Horizontal boring machines are built in many other designs, but they all have the same general arrangement as the machines illustrated and operate on the same principle, with the exception of special types intended for handling certain classes of work exclusively. The horizontal boring, drilling and milling machine is very efficient for certain classes of work because it enables all the machining operations on some parts to be completed at one setting. To illustrate, a casting which requires drilling, boring and milling at different places, can often be finished without disturbing its position on the platen after it is clamped in place. Frequently a comparatively small surface needs to be milled after a part has been bored. If this milling operation can be performed while the work is set up for boring, accurate results will be obtained (provided the machine is in good condition) and the time saved that would otherwise be required for re-setting the part on another machine. Some examples of work on which different operations are performed at the same setting will be referred to later. The horizontal boring machine also makes it possible to machine duplicate parts without the use of jigs, which is important, especially on large work, owing to the cost of jigs.
=Drilling and Boring--Cutters Used.=--Holes are drilled in a horizontal machine by simply inserting a drill of required size either directly in the spindle _S_ (see Fig. 1), or in a reducing socket, and then feeding the spindle outward either by hand or power. When a hole is to be bored, a boring-bar _B_{ 1}_ is inserted in the spindle and the cutter is attached to this bar. The latter is then fed through the hole as the cutter revolves. The distinction made by machinists between drilling and boring is as follows: A hole is said to be drilled when it is formed by sinking a drill into solid metal, whereas boring means the enlargement of a drilled or cored hole either by the use of a single boring tool, a double-ended cutter which operates on both sides of the hole, or a cutter-head having several tools.
There are various methods of attaching cutters to boring-bars and the cutters used vary for different classes of work. A simple style of cutter which is used widely for boring small holes is shown at _A_ in Fig. 3. The cutter _c_ is made from flat stock and the cutting is done by the front edges _e_ and _e_{ 1}_, which are beveled in opposite directions. The cutter is held in the bar by a taper wedge _w_ and it is centered by shoulders at _s_, so that the diameter of the hole will equal the length across the cutter. The outer corners at the front should be slightly rounded, as a sharp corner would be dulled quickly. These cutters are made in different sizes and also in sets for roughing and finishing. The roughing cutter bores holes to within about 1/32 inch of the finish size and it is then replaced by the finishing cutter. A cutter having rounded ends, as shown by the detail sketch _a_, is sometimes used for light finishing cuts. These rounded ends form the cutting edges and give a smooth finish.