Robotics in machine tending
By Arjun Prakash, Director, Effica Automation Ltd., Coimbatore
Multiple CNCs machines can be handled based on cycle time.
The advent of continuously falling prices in electronics and power electronics combined with the ever increasing cost and scarcity of labour has certainly pushed us into the age of robotics. The fact that China, a country where all economic factors were identical to that of India’s in the 1980s and a country that was known in the past decades as the world’s manufacturing hub due to its low labour costs, is today also the world’s largest consumer of industrial robots. This itself is a proof that we have already entered the age of robotics. They say people should think and machines must work. The youth of our country certainly have great aspirations and a job as menial as pick and place certainly does neither inspire them nor provide enough value addition to the end product to justify a fat paycheque. This is where robotics fits in with machine tending.
Most CNC machines installed in India are typically used for mass production where a few variants of the same type of part are manufactured for long periods of time with no changes in design or dimensions. A CNC machine operator typically even with some knowledge of programming the machine is reduced to the task of loading the machine and unloading the machine. This is certainly a waste of talent.
Part stacking magazines for various types of components.
Robotic machine tending
Robotic machine tending is the process of loading and unloading components for all types of industrial machines including CNC machining centers. The greatest advantage that is derived in the Indian context is the increased uptime from the CNC machine. Industrial Robots are generally not much faster than the human counterparts when they are timed on a single operation. But practically when you factor in human fatigue, refreshment breaks, inefficient night shifts and absenteeism in India, Robot users experience as much as 30 percent increase in machine uptime.
In projects where one robot caters to 3 CNC machines, this is equivalent of getting one more CNC machine. With robotic machine tending solutions in India falling below CNC machine prices, this translates to instant breakeven from machine uptime alone. The manpower savings is completely a bonus. This is of course only true when factors like machining cycle time, CNC machine cost, payload and required reach permit it, but in most cases a major portion of the breakeven is achieved from the machine uptime alone. But of course this does not mean that implementing a robotic machine tending project is as simple as writing out a cheque. There are many technical considerations that need to be given careful thought and analysis.
Burr can sometimes be a serious hindrance to automatic machine tending.
First, if a robot loads a CNC machine without human intervention, then we need to have a part stacking system so that the robot can operate at least a few hours without human presence before the entire stack is replenished again by a human. The same hold true at the unloading side, where machined components have to be emptied time and again. Since robots programmed to pick or place the component at fixed location within a few microns accuracy and with the correct orientation to grip, the part stacking magazines need to be designed in such a manner that they accommodate the part. This can often be a challenge with castings and other molded products where flash is a hindrance. The shape of the product is also a consideration where it is not possible to stack in the correct gripping orientation with good volumetric efficiency of stacking. This is important as robots have a predefined reach.
The other big hindrance can be the presence of burr after the machining operation. Burr is normally cleaned using a hand operated air gun manually. This can of course be automated by fitting an air jet on the robot, but the cycle time lost due to that should be kept in mind. If there is a possibility to fit permanent air nozzles close to the chucks and fixtures inside the CNC, where burr is typically a hindrance to the process, then it should be preferred. But sometimes even this does not solve the problems. In some types of steel and aluminium burr formation is often long coils and this gets stuck in chucks and fixture where heavy force is required to clear it. This should be solved either by tool geometry design (Chip breaking inserts) or by poke-yoke deflectors before the machine tending project is taken up.
Gripping in CNC machine tending is a straight forward process most of the time. Various types of two jaw and 3 jaw grippers are available. 3 jaw grippers are suitable for ID and OD gripping and 2 jaw grippers are suitable to hold long shafts. But again casting burr will lead to improper centering in 3 jaw grippers. This is a problem when the chuck center does not align with the part center. Even if sufficient clearance is provided in the chuck opening, enormous forces are subjected upon the robot wrist as the robot tries to resist the pulling related to the offset. This can be overcome using an offset compensator device.
A typical compliance unit to compensate for component offset with respect to chuck
Other considerations
Other considerations to be factored in are to ensure the safety of human operators. Since robotic accidents can even be fatal, it is important to completely cordon off the area with safety fences. Despite this, human access will be required, in the form of refilling and emptying magazine stacks. CNC machines also require frequent tool offset resetting and other minor changes. Doors and other openings need to be interlocked with safety switches and sensors.
A complete robotic cell tending to two CNC turning centers.
Conclusion
Finally having invested a great deal on the CNC machines and the machine tending project, it is always good to give some more thought to optimize the areas that tend to slow down the entire process. Non-machining activity like door open / close, chuck open close and where applicable tail stock and other accessories must be optimised for automatic operations and their speed can be considerably increased as sensors can be used to interlock their operation and prevent collision. The process of tool offset resetting should also be automated where it is very frequent, as no human should be present in an automated cell with 2 or 3 CNCs and one robot, or else the entire cell will come to a standstill while fixing one machine.
