Working model of Smeaton's boring mill, 1770

"This model which is based chiefly on the boring plant erected by John Smeaton at the Carron Ironworks in 1769, illustrates the best practice attained before the great advance in accuracy made by John Wilkinson in the Bersham boring mill.

Until the advent of the Newcomen atmospheric engine in 1 71 2 all boring appears to have been accomplished with simple augurs in the case of wood and with D bitts for brass and iron. Such bores rarely exceeded a few inches in diameter.

In Smeaton's mill the boring of large cylinders for the Newcomen engines was accomplished with the aid of a boring head which carried the cutting tools round its periphery. The head and its driving bar were unsupported except loosely at the driving end. Thus the weight of the head caused it to cut mainly along the bottom of the cylinder and, since it was guided solely by the casting itself, the resulting bore was neither circular nor straight. To rectify this to some extent the cylinder was bored in four settings, being turned through 90 degrees at each setting.

Power is derived from an undershot waterwheel which drives two spindles through spur gearing, Thus three speeds are available for boring, by making use of the waterwheel direct, or by either of the spindles. The cylinder is mounted on a trolley working on guide rails, and js drawn over the cutting head by a chain tackle and windlass. There is little or no rigidity for the cylinder and truck.