xiehe model

The method of thread cutting on CNC lathe machining is called single-point threading using indexable thread inserts. Since threading is both cutting and forming, the shape and size of the threading insert must correspond to that of the finished thread. By definition, single-point threading is the machining process of cutting a helical groove of a specific shape, with a uniform advance rate per revolution of the spindle. The uniformity of the thread is controlled by the programmed feed rate in Feed per revolution.

The feedrate for threading is always the lead of the thread, not the pitch. For single-start threads, lead and pitch are the same. Since single-point threading is a multi-step process, the CNC system provides spindle synchronization for each threading.

Thread Depth Calculation

Regardless of the threading method used, various calculations require thread depth. 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 American customary units)

Thread pitch = the distance between two corresponding points of adjacent threads.

On metric drawings, the pitch is specified as part of the thread designator.

Thread lead = the distance that the thread tool advances along the axis when the spindle rotates one revolution

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 the two available feed methods. Feed is the type of motion from one pass to the next. Three basic thread feed methods are shown in Figure 29:

1) Plunge method - also known as radial feed

2) Angular approach - also known as compound or side-feed

3) Corrected angle method - also known as modified composite (side) feed

The specified feed rate is usually chosen to achieve the best 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 infeed or modified compound infeed (angle method), provided the thread geometry allows for this approach. For example, square threads will require radial infeeds, while Acme threads will benefit from compound infeeds.

There are four methods available for composite feed threads:

1) Constant cutting amount

2) Constant depth of cut

3) Single edge cutting

4) Double-sided cutting

Radial feed

When conditions are right, radial infeed is one of the more common threading methods. It applies to cutting motions perpendicular to the diameter being cut. Each threaded hole diameter is specified for the X-axis, while the Z-axis origin remains constant. This method of feed is suitable for soft materials such as brass, certain aluminum grades, etc. In harder materials it may damage thread integrity and is not recommended.

The inevitable result of the radial feed movement is that the two blade edges work simultaneously. As the insert edges face each other, chips form at both edges simultaneously, causing problems that can be traced back to high temperatures, lack of coolant access and tool wear issues. If radial infeeds are causing poor thread quality, compound infeed methods can often solve the problem.

Composite 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 chips curl up, extending tool life.

With compound threading methods, deeper thread depths and fewer threads can be used for most threads. Compound feeds can be modified by providing a 1 to 2 degree gap on one edge to prevent chafing. The angle of the thread will be maintained by the angle of the threaded insert.

Thread operation

Many threading operations can be programmed for a typical. Some operations require special types of threading inserts, and some operations are only available if the control system is equipped.

Special (optional) functions can only be programmed:

Constant lead single-start thread (usually use G32 or G76)

Variable lead thread - increasing or decreasing (special option) (G34 and G35)

The G32 command is sometimes called "long-handed threading" because each tool movement is programmed as a block. Programs using the G32 can be long and nearly impossible to edit without major reprogramming. The G32 method, on the other hand, offers great flexibility and is often the only method that can be used, especially for special threads. The programming format of G32 requires at least four input blocks to start a single threading from the starting position:

Thread processing cycle (G76)

G76 is a multiple repeat cycle of threading and is the most common method used to generate most thread shapes. Similar to the roughing cycle, 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 approach.

Multithreading

Multi-start thread can be programmed by G32 or G76 thread processing command. The lead (and feedrate) of a multi-start thread is always the number of starts times the pitch. For example, a three-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 start around the cylinder, each thread must start at an equal angle.