Wednesday, July 13, 2011

Fire in the Hole




I like to use solid rivets. Emphasizing the joinery sometimes seems to dress up a piece which may otherwise be rather plain. They are a traditional or classical element whose use goes back to the Bronze Age.

Their nearly universal use in blacksmith tongs illustrates their advantage in being easily removable permitting repair and modifications.

If you do an image search for a rivet hammer, handheld type, not the air power hammer type, you will get the typical light cross peen variety. I’ve got a couple and rarely use them for anything especially rivets. A ball peen works best for me, at least in forming a rounded head. A flatter face is better for flush rivets or flat, low profile, ones. Maybe that’s what the rivet hammer is good for. The light weight which favors quick high velocity blows does make sense.

I only recently learned that in store-bought rivets the formed end is called the factory end and the one I form by upsetting, bucking, is called the shop end. That makes sense too. The shop end is also called the buck tail which seems to take a little more imagination to make sense. I suppose the factory end is the head and the shop end is the tail formed by bucking.

When bucking the factory end is supported by a rivet set to keep it steady and to preserve the head radius. I make mine by heating a block of steel and hammering in a carbon steel ball part way or using the press to do that. Various radius impressions can be done in a single block or individually. Incidentally some people seem to call the tool a rivet snap.

For most rivets I cut the shaft so the material available for upsetting is about 1.5 to 2.0 times the shaft diameter.

I usually slightly over-drill the rivet hole or slightly oversize the diameter with a punch or drift.

The out of usual context of this title occurred to me as I was heating a tenon in the process of peening. The punched hole was intentionally slightly larger than the diameter of the tenon so assembly would be easy but a tight fit was the goal. Directing the torch flame down the hole heated the base of the tenon, or rivet, so the following top blows would upset the shank to pretty well fill the hole. The next heats are directed at the tip of the tenon so it will mushroom to cover the hole and capture the bar. This is a small detail but sometimes important.

Another detail to consider with tenons, or rivets, and their corresponding fit holes is to make them both out-of-round, in other words, oval. There is extra rotational stability with this shape. Of, course it requires making an oval drift to prepare the holes. I’ve mostly used this on small items like delicate feet on small trivets.

In the process of making a lot of botanical stems I use a lot of 40d and 60d common nails. I use a cold stock shear to cut off the heads and gripper section. This leaves a lot of flat head rivet material. Common nails are tough and soft - I’m guessing about 1010 carbon steel alloy and work nicely as rivets.

I’m in the process of reworking some old inexpensive tongs which I picked up years ago in flea markets to form some more useful ones. I’ll likely have to make all those larger rivets myself. Fortunately I have some old rivet headers which I also found back in my flea market wandering days.

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