Cutting Methods -

Cutting Methods

The aim of this article is to set out the main equipment and methods of end and edge preparation for pipes and plates prior to welding. The advantages and disadvantages are discussed and the situations best suited to each method are described.

There are many reasons why it may be necessary to prepare the ends of tubes prior to an orbital welding operation, and even more methods of doing so. In order to understand the large variety of methods it is necessary to examine the reasons why such preparation is required. Preparation may be the final stage of the manufacturing process and, in addition to the cosmetic value achieved it also produces a finished product which will be more acceptable to the buyer of the goods. Accurate end preparation of tubes and pipes ensures that, where lengths of tubes and pipes are to be joined to each other or to other components, the squareness of the tube end will guarantee proper fit up and correct alignment on long runs. There are many types of preparation and these will vary according to the size of the component, wall thickness and the joining method to be employed. For example, a tube of 0.393” (10 mm) diameter with wall thickness of 0.118” (3 mm) may require simply a square, flat face for autogenous orbital TIG welding. A pipe of 30 inches diameter with a wall thickness of 2 inches may require an end preparation with a compound bevel, an accurate root face in addition to an internal bevel in order to ensure a consistent inside diameter. Most welding specifications call for a pre-determined end form which will vary according to the subsequent welding method. Welding is only one of many methods used to join tubular components and although this article concentrates on welding, accurate end preparation is also of great value in relation to some of the alternative joining methods such as threaded joints, pressure energized couplings, etc.. There is also a wide range of non-metallic materials from which tube and pipe is made and some of these undoubtedly benefit from high quality end preparation. However, this article concentrates upon metal tubes.

Methods of end preparation range from the simplest, eg. a hacksaw and file, to high technology CNC high volume production machinery which will turn out hundreds of finished tubes or pipes per hour. Some of these methods are presented and their advantages and disadvantages are now examined.

Hacksaw and file

This is obviously inexpensive in terms of equipment, but this is the only benefit that can be had from such an archaic way of working. The degree of accuracy and the speed at which ends can be prepared all but rule this out as a practical method of preparing weld joints. The high cost of labour also makes the benefits of this method dubious.

Grinding

Grinding is the most common of the low tech methods employed in the tube and pipe fabrication shops around the world. It is relatively inexpensive as the initial investment in equipment is considerably less than many of the alternative types of machinery currently available. It can be used on a wide selection of materials and it will, given sufficient time and a good operator, create most of the common weld preps in use at present. Grinding can be carried out in any workshop and also in-situ where circumstances allow. However where there is a risk of fire or explosion or for applications in such places as clean rooms, it is of no value. Grinding is also very noisy, being a major contributory factor to industrial deafness if no precautions are taken against the effects of the noise. It is dirty and sometimes dangerous and grinding dust can become airborne, and can be breathed in not only by the person doing the grinding but also by anyone else in the vicinity of the work. Once again, precautions can be taken against this risk. Hot sparks have been known to ignite flammable materials left close to the work site, and there is the added danger of setting the operator`s overalls alight as a result of a lack of attention when using grinders. Yet, again, there are precautions that can be taken to ensure that the operator does not end up as a fireball but, with his hat on to cover his hair, his gloves, goggles, face mask, leather clothing and breathing apparatus, he feels equipped more for a trip to the moon than a day in the fabrication shop. There are also some hidden costs in the use of grinders. The cost of grinding discs can be high, especially if the discs are designed for use on exotic materials. There is a high degree of maintenance required on grinding machines which is seldom taken into account when considering this method. Operator fatigue can also account for some unnecessary costs. No matter how enthusiastic the operator, fatigue will take its toll and productivity at the end of the working day will decrease. The accuracy of grinding is not optimum under any circumstances and, the more complicated the profile, the harder it is to achieve. There is also the risk of the inclusion of particles of the grinding disc on the pipe material, which can result in weld defects in the subsequent joining process.

Flame or plasma cutting

There are several ways of creating end preparations on tube and pipe using flame or plasma cutting methods. Many of them can be effective and provide acceptable surfaces for welding under certain circumstances. Hand held torches depend on the skill of the operator and consequently the degree of accuracy can be erratic. Manual flame/plasma cutting equipment costs are not prohibitive and these methods can be employed in a number of situations, both indoors and out, on site. There is the risk of fire and certain amount of mess is inherent when using either flame or plasma cutting. Mechanised methods of flame and plasma cutting have the advantage over hand held torches of a greater degree of accuracy, although there are still limitations to be taken into account. Flame cutting can only be used on carbon steel, and only the simplest of preparations can be achieved as the flame will only cut in a straight line. This provides an accurate bevel or a flat face, but nothing more sophisticated. This is adequate for some applications. Plasma cutting has the advantage over flame of being able to work on more exotic materials such as stainless steel, but it can be extremely messy and, on heavy materials, the cut tends to dissipate as it advances through the wall and it leaves an inaccurate finish. Both flame and plasma cutting induce a certain amount of heat into the material and this has to be taken into consideration. This heat input is thought by some engineers to be unacceptable.

Lather or boring machine

Employing a lathe or boring machine is an accurate and sometimes fast method compared to the others available. It also has the ability to produce complex profiles with a high level of repeatability. In a lathe, with the workpiece rotating, there are limitations as to the size and shape of the pieces to be prepared. A 12 meters length of tube spinning at high speed can be very hard to control. A boring machine has far fewer limitations, but there are many disadvantages common to both machines. Using these machines for end preparation ties up the machines and prevents them from being used for the many more complex operations of which they are capable. Not only is the machine taken out of circulation but also highly skilled operators, who could earn far more for his employers carrying out other tasks, is taken out of commission. Handling large or complicated pipes, possibly with bends or fittings welded into them, can be difficult and slow.

Hand held milling and nibbling machines

Hand held milling and nibbling machines vary in size and capacity from a small electric drill for lightweight plates of about 3mm thickness, to free standing machines that have the workpiece fed through by hand. Some of these machines carry a milling cutter that will remove large amounts of material very quickly. These leave a surface finish that is generally acceptable for most methods of welding. Other machines use a system similar to a miniaturised guillotine that nibbles the material away to a predetermined shape.

Purpose built end preparation machines

There are distinct areas where dedicated end preparation machinery can prove to be invaluable. The first is at the end of a tube or pipe production line, where the finished product arrives with rough ends. A customised end preparation system, matched to the speed of the line, will give the manufacturer a product which will look well finished and be more acceptable to a customer than a saw cut end. It also offers the producer the facility of sending out a product end finished to meet the exact requirements of the customer. This makes the product more acceptable to the customer as it removes the need for further processing.

The producer can offer its clients a choice of end profiles applicable to the joining process to be used in assembly. Similar facilities are available to the manufacturer of plate products, and the benefits accrued are largely those gained for tube and pipe. In some instances, for example where plate is being rolled into a tubular section, accurate machining to width is imperative as this will govern the diameter of the finished tube. Machines of this nature are usually built into production lines, together with rolling and welding equipment. For workshop activities, a vast range of both fixed and portable systems are available. The choice of using a fixed system, as opposed to a portable machine, largely depends on the volume of ends to be prepared and the speed at which it is to take place. If there is a large number of similar sized items to be prepared in a common profile, a fixed system should be preferred. This can be installed on a production line and work in harness with any other process that the components have to pass through. Where there is a variety of different tube or pipe sizes to be prepared or the numbers of end profile is lower, a portable system may be better suited to the situation.

Portable machines also have the advantage that, if the prepared pipes are to be assembled on site, the machinery can be taken to the job location and the same standards of accuracy can be achieved as it is in the workshop. Most of the previously described methods can be used where end preparation has to take place in-situ. However, the same limitations still apply depending on the job situation. Grinding and cutting by flame or plasma are precluded from any situation where there is a risk of fire or explosion. Nor are they of any value in clean room applications. Machines designed for in-situ use are usually light weight in relation to the dimensions that they are made to work on. They can be supplied with a variety of drive systems. There is a wide choice of methods for attaching the machine to the work, and several different cutting methods can be employed. Most of these machines are capable of producing accurate, repeatable end profiles, and do not usually require hot work, so they can be used in potentially dangerous environments.

The variety of equipment on the market for tube and pipe end preparation can range from a machine as simple as a grinder mounted in a jig which clamps the pipe to CNC machines which work at great speed and to a high degree of accuracy. There are situations where end preparations have to be created in an environment that precludes normal methods. This may be underwater or in an irradiated area on a nuclear installation, or possibly in a situation where a human operator cannot be employed due to inaccessibility or danger to life. For these applications remote control machines are needed. These range from simple long air hose and ball valve, to the complicated systems involving fibre optic mini television cameras and lights. The previously mentioned jig mounted grinder has few, if any advantages over the hand held version. There are machines that were originally designed to bevel plates and have been adapted for tubes and pipes. This will only work where the diameter is large enough to accommodate the hulk of the machine. The remainder of the machines fall into two main categories; inside mounted and outside mounted. In some situations, small diameters being the most obvious example, outside mounted machines are the only answer to the need for accurate end profiles.

There also exists the situation in which the inside of the tube may not be touched, for example on electrochemically polished tube where even a fingerprint can be disastrous. Some machines employ collects to grip the workpiece whilst others use a system of jaws similar to a chuck. For smaller sizes the collect system is the optimum solution as it is simple and accurate. Large collets are very expensive, therefore on the larger pipes the jaws system tends to find favour. Another subcategory of the old family is the chain type fixing, the biggest advantage of which is that it is practically without limit of diameter. Even large vessels can be bevelled using this method. The principal disadvantage of the chain method is that it is difficult to set up and, if care is not taken, the cutter may wander resulting in a spiral instead of a prepped end. Most old clamping machines also have the facility to part off, as well as bevel, which makes them a versatile tool. Machines mounted inside the pipe bore are usually quick and easy to set up and install. Nearly all of these use a mandrel, which has the facility to self-centre and self-align the machine with the axis in the correct position inside the pipe. Most employ form tools to create the desired shape on the pipe end. Many will perform more than one cutting operation simultaneously, ie. a compound bevel, counter-bore and root face. For thicker wall applications some machines use single point tooling with articulated tool holders which will accommodate varying angles and profiles. These types of machines can also be adapted to reinstate a damaged gasket seat on flanges and vessels.

Capital investment

The major drawback to the use of portable machine tools and sophisticated production system is that they require a large capital investment to establish them in the workplace. This can be a daunting prospect, particularly for small businesses which need to ensure their investments work hard for them and show a quick return on the outlay. The enhanced accuracy and repeatability acquired by using these machines means a reduction in the number of components needing to be scrapped, and an improvement in the number of resultant weld defects which need to be cut out or be repaired. Accurate end profiles mean that more advanced welding processes can be employed if necessary in order to speed up the overall fabrication process. Many automated welding systems need very accurate fit up, and are very unforgiving if there are inaccuracies in the weld preparation. If an estimator can judge more accurately the time taken to perform end profiles, then the total time allowed for the operation can be more easily quantified. This will lead to a more realistic reflection of the labour costs and subsequently the price quoted for the job.

 Conclusion

The selection of the equipment for tube end plate edge preparation, and the methods used depends largely upon the situation. It would be unrealistic to expect a small blacksmiths shop, which processes only limited amounts of pipework throughout the year, to invest thousands on machinery that would only be employed a few times. On the other hand, it is a huge potential benefit to major enterprises to have the best and most advanced equipment available to enhance the quality of their work, and hence their reputation, in a competitive marketplace.