Thanks to our friends OSG, we will be using their basic information throughout this chapter on basic drilling technology.
The first illustration below shows the different parts of the drill. It is important that you understand the terminology used when discussing drills. Throughout this chapter, we will be using this terminology often to describe basic theories of design and implementation of the drilling process.
Our discussion will be based on the new geometry of high-performance drills that most manufacturers currently use. Using standard twist drills on CNC machines is a thing of the past. New geometries, like the ones illustrated above and manufactured by OSG, yield substantial increases in productivity.
Some of the special features on the OSG high-performance drills are listed below:
1. There is no chisel in the center part of the drill. This provides a better biting action and reduces thrust requirements. This reflects principles of the radius point thinning Hosoi theory.
2. Reduced production time is achieved through higher feeds and speeds, unlike conventional drills where chips are broken into small pieces, and there is no need for step drilling.
3. The unique tool point design also eliminates center drilling.
Basic Drilling Technology – Unique point design
As you can see in Illustration 2 on the right, the chisel edge in the center section of the conventional drill is eliminated.
The OSG-designed drills have a chip pocket and smooth chip action along the radius cutting edge. Instead there is a radius point thinning. This provides better biting action than a conventional drill, where the chisel edge is easily crushed against the harder-to-drill material.
By increasing the point angle to 130°, instead of the conventional 118°, the drill creates small broken chips rather than the long, stringy chips created by conventional point drills.
In tests run by OSG and shown on the chart below, when drilling 1045 steel, substantial decreases in thrusts are used.
You can see in Illustration 5 that, by increasing the core diameter of the drill or cross-section, the rigidity of the drill is increased. This rigidity increases the accuracy of the holes drilled. In order to increase the rigidity of the drill, the manufacturer must be able to control the chips so they don’t clog up in the flutes.
Another key factor to evacuating chips from the drills is pre-and post finishing of the flute area.
Basic Drilling Technology – Chip Ejection
A conventional drill produces long curly chips, which can get tangled up in the spindle or work piece itself. With the unique OSG design, the chips are broken up if the feed is increased. On the other hand, conventional drilling feeds have to be increased so much to break up the chips that a substantial decrease in tool life usually results.
Illustration 8 shows the relationship that cutting speed and cutting fluids have on the life of the drill.
As you can see, when the cutting speed of the drill is increased, the number of holes drilled is reduced. In some cases, you may want to run a drill at a higher speed to reduce cycle time.
Illustration 9 shows how the life of the tool is greatly affected by the feed rate. A slower feed normally gives drills a longer life; as the feed is increased, the tool life will reduce. After reaching a certain point, tool life decrease will stop because, by increasing the feed, the number of revolutions needed to drill a hole is reduced. Underfeeding a drill will cause fast wear on the tool, while overfeeding of the tool will cause chipping.
Illustration 11 shows an accurate comparison of OSG high-performance drills to conventional drills. The accuracy of holes drilled with high-performance drills is increased due to the rigidity of the tools. Note that machine accuracy, tool-holding, and fixture conditions also affect the accuracy of holes.
Hole location can be affected by the flute length of the drill. The chart below shows the accuracy in positioning using a stub length drill and a Jobber length drill. As you can see, the longer length of the Jobber length drill has a definite effect on the accuracy in positioning.← Return to GTU main page