Not the whole of the hole.

Author:Stover, Mark
Position:Tooling around

We are very aware that in nearly all manufacturing environments, hole-making operations consume more machining hours than any other type of metal-cutting operation. There always seems to be an endless number of holes that must be drilled, reamed, tapped, bored, recessed, and so forth. This constant drone of holes can often lull us into a pattern of production processes, whereby we focus on getting the hole roughed in with a drill, and then address the component with secondary-operation tools to finish the various feature requirements as needed. This standard approach can possibly lead us to overlook an often underutilized method of producing holes--and one that can present some unique benefits and highly desirable byproducts. This hole-production method is called trepanning.

Trepanning produces a through-hole by axially cutting a peripheral circular groove that leaves a center core of uncut material. Trepanning is normally used on larger diameter holes. Most trepanning tools start at a diameter of at least 2" and in some cases can produce holes of 10" in diameter or larger. When compared with solid drilling, trepanning has many unique benefits.

One of the difficulties in producing large diameter holes is the significant horsepower requirements required to drive the large diameter drill. Trepanning tools require significantly lower horsepower than solid drills in most cases the power consumed will be less than half of what is needed for a solid drill. This lowered power consumption can expand the capacity of the available machine. For example, a 25hp turning center will probably struggle to drive a solid drill over 2.25" in diameter--whereas the same turning center can push a trepanning tool of 4.5" diameter or more. The same potential applies on machining centers. It should be noted that most of the time horizontal machining centers are preferred for trepanning tools (as opposed to VMCs) because of the need to manage the weight of the core during the trepanning operation.

Machine-cycle time is also an important consideration when evaluating trepanning. Compared with drilling, trepanning is easily just as fast an operation, and usually faster. And trepanning can result in significant cycle-time savings when compared to creating a large diameter hole by starter hole drilling and then multiple-pass rough boring.

Drilling large diameter holes produces a tremendous volume of chips. These chips have to be evacuated during the drilling process...

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