Oster Power Threading Handbook
Chaser Operating Theory
Threading of material with chasers is one of the most severe cutting operations in machining. Anyone who has cut threads on materials with a single point tool in a lathe will appreciate what is happening in a threading machine. With bolt or other straight threads, all of the material that is to be removed has been cut away by the time the first full tooth on the chaser has passed over the material. In taper type threads, the entire face of every segment is cutting simultaneously. Just before the die head opens at the completion of a two-inch taper pipe thread, the segments are removing the equivalent of a single cut 12 inches long and .002" deep! Larger machines take even larger cuts. In order to accomplish the operation and generate acceptable threads, all components of the threading system must be controlled. The design of the chasers is one of the important components in this system.
There are four important forces acting on each of the die segments during the threading operation. The direction of the forces are; (a) axially along the material, (b) radially outward from the material, (c) radially inward toward the material, and (d) tangential to the material in the direction of rotation of the material. Each of these forces must be controlled by the geometry of the dies and the design of the die head.
The axial forces acting on the die segments are those which result in the movement of the die head along the material during threading. These forces may be augmented by the use of a lead screw or by pressure exerted on the carriage, but the geometry which controls the forces acting between the die segment and the material must be built into the segments themselves in order to yield good results. Two characteristics of the die-set geometry are important to the control of theses axial forces. They are "move" and "helix angle" (see Definitions).
The move which is built into the die segments allows the cutting segment to "find" the groove which has been started by the previous segment and to take a deeper cut in the same groove. Move is built into the die segments at the factory and will not change during use or in subsequent grinding. This is also true of the helix angle. The helix angle required is based on the pitch and the diameter of the thread. As long as the move of segment is correct, the helix angle may be allowed to vary somewhat without serious results. This is what allows some dies to be used to thread more than one diameter of material. There are limits to the range of diameters that can be threaded by each die set. The range of diameters that can be threaded depends on the size of the material. In general, smaller diameters have a smaller range than larger diameters.
Working against the forces generated by the move and the helix is the force in the opposite direction (toward the beginning of the thread) generated by the throat of the dies. These forces are controlled by the angle of the throat and the relief angle of the throat. Generally, as the throat angle becomes more acute, the forces required to move the die over the material are reduced. The throat relief angle also creates a force which is radial in nature.
The forces acting radially outward on the die segment are created by pressure of the die profile surface on the material being cut. These forces are what keeps the die seated firmly against the scroll of the die head. The surface of the die which rubs against the material is called the "bearing". Dies are manufactured with a built-in bearing. As the die is ground in sharpening, part of the bearing is ground away. If the segments are ground too much, all of the bearing is lost and the dies are not held radially outward against the scroll. This results in the dies plunging into the material. Chattered, torn, or wavy threads are indications of not enough bearing. When a die is used, the profile surface of the die is worn. This increases the bearing. As the bearing increases, it wears into the cutting edge of the die and the dies dull. When the dies are reground and put in service, the bearing begins to wear to a new location on the profile face of the die (see Chaser Grinding).
The forces which tend to pull the dies into the materials are generated by the rake angle cutting the chip away from the material. Generally, the steeper the rake angle the greater the force pulling the die into the material. This force must be balanced by the forces described in the paragraph above or the die will plunge causing broken teeth, wavy or chattered threads or broken dies. Tough materials such as stainless steels and some alloy steels require dies with a sharper rake angle in order to prevent tearing of the threads. Unfortunately the sharper the rake angle, the faster the dies wear at the cutting edge. Therefore a balance must be achieved to insure both quality threads and suitable life of the dies.
The forces exerted tangentially to the material in the direction of rotation of the material tend to bend the die segments and push them away from the material. If any movement of the dies occurs because of theses forces, the geometry of the dies at the cutting edge and the bearing are no longer effective. For this reason the slots in the die head must be strong and of exactly the right size. Occasionally if large die heads are being used to thread small diameters, a die bushing is placed in the die head to help hold the dies in their proper location. If the slots in the die head are worn excessively, the dies do not contact the work piece at the right angle. Bearing is lost and the effective rake angle increases. This often results in chattering and difficulty with maintaining size.