Threading Tips – Coarse Pitch Thread Turning

There are a number of applications in thread turning for which the standard tooling available will not allow the forming of a thread due to a number of factors. The main factors are

a) the helix angle is so large that there is insufficient clearance for the threading insert to cut the thread form.

b) internal diameters too small for the standard insert and toolholder to enter.

Some Definitions

In order to understand some of the design criteria for tooling for threading coarse pitches, let us begin with some definitions.

The helix angle is defined by the angle formed between the slope of the thread groove and the line perpendicular to the axis of the thread, see figure 1 below.

The lead of the thread (this is the same as the pitch for a single start thread) is the distance between corresponding points on adjacent thread forms measured parallel to the axis of the thread. This can also be defined as tpi or threads per inch, which is the reciprocal of the pitch. For multi start threads, the lead is equal to the pitch multiplied by the number of starts

The thread drawing below is schematically represented as a spring. Thus we can see that for a given diameter, if we stretch the spring to increase the lead or pitch, then the helix angle of the original spring length “b” is now increased to “a”. Thus it is important in designing the tooling for coarse pitches to consider the effect of the increased helix angle.

The Influence of the Helix Angle in Threading Coarse Pitches

Since, by definition, a coarse pitch thread has a relatively large lead, and therefore large helix angle in relation to the diameter we must ensure that the design of the toolholder and insert allows sufficient clearance to avoid a rubbing or abrasive action on the cutting edge of the insert.

Typically, inserts for external threading applications are designed with a relief angle of 10º and for internal applications, 15º for the smaller inserts and 10º for the larger inserts. The relief angle is defined as the angle between the relief surface of the cutting insert and the workpiece.

These relief, or sometimes called clearance, angles permit the threading of a range of thread standards such a ISO, UN, BSW, ACME, BUTRESS and others. However, due to the varying angles of the thread profile in these standards, the relief angle on the cutting edge can vary from an acceptable 5º in the case of UN and ISO thread forms down to a critical 1º or 2º in the case of ACME or BUTTRESS forms.

Thus, the accurate calculation of the helix angle is required in order to optimize the design for these coarse pitch applications in order to ensure the design of the insert and toolholder give good cutting conditions with the necessary clearance angles.

In many cases a special anvil has to be used in conjunction with the toolholder to satisfy these cutting conditions. However in some cases, the size of the toolholder together with an anvil is too big to enter the internal diameter of the hole to be threaded. In these cases, a specially designed seating is machined directly in the toolholder so that the insert is then mounted at the correct helix angle directly.

In the case of very small diameter holes it may not be possible to enter the toolholder and insert combination even when the toolholder is designed for use even without an anvil. In these cases a solution is found by using the insert in the “U” form rather than the standard style in order to reduce the tool and insert combination in diameter, thus allowing entry into smaller holes.

This “U” form is also used when a large or deep tooth form must be ground on the inserts. The standard style insert is not always suitable for grinding large tooth forms, the “U” style being more flexible in these applications. However, the “U: form cannot reach to a shoulder and the component design must take this into account.

Post time: 02-06-2017