The method of thread cutting on CNC machining on a CNC lathe is called single-point threading using indexable threaded inserts. Since threading is both cutting and forming, the shape and size of the threaded insert must correspond to the shape and size of the finished thread. By definition, single-point threading is the process of cutting a spiral flute of a specific shape, with each rotation of the spindle advancing at a uniform speed. The uniformity of the thread is controlled by the programmed feed rate in the feed rate per revolution. Threading
The feed rate is always the lead of the thread, not the pitch. For single-head threads, the lead and pitch are the same. Since single-point threading is a multi-pass operation, the CNC machining system provides spindle synchronization for each threading.
Thread Depth
CalculationsRegardless of the threading method used, thread depth is required for various calculations. It can be calculated from these common formulas (TPI is the number of threads per inch): Outer V-Thread (60 degrees in metric or US customary units): Inner V-thread (60 degrees in metric or US customary units) Thread pitch = distance between two corresponding points of adjacent threads. In metric drawings, the pitch is specified as part of the thread designation. Threaded lead = The distance that the threaded tool travels along the shaft in one rotation of the spindle
Feed
MethodThe way threaded cutters enter the material can be programmed in a variety of ways, using the two available feed methods. Feed is the type of motion that is passed from one pass to the next. 1) Cutting method – also known as radial feed 2) Angular method – also known as compound or side feed 3) Corrected angle method – also known as corrective compound (side) feed
A specified feed is usually selected to achieve the optimal cutting conditions for the insert edge in a given material. With the exception of some very fine leads and soft materials, most thread cuts will benefit from a compound feed or an improved compound feed (angle approach), provided that the thread geometry allows for this approach. For example, a square thread will require a radial feed, while an Acme thread will benefit from a compound feed.
There are four methods that can be used for composite feed threads:
1) Constant stock removal 2) Constant depth of cut 3) Single-edge cutting 4) Double-sided cutting
Radial feedIf
The conditions are right, radial feed is one of the more common methods of threading. It is applied to the cutting motion perpendicular to the diameter being cut. The diameter of each threaded hole is specified as the X-axis, while the Z-axis start remains the same. This approach is suitable for soft materials such as brass, certain aluminum grades, etc. In harder materials, it may compromise thread integrity and is not recommended.
The inevitable consequence of the radial feed movement is that the two insert edges work at the same time. As the insert edges are opposite each other, chips form at both edges at the same time, leading to problems that can be traced back to high temperatures, lack of coolant access and tool wear issues. If the radial feed results in poor thread quality, the compound feed method can often solve the problem.
Compound feeding
The compound feeding method – also known as the flanking feeding method – works differently. Instead of feeding the thread tool perpendicular to the diameter of the part, triangulation moves the position of each pass to the new Z position. This method results in threading, where most of the cutting takes place at one edge. Since only one insert edge does most of the work, the heat generated can be dissipated from the tool edge while the chips are curled, extending tool life.
With the composite threading method, it is possible to use a deeper thread depth and fewer threads for most threads. The composite 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 held by the angle of the thread insert.