How to Peen a Rivet
Making medieval body armour is a study in how to fasten plates of metal or leather around the human body. This is often done with rivets. Thus it is not surprising that one of the most common questions that comes up in armouring discussions is, "How do you peen a rivet?"
I learned to peen a rivet from (now) King Artan McDarough of Forth Castle. Artan's method is different from most other people. Instead of using the ball end of the ball-peen hammer, he uses the flat end. The ball is only used a couple of times in special situations.
Here is my interpretation of Artan's method for peening rivets:
Rivets are used to join to pieces of material together. You can fasten plates to plates, leather to plates, leather to leather, plates to fabric, etc. In this particular case, we are going to rivet a plate to a piece of cloth.
Here is what we start out with:
We have a couple of plates of metal with a hole punched in it, a rivet, a couple of burrs, a leather washer, and a piece of fabric.
The first thing we need to do is put a hole in the fabric. Holes put into fabric should be pierced, not punched. Punching involves removing a small disc of material from the fabric. This results in numerous cut threads running through the weave of the fabric. These cut threads may unravel, which is of course a bad thing. Piercing through the fabric with a pointy object instead spreads the threads of the weave, leaving them all intact.
When riveting fabric or leather, it is a good idea to put a special washer, called a burr, under the finished head of the rivet. This helps distribute the load around the head of the rivet and makes it harder for the rivet to pull through the material. Generally, burrs are not needed when the head is against metal. When I rivet fabric, in addition to the metal burr I also put a leather washer between the burr and the fabric. This prevents the metal burr from cutting into the material.
A note on material selection for rivets. Iron or steel rivets are appropriate for metal to metal joins. However, for leather or fabric to metal joins, use copper or brass rivets. The corrosion (rust) of steel rivets can cause organic materials to rot. The oxidation of copper and brass are not as harmful.
Now we have all the components of the join in place. Make sure you haven't forgotten something before you start peening - nothing is more frustrating than to finish peening the perfect rivet head only to discover you forgot to put the burr or washer in place. Then you get to drill out the rivet and do it again!
Sometimes your rivet shank will be too long. It does not take much rivet shank standing proud to form into a good, strong rivet head. In this case, too much shank is standing proud.
To fix this, simply clip the rivet shank short with a pair of end nippers, side dykes, or mini bolt cutters.
Now we are ready to start peening the rivet. To peen, you need a ball peen hammer and a rivet peening block. For most rivets, you do not need a large hammer. In fact, I recommend starting out with a small hammer to use for learning technique. Rivet peening blocks can be made out of any handy block of metal. A block of copper makes an excellent peening block, as it deforms with the rivet head and won't mar it. I didn't have a block of copper, so I ground a depression into an auto-body dolly.
When peening a rivet, one must resist the natural temptation to treat it like a nail. The idea is not to simply hammer away at the face of the rivet shank, smashing it into oblivion. Peening a rivet head is actually a careful metal forming operation. You are, in actuality, rolling an edge on a small cylinder of metal.
To start with, give the rivet shank a couple of direct strikes straight down, along the axis of the rivet shank, using the flat face of the hammer. This flattens the face of the shank, and also causes the shank to swell, lodging it in the hole through the plate you are riveting to the fabric. This makes the rivet "stick" in the hole a bit, which is handy, because otherwise it will tend to "dance around" as you try and peen it.
Here is where my technique differs from most. Most people I know switch to the ball peen end of the hammer to form the rivet head. This is understandable, as the ball contacts the new-forming rivet head with nearly point-contact, and spreads the metal very rapidly. Unfortunately, it also tends to crater the rivet head, making it look like something somewhere between the surface of a golf ball and the surface of the moon. Furthermore, a careless strike near the edge of the shank will smear or tear the rivet head you are trying to make.
Instead, I use the flat face of the ball peen hammer to form the rivet head. I call my technique "walking around the rivet", because I strike the edges of the rivet shank at an angle, working my way all around the shank. This slowly folds over the edge of the shank.
Here you can see the results of the first pass of "walking around" the rivet:
Note how your newly-forming rivet head has a faceted look to it, like a gemstone. This is caused by the flat face of the hammer. As you finish the head, you will planish it to a nice, even finish.
Sometimes the rivet head will not swell as much as you would like, especially with harder rivets (like steel). In this case, I do use the ball end of the hammer. I strike directly to the center of the rivet shank - this causes it to spread evenly in all directions, giving me more material towards the outer edges to roll over with the flat face of the hammer.
Here you can see me using the ball end of the hammer to do this:
Note the dimples in the center of the rivet face where I struck it with the ball end of the hammer. This has had the effect of moving material away from the center of the shank, widening the rivet head. In the side view you can see the rivet head beginning to form!
Now I resume "walking around" the edge of the rivet head with the flat face of the hammer. Here are the results of more "walking":
Keep going until you have a nice, even head. If you do it right, you should end up with a nicely planished, flat or slightly domed rivet head. The rivet head that you have formed should look as nice as the one made by the factory on the other side. You want to make sure that you have mushroomed the head enough that the materials caught between the rivet heads are firmly sandwiched in between. Once the rivet is finished, there should be no movement or rotation of the joined materials. Of course, this type of rivet join is for firm, permanent joining. Rivets used for articulation are not set as tightly, as this would hamper articulation! Setting rivets in articulations is a bit trickier than setting "static" rivets, and is the subject for another essay (by someone more qualified than I!)
You will note that my rivet head cracked a bit. Well, your rivet heads are on the inside of the armour out of sight, right? (grin).
That's it! Just remember that it doesn't take a whole lot of force to peen a rivet. Go easy, take your time, and do a good job!