Rivetting

[debequem]

Anyone know of or can explain how to rivet aluminum sheet metal together? I am not talking about pop rivet guns, but actual aircraft type of riveting as done in production.

What kind of tools do they use and what process is used?

Thanks,

Marv

[winchman]

Here's a link to a selection of rivet tools:
http://www.aircraftspruce.com/catalo...menus/to20.php

There are two ways to install rivets. The first is with a squeezer, which has two anvils. One fits the head of the rivet. The other is usually flat. The squeezer is used to compress the part of the rivet that protrudes beyond the material to be held together. This is a one-man operation, but it's limited to parts which will fit in the throat of the squeezer. They make hydraulic squeezers that will smash some pretty big rivets, but the hand squeezers are limited to 3/16" or so.

The second method is with a rivet gun, which is an air powered hammer. The hammer is usually placed on the head of the rivet and a "bucking bar" is held on the end to be upset. The process is really pretty easy, but it takes a while to learn how to hold the bucking bar square to the rivet. If it's not square, the rivet will be deformed off to one side. Two people are usually needed to drive rivets in this manner, one inside with the bar while the other is outside with the rivet gun. There are dozens of specially shaped bucking bars to fit just about every imaginable situation. It's VERY noisy, so ear protection is a must.

Roger

[Evan]

Roger has it right. A few more things to add. The most common sizes used are 3/32, 1/8 and 5/32 diameter. Length will vary according to material being fastened. The tail of the rivet must extend at least one rivet diameter but not more than 1.5 diameter from the work. To seat a rivet prior to bucking it it is useful to have a bucking bar with an approriate size hole drilled in it. This is placed over the tail of the rivet and a few taps from the air hammer will ensure that all layers are tight.

Aircraft rivets are made from 2024 alloy and are in a T3 temper as supplied. Upon driving they go to a T6 temper as they work harden.

If you have a trouble spot where it is very difficult to reach the back side of the rivet you may soften the rivet first to make bucking easier. The rivets are dyed, usually yellow, pink or purple and sometimes green. The dye is an indicator. You may heat the rivet with a propane torch and at the correct temperature the dye vanishes. The rivet is now in a dead soft annealed state and drives like butter. Driving it will work harden it and it will reach nearly full strength in a couple of days. Rivets may only be treated this way once and will gradually harden in about 24 to 48 hours even if not driven. If you have a batch of rivets heat treated to soften them the working soft life may be extended by keeping them in a freezer. If they are allowed to go hard they may still be used as normal but should not be used when a non treated rivet will do.

Riveting on thin metal such as .020 2024 alclad requires a short stroke hammer to avoid dimpling of the skin. Don't try using a muffler chisle gun, it hits too hard.

On heavy aircraft rivets are not usually used for sizes above 3/16. The next size up is 1/4 and for that size of fastener a special type of bolt called a Hilock is used. It has a flat head and a precision ground shank with threads on the end. The hole for it is drilled with a stepped drill bit that makes a pilot hole and then reams the hole to exact size. The nut is made of aluminum and has a round threaded body with a hex head crown. The hex head crown snaps off when the correct torque is reached. They are removed by drilling.

Also available for locations where it is impossible to buck the tail of the rivet is a special form of pop rivet called a cherry rivet. It works in a similar fashion to a pop rivet but contains a solid core that contributes to the strength. If cherry rivets are used the rivet pattern must be made slightly denser to compensate for lower strength. In some cases a twin row of rivets is better with the two rows staggered.

All holes must be deburred before assembly. No scratches are permitted on aircraft sheet metal. The metal must not be marked with pencil.

The reason for the no scratches rule is that 2024 alclad aircraft alloy is a special aluminum sandwich. The 2024 alloy has poor corrosion resistance so to protect it it is covered with a thin cladding of pure aluminum on both sides. This cladding is only about .0015 thick and if scratched exposes the alloy. This can result in stress corrosion eventually producing a crack leading to structural failure. On most aircraft nearly all parts are load bearing in some fashion.

One more thing, holes for aircraft rivets are drilled using numbered drills. For 3/32 use a #41, for 1/8 use a #30, for 5/32 use a #20 and for 3/16 use a #11.


Another thing you will need is a supply of temporary fasteners called Clecos. These are a special spring loaded reusable fastener that is inserted into the rivet holes to maintain alignment of the sheet metal during assembly. They are fairly expensive and require a special pair of pliers to use. Clecos are like clamps, you can never have too many. They come in the same sizes as the rivets and are color coded. Grey is 3/32, copper is 1/8 and black is 5/32. You can also use nuts and bolts with fiber washers for temporary assembly.



[This message has been edited by Evan (edited 11-24-2003).]

[wierdscience]

Evan,neat!I always wondered why the a/c rivets we have around the shop are color coded,now I know.

We have some rivets that have a flat head,look like passivated stainless plus they have a 8-32 thread with the threaded end having a hex recess,any ideas what these are for?

[Evan]

Wierd,

That's a Hilock fastener. The Hilocks are supposed to be a slight interference fit in the hole which prevents them from spinning when the nut is installed. After the hole is drilled the hilock is driven in with a rivet gun using a flat snap as for flush rivets, then the nut is installed. But if it does spin a bit you can put a hex wrench in the end to hold it while it is torqued down until the hex head snaps off. Normally the head is driven (torqued) with a 3/8 power air ratchet. I have one of those, very handy tool. The allen key is used with a manual wrench when necessary. If you look closely at a Hilock you will notice that the part just at the end of the threads has a radius to the main shank. That is designed to polish the hole as it is driven in.

Speaking of flush rivets, that is one area that really is a test of your ability. Flush rivets give significant advantages in drag reduction but require greatly increased labour and greatly increased chance of screwing the job. If you are hand countersinking all it takes is one hole countersunk too far to wreck the piece. In some cases you can go to the next larger size of rivet to salvage the work, but not always. The best way is to use a countersink with a depth stop. I never did that and never trashed any thing that mattered, you really have to focus.

I think my aircraft metal work training is why I like making precision devices. Not many machinists are accustomed to working on metal eight or ten feet long with cumulative tolerance not to exceed .032". Thats equivalent to working to .0002 per inch on a routine basis. Even a slight temperature change in the shop can mess up your measurements. Someone opens the hangar door to move a plane and suddenly everything is out of whack.

[This message has been edited by Evan (edited 11-25-2003).]

[wierdscience]

Flush rivets,I have worked with them,not on aircraft but on Spartan travel trailers,something like 2800 of the little buggers in a 28' trailer,loads of fun!Friend of mine had one he was restoring,the previous owner lost the wheel skirts,we fabed a new set from prints.

[Evan]

The old DC3 had all regular buttonhead rivets from the factory. Someone went to the trouble of replacing the front third (leading edge) of the wing's rivets with flush instead. It went 12 knots faster at the same power level. I wonder how much time, fuel and money that would have saved over the operational life of that plane?

[wierdscience]

Its amazing how all the "little"things add up,my boss had us build x-metal sides for his one-ton truck,I told him that it might cut his mileage,boy did it,about 5 mpg,all those thousands of diamond shaped holes add up to lots of drag.

Remember too the space shuttle external tank how they used to be painted white?Seems like the payload gain was something like 3500 lbs after they stopped painting it,and when you think of it why paint something thats just gonna burn up anyway?

[Evan]

The paint on a 747 weighs about 650 pounds.

[CCWKen]

The paint on an average size new car weights about 3 pounds. No wonder they don't last.