The method of thread cutting on CNC lathes is called single-point threading using indexable thread inserts. Since threading is both cutting and forming, the shape and size of the thread insert must match the shape and size of the finished thread.
Dimensions correspond. By definition, single-point threading is a process of cutting spiral grooves of a specific shape with a uniform forward speed for each revolution of the spindle. The uniformity of the thread is controlled by the programmed feed rate in Feed rate per revolution. Thread processing
The feed rate is always the lead of the thread, not the pitch. For single-start threads, lead and pitch are the same. Since single-point thread processing is a multi-pass process, the CNC machining system provides spindle synchronization for each thread processing.
CNC lathe processing
Thread depth calculation
Regardless of the threading method used, thread depth is required for various calculations. It can be calculated from these common formulas (TPI is threads per inch):
External V-thread (60 degrees in metric or US customary units):
Internal V-thread (60 degrees in metric or US customary units)
Thread pitch = the distance between two corresponding points of adjacent threads.
In metric drawings, the thread pitch is specified as part of the thread designation.
Thread lead = the distance the threading tool advances along the axis during one revolution of the spindle
The spindle speed is always programmed in direct r/min mode (G97), not in constant surface speed mode G96.
Feeding method
The way the threading tool enters the material can be programmed in a variety of ways, using two available feed methods. A feed is a type of motion from one pass to the next. The three basic thread feeding methods are shown in Figure 29:
1) Plunge method - also called radial feed
2) Angular approach - also known as composite or side feed
3) Modified angle method - also known as modified compound (side) feed
A specified feed rate is usually chosen to achieve optimal cutting conditions for the insert edge in a given material. With the exception of some very fine leads and soft materials, most thread cutting will benefit from compound feed or modified compound feed (angle method), provided the thread geometry allows this method. For example, square threads will require radial feeds, while Acme threads will benefit from compound feeds.
There are four methods available for composite feed threads:
1) Constant cutting amount
2) Constant cutting depth
3) Single edge cutting
4) Double-sided cutting
CNC lathe processing parts
Radial feed
If conditions are right, radial feed is one of the more common methods of threading. It applies cutting motion perpendicular to the diameter being cut. Each threaded hole diameter is specified for the X-axis, while the Z-axis starting point remains unchanged. This feeding method is suitable for
Soft materials such as brass, certain aluminum grades, etc. In harder materials it may damage thread integrity and is not recommended.
A corollary of radial feed motion is that both blade edges work simultaneously. Because the insert edges face each other, chips form at both edges simultaneously, causing problems that can be traced to high temperatures, lack of coolant passages, and tool wear issues. If radial feeds result in poor thread quality, a compound feed approach can often solve the problem.
Compound feed
The compound feed method - also known as the flank feed method - works differently. Instead of feeding the threading tool perpendicular to the part diameter, trigonometric calculations move the position of each pass to a new Z position. This method results in threading where most of the cutting occurs on one edge. Since only one insert edge does most of the work, the heat generated can be dissipated away from the tool edge while the chips curl, extending tool life.
With the composite threading method, you can use deeper thread depth and fewer threads for most threads. Compound feeds can be modified by providing 1 to 2 degrees of clearance on one edge to prevent rubbing. The angle of the thread will be maintained by the angle of the thread insert.
Thread operation
Many threading operations can be programmed for typical CNC lathe machining. Some operations require special types of threading inserts, and some operations can only be programmed if the control system is equipped with special (optional) features:
Constant lead single start thread (usually G32 or G76 used)
Variable lead thread - increase or decrease (special option) (G34 and G35)
The G32 instruction is sometimes called "long hand threading" because each tool movement is programmed as a block. Programs using the G32 can be long and nearly impossible to edit without significant reprogramming. The G32 method, on the other hand, offers great flexibility and is often the only method that can be used, especially for specialty threads. The programming format of G32 requires at least four input blocks to start a single thread machining from the starting position:
Thread machining cycle (G76)
G76 is a multi-repetition cycle of threading and is the most common method for generating most thread shapes. Similar to the roughing cycle, the G76 is available in two versions 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 not available in the one-block method.
Multithreading
Multi-start threads can be programmed using G32 or G76 threading instructions. The lead (and feed rate) of a multi-start thread is always the number of starts multiplied by the pitch. For example, a tri-start 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,