TECHNICAL ARTICLE:
| High Production Cutting of Tube Layers Using Carbide Tipped Circular Sawing Machines |
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| By: B Traunsteiner, Framag Ind GmbH, Austria | ||||||||||||
Part 1: Introduction and History Introduction The profitability of nearly all heavy industrial plants worldwide is in a precarious situation as there are increasing instances of plant closures, mergers and take-overs. In order to survive and to save jobs, pipe mills will need to analyse their present processes and look for methods to achieve high productivity and throughput. The clear goal is to reduce cost. In order to achieve this task, it becomes necessary to develop new ways to sever continuous cast billets for pipe manufacturing and subsequent cutting the pipes to length, efficiently and economically. The emphasis is productivity, throughput and low tooling cost.
Abrasive Saws Instances of abrasive saws still in use today have become increasingly rare. The tooling and machine maintenance costs are extremely high, especially with alloy steel, while the severed surfaces are of poor quality and require frequent and expensive machine adjustment. Another big problem is the grinding dust that covers the entire production area and causes health and machine wear problems especially when mixed with oil. Hot Friction Cutting This method is still frequently in use especially in outgrown production facilities with limited cooling beds that are less than 100m long. Space limitations do not allow exchanging this old fashioned method with modern equipment. The tool cost and the equipment maintenance of hot friction sawing is high and frequent blade change due to the low tool life is time consuming. The inaccuracy in length and poor severed surfaces require additional adjustments and processes. Flame Cutting For environmental reasons, this process is not used for pipe cutting any longer. HSS Cold Sawing Due to the slow surface speed of HSS blades, the cutting cycle is long and therefore requires more machines with the necessary handling equipment. Despite the good quality of the cut surfaces, this method is usually not considered because it requires too much space and handling equipment. Band Saws Similar to the HSS cold saws, the band saws are just too slow. The cutting accuracy is poor and the necessary space and handling equipment requirements exclude this method. This overview shows that a new “State of the Art” method is required to sever steel more economically.
The Solution The answer to such problems is sawing with circular carbide tipped saw blades in pipe mills in connection with length measuring stops. Short cycle times, high quality of the cut surface, economic tooling cost, good environmental conditions, acceptable space demand, low maintenance requirements and user ease makes sawing with carbide tipped circular saw blades most effective. Tube mills generally produce oil field pipes, low alloy steel and chrome steel pipes, general-purpose pipes and tubes and tubing stock for car manufacturing such as axle tubes, etc. With the seamless tube mill, the following general specifications can be achieved.
Auxiliary Clamping Device The pipes are transferred in layers out of the end of the cooling bed and transported on roller conveyors to the saw systems. Generally, the flow is opposite to the roll direction. Especially with small pipe diameters and long pipe lengths, it is important to prevent the pipes from overlaying in the saw in-feed section. This can be done with an auxiliary clamping device on the infeed conveyor consisting of a horizontal and vertical clamp located in front of the saw blade. This clamping device is also advantageous for larger pipe diameter to support the horizontal clamping device of the saw. Scrap Removal The removal of the trim cut scrap (the end of the pipe after rolling becomes the leading end of the pipe for the saw process), is accomplished by advancing the pipe layer into the saw. The trim cut is then lined up at the scrap stop gate to establish the trim cut length, which is then followed by the saw cutting cycle. During the forwarding of the pipe layer the power roller of the scrap tipping table is raised. Prior to activating the vertical clamp, this power roller is hydraulically lowered to prevent damage to the roller bearings from the vertical clamping force on the pipe layer. Normally, a duplicate scrap-tipping table is arranged on both sides of the saw blade for the trim cut and crop end. The tipping of the table is actuated by a hydraulic cylinder and tips downward in the flow direction of the pipes for the trim cut and downward against the flow of the pipes for crop end scrap. If due to the rolling process, the crop ends could have different lengths, a length stop can be provided on the infeed side of the saw, which is lowered as soon as the pipe layer has passed. The pipe layer flow is reversed and the crop ends back up to line up on the stop. Subsequently, the table is tipped and discharges the crop ends to the scrap conveyor. Both tipping tables are mounted to the machine base. Clamping System It consists of vertical and horizontal clamps. The vertical clamps on both sides of the saw blade are either a mechanical rack and pinion system, or are actuated by hydraulic cylinders. Integrated into the vertical clamps is a transducer that measures the diameter of the pipes to verify the correct diameter input into the computer. This prevents blade damage when the saw head is rapidly advanced toward the pipes and must switch to the feed cycle before entering the pipe layers. In order to guarantee a parallel clamping of the pipe layers, the vertical clamps close firstly at low pressure followed by horizontal clamping at full pressure and subsequent full pressure increase of the vertical clamps. The clamping pressure is variable to suit the material and wall thickness, etc. which is calculated and adjusted to suit the integrated computer to avoid deformation of the pipes. The horizontal clamps force the layer against a positive fixed jaw. To double-check the layer width, an integrated transducer in the horizontal clamp cylinder compares the inputted value with the actual value to avoid mistakes. To achieve a separation for safe return of the saw blade, the auxiliary clamping system is equipped with a hydraulically operated retracting system. The layer (in front of the saw) is still clamped after the cut and retracted against the flow direction. Saw Specification Saw sizes are based on pipe diameters and layer widths. Generally layer widths of 600 and 1,000mm are common to allow the use of standard saw blade diameters of 1,000 to 1,600mm. Vertical saws have proven to be the best suitable machines to cut pipe layers. With these saws, the saw head is fed vertically downward on linear lines through the layer and thus requires the shortest saw cycle. The feed system consists of a Servomotor, timing belt drive and ball screw and varies in size according to the machine capacity. A properly designed, powerful hydraulic cylinder forces the head upward against the weight of the head.
This way, the balls in the ball nut are constantly being forced against the upper flank of the ball screw during the cutting cycle to eliminate vibrations and stabilising the sawing process. Additionally, this hydraulic cylinder is a safety feature as it prevents the gearbox from coming down unattended after a break in the timing belt. The saw blade is driven by an AC motor and poly-V- belts, with a belt ratio of 2-2.5, and acts as a safety coupling between the motor and gear head. The big pulley acts as a disk fly wheel, and this fly wheel action is extremely important to reduce vibrations that are induced into the saw blade when the carbide teeth enter the material, thus momentarily increasing the torque in the gear box. Timing belts are not accepted due to the noise when the belt teeth enter and exit the pulley grooves. The gear head is solid, not split, with three sets of anti-backlash hardened and ground helical gears. To prevent vibrations, an additional gear is arranged in the third gear set to make the drive backlash free. To improve tool life, the saw blade is backed off the cut surface after the pipe layer is moved away and before the head is retracted. This is accomplished by retracting the saw spindle including the blade dampener with a hydraulic cylinder, arranged on the rear end of the gear head (separation). The saw blade dampener consists of two units arranged 180º apart on the horizontal plane of the blade and is solidly mounted to the gear head. Depending on the saw blade diameter and saw blade thickness, it can be manually adjusted to fit. Since the dampeners move with the saw blade, a constant dampening effect, regardless of the position of the blade in the cut, is guaranteed. It is of importance that the dampeners in front of and behind the saw blade are big enough and adjusted at a maximum distance of 0.1mm to the saw blade. This guarantees elimination of vibrations. For saw blade changes they can be opened easily. Adjustments are required only when the saw blade dimensions are changed. During the rapid advance movement of the head before the cut and the rapid return after the cut, the spindle speed is reduced about 30 per cent to lower the vibration noise when the saw blade is idling, especially with large saw blades. In order to obtain optimal cutting conditions it is important to spray a minimal micro mist lubricant on the teeth. This is accomplished with nozzles, which are fed by a micro lube unit with flow control mounted on the head. Proper spray volume and directional pointing of the nozzles is important for optimal spray concentration and minimal use of coolant. Another important consideration for economic sawing is the cooling of the blade with compressed air. This is accomplished with two pairs of nozzles, on both sides of the saw blades. The chips, which are produced during the cutting process, have to be deflected on both sides of the saw blade by well arranged chip chutes to the chip conveyor, which is usually located below the sawing machine. All important points of the machine are lubricated by a central lube system. It is a fact that any reduction or elimination of vibrations during the sawing process reduces the wear of machine components, the noise level, and increases the tool life. This is the major goal to reduce tool cost. The special composite under the trade name Hydropol®, which is filled in the machine base and column, is a major component in vibration control.
Based on this experience, it enables us to build a sawing system which is designed from the base up to eliminate vibrations and to minimise the vibrations induced by the physical sawing action.Also extremely important for the economical operation of a sawing system is operator and maintenance ease. Operators must be able to learn the controls quickly and must be able to easily transfer their experience and communicate with others without additional training. Operators of sawing systems are well advised to realise a modem connection to the manufacturer in order to make program changes quickly.
Length Measuring In order to avoid secondary machining, it is extremely important to saw accurately to length by means of a length stop. It consists of a base with linear guides, a clamp rail and the adjustable stop carriage. The base is filled with Hydropol® similar to the machine base. Based on the rigid design with the vibration absorbing capabilities as previously outlined, excellent accuracy of ±5mm with finished length of 25m can be achieved. This also requires temperature compensation. By non-contact measuring of the temperature shortly before these pipes are sawed, and subsequent calculation of correct length with the machine computer, accurate lengths can be cut even at varying pipe temperatures by automatically adjusting the length stop. The stop carriage is adjusted by means of a rack and pinion servo drive. The traverse rate of the stop carriage is about 6-60m/min. Mounted on the stop carriage is a hydraulically dampened swivel stop. The dampening affect is the same for all layer weights. After the dampener action, a hydraulic cylinder pushes the stop and the pipe layer back to its preset position to guarantee the required accuracy. A cable carrier provides support for the necessary hydraulic hoses, control wires and lubrication hoses from the central lubrication system to the stop carriage. The control adjustments of the stop carriage are undertaken during the saw cycle to avoid unnecessary loss of cycle time. Machine Control The SOFT-PLC control is considered highly innovative and futuristic. It controls not only the entire saw system linked to the plant’s main server, but allows the continuous storage of specific data. Especially worthwhile to mention is the capturing of complete tool life data including process parameters, administration of saw blades and material to be cut, diagnostic recognition of the actual system, and failure diagnostic with the remote failure correction capability. A commercially available industrial computer is used with the specific software package. It becomes increasingly necessary to also control the decision process of saw blade change. We already have practical experience that shows by constantly checking and controlling parameters (such as current usage of the feed servo motor, torque requirement of the saw blade for each different material specification, and material size), we can calculate when a saw blade has to be changed. Such data can be stored in the computer and used to determine the optimal blade change time. With this feature, we have an excellent tool for preventative maintenance and to maximise blade life.
Saw Blades Next to the quality of the saw machine itself the saw blade is a decisive factor. Saw blades tend to vibrate, depending on their structure, thickness, diameter of material to be cut, etc. Primarily the saw blade manufacturers provide means to reduce these damaging vibrations (holes, grooves, etc.). Nevertheless it is important that the saw itself avoids the origination of vibrations. Only the right combination of saw blades, saw machines, and cutting parameters with reference to the material to be cut, will guarantee success. There is no rule based on practical experience to find this combination. Balanced solutions for the saw operator have to be developed by the saw manufacturer. It is a fact that only close cooperation between the saw system manufacturer, the blade manufacturer and the users of the system will find the right answers. There are many opinions regarding the optimal time for blade change and every user has to come up with the best answer for his process. (See section controls)
Before a Project Before specifying a saw system, the saw manufacturer and the saw system’s user have to get together to plan for the required space, the logistic of material flow and other considerations. Technical specifications expected by the users and promises of fulfilment by the supplier will not result in an optimum solution. Excellent saw systems and further innovations in this field could only be created by cooperation of all parties involved, the designer and builder, the saw blade manufacturer, the owner and operator, and the personnel who run the saw system. With joint effort, carbide sawing will become a deciding factor in economic pipe manufacturing. Critical Factors of a Circular Saw For the saw system manufacturer, the following points are critical in order to come up with an optimum solution.
If severed with carbide tipped circular saws, the length accuracy of the incoming material and integrated material feed is ±1mm. This is crucial when considering 0.3mm is the average deviation of billet diameters a length that leads to a final pipe length deviation of 1m. In contrast, flame cutting leads to a much higher loss of material. Thus by changing to carbide tipped circular saws a potential payback time of 2 years can be achieved. In addition, further machining of cut pipes enables carbide tipped circular saws to produce high cut surface quality and high length accuracy. Thus enormous cost saving can be realised because no further machining is required. 80 per cent of the tube production can be sold immediately, resulting in a payback time of 2-3 years. In comparison to abrasive cutting, carbide tipped circular saws lead to tool cost savings of 20 per cent. If high quality carbide tipped circular saws are used as described, another 10-15 per cent tool cost savings can be achieved. With such potential cost savings and operational possibilities, carbide tipped cold circular sawing is the most economic method to sever incoming material and pipes in tube mills. |
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