A gundrill starts its cut off-center and is not center cutting where the tip apex attempts to enter the workpiece. If circumstances do not allow for a gundrill bushing, an accurate pre-drilled hole can accomplish the same result. In this case, make sure the point geometry of the twist drill in the bottom of the hole allows the point of the gundrill to bury itself first, before the outer corner of the drill comes into contact with the bottom of the hole.

Once a hole is well started, it becomes the bushing for the tool during the remainder of the drilling, since the gundrill is self-piloting. If the part is rotating, it is preferable for the bushing to rotate with it. If the part is stationary, the bushing should be rigidly clamped to the bushing support.

The bushing axis must be concentric and in true alignment with the spindle axis. If you need to check your spindle to bushing alignment, we will be happy to send you complete instructions and a drawing for the parts you will need to accomplish it.

The bushing support must be part of the machine set-up in order to eliminate constant bushing realignment, which can occur if mounted in a removable fixture. The bushing should be fixed, when possible, rather than adjustable.

The bushing front section, where it touches the workpiece, has a normal tolerance of +.0002, -.0000. Length of this close tolerance section need only be one diameter from the front edge. The rest of the bushing may be relieved but the bushing must be concentric to the body diameter - with good push-fit in the support liner.

The bushing bore should have no chamfer where touching the workpiece. It should contact the work with a sharp ID edge to avoid chips packing in an open space. It is advisable to crown the bushing face so that close contact with the workpiece is established.

Bushings are normally made of hardened steel, but for the smaller sizes it is cheaper and easier to manufacture inserts which are a press fit into liner bushings. On high production machines these inserts can be made of carbide.

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