Thread Milling Coarse Pitches

Thread milling, still in some minds a relatively “new black box” technology, continues to develop with further applications being added to an already exciting and productive method of machining.

With the continuous development in CNC machining centers and the resulting expansion in sales of these machine tools, manufacturers now have great opportunities to reduce manufacturing costs. Most machine centers are designed for applying helical interpolation (an essential feature for thread milling) in both clockwise and counterclockwise directions, so that left and right hand threads can be machined with the same cutter.

Development in software aids to programming take the uncertainty out of the decisions to use thread milling as a key component, not only to reduce the direct cost of producing threads but also significant costs in unnecessary handling and set up costs.

Still today, tapping many types of hole is fraught with problems, the biggest of these being the potential risk of scrap or rework should the tapping operation result in breakage of the tap or due to wear, the thread is undersize. The flexibility of the thread milling operation with indexable inserts eliminates these potential problems entirely.

Advantages of Thread Milling

– short cycle times due to rapid feed rates and high cutting speeds

– no thread relief groove needs to be machined, thus saving an unnecessary operation

– for a given set of tooling, there is no limit to the maximum bore diameter that can threaded.

– complete machining can be finished in a loading up thus eliminating costly set up charges

– the same toolholder can be used for both internal and external threads

– the same insert can be used for both left hand and right hand threads.

– inserts are interchangeable, if it is necessary to replace an insert there is no need to reprogram the machine tool.

– in the case of BSW, BSP, BSPT, NPT, NPTF the same insert will produce both internal and external threads.

– since a small range of tooling covers a very wide range of applications, the amount of money tied up in stock can be significantly reduced.

– the quality of the threads have a high surface finish

– the technique is suitable for machining hard materials.

– chips are always short, thus there are no problems in chip control.

– threading of large diameters requires no extra horse power compared to smaller diameters.

– due to very low cutting forces, components with thin walls can be machined without risk of component distortion.

– as wear takes place on the insert, compensation in the program can be carried out without any waste of time.

What is Helical Interpolation?

The pre requisite for generating a thread form with a thread milling indexable inserts is the CNC machine tool function for guiding tool movement along a helical path.

Helical Interpolation creates a tool movement in a helical path from point A to point B on the surface of an imaginary cylinder. This helical path involves the simultaneous movement in axes X , Y, and Z. In thread milling, the circular movement of the X and Y axis creates the thread diameter while the simultaneous linear change in the Z axis creates the pitch of the thread.

Thread Distortion

Thread milling inherently creates a level of thread distortion, however, in all the standard operations, this distortion is designed and confined within the limits of the tolerance of the thread standard.

Why do we get distortion? The thread is formed by applying a tool with a rectilinear profile of its teeth and the resultant profile being curvilinear. In other words, if we want to mill a slot in a piece of metal using a T-slot cutter, the width of the groove is a direct result of the width of the cutter being used.

If, however, the slot to be machined is inclined to the normal axis of machining, then the width of the groove becomes wider than the width of the T-slot cutter itself. Thus the slot is distorted.

With the complex form of thread inserts, this distortion can affect the radii of the thread form, the depth, the flank angles, flank form and width of the thread form.

Threading of coarse pitches creates the potential hazard of distortion and therefore, special designs of tooling must be made in order to compensate for each of these distortion factors.

What Are Coarse Pitch Threads?

Coarse pitch threads are a combination of a small thread diameter and a relatively large pitch. Since the thread milling operation is based on 3-axis simultaneous movement, so the profile shape machined on the workpiece is not an exact copy of the insert profile. In other words the profile is generated as in hobbing operations and copied as is the case with thread turning operations.

What Causes Distortion?

In addition to combination of thread pitch and diameter, two other factors can cause distortion; the diameter of the tool used for generating the profile and also the flank angles of the thread form.

The potential for distortion is usually confined to threading internal holes although thread standards such as ACME and TRAPEZE with flank angles of 29 deg and 30 deg respectively must be checked on a case by case basis.

In general,

a) as the pitch of the thread increases with all other parameters constant, then the level of distortion increases.

b) as the profile angle of the thread decreases, as with ACME, for example, the level of distortion increases

c) as the pitch diameter decreases so the level of distortion increases

d) and finally, as the diameter of the toolholder being used increases, so does the level of distortion. For internal threads, a general rule is that when the ratio of the cutting tool diameter and the minor diameter is 70% or above, the risk of profile distortion is high.

Most manufacturers of thread mill tooling will specify minimum bore diameters for their toolholders and it is important to use these reference table

Mathematical modeling techniques allow simulation of the effects of these parameters in order to provide tooling to solve machining problems which previously could only be machined by turning or tapping.

Whilst a major advantage of the existing tooling is its flexibility to cope with a wide of diameters, in the case of the coarse pitch applications, the toolholder and insert combination is dedicated to a limited number of threading applications and care must be taken to refer to the manufacturers recommendations.

In conclusion, further applications can no doubt be found for this highly cost effective technique. It is always recommended to contact your supplier if you don’t find a solution in their catalog. You never know what tricks they may have up their sleeve to provide you with solutions you thought may not be possible.

Author: Stuart Palmer – Marketing Consultant

VARGUS – At the forefront of the tooling industry for more than 40 years. Vargus is a world-leading manufacturer of high-quality cutting and deburring tools for the metals and plastics industry.

Established in 1960 in Nahariya, Israel, VARGUS is the tooling division of the NEUMO Ehrenberg Group, one of Europe’s largest privately owned manufacturers and distributors of stainless steel products and metal cutting tools.

Since the company’s inception, VARGUS engineers have pioneered breakthrough tooling solutions such as the first triangular laydown threading insert, the first indexable thread milling system, and the original hand-deburring system.

This tradition of innovation continues with industry-leading solutions from the company’s two best known product lines – VARDEX threading solutions and SHAVIV hand-deburring solutions.

Today VARDEX is the undisputed market leading threading program, with tens of thousands of thread turning and thread milling tooling solutions, available from stock or tailor-made special tools. VARGUS also makes the MINIPRO line of miniature tooling for machining small-diameter parts, and a range of cutting tools for boring, groove turning and milling applications. The SHAVIV line of hand-deburring tools rounds out the company’s offerings with professional solutions for finishing metal and plastic components.

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