Boring using Boring Milling Machines
Boring milling machines come in several sizes and styles. On small workpieces, boring operations can be done using a lathe while on large workpieces; the creation of accurate holes is done through boring milling machines. Workpieces usually vary in size from the more common 1 to 4 meters to those that can be referred to as extremely large at 20 meters which would require about 200 horsepower for the job. Since boring is a process that is meant to decrease product tolerances on pre-existing holes, different considerations in design are often required. One of the considerations is deflection of the cutting tool in large length-to-bore diameters. Another is the preference for through holes over blind holes that do not traverse through the thickness of the workpiece. A blind hole is more challenging than a through hole that completely goes all way through the material. Deep hole boring is also especially challenging but developments in technology have allowed for deep holes to be bored with greater accuracy.
Boring using a lathe
On a lathe, boring is accomplished in two ways: by mounting a holder and a boring tool bar with a cutter bit on a tool post while revolving the workpiece and by mounting the workpiece in a fixed position on the carriage while revolving the boring tool and the cutter bit in a chuck that is attached to a headstock spindle. The cutter bit which is used in boring is similar what is used for external turning on a lathe. The boring tools bars are supplied in different sizes and types to hold the cutter bits. Depending on the nature of the workpiece that is being bored, the cutter bits can be held by the boring tool bar at 90”, 30” or 45” angles. Generally, boring is accomplished using a 90-inch cutter bit. It is more desirable for the boring tool bat to be as large possible so that it will not interfere with the walls of the hole. Greater attention should be given to the end clearance angle and the back rake angle due to the curvature of the hole.
Boring is essential because drilled holes are not always perfect. Sometimes there are certain imperfections in the materials that will cause the drill to move out of alignment. When accuracy is required by the application, drilled holes are undersized so that they can be bored or reamed to their proper dimensions. Another benefit provided by boring is truing larger holes in flat workpieces. The hole is cut undersize through a band saw or a trepanning tool after which it is trued to the required dimensions through boring.