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Jig Boring
Jig boring is used to accurately enlarge existing holes and make their diameters highly accurate. Jig boring is used for holes that need to have diameter and total runout controlled to a high degree. Typically, a part has holes machined on regular equipment and then the part is transferred to a dedicated jig boring machine for final operations on the especially accurate holes. Jig boring can also maintain high accuracy between multiple holes or holes and surfaces. Tolerances can be held readily within ±.005 mm (±0.0002 inches). Dedicated jig boring machines are designed to machine holes with the tightest tolerances possible with a machine tool.

When designing a part with holes, it is important to determine what holes must be jig bored. The reason for this is that jig boring requires extra time and attention, and the jig boring machine at the machine shop may have a back log of jobs. Jig boring can therefore have a big impact on the lead time of a part. A cross section of a hole being jig bored is shown below.

Standard boring can be carried out on a mill fitted with a boring head or on a lathe. Boring is most accurate on a lathe since a lathe is dedicated to solids of revolution (axially symmetric parts).

Gun Drilling
For long holes such as those found in gun bores, gun drills are used. The length of the hole requires that coolant be delivered through the shaft of the gun drill to the cutting front. The coolant also serves to eject chips from the cutting area and to move them back and out of the hole entrance. The figures below illustrate a gun drill and the cutting/cooling configuration.




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