Hammers have two parts--the hammer handle and hammer head. Beyond that are subtleties that only engineers and blacksmiths might appreciate. Every carpenter, every machinist, and every home handy person has at least one hammer in the tool kit.
Use the face of the hammerhead--the heaviest striking surface--for most work. Sledge hammers have two identical faces. Most other hammer types have only one face; the opposite side of the hammerhead is called the peen. The face of a hammer is slightly convex or crowned; too much crown causes nails to skate across the face when struck. Some framing hammers have a cross-hatched face to reduce slipping and allow heavy blows. Handle sockets or eyes have a slight taper, wider at the top than the bottom.
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Peens come in many styles and are designedfor many different purposes. Machinists' hammers have round or ball peens; one use is to expand the head of a solid rivet. The common blacksmith's hammer has a cross peen used for lengthening long stock. Carpenters' hammers include a claw peen for pulling nails.
Many hammers still use handles of hickory or ash. Fiberglass handles may be bonded to the hammerhead with strong adhesives or strap fastenings. Steel handles are forged from the same continuous piece of steel as the hammerhead and covered with a cushioning grip.
The older wooden handles are fixed in the eye of the hammerhead with a double wedge. The first wedge--a wooden wedge--follows the long axis and expands the handle to the sides of the eye. The second wedge--of steel--splits the wooden wedge crossways and expands the handle toward the ends of the eye.
An unseen but essential quality of every hammerhead is temper. Eye walls should be tempered much softer than face or peen. Striking faces must be hard enough to resist denting. Incorrectly tempered hammers can shatter like glass or dent so quickly that they leave scars in the work.
A few companies produce hammers they claim are unbreakable, with handles that won't bend or break or become loose. These claims should be viewed with skepticism. The basic design of these products is the same as the conventional one--hammerhead and handle. No adhesive is permanent. Steel handles can bend or break. Fiberglass can still be damaged by a missed blow.