RIVETING

 

I was informed that I really hadn't discussed riveting technique too much on this site ... I agree and figured that I'd add my $.02 on this issue. Firstly, I must say that I'm not overly experienced on this and most of what I'm saying is from the school of hard knocks (and yes I've also drilled out my share of rivets that I wasn't happy with). Besides AC 43.13-1A , there are several good books and articles on riveting and I encourage the viewer to also try to find and read these if they're a newbie with a rivet gun. Most of the books also show examples of how to identify a properly set rivet and also the various common mistakes. If one isn't experienced, its also important to do some testing and practice on scrap pieces in order to get the "feel" of the process. I still find it takes a few rivets at the beginning of a session to really feel comfortable with it again.

Starting in August 2005, Kitplanes started a sheetmetal series of articles by Dan Checkoway and there are plenty of good tips in there. Another good article with some interesting tips is located here.

 

TOOLS
HOLE PREPARATION
RIVET SELECTION
WET RIVETING
UNIVERSAL HEAD RIVETS
FLUSH HEAD RIVETS
REMOVING RIVETS
RIVETING TRICKS


 

TOOLS

I've already added most of these items to the "Tool" section of this website, but I thought I'd add a few more comments here.

 


 

HOLE PREPARATION

Obviously the more pristine the holes are, the better the riveting job will be. I've found that I'm primarily using 1/8" rivets, but I choose to do most of my initial fabrication using a #40 drill and 3/32" (silver) clecos. Once I've got the pieces completely fabricated and ready for final assembly, I then re-drill the holes with a #30 drill for the 1/8" holes. Usually the only time I'm using 1/8" clecos (copper) is as I'm re-drilling the holes and then during the actual riveting. This makes a very clean / round hole which can also be slightly adjusted during the re-drilling for any discrepency. Although I have a speed deburring tool which works great, especially on steel, I usually debur the holes in aluminum by hand using an old 3/8" drill. There is no need to actually chamfer the hole as one is only trying to just remove the burr left on both sides from the drilling.

For a universal head rivet, the hole is then complete. For a flush head rivet the hole then needs to be either countersunk or dimpled; the size of the rivet and thickness of the material will determine which technique should be used. Although there are tables available, my rule-of-thumb has been that I'll countersink 3/32" rivets in .032" or thicker material and 1/8" rivets in .063" or thicker material. Otherwise I'll dimple the surfaces i.e. 1/8" rivets in .032" material. One technique I've used where I can get access, is to simultaneously dimple both surfaces which yields a perfectly flush mating surface. This can also be used where one hole may be slightly ovalled since the resulting dimples will lock the panels in their final position.

When countersinking, it's worth a few extra minutes to carefully set up a microstop using test holes in scrap material such that the rivets will be truly flush or possibly about .001" recessed. On thinner material, I've found that it's important for me to use a backing block for the cutter's pilot or else I tend to slightly oval the hole. Perhaps it's just me, but I also find that after using the microstop on a row of holes I then go back and drop a rivet in each hole to test it ... perhaps one in five needs to be slightly redone.

Tip: I find that some of my clecos leave a slight marring around the holes i.e. a ring about 3/8" diameter. During fabrication this isn't a big deal but when preparing to rivet alodined or primed surfaces, this can be frustrating. I chose to get a 25 pack of "No-Mar" Reusable Cleco Boots and these eliminate the problem as they still give the cleco a very effective clamping pressure but there is a soft polyurethane face against the work surface. I use these exclusively in areas where I'm doing final riveting and they work great for me.

Tip: Holes for 8-32 screws are drilled with #19 drill for a relatively close tolerance fit. Although they're undersize, 5/32" clecos (black) can be used to hold these pieces together if they've been carefully aligned.

 


 

RIVET SELECTION

There are only a few places where pulled rivets are required; the horizontal stabilizer, the doors and possibly some of the corner trims. Since most handymen are familiar with "pop" rivets, I won't say a lot about these. However, one should consider using high quality aviation pulled rivets which won't allow the mandrel to fall out of the formed head and are available in much higher strengths than the hardware store variety.
Anecdote: I recently had a chance to use and compare some Cherry stainless versus hardware aluminum rivets. There was an amazing difference in the amount of extra force that was required to pull the stainless rivets. I also used some Cherry Max rivets which do a great job and it's easy to understand why they command a premium price.

The majority of the rivets used will be of the solid variety. Obviously there is the choice of universal head or flush rivets, but there is also the choice of size, length and material. For nutplates, one uses 3/32" flush head rivets and for pretty well everything else I'm using 1/8" rivets. Since we're working with metal, one can use the standard AN426AD and AN470AD rivets whereas if one is riveting on composites, the softer "A" instead of "AD" rivets are normally used. That just leaves the choice of length.

The general rule of thumb is that there should be 1.5 times the diameter that extends beyond the mated surfaces before the rivet is bucked or squeezed. Too short and the rivet won't get it's full strength and grip, whereas too long tends to cause problems such as tilted rivets. While most of the required rivets will be the same length, there are several places where longer rivets are required. This can be accommodated by either having all of the various rivet lengths available, or what I did was to get the most common rivets (for 2 * .032" material) and then some 1/2" rivets that can be cut to the correct length to handle different sized material.

There seems to be some debate about the strength of universal vs. flush rivets. One reference that I read seemed to indicate that universal head rivets made a stronger joint. However, I also read a very informative article where the author made a series of riveted test coupons and then analyzed their shear failure in a properly equipped lab. While none of the riveted coupons were as strong as the base metal (i.e. non-riveted strip), the flush riveted coupons were up to 25% stronger. As one thinks about this, they have to realize that the two dimpled holes form an interlocking mechanism that provides additional shear strength over and above the strength of the actual rivet material.

 


 

WET RIVETING

If one reads the various Mil-spec documents they talk a lot about surface preparation, such as alodine, and then the methods of joining faying surfaces. Exterior exposed joints are supposed to be done with a coat of sealant between them while interior joints can be done with primer on both surfaces. Where possible, I chose to go with the home-builder's half-way solution which is to join the surfaces with a coat of wet primer.

I'm sure others will be able to come up with a better technique, but here's how I did it. When riveting, I usually keep a small amount of mixed zinc chromate primer handy. Since I'm using a two-part epoxy formulation, I have a bunch of the small 1 ounce clear plastic medical cups which I use to mix up only about 1/4 ounce at a time. I keep a small cheap hobby paintbrush handy and also some of the narrow foam brushes which I bought in bulk. Just before riveting the surfaces, I spread a thin layer of primer on both surfaces and then cleco them together. Since I tend to apply the primer to a wider area than absolutely required, MEK makes short work of cleaning off the excess and gives a bit of time for the clamping pressure to squeeze out any excess primer. The part is then riveted and any excess primer is cleaned off once again. If one was really trying to get total corrosion protection, they could also dip the rivets into the primer just before inserting them.

Tip: It's always bothered me that in this high-tech age we still get out paint in cans with no easy way to pour from them without filling the rim with product, especially when I use the can often but for just small quantities. I have a small pint can of my two-part primer near the area where I do my riveting and find that I use a piece of about 3/8" square wood stock to stir it each time I open the can. By placing the mixing cup right beside the can and twirling the mixing stick, I can then use the drips from the stick to put the required amount of the thick Part A into the cup ... takes about 3 or 4 dip-and-drip sequences. The Part B catalyst is very thin and I have it in a small capped bottle which I can then easily pour/measure into the mixing cup. This works great for very small batches (about 1/4 ounce) and for slightly larger ones I've also used disposable plastic spoons to dip into the can without having to pour. Since the thinner Part B initially floats above the Part A, I've found that if I pour in too much of the Part B then it's a simple matter of dipping in a small piece of shop towel and it will wick up the excess.

 


 

UNIVERSAL HEAD RIVETS

Perhaps it's my set, but I found it was very easy for me to put a smiley into the work surface right beside the head and I found that I had to be very careful when using a universal head set to make sure that the set was perfectly at 90 to the surface before riveting. I did find an easy solution to this in the form of using "Snap-Soc" rivet set caps. These are essentially a plastic cap that slides over the end of the set and don't seem to interfere with the riveting action but do seem to stop most of the damage to the rivet head and/or the surrounding area.

I don't do it, but I have heard that if one puts a piece of masking tape on the end of the set that it will help to prevent marring of the rivet. It obviously needs to be changed relatively often and my concern would be that it also would transfer it's adhesive to the set which would then need to be cleaned off as the tape is changed.

 


 

FLUSH HEAD RIVETS

I ended up using a lot more flush head rivets than I did universal heads. In fact, I chose to use flush head rivets on the exterior panels as it only took a couple of hours to dimple all the panels and I much prefer the cleaner look and the fact that the panels lock themselves together.

Where possible, my first preference is to use the hand squeezer on rivets. This gives a controlled setting action where I can carefully watch to see if anything is going amiss. While it takes a little bit of muscle effort, the results are normally quite consistent and I'm satisfied with the results. If I have to use the rivet gun, my preference is to do it with the special back-rivet set which automatically applies pressure to the two work pieces to hold them together. Most of my back riveting turned out very well and I can't remember having to drill any of them out. Unfortunately, it's often not possible to get the back-rivet set into position and one then has to resort to the traditional flat set and bucking bar. This works, but similar to universal head rivets, one has to be extremely careful to put more pressure against the lower work surface rather than against the bucking bar in order to resist the pressure from the set.

I don't know how to emphasize it enough, but if one is working with a square-faced non-swivel set then it is imperitive to keep the set perfectly flat on the work surface. If there is any "tipping" of the set, it will surely create a noticeable dent in the surrounding surface that can only be hidden using filler material. When working solo, I find that I actually spend as long, or longer, verifying that the set is perfectly flush than I do positioning the bucking bar. Once one has squeezed the trigger, it's too late to correct it! I'd much rather take an extra second or two to verify the alignment than get out the filler material, mix it up, apply it, wait for it to dry and then try to sand it smooth ... on a pre-painted surface it's even a more tedious process.

When joining two dimpled sheets with flush rivets, the rivet tends to be relatively flush after bucking; too much edging and the material will appear dented, but the rivet is still flush. However, I had more difficulty on thicker materials which had been countersunk. Although I was quite careful with the countersink depth, it seemed that after the pieces had been alodined, riveted, primed and painted, the rivet was not nearly as flush as I thought and there was sometimes a noticeable bump/dip in the painted surface. I now understand the purpose of a rivet shaver.

When working with flush rivets, I'm a real fan of the special riveting tape that is a thin clear mylar and has the green adhesive strips down each side. This holds the rivet in place and also prevents most of the marring to the rivet head and the surrounding area. Whatever the adhesive is, I don't know, but I haven't had any cases where it left a residue on the working surface. A single piece about an inch long can be repeatedly used many times ... perhaps up to a dozen or so times. When working solo, I tend to cut five or ten pieces and then start using / re-using them as required. Once they've all been used, I then take a quick break from the riveting process as I remove them before starting over again. When riveting with a partner, it really speeds up the bucking process if one first puts rivets and tape into/onto alternating holes before starting. One can then quickly shoot the line of rivets without having to fuss with pulling clecos, selecting rivet lengths, installing them and then bucking them individually. The one negative of riveting tape is that it is a slippery mylar-like material and it is easier for the rivet set to wander if the gun is not being held firmly.

Dimpling of sheet metal for flush rivets bends the metal and slightly stretches the holes. I've found that a 1/8" drill bit actually makes a slightly better dimpled hole than the more normal #30 drill bit which is .0035" larger.

 


 

REMOVING RIVETS

We'd all like to believe that this isn't required, but I'll admit that I've drilled out my share of rivets that I wasn't happy with. Before doing this, I think it is important to confirm that it really is required as it is very difficult to keep the holes perfect during the drilling and clean-up sequence. If there are only minor irregularities, it is probably better to just leave the rivet rather than risk making it worse on the drilling / re-riveting sequence.

The objective when drilling out a rivet is to just drill out the head without actually going into the work piece's holes. Once the head has been drilled out, it can be snapped off with a punch (or the base of a second drill) and the rivet's stem is then removed with a punch. Note that the work piece MUST BE suported before using the punch or else it will be deformed.

The one tip I have in this area is ... GO SLOWLY until one is familiar with this process. It is a lot easier to take an extra minute or two to verify that the drill really is centered and won't damage the hole rather than messing up the hole(s) and having to either go to a larger size rivet or re-fabricate a piece. I find that I still stop after the drill has gone in about the depth of the angled cutting faces and double check that the drill is still centered. If it has wandered a bit, then I can angle the drill towards where I imagine the top of the work piece's hole to be.

 


 

RIVETING TRICKS

I think one of the biggest "tricks" would be thinking ahead when one is drilling rivet holes and consciously viusalizing how one would actually rivet the parts together during final assembly. Early on, I did several holes that if they had been shifted less than a 1/4" would have made the bucking process much easier. Visually and spacing wise they worked extremely well, but the tail of the rivet was in close proximety to another rivet from a different angle. There's also the situation where a rivet is close to a lip or obstruction that makes it very awkward to get the gun/set or a bucking bar into position. In fact, I had a couple of rivets that were impossible to buck until I made a custom set and also a couple that required a custom bucking bar. I'll also admit that I even ended up having to use a couple of pulled rivets since I couldn't get a bucking bar behind them due to obstructions.

In addition to planning ahead for the actual riveting operation, there is also an advantage to planning ahead for painting. After fabricating and preparing the various sub-assembly parts, I have a natural tendancy to want to rivet the pieces together and then move on to the next piece. Certainly this is the natural flow and can expedite the process when parts are done in batches. However, there are a few places where this might cause problems later on when one is going ahead with the painting. Perhaps it's my lack of experience with a spray gun, but I had a hard time visualising how I could get a good even coat of paint in the deepest parts of the two side panels that go between the seats and the door sills since there really isn't an angle that allows for a good spray pattern. I ended up choosing to paint the two parts of each panel separately and then riveting them together after painting. A quick touch-up with a brush on the bare rivet heads and the panels were complete. Similarly, it would be a very difficult task to paint the various interior surfaces of the hull sheeting after they've been riveted in place; I chose to pre-paint these surfaces as well.

 

There are several places, such as on the underside of the cabin, where two panels meet in a flush lap joint. When the rivets are bucked it can cause the edge of the outer panel to slightly raise up and the joint no longer appears "tight". The solution to this is to lightly go over the outer edge with an edge rolling tool before joining the two panels. It should be noted that this edging does not have to be so hard that it results in a pronounced angle and in fact it only requires a very slight bend. If the piece is over-edged, it will result in noticeable denting when the rivets are driven.

 

In some areas, the bucking bar has to be very close to either pre-painted or anodized pieces. I find that if I wrap some duct tape around the bar, except on the working face, it acts as a bit of a cushion and prevents marring the adjoining surfaces. In a few places I also found that a bit of tape on the surrounding surfaces also helped.

 

Much of the riveting can be done solo, but this does make it somewhat harder and more awkward. When riveting smaller and light assemblies, it is sometimes hard to figure out how to hold the bucking bar and rivet gun while also holding the parts steady. It is important that one can apply more pressure to the gun than the bucking bar, otherwise the rivet will be forced out of the hole before the bucking even begins. I've found that I can usually roll a couple fingers around the edge of the bucking bar and these can be used to both stabilize the bar / work and also to apply the back pressure to the work. It's also possible sometimes to mount the bucking bar (or a cupped set) in a vice and then have a second hand free to stabilize the parts.

 

Riveting with a partner adds an extra hand on both sides of the work. Obviously there is a need for good communication and a bit of practice on scraps is in order. For the person with the gun, they can use the extra hand on the set to totally stabilize it and prevent it from wandering. I know I was leery of doing this until I first tried it ... just try it alone using the gun and a piece of 2x4 lumber; sure there's a little bit of vibration, but I still have all my fingers.

 

I find when working solo that I like to apply more pressure to the rivet gun and the backside of the surface than I'd originally thought was needed. The reason is simply to prevent the set from wandering once the trigger is pulled. In the event that the set does start to wander, I have a natural tendancy to want to lift the gun even before I've fully let off the trigger. While this is sometimes an advantage on universal head rivets, it can create a real mess on flush rivets if the set is the slightest bit cocked. Definitely a natural tendancy that I've been trying to break and the best solution is to never let the set wander in the first place.

 

There are a couple of places where it is very difficult to get the bucking bar into position. If there is any question of whether there is good contact, then do NOT squeeze the trigger ... if the rivet is not properly backed with the bar then material will deform from the force of the gun and it is virtually impossible to remove this oopsie without removing the whole panel. With a good teasing trigger it is possible to give just one very light tap to verify that the bar is properly positioned before giving the gun a full shot.

 

There are a couple of places where I had to place rivets such that the upset head is in an area where I'd prefer to have smooth surfaces (ie. close to wire bundles). Unfortunately, normal bucking of a rivet causes a flattened surface with relatively sharp edges. This can be handled one of two ways:

  1. For a countersunk rivet, instead of using a flat set and bucking bar, by back riveting using a bucking bar on the flush head and a universal head set to form a rounded head.

  2. On a universal head rivet, first doing most of the bucking process and then coming back using two universal head sets. This can sometimes be done by mounting one set in a vice and laying the formed head into it and then using the other set in the rivet gun.

 

I found a few places where I wanted to have smooth surfaces on both sides of riveted material and also a couple of holes that needed to be filled. Although it really isn't appropriate for structural joints, I was shown a trick that works for this situation. One piece (or side) of the material is countersunk in the normal manner and then the other piece (or side) is also countersunk where the shop head will be formed. By selecting a flush head rivet that just protrudes perhaps 1/16" or so (on a 3/32" rivet), the rivet can then be bucked / squeezed and the tail should be flush with material. Again, this is not a proper structural rivet but it can be used for cosmetic purposes or where the loads are pushing the two pieces of material together.

 

I was very careful using the countersink when preparing the holes for flush head rivets, but I found that not all of my rivets sat perfectly flush when I was finally ready to join the parts (ie. maybe a couple of thousanths of an inch proud). Perhaps this is due to the Alumiprep and Alodine sequence, wet riveting the surfaces, slight variations in the rivets or a very slight misalignment in the holes. Regardless, I finally found a trick that works for me in many cases. After inserting the rivet, I then try to back up the lower piece (not the rivet) and then give it a couple of quick light taps with the rivet gun which tends to fully seat the rivet. I then go ahead and actually set the rivet using either the bucking bar or squeezer.

 

There were a few places, notably around the center side ribs, where the two (or three) sheets of metal did not want to sit flush while the rivet was inserted and bucked. The solution to this is to take a piece of leather or gasket material that is roughly the thickness of what should be the protruding part of the rivet and punch a hole in this material. It is then slipped over the rivet end and the bucking bar rests against this material. As the rivet is driven, the material forces the panels to be pushed together before the rivet expands. After the bucking is complete, the material is simply slid off the expanded rivet.

 

There were a few areas on my pre-fitted cab where factory-supplied rivet holes had been drilled such that they just caught the edge of the underlying material. While the dimpling of these holes was tricky, it helps to position the pieces but does not aid in their ability to be clamped together as the rivet expands. The solution here is to use a slightly longer rivet and to add a rivet washer so that the edged piece is clamped between the outer sheet and the rivet washer which is actually holding the expanded rivet and taking the force. These rivet washers also worked well on the soft corner door frames where some of the holes had been slightly ovalled and I was concerned about how the rivets would eventually "work" in the soft material and become loose.

 

If a rivet doesn't seem to be setting properly, STOP IMMEDIATELY! Sometimes it is possible to correct the situation without having to drill out a rivet and if the rivet has to be drilled out it is much easier to do so when it hasn't been fully bucked. A common problem I had initially, especially doing flush rivets without help, was not pushing hard enough against the lower surface which then causes the rivet to "rise" in the hole due to the force from the bucking bar. If this is detected quickly enough, it is sometimes possible to back up the lower surface and re-seat the rivet with a couple of light taps from the rivet gun. If it's not caught quickly enough, further action won't seat it and it will just make it more difficult to drill it out.

 

Some days it just seems that riveting won't work properly for me ... why I don't know, but it seems things are conspiring against me and I end up having to drill out a lot of them. If possible, I find that it's best for me to just stop for awhile and give it a rest. A couple of times I've then realized why I was having problems and other times it seems like when I restart the process that I haven't changed anything but magically it's working again. Obviously more experience with riveting would help with this anomoly but it's interesting to note that I've also worked with someone who has done a lot of riveting and they also experience this on occasion. I've also had a case where the gun's regulator had changed to full pressure which resulted in a rivet that was way over-bucked before the gun slipped and dented the surrounding material. I figured the piece was ruined, but by leaving it for a day I was able to see how this piece could easily be salvaged without a lot of effort.

 

I used a few pulled rivets, both the countersunk and protruding head style. Since I hate to find mandrel heads rattling around, I use a small bit of JB-Weld in the hole up against the mandrel which is applied with a toothpick. By completely filling the holes on pulled rivets in visible areas, this also gives a much better visual appearance.

 


 

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Last updated: September 22, 2011