Accuracy of Drilled Holes—quoted from Machinery's Handbook 27th Edition
Normally the diameter of drilled holes is not given a tolerance;
the size of the hole is expected to be as close to the drill size as can be obtained.
The accuracy of holes drilled with a two-fluted twist drill is influenced by many factors,
which include: the accuracy of the drill point; the size of the drill; length and shape of the
chisel edge; whether or not a bushing is used to guide the drill; the work material; length
of the drill; runout of the spindle and the chuck; rigidity of the machine tool, workpiece,
and the setup; and also the cutting fluid used, if any.
The diameter of the drilled holes will be oversize in most materials. The table Oversize
Diameters in Drilling on page 885 provides the results of tests reported by The United
States Cutting Tool Institute in which the diameters of over 2800 holes drilled in steel and
cast iron were measured. The values in this table indicate what might be expected under
average shop conditions; however, when the drill point is accurately ground and the other
machining conditions are correct, the resulting hole size is more likely to be between the
mean and average minimum values given in this table. If the drill is ground and used incorrectly,
holes that are even larger than the average maximum values can result.
Some conditions will cause the drilled hole to be undersize. For example, holes drilled in
light metals and in other materials having a high coefficient of thermal expansion such as
plastics, may contract to a size that is smaller than the diameter of the drill as the material
surrounding the hole is cooled after having been heated by the drilling. The elastic action
of the material surrounding the hole may also cause the drilled hole to be undersize when
drilling high strength materials with a drill that is dull at its outer corner.
The accuracy of the drill point has a great effect on the accuracy of the drilled hole. An
inaccurately ground twist drill will produce holes that are excessively over-size. The drill
point must be symmetrical; i.e., the point angles must be equal, as well as the lip lengths
and the axial height of the lips. Any alterations to the lips or to the chisel edge, such as thinning
the web, must be done carefully to preserve the symmetry of the drill point. Adequate
relief should be provided behind the chisel edge to prevent heel drag. On conventionally
ground drill points this relief can be estimated by the chisel edge angle.
When drilling a hole, as the drill point starts to enter the workpiece, the drill will be unstable
and will tend to wander. Then as the body of the drill enters the hole the drill will tend
to stabilize. The result of this action is a tendency to drill a bellmouth shape in the hole at
the entrance and perhaps beyond. Factors contributing to bellmouthing are: an unsymmetrically
ground drill point; a large chisel edge length; inadequate relief behind the chisel
edge; runout of the spindle and the chuck; using a slender drill that will bend easily; and
lack of rigidity of the machine tool, workpiece, or the setup. Correcting these conditions as
required will reduce the tendency for bellmouthing to occur and improve the accuracy of
the hole diameter and its straightness. Starting the hole with a short stiff drill, such as a center
drill, will quickly stabilize the drill that follows and reduce or eliminate bellmouthing;
this procedure should always be used when drilling in a lathe, where the work is rotating.
Bellmouthing can also be eliminated almost entirely and the accuracy of the hole improved
by using a close fitting drill jig bushing placed close to the workpiece. Although specific
recommendations cannot be made, many cutting fluids will help to increase the accuracy
of the diameters of drilled holes. Double margin twist drills, available in the smaller sizes,
will drill a more accurate hole than conventional twist drills having only a single margin at
the leading edge of the land. The second land, located on the trailing edge of each land, provides
greater stability in the drill bushing and in the hole. These drills are especially useful
in drilling intersecting off-center holes. Single and double margin step drills, also available
in the smaller sizes, will produce very accurate drilled holes, which are usually less than
0.002 inch larger than the drill size.
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