THREAD MILLING                 

ORBITAL THREAD MILLING

The method of “orbital thread milling” is becoming very popular in Australia nowadays, to cut threads in production very quickly, usually in just one pass in & out. Thread milling is using a solid carbide cutter looking & used like an endmill with fluted serrations and actually milling in a CNC machine centre to machine threaded holes, small as M1.6mm & much larger. For larger threads indexable threading inserts can be used.

Thread milling requires simple programming on a CNC machine centre to correspond with the pitch of the thread. Feeding & ramping in the threadmill cutter downwards the Z axis to one pitch, then ramping out to the centre of hole once depth has been reached, then exit the thread mill cutter vertically up.

To produce internal threads, drill the minor thread diameter to its appropriate size. Then, position the thread mill to the required depth. Next, mill either the 'X' or 'Y' axis to the required thread pitch diameter. With small sizes and with difficult to cut material, it may be necessary to remove the material in several passes. It is always best to "arc-in" and "arc-out" when thread milling. Any "arc-in" and "arc-out" movements must have a corresponding 'Z'-axis motion during the 'X-Y' circular moves. For example, if the "arc-in" is over 90 degrees, the 'Z'-axis departure must be 1/4 of the thread pitch. (90 degrees is 1/4 of a circle). A right-hand thread is produced by orbiting in a counterclockwise direction while bringing the 'Z'-axis up one pitch per 360 degrees.

A left-hand thread is produced by orbiting in a clockwise direction while bringing the 'Z' axis up one pitch per 360 degrees. The entire process can be achieved by interpolating in a downward direction and reversing the orbit direction.

External threads must have the major diameter milled to size before the thread mill is used. Right-hand threads are cut by interpolating up and in a counterclockwise direction. The same threads can be cut by interpolating down and changing the orbit direction.

NPT threads are usually produced while interpolating the tool in a downward direction. Since these tools are crest cutting, it is not absolutely necessary to ream the internal minor diameter or mill the external diameter to size. However, it is highly advisable to do so since the tools will have much less material to remove. If the tool is to be interpolated in an upward direction, spiral interpolation must be used.

The same surface speed can be used for thread mills as for end mills of the same size. The feed rate must be slower, however, since thread milling often involves unfavorable length-to-diameter ratios. Also, keep in mind that the thread mills have more surface area contact than an end mill of equal length. Most CNC mills are programmed in inches per minute which is applied at the centerline of the spindle. In internal applications, the outside diameter of the tool will be traveling faster than the centerline of the tool. The reverse is true for external applications. It is best to start out conservatively with feed rates and the number of passes required and adjust upward per good machining practice.

Understand the Many Advantages of Thread Milling

Thread milling has become an increasingly popular alternative to tapping.  For example, thread milling is a great choice over tapping when:

 Your workpiece is very large and cannot be easily mounted on a lathe
 Your workpiece is non-rotatable or parts are unsymmetrical
 You want to want to complete the drilling and threading operation in one clamping
 You have a power limitation on your milling machining
 You have chip-flow problems
 A high quality surface finish is required on your thread

 You want to reduce your tooling costs.

Thread milling by interpolation

» Thread milling in one pass
» Short machining time
» Thread diameter accuracy adjustment
» Thread milling next to the bottom of blind hole
» Excellent and controlled thread surface finish
» No problem with broken taps
» One tool is suitable for various thread milling profile inserts
» Large variety of tool diameters