Buck that.....

One of the new skills I'm learning is how to "buck" rivets.

Bucked rivets, also referred to as driven or solid rivets, have been used as the primary fastener type in the construction of aircraft for decades.  These solid rivets are light-weight, strong and inexpensive fasteners. That’s why they and pulled rivets were chosen for aluminum aircraft construction.

A bucked rivet is a round fastener that attaches two or more pieces of metal together. The rivet is driven by a pneumatic rivet "gun" with an attached rivet "set" shaped according to the shape of the manufactured head of the rivet. The rivet's "tail" (blunt end) is backed up by a "bucking bar" that acts as an anvil while the rivet gun and set are acting as a hammer.

​As rivets are driven the tail of the rivet is transformed (technically called “upset”) and two things are accomplished.  First, the rivet shortens in length and the exposed tail bulges outward to morph into what is called the shop head.   Secondly, the shank diameter swells in the hole to fill it entirely. A bucked rivet holds the metal pieces in compression (like your thumb and forefinger holding two pieces of paper together) and in shear (does not allow the pieces of metal to slide around in relationship to each other. Typically multiple rivets are used to hold objects together and the combined strength of all of the rivets have tremendous holding power. Although riveted construction is permanent in nature, it can be easily repaired by drilling the existing rivets out, making any necessary repairs and re-riveting with the same or a slightly larger size of rivet.

Although it will take some time and practice to learn how to do this, it really isn’t a terribly difficult skill to acquire.

First thing is the rivet gun.  The rivet gun is like a hand held pneumatic jack hammer for driving rivets and could easily be confused with an air hammer. However, there is a significant difference between these tools as the rivet gun’s impact can be controlled by varying the amount of squeeze on the trigger, whereas an air hammer is on full or off. The gun incorporates a removable coil spring that both holds and retains the rivet set securely to the rivet gun and acts as a return spring each time the rivet gun drives the rivet set outward:

The rivet set is the business end of the rivet gun. You can have one rivet gun and any number of rivet sets to accommodate the shape required for the contour of the manufactured head of the rivets being used.  From what I can gather in the plans, the vast majority of bucked rivets in the Zenair aircraft are of the domed head type, so the rivet set I need looks like this:

The bucking bar is the other half of the equation.  Just about anything of a hardened material can be used and like the rivets themselves they come in all shapes and sizes, allowing access into tight or blind spaces where required.  The brass cylinder on the left in the photo above can be used as a bucking "bar", as can the red cylinder propping up the rivet gun for the photo.  Sometimes they are a square block, it doesn't matter as long as it can be held firmly and flat against the tail of the rivet being driven.

Like any fastener, rivets come in any shape and size you desire, and the plans are very specific about the specs required at each location.  Usually, the length is measured from the flat underside of the rivet head to the tail and is noted in sixteenths of an inch.

Sometimes, rivets that are too long can be trimmed to the correct length using a rivet cutter.  A rivet cutter is like a scissor or shear that can cut rivets of various sizes to specific lengths. The 1/4'” length rivets cannot be cut any shorter with this tool as this the minimum length of cutting. However, the rivet cutter can cut longer rivets (like the 1/2” rivets) to 4/16” (AKA ¼”), 5/16”, 6/16” (or 3/8”) and 7/16”. The rivet cutter has a series of length setting spacers that can be rotated above the selected hole that is appropriate to the rivet diameter being cut. Then, tighten the knurled knob to keep the length setting you desire while cutting a number of rivets:

An A4 rivet being prepared for trimming to correct length. This was just for the picture, but I'd want to be more careful using the spacers. I actually have three, not two under the rivet head like I intended! This would make the rivet too long by 1/16th.

As with any new skill, practice makes sense.  I took a couple of scrap pieces of aluminum, match drilled some holes and got right to bucking some practice rivets.

Clecoed together as would be standard practice on a real rivet assembly process. Here I'm using a bench vise to hold the work, but it could be just as easily on the airframe. You can see the gold coloured A4 rivet placed in the hole at the centre. This is the rivet gun and set side of the job.

Backside of the rivet protruding through the hole. This is the side the bucking bar is held against.

Because this is a two handed job (sometimes a two person job) it was impossible for me to get pictures of the rivet gun and bucking bar in action, and I forgot to get a picture of the after bucking.  I'll post some soon, but I'm happy with my first attempts and Ron says my results are to an acceptable level for construction.  Again, it's not a hard skill, but an important one to get right.

Setting aside the rivet gun for a bit, I focused on match drilling and fitting (and of course deburing) the ring root assembly in anticipation of final riveting (both pulled and bucked rivets).

With everything clamped temporarily in place, I noticed that the wing root rib upper flange wasn't matching up properly:

As always, when something doesn't look right, refer to the plans.

The plans sometimes don't describe things as well as they should.  Part of that is on me as a new builder.  In this case, the plans advise that the flange is only bent to 30 degrees at the front, gently increasing to 90 degrees towards the rear.  This makes sense as the wing root skin that it supports curves in the same way.  So I'll have to bend it back.  Hammering it isn't an option, but using another of Ron's custom made tools is:

Running seam pliers, made from a pair of visegrips with welded on nose pieces.

With a bit of gentle work and some minor trimming, the root rib now fits as expected:

Much better!

Next step was figuring out how to drill the rib, spar web, web doubler and nose rib in prep for final riveting.

Much like the wing attach fitting, this involved back drilling the top hole through the web into the rib, cleco, drill the bottom hole, cleco and finally the middle hole, being very careful not to ruin the previous holes:

The beauty of clecos is that they can be used from either side. This left me room for the drill to complete the middle hole.

Remove the root rib, debur and repeat the process for the root nose rib, back drilling from behind the spar.  Remove, debur and fit everything together again:

Final riveting of the root ribs will be done once the rear spar fitting is complete and test fitted.  I made progress in this regard too by match drilling the rear spar doubler to the rear spar:

Pictured just after drilling and clecoing the first two holes. This ensures good alignment of all the holes when match driliing.

As I stated before, the vast majority of rivets used in Zenair aircraft designs are pulled rivets as the sheet metal construction fastener of choice.  The spar web doubler was ready for final pulled rivet setting.

The larger A5 rivets are called out in the plans for this assembly.  Although they can be pulled by a hand rivet tool, it is much easier and consistent to use the pneumatic rivet gun.  It uses the same mechanics as the hand riveter, but is much quicker and completes the "pull" in one shot, alleviating potential issues with partially completed rivets (where the rivet isn't fully pulled before the mandrel breaks off).

The pneumatic rivet gun used for pulled rivets. By the end of my build we will become best friends!

The pneumatic rivet gun made short work of setting the A5 rivets:

Pulled A5 rivets look real nice. When I took this picture, I was starting to test fit the solid rivets for length in the wing attach fitting. They run along the bottom where the clecos in the picture are securing the spar web to the spar cap.

Backside of spar web doubler - looks like a crop of mushrooms.  It's pleasing to see how much rigidity this gives the structure, something  ignored by the previous builder.  If you look close you can also see the tails of some of the yet to be bucked solid rivets where they poke through the spar cap:

Progress...  Bucking and pulling rivets is fun (that opinion may change after doing thousands of them).  I'll post some pictures and maybe even some video when I get a chance.  Stay tuned :)