will enable me to remove the panel easily.

 

As a picture tells a thousand words, the following 2 show how far the screw goes in before becoming too tight to turn.

 

 

[facthunter]

IF it starts off quite free then tightens it's usually the number of tpi is difference What is it coated with?  OIL one and run it through  from both ends and examine the thread for distortion/uneven polishing. Nev

 

 

[old man emu]

to attach a 0.040" instrument panel to the standard 0.025" panel

 

0.040 + 0.025 = 0.065"  which is approximately 1/16" (0.0625) or 2/32" or two threads.

 

What is the length of the threaded section of the nutplate? Let's call if "X"

 

The minimum length of the screw needs to be "X" because the grip length of the 10R7 is 1/16" which will be taken up the thicknesses of the two sheets it is going through.

 

The shortest overall length for an AN525-10R* screw is 3/8". AN525's are for structural work and have a tensile strength of 125,000 psi. What you are wanting to fasten together is not structural, so you could go to an AN526 non structural screw with a tensile strength of 55,000 psi. Also the 536's come in 1/4" and 5/16" overall length. The screws are either low carbon, Cadmium I plated (silvery) or stainless steel (shiny). The SS screws (AN526C) have a tensile strength of 80,000 psi, which is of little consequence for your use.

 

Don't forget that these nutplates are self-locking. The threaded section is oval in shape. That is what is making it hard for you to get the 10R7 length all the way through.

 

 

[onetrack]

Marty, get hold of a thread pitch gauge and check the nut against the screw. Despite the full traceability, there's always the chance of a mix-up. Traceability merely finds the person responsible.

 

The short distance the screw is going in, before it binds, indicates a serious thread mismatch.

 

 

[Yenn]

You could also try a number 6 screw to se if it screws in correctly. from memory the diameter difference between 8 and 10 should be .006"

 

It should screw in easily and still be sloppy when tight, with a touch more torque it should pull out.

 

Does the nutplate have its number stamped on it?

 

 

[M61A1]

You could also try a number 6 screw to se if it screws in correctly. from memory the diameter difference between 8 and 10 should be .006"

 

It should screw in easily and still be sloppy when tight, with a touch more torque it should pull out.

 

Does the nutplate have its number stamped on it?

 

A -6 should fall through a -10 nutplate.

 

-10s are about 0.190" , a _8 is about 0.161" and a -6 about 0.130".

 

It sounds like an nutplate quality issue, if it starts easily then gets really tight at the end. 

 

Many of the ones I use at work with close tolerance screws require grease or anti-seize when new or they are excessively tight .

 

By excessive I mean run on torque far exceeds the listed maximum torque for the fastener.

 

 

[Bruce Tuncks]

You are right to complain Marty. What I would do though is to run a tap through them and then use a bit of weak loctite on the bolts. The only purpose of the system is to stop vibration-loosening and this will not happen without an off-torque and even if an off-torque was present, the loctite would stop any loosening.

 

That tapered metal-interference tightening idea predated loctite I reckon, and it is geometrically so tricky that your problem is not hard to imagine.

 

 

[facthunter]

Sometimes they distort (ovalise) the last couple of threads like a metal locknut in principle. You will pick that if you thread it from the other end. I wouldn't be running a tap through it. That usually gives an excessively loose thread and the finish can be a bit ordinary if the metals a bit tough. Which it often is. You want good thread fits anywhere on a plane They don't make vibrationless ones .Nev

 

 

[kgwilson]

I think I'd just get better quality nut plates. Problem solved.

 

 

[facthunter]

Where do you find that brand. ? They might actually be OK.  though. Today I get problems with threads like I've never had before. Some of the stuff is real rubbish. Nev