Modern Threading Technology

(with acknowledgment to Mike Kanagowski, General Manager, VNE Corp, Wi, a sister company of Vargus who contributed some of the material) ร้อยไหมจมูก

Introduction

If we are honest with ourselves, manufacturing engineers looking for increased productivity, spend a lot of time looking for optimizing tool set ups, choosing correct cutting tool grades for a given workpiece and finding the maximum feed and speed conditions in turning and milling applications. They do not necessarily spend the same of time in optimizing threading operations since there is still an aura of “black box” attitudes concerning this operation.

Threading technology today has advanced in parallel with turning and milling improvements as far as tool grades and coatings, however advance in the design of inserts for threading chip control and the rapid strides in thread milling technology, give the manufacturing engineers a much wider choice for optimizing productivity.

Thread Turning

There are over 40 types of internationally accepted thread standards, some rarely used others much more popular. In addition, many countries have established variations on the international standards for their specific manufacturing requirements.

Primarily the threads are used in four categories:-

Fasting: nuts and bolts

Containing: lids of jars, gas caps, etc

Connecting: fittings and pipe couplings

Actuating: lead screws to transfer power and motion.

The ISO and UN standards are widely used in all industries, the other popular standards have more specific applications: –

BSW Gas and water fittings

NPT- Pipe fittings

BSPT- Gas and water fittings

ACME- Moving parts

Metric buttress- Moving parts in machine tool construction

Trapeze- Moving parts

Round – Tube fittings for food and chemical industries

UNJ & MJ- Aircraft industries

API- Oil industry

A little over half of the thread forms are based on what we will call the 60º Vee geometry and only differ in such factors as the size of the tolerances and root and crest radii.

Threading versus Turning

Threading operations are much more demanding than straight forward turning operations. Cutting forces are in general higher in threading and the cutting nose radius of the insert smaller and thus weaker.

Comparing the feed rate for turning and threading, we see that in threading, the feed rate must correspond exactly to the pitch of the thread. In the case of an 8 TPI thread, the tool must travel at a feed rate of 0.125 inch/revolution. The nose radius of the threading insert is typically 0.015 “. In the case of turning, the normal feed rate is 0.012 inch/ revolution with a standard radius of 0.032 “. From this example we see that threading feed rates are typically 10 times greater than turning. Correspondingly, the cutting forces at the tip of the threading insert can be anywhere from 100 to 1000 times greater than those for straight turning operations. Thus the nose radius of a threading insert plays a vital role in threading and its dimension is strictly limited by the allowable radius at the root of the thread form as defined in the relevant standard. Unlike turning where the material can be sheared, if, in the case of threading, material is “pushed” then thread distortion will be occur.

Further, since the thread is formed by carrying out a number of passes over the length of the thread, the leadscrew of the cross slide is working excessively hard, stopping and starting, moving forwards and backwards and this factor alone results in a limitation in optimization potential.

Partial Profile versus Full Profile Inserts

Partial profile inserts, sometimes referred to as “non topping” inserts cut the thread groove without topping or cresting the thread. These inserts allow production of a wide range of threads, however the nose radius of the insert ( the most vulnerable part of the insert) must be small enough to produce the smallest pitch. The depth of thread is also affected by the small nose radius. For example for a 8 TPI thread, a partial profile insert requires a thread depth of 0.108″ while the same thread with a full profile insert will be no deeper than the specified 0.81″. Thus a stronger thread is produced with a full profile insert and further, up to four less passes in producing the thread.

Multi Tooth Inserts

Multi tooth inserts are designed with a number of teeth so that each one cuts deeper into the thread groove than the previous tooth. Thus the number of passes required to produce a thread can be reduced by up to 80%. The tool life of these inserts is considerably longer than single point inserts since the final tooth is only machining a half or a third of the metal removal of a given thread.

These inserts obviously can give a big push to improve productivity, however, due to the higher cutting forces they are not recommended for thin walled parts as chatter can result. The design of the workpiece should have a sufficient amount of thread relief or run out to allow all the teeth to exit the cut.

Infeed Per Pass

The depth of cut or infeed per pass is critical in threading because each successive pass engages a larger portion of the cutting edge than the preceding pass. If a constant infeed per pass is defined, forces and metal removal rates increase dramatically on each pass.

Producing a 60º thread form using a constant 0.010″ infeed per pass will result in the second pass removing three times the amount of metal as the first pass. For each succeeding pass the amount of metal removed grows exponentially. Thus the pressure on the nose radius increases accordingly. The depth of cut should reduced on each pass in order to achieve more realistic cutting forces.

Infeed Methods

a) Radial – not recommended for general use

Whilst, controversially, this method is probably the most common method of producing threads, it is the least recommended. Since the tool is fed radially (perpendicularly to the workpiece centerline) metal is removed from both sides of the thread flanks, giving a V shaped chip. This form of chip is difficult to break this chip flow can be a problem. Further, since both sides of the insert nose are subject to high heat and pressure, tool life will generally be shorter than other infeed methods.