What Is a Center Drill?
A center drill is a type of cutting tool used in machine tools.
Although center drills are used for positioning before drilling a pre-drilled hole, they are often used for purposes other than positioning due to their versatility.
A locating hole is a hole used to support a workpiece when machining a cylindrical workpiece (shafts) on a lathe or cylindrical grinder. Since the locating hole is the base of the pre-drilling process, it must be highly accurate and free from bends and distortions. Therefore, the center drill is more rigid than an ordinary drill and is less prone to misalignment during drilling.
Uses of Center Drills
Center drills are used to drill a center hole in the end face of a cylindrical workpiece (shaft). In addition to drilling center holes, center drills are used in the following situations:
1. Positioning Hole Drilling
A center drill is used to drill a hole precisely when drilling. When starting drilling, the tip of the drill may be shaky, and in such cases, a center drill can be used to prevent shaking.
2. Hole Chamfering
Center drills are available with 60°, 75°, 90°, and R-shaped edges, which can be used for chamfering.
3. Drilling Holes in Plates
Through-holes can be drilled into thin plates. This eliminates the need for a pre-drilling process and allows chamfering after drilling in one step, thus shortening the work time.
Types of Center Drills
Center drills have a short overall length, a thick shank, and high rigidity. When machining with an ordinary drill, the tip of the blade shakes when the blade bites into the workpiece, affecting the accuracy of the drilling position. Center drills with high rigidity have very little blade shake. Types of center drills are as follows:
1. Type A
The short, small-diameter portion of the cutting edge is followed by a 60° taper portion. The tapered part is also available in 75° and 90° versions.
2. Type B
A short, small-diameter section of the cutting edge is followed by a 60° taper section, which is followed by a large chamfer cutting edge as the third step. This serves to protect the center hole during machining and setup.
3. Type C
The short, small-diameter portion of the cutting edge is followed by a 60° tapered portion, and then a blade is attached to the third step in the form of a counterbored edge. The counterbored portion serves to protect the center hole in the same way as the chamfer of Type B.
4. Type R
The taper portion next to the short small-diameter portion of the cutting edge has an R. Because of the R, the contact with the center is a linear contact. Even when the angle of the taper does not match or the shaft center is off-center, linear contact can be used to support the workpiece. It is often used for precision machining of small workpieces.
How to Use a Center Drill
It is important to pay attention to the depth of the center drill when using a center drill for positioning.
If the center drill depth is too shallow, the accuracy of the pre-drilled hole will be reduced and chamfering will have to be performed again after the pre-drilled hole is drilled, which is not efficient.
On the other hand, if the center drill is too deep, the load on the center drill will increase, and in the worst case, the center drill may break if it is drilled too deep beyond the taper section, so be sure to complete drilling in the middle of the taper section.
In addition, if you calculate the machining depth so that the chamfer portion remains in consideration of the diameter of the pre-drilled hole and then perform positioning drilling, the chamfering work after drilling can be eliminated, allowing for more efficient hole drilling.
Other Information on Center Drills
Center Drill Rotation Speed
The rotation speed of a center drill is generally 1,000 rpm.
For widely used steel materials such as S45C and SS400, a speed of around 1,000 rpm is fine, but when drilling very hard steel materials such as die steel, the rpm should be reduced to around 500 rpm and the feed rate should be slowed down. When the material is hard, a cemented carbide center drill may be used.
The material used for center drills is generally HSS (high-speed steel), but cemented carbide can also be used for machining hard steel materials such as die steel.