Tools used in drilling operations represent nearly 25% of all the tools being used in the world. There are those operations that are strictly drilling operations, but we also use drilling machines to perform other operations such as reaming, tapping, countersinking and counterboring. The rules and principles of cutting speeds and RPM calculations apply to all of the operations being performed on drilling machines. An example of this would be reaming. Reaming is done at half the speed and twice the feed as drilling. This rule still applies on the drill press as it does on the milling machine or the lathe. Pay very close attention to the information introduced in this unit and other units dealing with cutting speeds because cutting speeds have the greatest impact on tool life.
Cutting speed is the speed at the outside edge of the tool as it is cutting. This is also known as surface speed. Surface speed, surface footage, and surface area are all directly related. If two tools of different sizes are turning at the same revolutions per minute (RPM), the larger tool has a greater surface speed. Surface speed is measured in surface feet per minute (SFPM). All cutting tools work on the surface footage principle. Cutting speeds depend primarily on the kind of material you are cutting and the kind of cutting tool you are using. The hardness of the work material has a great deal to do with the recommended cutting speed. The harder the work material, the slower the cutting speed. The softer the work material, the faster the recommended cutting speed (Figure 1).The hardness of the cutting tool material will also have a great deal to do with the recommended cutting speed. The harder the drill, the faster the cutting speed (Figure 2). The softer the drill, the slower the recommended cutting speed.The three factors, cutting speed, feedrate and depth of cut, are known as cutting conditions. Cutting conditions are determined by the machinability rating of the material. Machinability is the comparing of materials on their ability to be machined. From machinability ratings we can derive recommended cutting speeds. Recommended cutting speeds are given in charts. These charts can be found in the Machinery’s Handbook, textbook, or a chart given to you by your tool salesperson. In Table 3 you will find a typical recommended cutting speed chart for drilling
