used doesn't need to be particularly hard or strong as there is relatively little difference between the stiffnesses of most steel alloys. Holders used on lathes Bit holder and toolpost The toolpost is the part of a metalworking lathe which either holds the tool bit directly or holds a toolholder which contains the tool bit. There are a great variety of designs for toolposts (including basic toolposts, rocker toolposts, quick-change toolposts, and toolpost turrets) and toolholders (with varying geometry and features). Box tool A box tool is mounted on the turret of a turret lathe or screw machine. It is essentially a toolpost that brings its follower rest along with it. A tool bit (or several tool bits) and a compact follower rest (usually V-shaped or with two rollers) are mounted opposite each other in a body which surrounds the workpiece (forms a "box" around it). As the tool bit puts a lateral deflecting force on the workpiece, the follower rest opposes it, providing rigidity. A different and popular type of box tool uses two rollers rather than a follower rest. One roller is called a "sizing roller" and the other roller is called a "burnishing roller". The rollers turn with the stock to reduce scarring on the finished turn. Opposing tool bits may be used (instead of a rest) to cancel each other's deflecting forces (called a "balanced turning tool"), in which case the box tool begins to overlap in form, function, and identity with a hollow mill. Holders used on shapers, slotters, and planers Clapper box Shapers, slotters, and planers often employ a kind of toolholder called a clapper box that swings freely on the return stroke of the ram or bed. On the next cutting stroke, it "claps" back into cutting position. Its movement is analogous to that of a butterfly-style check valve. Holders used on milling machines Fly cutters Fly cutters are a type of milling cutter in which one or two tool bits are mounted. The bits spin around with the rotation of the spindle, taking facing cuts. Fly cutters are an application of tool bits where the bits are part of a rotary unit (whereas most other tool bit use is linear). History Tool bits have been used for centuries, yet their further technological development continues even today. Before about 1900, almost all tool bits were made by their users, and many machine shops