Allied drills are one of the machine tools used for high tech gear grinding and machining to achieve a unique geometry. The machining process involves gear grinding, hobbing, and drilling but it is different from other machining operations because it a combination of cutting and metal extrusion at the chisel end found at the center of the drill. There is a high thrust force on the allied drill that causes a feeding motion to extrude metal under the chisel edge. Afterwards, there is the tendency to shear due to the action of negative rake angle tool.
Important features of the allied drill
Machine settings that are used in the gear grinding and drilling operation reveal certain features that have to be carefully considered. First of all, depth of cut is a fundamental dimension in other cutting processes but it corresponds more closely to a drill radius. The width of a chip that has not been deformed corresponds to the length of the drill lip which is dependent on point angle as well as drill size. In any given setup, the width of the chip has that has undergone deformation is a constant in the gear grinding and drilling operation. Another important feature of the Allied drill is feed dimension that is specified for drilling. Feed dimension means the feed per revolution of the spindle. For the more common two-flute drill, it is half a feed per revolution. The thickness of the un-deformed chip differs from the feed per lip depending upon the point angle.
Various Features of Allied Drill that make it different from other Cutting Tools
The spindle is another constant in the drilling operation. Cutting speed is normally computed from the outside of the diameter but it varies along the gear grinding edge. At the center of the chisel edge, cutting speed can be zero but at any point on the lip, it can be proportional to the radius of that point. An important characteristic of drilling is the variation of cutting speed along the cutting edges. Once cutting speed has been selected for a particular workpiece, material and condition, an appropriate feedrate must be established to maximize productivity and at the same time maintain chip control. In the gear grinding operation, feed is expressed in inches per revolution or the distance the drill moves in inches for each revolution of the drill. Manufacturers often recommend the appropriate gear grinding speed and feed for various materials that need machining.
If you will take a closer look at the drilling operation, you will notice that once the allied drill engages with the gear grinding workpiece, the contact is continuous until the allied drill breaks through the bottom of the workpiece or until it is withdrawn from the hole that was made. In this situation, drilling closely resembles turning and sometimes milling. Shortly after the contact between the drill and the workpiece, you can expect continuous cutting; however, drill wear also begins. Sometimes, instead of progressing at a constant rate, wear accelerates continuously and as it progresses to cause more heat that makes wear faster. Operators often restore the drill to a new condition by removing the worn out area so that the drill can continue on working.