Referencing the old handle for rough sizing, I sketched out a new handle design on a scrap piece of ash from an earlier project. Knowing I didn't want a straight handle shape, I printed some pictures of hatchet handles as reference and sketched onto the wood until i had a profile I was happy with.
With the design penciled on the wood I went to the bandsaw and cut the rough shape out.
After the rough shape has been cut I compared the new rough shape to the old handle for sizing, ensuring that my handle was approximately the same length as the original and meets the new design I was going for. With everything checking out I could move on to shaping the rough handle into something more ergonomic.
Depending on the scrap you have, you may have to reduce the width of your handle. You can rip your scrap piece before you start roughing out the profile, but I chose to do it after.
Once I has happy with the sizing I could start shaping the handle on the bandsaw.
The eye of the hammer is where the handle is inserted into the head. I made a few measurements of the eye opening from the head and transferred it to the eye of the handle.
With the head fit to the handle I made marks where I could trim the handle profile down near the head.
Back at the bandsaw the shape was refined further, careful not to remove too much material as the final shape will be refined by hand with the rasp.
With the majority of the bulk removed from the handle and the rough shape made a hand rasp was used to smooth out the contours of the handle and give it the final shape.
After shaping with the rasp I compared the new handle with the design intent and the old hammer handle. My handle was slightly longer and had a more ergonomic shape.
The fine end of the rasp leaves a decent finish, but you can really make the grain of your wood handle stand out by sanding it smooth.
With the handle shape finished the wedge slit can be cut into the handle eye.
To ensure the head stayed on the handle we'll drive a metal wedge into the eye slit when the head is installed. This steel wedge was made from a scrap of steel bar stock that was about as wide as the eye. I ground a taper in one end of the steel wedge with an angle grinder.
All done.