The method of thread cutting on CNC lathe is called single point thread machining with indexable thread inserts. As thread processing is both cutting and shaping, the shape and size of the thread insert must be consistent with the shape and size of the finished thread
The dimensions correspond. According to the definition, single point thread machining is the process of cutting spiral grooves of a specific shape. Every time the spindle rotates for a circle, the forward speed is uniform. The thread uniformity is controlled by the programmed feed rate in the feed rate per revolution.
Threading The feed rate of is always the lead of the thread, not the pitch. For single head threads, the lead and pitch are the same. Since single point thread machining is a multi process, the CNC system provides spindle synchronization for each thread machining.
CNC lathe
Thread depth calculation
No matter what thread processing method is used, thread depth is required for various calculations. It can be calculated from these common formulas (TPI is the number of threads per inch):
External V-thread (metric or American customary unit is 60 degrees):
Internal V-thread (metric or American customary unit is 60 degrees)
Thread pitch=distance between two corresponding points of adjacent threads.
In metric drawings, the pitch is specified as part of the thread designation.
Thread lead=the distance that the thread tool advances along the axis when the spindle rotates for one revolution
The spindle speed is always programmed in direct r/min mode (G97), not in constant surface speed mode G96.
Feeding mode
The way the thread cutter enters the material can be programmed in a variety of ways, using two available feed methods. Feed is a type of motion transferred from one time to the next. Three basic thread feeding methods are shown in Figure 29:
1) Cut in method - also known as radial feed
2) Angular method - also known as compound or side feed
3) Modified angle method - also known as modified compound (side) feed
Usually, the specified feed rate is selected to achieve the best cutting conditions of the blade edge in a given material. With the exception of some very fine leads and soft materials, most thread cutting will benefit from a compound feed or an improved compound feed (angle method), provided that the thread geometry allows this method. For example, square threads will require radial feed, while Acme threads will benefit from compound feed.
Four methods can be used for compound feed thread:
1) Constant cutting amount
2) Constant cutting depth
3) Single edge cutting
4) Double-sided cutting
CNC lathe processing parts
Radial feed
If the conditions are suitable, radial feed is one of the more common thread processing methods. It applies to the cutting motion perpendicular to the diameter being cut. The diameter of each threaded hole is specified as the X axis, while the starting point of the Z axis remains unchanged. This feed method is applicable to
Soft materials, such as brass, some aluminum grades, etc. In harder materials, it may damage thread integrity and is not recommended.
The inevitable result of the radial feed motion is that two blade edges work at the same time. Since the blade edges are opposite to each other, chips are formed at both edges at the same time, which leads to problems that can be traced back to high temperature, lack of coolant path and tool wear. If the radial feed causes poor thread quality, the compound feed method can usually solve the problem.
Compound feed
The compound feed method - also known as the flank feed method - works differently. Instead of feeding the thread tool perpendicular to the part diameter, the position passed each time is moved to the new Z position by triangulation. This method results in thread machining, where most of the cutting occurs at one edge. Since only one blade edge completes most of the work, the heat generated can be dissipated from the tool edge, and the cutting chips curl, thus extending the tool life.
Using the compound thread processing method, you can use a deeper thread depth and fewer threads for most threads. The compound feed can be modified by providing a gap of 1 to 2 degrees on one edge to prevent friction. The angle of the thread will be maintained by the angle of the thread insert.
Thread operation
Many thread processing operations can be programmed for typical NC lathe machining. Some operations require special types of thread inserts and some operations can only be programmed if the control system is equipped with special (optional) functions:
Constant lead single head thread (usually G32 or G76)
Variable lead threads - increase or decrease (special option) (G34 and G35)
The G32 command is sometimes referred to as "long hand threading" because each tool movement is programmed as a block. Programs using G32 can be long and almost impossible to edit without major reprogramming. On the other hand, the G32 method provides great flexibility and is usually the only method available, especially for special threads. The programming format of G32 requires at least four input program segments to start a single thread machining from the starting position:
Threading cycle (G76)
G76 is a repeated cycle of thread processing, and is the most commonly used method to generate most thread shapes. Similar to the roughing cycle, there are two versions of G76 depending on the control system used. For older controls, use the single block format, and for newer controls, use the two block format. The two block format provides additional settings that are not available in the one block method.
Multithreading
Multi head threads can be programmed using G32 or G76 thread machining instructions. The lead (and feed rate) of a multiple thread is always the number of starts multiplied by the pitch. For example, a three head thread with a pitch of 0.0625 (16 TPI) would be 0.1875 (F0.1875). In order to achieve the correct distribution of each starting point around the cylinder, each thread must start at an equal angle,
