Threading thoughts

According to Dormer Tools (0870 850 4466), probably the most commonly accepted method of producing a thread is by cutting the workpiece material with a thread cutting tap or die. But one alternative method that offers significant benefits over traditional methods, although not new, remains a relatively unexploited resource, says the company – that of thread forming. Threading with fluteless taps – also known as cold forming or roll taping – involves the creation of a thread by forming the material, rather than cutting it, as with other taps. Image: Thread forming has significant benefits over tradition tapping, says Dormer Tools A fluteless tap is essentially a cutting tap without the flutes. Various designs of fluteless taps exist, but all follow the same basic concept – a thread is ground onto a blank with a specific geometry that introduces forming lobes at the same time or, conversely, the cylindrical thread is relieved at various positions around the circumference to leave the forming lobes. These lobes, spaced periodically around the tap, carry out the forming process, creating the thread as the tool is advanced into the hole. One of the key differences between this process and thread cutting is that it does not produce chips. Consequently, there is no need to periodically back out the tap to clear away swarf, which, in a cutting tap, can jam and break the tap, resulting in costly machine downtime. Tests have shown the process of thread forming increases not only the yield strength of the thread, but also its surface finish, hardness and wear resistance. As a result, higher cutting speeds are recommended for forming taps than for the corresponding cutting taps in the same material. As with other tooling applications, coatings play a major part in the development and increased productivity of fluteless tap applications. Titanium nitride (TiN) is the default choice in carbon steels up to high alloy steels, and is recommended for stainless steel. Chromium nitride (CrN) is an excellent coating for soft steel materials, and is also a good alternative in aluminium and titanium and nickel alloys, providing a low tendency for material welding. A key element recommended for successful fluteless tapping is coolant that has an extreme pressure additive – Dormer Supercut, for example. This is because any breakdown of the lubricant during tapping will allow 'metal to metal' contact, with possible breakage and/or subsequent reduction in performance. ENERGY CONSIDERATIONS The benefits of thread milling using carbide insert tooling are underlined by Vargus Tooling (01952 583 222), although not necessarily the strengths traditionally associated with the process. With machine builders constantly reducing the amount of power required for machining – in line with the global desire to reduce carbon emissions – there is a strong argument that users should also investigate how alternative methods of machining can add to the savings. Most production engineers will be aware of the cost-saving benefits of creating screw threads by thread milling rather than by the traditional route of drilling and tapping – in terms of reduced overall cycle times and minimised tool costs, but Vargus Tooling UK's general manager, Andy Smith, points out that thread milling can also make a big difference to machine tool energy consumption levels and, therefore, users' electricity bills. Image: Thread milling is energy efficient, highlights Vargus "Inherent in the design and application of these tools is the small contact area between tool and workpiece, compared to alternative methods, which can be beneficial not only in terms of thread quality, but also in reduced levels of power requirements," he says. "As a result, the overall 'energy cost' savings with thread milling can be up to 30%, compared to drilling and tapping. "When you combine that with the time (and inherent energy) savings of reduced overall machining times, plus the lower tooling costs, the bottom line benefits can be substantial," he offers. WIZARD IDEA FOR THREAD MILLING Not all machining centres/milling machines incorporate a helical milling sub-routine, so Vargus developed its easy-to-use, Wizard-based software package Vargus TM Gen that automatically generates programs, often in less than 60 seconds. After specifying an internal or external and left- or right-hand thread, and selecting thread standard, the operator defines the pitch (mm or TPI), then thread nominal diameter, length and workpiece material. Once the method of machining (climb or conventional) is chosen, the software presents the appropriate range of tools and inserts available. A tooling combination is then selected and a review allows the operator to alter/optimise carbide grades, speeds and feeds, as well as cycle times, by choosing alternative tooling, and speeds and feeds. The result is a computer-generated CNC program. Importantly, says Vargus, its inserts are interchangeable, so, if it is necessary to replace an insert, there is no need to re-program the machine. Turning to a somewhat more esoteric process for, in particular, threaded shafts, thread whirling, this technique is being sought as a solution to the production of threads on tough materials. For example, medical implant components, such as corrosion- and wear-resistant titanium, medical grade stainless steels and special alloys, can present considerable challenges. Tooling giant Sandvik Coromant (www.sandvik.coromant.com) says its carbide insert CoroMill 325 range can provide an answer. Image: Thread whirling is a solution to the production of threads on tough materials The company says thread whirling is increasingly preferred to conventional single-point threading techniques, thanks to its speed – the smooth, tangential cutting action minimises cutting forces to give high metal removal rates. As an example, one surgical component manufacturer was struggling with short tool life when producing 66 mm long threads on cortical bone screws made from wrought titanium alloy on its sliding-head machine. The problem was being compounded, due to logistics problems in ordering and regrinding the thread whirling inserts. However, by switching to a 12 mm diameter CoroMill 325 with six HA, GC1105 grade inserts, tests showed a staggering 279% tool life increase – without any changes to cutting data. Furthermore, the previously required grinding/polishing operation after threading was eliminated, due to the high quality finished surface produced. SINGLE-POINT DEVELOPMENT Finally, turning to single-point threading and Seco Tools (01789 764341) has come up with an innovation for one particular task. The company's single-pass pull threading process can, it says, take as much as 40 seconds off the typical one-minute cycle time required for putting the internal thread on an oil pipe coupling, and it also improves chip control. Coupling threading has traditionally involved multiple-pass push-threading with right-hand tooling systems. Generating internal threads in this way, however, results in long cycle times, largely due to inefficient handling of chips. The Seco-developed process applies a left-hand tooling system, using multi-tooth, chaser-style threading inserts that are pulled through to direct the cutting forces into the beds of the turning centres used to machine the couplings. These inserts are extremely reliable and wear resistant, enabling higher cutting speeds to be employed. But, to take best advantage of the single-pass threading process newer, rigid machines equipped with powerful (22 kW/38 kW) spindle motors and well-made, strong chuck jaws are recommended. Poor performing or even retrofitted outdated machines are unlikely to deliver a reliable or repeatable process. Seco also recommends that machines have at least 70 bar high pressure coolant capability to break down large, stringy chips effectively. Box tem 1 Thread-forming benefits Advantages compared with cutting taps, according to Dormer Tools, include: [] Cold forming is faster than ordinary thread cutting; [] Longer tool life; [] One type of tool can be used in a range of different materials, and for both through and blind holes; [] Cold forming taps have a stable design, which gives lower risk of breakage; [] Threads to the correct tolerance are guaranteed; [] No chips; [] Stronger thread (higher stripping strength), compared to thread obtained by cutting (up to 100% more); and [] Lower surface roughness than by cutting. Box item 2 Thread milling is energy-efficient The small contact area in thread milling delivers a number of benefits, says Vargus Tooling: [] Faster cycle times (on average at least 10% faster, dependant on diameter and material) reduce overall machining times (and, therefore, further reduce power demands); [] Less strain and demand on the machine spindle - increasing spindle life and reducing the need for replacement (reducing the energy required in the manufacture of such parts); [] No stopping and starting of the spindle (further power savings); [] No 'pecking' on high yield materials (again, no stopping and starting of the spindle). Other benefits against drilling/tapping on machining centres are: [] Shorter machining cycles due to the combination of operations – for instance, the same tool can often be used for generating internal and external, and right- and left-hand threads; or for threading and chamfering; [] Reduced cycle and set-up times via single set-up working (reduced load/unload times, too), following finish milling operations, for example; [] Improved surface finish; and [] Improved swarf control via the generation of short/small chips. Box item 3 Faster, better thread forming Walter Tool's (01527 839450) new Protodyn HSC solid carbide thread former enables high quality threads to be produced at speeds of up to twice as fast as traditional HSS tools, and with extended tool life of more than three times conventional counterparts. For one formed component, the number of threads produced using a Protodyn HSC thread former was 90,000, compared to 20,000 - a 350% increase in tool life, while a time saving of two hours was seen. For a connecting rod, tool life was extended by 300%, compared to the previously used thread former; and the number of threads produced on a crankshaft increased by almost four times per tool, while tool life was increased by 280% and machining time reduced by 25%. Box item 4 Seco threadmilling tools and software Seco Tools (UK) has introduced a new range of reliable and flexible solid thread milling cutters called Threadmaster (TM), together with its Threading Wizard software. Image: Seco Tools has introduced tools and software The range of cutters is designed to produce high precision threads on what are often high cost components, and helps manufacturers reduce tool inventory costs (one tool can be used to mill several thread sizes), and benefit from a stable and reliable machining process. The TM thread milling range makes threads within narrow tolerances and without burrs. Compared to threading with a tap, milling threads with a TM is more reliable, because the risk of broken taps is avoided. The TM range of solid thread milling cutters is suitable for a number of applications and materials, including hardened materials, cast iron, steels and aluminium alloys. It covers dimensions from M1 by 0.25 up to M20 by 2.5 and has cutters for UN/C, W and NPT/F threads. The software helps the correct identification and selection of the most appropriate holder and insert, identifies the best operating parameters and then downloads this information to the CNC machine. It is available free-of-charge and can be downloaded from www.secotools.com/customerzone. Extended article from here Thread milling at smaller diameters and more While threadmilling can be used to produce external and internal thread forms, the majority of applications are to replace the use of solid taps and, while producing threaded holes, particularly at smaller diameters, the use of solid taps remains a popular choice, the versatility of threadmilling is attracting growing number of followers, especially where the production mix involves numerous thread sizes in a choice of materials, says WNT. Image: Thread milling offers benefits when a component has multiple thread sizes Threadmilling takes advantage of the control available on modern CNC machine tools to move a cutter, accurately, in multiple axes at the same time. To create a thread using a milling technique, you need simultaneous movement in three axes (X, Y and Z) and a cutter that is ground or formed to the flank angle of the thread to be cut, with the necessary clearances to overcome the pitch of the thread being machined. This also allows the same cutter to be used to produce left or right-handed threads, reducing the tooling inventory and operating costs for manufacturing businesses. The basic process for threadmilling using a cutter with multiple cutting edges is to position the cutter in the hole, along the machine's spindle axis at a depth equal to the length of thread required. The machine control then begins to feed the cutter out to the nominal diameter of the thread, while making a full 360 degree rotation of the bore and at the same time moving one thread pitch in distance along the z-axis (machine spindle axis). For deeper threads, the cutter can be repositioned and the process repeated, or a cutter with a single cutting edge can be employed. The combination of movement is described as helical interpolation, and without that capability a machine cannot be used for thread milling. Threadmilling cutters are available in many formats, including solid carbide and indexable insert varieties, with single-point or multi-point cutting edges, and provide a viable alternative to tapping. This is particularly true in the case of large diameter bores that require threading, where the purchase of a specific tap to complete the job is both costly and restrictive, as the tap can only be used for that one size of thread, whereas a thread milling cutter can be used on any thread diameter, as long as the pitch (in the case of multi tooth cutters) and form are the same. Another big advantage when threadmilling larger bores - say, above 25 mm - is the fact that the horsepower requirement is greatly reduced, due to the fact that there is less cutter in contact with the material. This allows smaller, lower powered, machines to achieve threads that would be unachievable using taps. Smaller diameters can also benefit from threadmilling, and WNT has recognised this with the introduction of cutters of producing threads at M1.6 in size as part of its Micromill range. A further advantage to threadmilling smaller diameter holes is the elimination of the risk of tap breakage and the associated costs of removing a broken tap or, scrapping a valuable component. The cutting forces when threadmilling are minimal, swarf control can be optimised and through tool coolant is standard on the majority of the WNT range. This leaves threads somewhere in between, say from M6 to M24. Here WNT has its type UNI solid carbide circular threadmilling tools. These solid carbide cutters look very similar to a conventional tap, but perform multiple operations starting by creating a chamfer on the face of the component, then machining the core hole and finally machining the thread form. This type of tool is restricted to machines that have the capability for screw interpolation and with sufficient through spindle coolant capacity. So in conclusion, why choose threadmilling over tapping? • Threadmilling can produce different sized threads with the same tool, thus reducing cost and inventory • Simple toolholding (no specialised tap holders required) • No axial deformation of threads (ie better quality) • Greatly reduced deformation when threading thin walled components • Clean thread flanks are produced • Reduced cycle times • No/or limited risk of scrap components if a tool should break • Reduced stress on machine spindle as cutting forces are lighter and there is no need for reversing the spindle direction • Improved performance in difficult to machine materials User tips Modern machine controls provide two different options for the control of the feed motion when threadmilling. In the first instance, the control manages the motion at the diameter of the tool. The second control option the feed is determined at the centreline of the tool. A user has to determine which method is being applied in order to get the ideal thread form. To do this the user has to follow this procedure: Firstly enter the threadmilling routine into your control, at the same time add a 'safety margin' to ensure that the tool will cut fresh air. Now run the program and confirm the cycle time and check this measured time against the calculated cycle time. If the actual time is longer than that calculated the machine is controlling the feed along the tool centreline, and conversely, if the time is shorter then the feed is being controlled at the diameter of the cutter. To calculate the peripheral feedrate in mm/min use this formula: Vf = n • fz • z To calculate centreline feedrate in mm/min use this formula: Vfm = Fz•(D-Dw)/D Where: Dw = effective diameter (mm) N = RPM Fz = feed per tooth (mm) Z = number of cutting edges (radial) D = nominal thread diameter or external profile diameter (mm) Dm = Tool centreline diameter (D-Dw) (mm) First published in Machinery, October 2012