A Word About Boring Heads

The basic boring head is made up of three components: the body, the bar holder and the graduated micrometer screw (dial screw). the body is usually threaded, permitting the use of the boring head on most machine tools. The bar holder contains a dovetail that is mated to a dovetail slot running the full width of the body. A web, machined on the underside of the bar holder, rides in a slot ground into the head section of the dial screw. As the dial screw is advanced or retracted, it moves the bar holder along with it--permitting controlled adjustments of .0005, or less.

To duplicate the large range of hole sizes that can be made with a boring head, a shop would have to be stocked with many special boring bars capable of boring the one size hole they were designed for. As a rule, this type of special consists of two, or more flutes. However, multi-flute tools present a problem if all flutes are not absolutely identical, even down to flute edge wear.  The  slightest variation could produce a less than perfect hole.

On the other hand, a boring head, depends on a single-point tool to produce a hole. The advantage of this type of tool lies in its ability to form a perfect circle as it rotates around the centerline of the machine tool spindle. And because in a single-point tool the cutting edge is constantly equa-distant from the centerline, the hole it bores is true.

Suggestions For Better Boring Head Performance

1. Cutting edge of tool must be on centerline.
2. Tool sharpness for boring is more critical than for O.D. work.
3. Keep tool overhang to a minimum.
4. Use power feed whenever available.
5. Make allowances for "spring" or deflection when taking a heavy cut. Additional material will be removed on finish cut even though an additional adjustment has not been made.
6. When using carbide, avoid reversing or stopping the spindle in the middle of a cut. This can cause chipping or breakage.
7. For best surface finish spindle should be turning while tool is being retracted from the bore.
8. Avoid "bottoming-out" in a blind hole. A boring tool is not designed for end cutting.

Drill Drift

Drill drift can be the result of a poorly ground multi-flute tool or may occur when one of the flutes becomes dull. As the cutting edge breaks down, greater pressures are exerted on the dull flute. This forces the tool to drift in the direction of the freer cutting flute. A well ground reamer may correct some of this condition. However becaues it is also a multi-flute tool, it will have a tendency to follow the original hole. Drift can be corrected only with a single-point tool such as used in a boring head.

True Position

One of the great advantages of using a boring head with a single-point tool is its ability to establish the true position of a hole or a series of holes. This is possible only because a single-point tool can remove more stock from one side of a hole than the other.

In the illustration, the original drilled hole is out of print tolerance. In other words, the centerline distance between the holes is not correct. By simply dialing in the correct centerline distance the hole can be re-bored to its true position. However, this operation can only be successful if there is sufficient stock remaining in the hole for a 100% cleanup.