Machinery's Handbook, 31st Edition
CNC THREAD CUTTING 1375 Thread Lead = the distance the threading tool will advance along an axis in one revolution of the spindle (4) Spindle speed is always programmed in direct r/min mode (G97) not in the constant surface speed mode G96. Infeed Methods.— The way a threading tool enters into the material can be programmed in several ways, using two infeed methods available. Infeed is the type of motion from one pass to the next. Three basic thread infeed methods are shown in Fig. 29: Feed Rate Lead = TPI Numberof Starts Numberof Starts Pitch # = =
1-2˚
Compound infeed
Radial infeed
Modified compound infeed
Fig. 29. Thread Infeed Methods.
1) Plunge method—also known as the radial infeed 2) Angular method—also known as the compound or flank infeed 3) Modified angular method—also known as the modified compound (flank) infeed Specified infeed is typically selected to achieve the best cutting conditions for the insert edge in a given material. With the exception of some very fine leads and soft material, the majority of threading cuts will benefit from a compound infeed or modified compound infeed (angular method), providing the thread geometry allows this method. For example, a square thread will require a radial infeed, while an Acme thread will benefit from com pound infeed. Four methods can be applied for compound infeed threading: Radial Infeed.— Radial infeed is one of the more common threading methods, if the con ditions are suitable. It is applied to a cutting motion that is perpendicular to the diameter being cut. Each threading pass diameter is specified as the X-axis, while the Z-axis start point remains unchanged. This infeed method is suitable for soft materials, such as brass, certain aluminum grades, etc. In harder materials it could damage the thread integrity and is not recommended. The inevitable outcome of a radial infeed motion is that both insert edges are working simultaneously. Since the insert edges are opposite of each other, the chips are formed at both edges at the same time, causing problems that can be traced to high temperatures, lack of coolant access, and tool wear problems. If the radial infeed results in a poor quality thread, a compound infeed method will usually solve the problem. Compound Infeed.— Compound infeed method—also known as a flank infeed method—works on a different principle. Instead of feeding the threading tool perpen- dicularly towards the part diameter, the position for each pass is shifted by trigonometric calculation to a new Z-position. This approach results in a threading pass where most of the cutting takes place at one edge. As only one insert edge does most of the work, the generated heat can dissipate away from the tool edge while the cutting chips curl away, which extends the tool life. 1) Constant cutting amount 2) Constant cutting depth 3) One edge cutting 4) Both edges cutting
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