If you've ever broken a really nice wooden-handled hammer or other hand woodworking tool, you know how frustrating it can be have to throw the tool away and buy another. Why trash it when it is so easy and fun to make your own hammer handles?
If you have a lathe and a few gouges, you can turn your own handles out of ash, hickory, cherry, oak, maple or any other kind of hardwood.
First, on a square blank of stock, make marks on each end from corner-to-corner to form an "x". The center of the x will be the center of the blank.
Second, center the blank at these opposite points in your lathe and round the stock to the maximum diameter of the handle (the widest point of the oval shape of the handle).
Third, remove the stock from the lathe and measure two offset distances on each end of the blank (as shown by the small circles in the image above). The offset marks should be equidistant from the center along one axis only, but the distance will be determined the desired narrow-axis of the oval. In other words, the wider the offsets, the narrower the width of the handle. If you place each offset 1/4" away from the center, your handle will be 1/2" narrower along the narrow axis than along the wide axis.
Once you've placed identical symmetric offset marks on each end, place the blank into the lathe on one of the offsets (on each end). Turn on the lathe at a relatively slow speed and turn the blank to the desired shape. This will create half of the oval.
When you're satisfied with the shape of this half of the oval, move the handle stock to the opposite offsets and complete the other half of the oval.
Once the two sides of the oval match relatively closely, you'll likely need to do some hand sanding to smooth out the rough edges. If you don't have too drastic of a difference between the two axes of the oval shape, you may be able to sand the handle by placing it back into the lathe on center and sand it in the lathe.
After completing the handle, use a chisel to adjust the fit to the shape and size of the adze eye of the hammer's head. The handle should fit smoothly and easily, and should extend just past the adze eye by about 1/8".
With a band saw or hand saw, make a narrow-kerf cut of about 1" length through the adze end of the handle to accommodate a hardwood wedge. This will hold the head onto the handle. Cut a wedge the same width as the cut. The wedge should be about 1" long and only about 1/8" at the fat end.
Place the handle into the adze eye, then drive the wedge into the slot. The pressure created by the wedge will hold the head onto the handle. Trim any excess off of the wedge as necessary.