The shears of lathes require to be as rigid as possible, because the pressure of the cut, as well as the weight of the carriage, slide rest, and tailstock, and of the work, tends to bend and twist them.
The pressure of the dead centre against the end of the work considered individually, is in a direction to bend the lathe shears upward, but the weight of the work itself acts in an opposite direction.
The strain due to the cut falls in a direction variable with the shape of the cutting tool, but mainly in a direction towards the operator, and, therefore, tending to twist the shears. To resist these strains, lathe shears are usually given the I form shown in the cuts.
Fig. 631.
Fig. 632.
[Figs. 631] and [632] represent the ribbing in the Putnam Tool Company’s lathe; a middle rib running the entire length, which greatly stiffens it.