The final wire spring to cover is a torsion spring. In its simplest incarnation a torsion spring is just a stick of music wire with a few twists, usually 2-5, wound on a mandrel, with the ends left straight. That version is so uninteresting that we’ll proceed directly to a more interesting incarnation: AR and AK hammer springs. (The AR series rifles also use a torsion spring for trigger return). This sort of spring is really two torsion springs with a linkage in between that engages the hammer.
For those that are unfamiliar with such a spring, here is the spring from an AK-47:
The AK-47 is one of a few designs that have the additional feature of being made from material which is actually a pair of twisted wires. For our purposes here, we’ll wind a torsion spring from a single piece of music wire to replace a factory AK spring.
There are three key measurements that we need:
- The distance from the center of the ‘axle’ or pivot pin, to the area on the hammer where the spring arm should apply its force.
- The width (plus a small allowance) of the hammer, and
- The diameter of the bosses on the hammer that the spring windings will rest on.
The first dimension is a direct measurement. The second is the width of the hammer plus 0.01″-0.02″. The final dimension is easily ignored by using a mandrel the diameter of the bosses on the hammer. By doing so, the spring will ‘unwind’ enough after forming to ensure a loose enough fit that there is room for it to tighten as the spring is tensioned.
Making a hammer spring requires special fixturing. In essence the fixture is an axle (mandrel) of the correct diameter, around which rotates a form, of the desired width, which holds a bite (the place where the spring material doubles back) at the appropriate radius. Add to this something to straighten / tension the wire before it is wound, and you have everything you need.
There are many ways to make a fixture that satisfies these requirements. I’ll make mine as three units: collars to account for the diameter of the bosses, a form which factors in the hammer width and bearing surface, and a tool to maintain tension and straighten the wire as it’s being wound. As an arbitrary choice, I’ll use a 1/4″ axle for the assembly to work on. It’s worth noting that the easiest way to do this might involve using the hammer itself as a form and collars with only the addition of a tensioning tool, but a custom tool will give a little more control.
Start by making the form. The form for this AK replacement spring will be 0.300″ wide (that’s the width of the AK hammer (0.270″) pluse a 0.030″ allowance. It will trap the “bite” of the wire at 0.575″ from the center of rotation. The center of rotation will be a 1/4″ hole to fit the axle.
I square up a piece of material that’s close to the correct dimensions, thin it to 0.300″. At that point I’ll layout two lines – the axis on which the axle hole (1/4″) will be drilled and the line that must trap the bite 0.575″ away.
I’ll then saw a notch to capture the “bite” – the notch must be wide enough to accommodate the wire I’ll use – extra width won’t hurt. You can double up blades on a hacksaw to do this, if necessary.
With the notch cut, I’ll go to the mill or drill press and drill the axle hole – that’s it for the form.
Next, I’ll make the collars. I’ll start by drilling some stock 1/4″ for the axle, then turning it to the diameter of the hammer bosses (0.390″). That done, I’ll part off two pieces 1/2″ long. (The windings should only take up 1/4″ or so, so a great deal of width is not required).
Next a tensioning tool is needed. It needs to capture, and tension, two legs of the spring, rather than just a tail as in previous spring types. A single screw adjustment ensures that tension is even over both legs. I made mine from an ‘L’ shaped piece of 3/8″ stock, so I could mount it in a standard tool holder. A small bar of 1/4″ strap serves as the tensioning device. Tension may be adjusted by the single 1/4″-20 screw.
A lathe is not at all necessary to wind a hammer spring, but it’s convenient in this case. Anything that can hold the tensioner and axle will do. The spring is started by making a bite in the spring that is the width of the form. The legs are placed through the tensioner and the bite is placed in the notch on the form. The collars are held in place by whatever means are expedient. (My means look especially improvised – but they work!)
Unlike previous springs, this one must be wound the exact number of turns desired as the straight tails are part of the final product. By rotating the form, the coils are wound.
Complete the winding by snipping the ends of the spring stock and removing the raw spring.
I took a practice run and produced a mediocre spring, the next was what I wanted. For the AK spring, the tails have a bend after the straight portion – in this way they serve double duty, fulfilling the function of the trigger return spring as well as the hammer spring. Bend the tails, trim, then bend the trimmed ends. The final spring fairly resembles the spring it is patterned after. Here it is, installed on an AK hammer:
Temper the spring in the usual way, by baking at 450* for an hour.
If you’ve built AK ‘kits’ then you’ve probably replaced the factory trigger with US made parts to be in compliance with 922r – winding springs for the old parts makes them usable in other projects (bearing in mind that 922r still applies – you may not use more than 10 ‘counting’ parts in a firearm).