Cutting tools on drilling machines

Various cutting tools besides twist drills are used on a drilling machine, and some of them are described below. 

Combination drills known as Subland drills combine a number of operations in a single tool e.g. drill and ream, drill two diameters, drill and chamfer, drill and spotface, drill and counterbore. Fig. 8.10. Each cutting edge has a separate land and flute. Fig. 8.11, which enables cutting to take place and resharpening to be easily carried out. 

A reamer is used to produce a hole of greater accuracy than can be obtained using a drill. The hole is drilled undersize by an amount depending upon the diameter (see Table 8.2) the required finished size is then obtained with the reamer. Care should be taken with the position and alignment of the drilled hole, since the reamer will correct size, roundness and surface finish but will not correct errors in alignment. 

As a general rule, reaming is carried out at half the speed used for drilling. 

Flutes on reamers usually have a left-hand spiral or helix, opposite to that of a drill. This pushes the metal chips ahead of the reamer rather than back up the flutes and prevents scratching the bore. This feature also prevents the reamer from screwing itself into the hole, which would tend to happen with a right-hand helix. 

Reamers are made from high-speed steel in sizes up to 50mm diameter in a variety of types, the most common being the machine reamer. 

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