Nuclear Fuel Complex uses pilgering as one of the operations in production of seamless pressure tubes, calendria tubes and fuel tubes for pressurised heavy-water reactor & prototype fast breeder reactors. To accommodate different sizes of these structural components, different mills are in use. 6-axis (prior to introduction of motion controller) die forming machine is a special purpose machine, used to fabricate tooling for rolling mills, which in turn are utilised for production of pressure tubes & calendria tubes. Control system of this machine was based on TTL, simultaneously controlling position of 6 axes, in accordance with the cut-profile generated by a master hardware template in conjunction with a linear sensor. This system is modernised with introduction of motion controller & CAM builder software along with digital servo drives to run Virtual Master and control the position of axes accordingly, while reducing number of axes to 5. This system offers benefits of reduction in design to delivery time, improvement in quality of end product, better availability & maintainability.
The cold mill pilgering process uses ring dies and a tapered mandrel to reduce tube cross sections in one process step. Ring dies are rotated on its axis while given back & forth movement also. Due to its profile, the dies compress the tube against mandrel, resulting in reduction process, when it travels in forward direction. The space in the groove cut, accommodating tube over mandrel, in the die is made progressively smaller (called profile) causing the die to work on tube, thereby, reducing its diameter and wall thickness.
The legacy mechanism
The profile of cut is generally a smooth curve found from calculation of various design parameters. The curve of profile-of cut from 0° to 360° on the ring is first divided into about 50 points on it and each point on the curve is joined with neighbouring points with linear approximation. These linear approximations are then transformed onto a linear metallic template. In the graph, height at any point (Y-axis) represents the value of curve (depth of cut) at that point on X-axis, i.e. location on periphery of the die. During cutting, the template is travelled at a constant speed (synchronised with rotary speed of dies like 10° per min or so) by the control system and is made to press a linear sensor with 2 μm resolution.
The sensor output is of pulse type. Travel of template from one point to another in X-direction, results in push or retraction of sensor’s roller equal to relative difference of height values at those two points. This push or retraction generates pulses, which are then detected & processed by TTL to produce required Y values (depth of cut), while X-axis is tracked by the encoder mounted on servo motor responsible for motion of the template.
Cutting on both the dies is achieved simultaneously. Two dies (forming two rotary axes) are mounted on two movable beds (forming two linear axes). In between the dies, there remains a cutting tool rotating at high rpm. The cut on the periphery of dies is achieved by inducing eccentricity (The 5th axis) in the rotating tool. The depth of cut is proportional to eccentricity in the tool, which in turn is decided by the set point pulses generated by template & sensor.
Also rotated are dies at a configurable feed rate from 5° per min to 90° per minute. This feed rate controls the linear speed of travel of template (the 6th-axis) too, to synchronise X and Y values of the curve. One complete operation requires rotation of dies from 0° to 360° and template’s travel from start to end point; all the three movements are synchronised with each other.
Original control system was realised with TTL based circuits, which control the position of all the axes. Dies rotation and template drive axes are run in position synchronism at a selectable feed rate (e.g. 5°/min). Beds and eccentricity axis position depends on the cam, which in turn depend on the groove profile. The shaded portion, is implemented in TTL controller and servo drives accept speed set point from the controller.
Dimensional accuracy of any tooling decides quality of the end product. In this case, since a curve is approximated by linear interpolation between 50 different points, proximity of each point with another is of utmost importance. However, due to physical constraints on fabrication process of template, only alternate points were considered for template fabrication; leading to an upper limitation on quality. Secondly, template fabrication itself, keeping close tolerances in view, was a time consuming & involved process. Also, complexity of circuits due to intricate control requirements resulted in very high Mean Time To Repair (MTTR) in case of occurrence of a breakdown.
New control system: Architecture of the modernised control system is depicted in Figure, as shown, it comprises of motion controller with built-in PLC, AC servo motors with servo drives and high resolution encoders for accurate positioning. The system architecture is based on high speed Ethernet-based SERCOS-III communication bus, connecting all the drives with motion controller. The motion profile is developed in the CAM-builder software in HMI and downloaded to motion controller, from where position set points with interpolation are transferred to respective drives. The system runs one virtual master in the motion controller and all axes are run in synchronism with respect to the position of the virtual master. This virtual master replaces hardware template & linear sensor combo, thereby reducing number of axes to 5.
The existing control system has been retrofitted with latest, industry standard control system having state-of-the-art diagnostic and development features. New system provides a convenient way for manufacturing the dies while offering following advantages:
Due to e-cam, inaccuracies in fabrication of template, dependence on skill of operator and inaccuracies of linear sensor generating the profile are completely avoided.
Accurate generation of profile to even micron level, as generated and communicated position values to drives in digital form.
High resolution encoders, thereby improving accuracy of final product
Smoother profile: Up to 1024 points against present practice of 51 points, providing feasibility of smoother profile on the dies.
Cam-Builder Utility: making it possible to view the profile in graphical form, on HMI, while entering profile data.
Productivity: Faster design-to-delivery times, as time taken to prepare hardware template is eliminated.
Compensation Of Mechanical Errors: Electronic backlash compensation of linear axes.
Energy Saving: Deletion of an axis, serving as Template drive.
Improved Safety: due to provision of software limit switches for all axes, and additional interlocks based on die dimensions.
Absolute Measuring System: requiring no homing of axes at each power up. Axes positions remembered even after power Off and ON.
Convenience Of Operation: Latest Touch-screen HMI based operations with real time trending of axes position for better information to operator.