Forging iron requires hard hitting and good hammer control. Having a well designed and efficient hammer is essential if you want to produce good work and last the entire day without your arm becoming too tired and/or sore. The weight of the hammer head, the shape of the hammer head, and the shape of the handle all need to be considered when making the perfect forging hammer.
Some would say the heavier the hammer the better. But that’s not necessarily true. When considering the formula for kinetic energy, it is seen that acceleration is the more important factor over mass in producing energy. Other factors being equal, a faster swing at a lower weight is better than a slower swing at a heavier weight.
Obviously you will need a heavier hammer for larger material, but if you’re mostly making average size home decor and tool projects, it is good to have one “go to” hammer which can handle 90% of those projects. In my opinion, a well balanced 2.5 pound hammer is the best weight for this.
The balance of the hammer refers to the amount of weight on each side of the handle. A perfectly balanced hammer will have equal weight on each side. A front heavy hammer will have more weight on the main face side. A Japanese style hammer has all the weight on one side.
I personally prefer a slightly front heavy hammer as it produces a dropping effect and (possibly) helps with acceleration. But that idea is probably just in my mind. I am not a physicist.
The most common forging hammer will have a cross peen on the back side of the hammer. The cross peen is used for spreading out material. You can orientate the peen in others ways to suit the type of spreading you want to do. The peen should be quite flat with rounded edges. With my hammer (pictured above) I made the peen quite wide as I find this works better for what I want it to do. But of course, the narrower the peen, the narrower the fuller it will produce.
The face of the hammer should be close to flat with a slight crown. The edges of the face should be rounded as sharp corners will show in the work piece. Some guys like to work with a rounding hammer which has a pronounced rounded face, like a squished ball. This shape can be helpful when drawing out material. But the peen of the hammer and/or the corner of the anvil work well for that as well.
The gripping part of the handle should fit your hand comfortably. I like my fingers to completely reach around the hammer so that they just touch the ball of my thumb. You don’t want to have to grip the hammer too tightly as that will cause fatigue and soreness in your arm. You should mainly grip the handle with your index finger, middle finger, and thumb. This allows the hammer to pivot as you swing causing a whipping effect as you bring the hammer down. The handle should not be shaped round, but rather with a square/oval shape. This will better conform to your grip.
I shave down the handle between the grip and the hammer head to a relatively small circumference. This creates a spring effect in the handle and helps dissipate the shock of the hammer blows so the vibrations don’t travel up to your arm.
So this is the perfect forging hammer: A good weight you can work with all day; a properly shaped face and peen; balanced on the handle to your liking; a handle which fits your hand comfortably; and a handle which correctly dissipates shock vibrations from reaching your arm.