Thread Milling Basics: Understanding Helical Interpolation in a CNC Mill

Thread Milling is a method for producing a thread by a milling operation.

The most common way to produce a thread is still by tapping and turning but today we see more and more milling and this is because CNC milling machines with three simultaneous axes are very popular. These can now be found in every small workshop.

To perform a Thread Milling operation, a helical interpolation movement is required. Helical interpolation is a CNC function producing tool movement along a helical path. This helical motion combines circular movement in one plane (x,y coordinate) with a simultaneous linear motion in a plane perpendicular to the first (z coordinate).

Any three axis mill that is capable of helical interpolation can be used for thread milling. Again, helical interpolation involves three axes moving simultaneously. Two axes, 'X' and 'Y', move in a circular motion while the 'Z' axis moves in a linear motion. For example, the path from point A to point B  on the periphery of the cylinder combines a circular movement in the 'X-Y' plane with linear movement along the 'Z' axis. The 'X' and 'Y' circular motion will determine the diameter of the thread. The 'Z' axis linear motion will cut the pitch (or lead) of the thread.

How to use a Thread Mill

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 feet per minute 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.

Vargus Thread Milling

Vargus thread milling tools are based on indexable multitooth inserts. The cutter rotates around itself at high speed and at the same time moves along the helical path. All the teeth are machined simultaneously so every tooth creates one pitch. At the end of the operation all pitches are combined into one complete thread and that by one pass only. This result is achieved with Vardex high accuracy inserts and use of a CNC milling machine.

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