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Advantages of Milling Robots
Today, we are finding more industrial robots performing milling and trimming operations. In the past, particularly in the case of milling, traditionally were carried out by CNC machines.
There are two major advantages of using a six-axis robot:
The first ratio – “envelope against price” is low. In other words, it is possible to mill big things for not too much money.
The second is the ability of the robot to mill from different directions including undercutting. If the robot is combined with an integrated external axis to provide a rotating turntable or positioner this allows the most complex shapes to be milled.
We must however mention at this stage the two considerations to take into account with robotic milling and to a lesser degree trimming.
Two considerations to take into account with robotic milling and to a lesser degree trimming
Firstly, the accuracy of milling operations carried out from a program created via an offline source. Basically, there is a difference between jogging the robot by eye using the teach pendant and saving a position compared to generating the position offline.
If we use the teach pendant then we are asking the robot when it runs the program to go back to the position that we just saved and in this case, we are working with the “repeatability” of the robot, typically around +/- 0.15mm.
However, in the case of an offline program, the position has been calculated mathematically from a datum point and we are expecting the robot to move mechanically to that point. In this case, we are working with the “accuracy” of the robot, typically around +/- 1.0mm.
The ability of the robot to accurately move to a calculated point from an offline source will also bring in the tolerances involved from:
(1) teaching the TCP or tool centre point,
(2) the definition of the local coordinate system, and
(3) the tolerance of the placing of the part (in the case of trimming or stock in the case of milling) on the fixture.
Secondly, we must consider the rigidity of the robot. If we compare to a CNC machine the robot is going to be less rigid. Why? Because of the mechanical construction to achieve the greater reach tends towards a machine that might have some deflection particularly when the robot is stretched right out. All this means that the robot cannot generate high cutting forces, so we don’t normally consider robots for milling hard materials like steel etc.
Main Elements that make up the majority of Milling Cells
Having talked about the advantages and considerations to take into account let’s have a look at the main elements that make up the majority of robotic milling cells.
Firstly, we have the six-axis robot itself. Considerations here are for payload, envelope, and rigidity. These considerations usually point to a largish robot where the payload will invariably be much greater than the weight of the spindle that will be carrying out the cutting.
However, it will be both the envelope and rigidity aspect that will point to the robot choice rather than payload in this case. Because the tool paths will be created offline and no doubt consists of a large number of points reaching into the hundreds of thousands.
It is important to bear in mind that the controller needs the capacity to hold large programs. For example, reaching as much as 20 or 30 megabytes for large milling projects.
Milling Robots tool changer
Mounted near the robot but out of the work area will be mounted a tool rack which will have plastic forks to hold each of the cutters in their respective tool holders. In addition, a proximity switch wired to the robot to indicate that a tool holder is present in the rack. When a tool change is called for the robot will come over to the tool rack to exchange tools.
Consider whether an external axis is Required for the Robot Cell
Allied with the robot we must consider whether an external axis is needed. This will largely be dictated by the need to re-orientate the stock to allow the robot to work be-hind or to the side of the stock rather than having the robot “reaching” thereby allow-ing the robot to work in a more rigid configuration of it’s axes.Fitted to the robot will be some form of spindle to work the cutter. Invariably this will incorporate a tool changer to allow the use of different cutters through the milling operations. The automatic exchange of the cutters takes place as part of the cutting program sequence. The spindle will need to be of sufficient power to work the largest cutter envisaged. Allied with the spindle will be an inverter to vary the cutter rotation speeds. The cutter speed will be under the control of the robot controller usually via an analogue output cabled to the inverter speed input.
More Elements to consider when investing in Milling Robot Cell
- To control the complete cell a safety panel with cell control pushbuttons will be installed which is electrically connected to the robot controller.
- For mounting the part or material to be milled will usually be some form of a table.
- The cell will need some form of guarding which will form a room or enclosure to make sure that nobody is present whilst the robot is working in automatic.
- Lastly, we would normally have some form of offline programming package to generate the milling paths from CAD data. This software is autonomous to the robot.
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Turnkey Industrial Robot Solutions
Phoenix integrates and installs complete robotic milling solutions from initial design and consultation to manufacture. Also, we offer robot training, robot programming, robot simulation, commissioning, and technical support. Whether you decide to configure your complete milling robot cell, around a new robot or reconditioned robot.