March 21, 2014
Simulating the machining process before the job hits the shop floor reduces errors, avoids collisions.
Trial-and-error machining has, in most instances, gone the way of the dodo and highvolume, low-mix production. Shops today need to start making chips— and quality, conforming parts—as soon as the green light goes on.
Increased computing power in today’s PCs has enabled more detailed simulations to be created in much shorter times. This now makes it practical to try out different tool path strategies before choosing the one that is most efficient. On the machine, improvements in control technology make the machine tool control less likely to slow down the process when simulation is run on the actual machine.
New robot controls even allow programmers to accurately detail the exact size, shape, and motion of every part of a robotic workcell. This creates accurate simulations that increase the ability to detect collisions and minutely adjust a robot’s joint angles.
This makes the forecasting of processing times from simulation more likely to accurately reflect actual machine time.
While offline programming and simulation have been around for many years, many shops are still not taking advantage of the full power of this software.
A suite of simulation software can help shops to do the following:
While all of these are important, it is a collision that can cause the most damage and have the most impact on a machining process.
“Simulation alerts the programmer to any collisions that might occur during machining. Collisions are expensive, not just because of the damage to the part, but also because of the possibility of breaking a tool or damaging the machine tool spindle or other parts,” explained Peter Dickin, international marketing manager for Delcam.
With most traditional CNC mills and lathes, modern software will never produce collisions. But as processes become more complex, for example, 5-axis or turn-mill machining, the movement calculations become more challenging and the possibility for collision increases.
Also, these processes create more complex and therefore valuable parts, so the cost of a damaged part increases.
"It is much cheaper to have a collision on the computer during simulation than to have a collision on the machine tool, especially if it occurs just before the part is finished," said Dickin.
Avoiding collisions saves time, money, and materials if the part is scrapped or if the cutter is broken, while more serious collisions can cause very expensive damage to the machine tool.
There are three opportunities for collisions to occur during the machining process. They are:
As more companies seek to drive down per-part costs, automation and lights-out production are becoming the norm. In this type of machining process, avoiding collisions is very important.
“Simulation allows you to see the exact path of the robot and end-of-arm tool to create the most efficient and accurate movement, as well as optimum part placement in the workcell. Simulation also allows us to identify any collisions between the robot and the workcell, part, or other components,” said Michael Starry, robotic applications engineer for In- House Solutions.
With complex parts the ability to maneuver the end-of-arm tooling into more difficult positions becomes critical and challenging. Often only millimeters of space are between the toolholder and part features.
“With simulation we can look closely to verify and change the program as needed to avoid any collisions in the cut path itself,” said Starry.
Simulating the entire process, including loading and unloading, shows exactly what will happen in each portion of the process.
“From the ability to simulate an operator loading a part and pressing the cycle-start button through any cutting, including robotic, we can visualize the entire process,” said Starry. “Verification of collisions, movements, placement of parts, and accuracy of the entire cell allows us to optimize the cell, which increases production and improves efficiency.”
Simulation also can be used to compare the cycle times for various tool path strategies and compare different part orientations on the machine tool bed. The programmer can then choose the most efficient strategy and part position.
It should be noted, however, that the predicted times may not be entirely accurate down to the exact second because of all of the variables in play during the machining process.
“The relative accuracy usually is very good,” said Dickin. “For example, if one strategy is shown to be 10 percent faster on the computer, it will be 10 percent faster on the machine.”
This comparison is possible, in part, because of the accurate modeling new simulation software provides. Most software includes highly accurate machine tool models. Developers work closely with machine tool OEMs to ensure correct computer models of new machines are available in the software as soon as they are introduced.
One important step is to make sure that the options chosen in the computer program are exactly the same as the settings used on the machine.
“Any differences can easily lead to a tool path being rated safe in the software and then causing a collision on the machine,” said Dickin.
CAM systems that are constantly changing to keep up with new machine tool and cutting tool technology also allow the newest machining theories to be used.
“This is essential because machines are becoming increasingly complex, and cutting tools are able to operate at speeds and cutting depths that would have been unthinkable even five years ago,” said Dickin.
One major change has been the introduction of high-efficiency area clearance tool paths to replace conventional raster roughing.
“These tool paths are much more complex and often include trochoidal moves to remove material in slots and internal corners more effectively,” said Dickin.
Simulation can be used to avoid moves that are undesirable, even if they will not cause a collision. Examples are sudden changes in direction and axis reversals, which can damage the cutter and leave marks on the surface of the part.
Simulation can also be used to help avoid moves that would cause excessive wear on the tool or moves that would damage the tool.
“Through simulation we can see if there will be a collision between the part or its fixturing and the noncutting area of a tool prior to running the program at the machine,” said Starry.
Productivity is improved because spindle on-times are increased. Preventing collisions means delays in production caused by the machine being down are eliminated.
Simulation also can identify air moves exactly as they happen in the real-world work environment, which allows programmers to optimize the entire tool path, cutting cycle, and robot motion. By doing so, spindle ontime can be improved significantly.
Surface finish is improved because undesirable moves, such as axis reversals that can leave marks on the surface of the part, can be eliminated before they occur. Simulations give an accurate reflection of the finish that will be left on the part, and the programmer can then decide if it is acceptable, or if another operation is needed to improve the finish.
Founded in 1986, Canadian Industrial Machinery is a full-service magazine supporting Canada's metalworking manufacturing sector. CIM is Canada's leading national monthly magazine dedicated to the metalworking and fabricating industry, reaching nearly 17,000 industry professionals every month. CIM also produces the annual Metal Manufacturers' Purchasing Guide (MMPG).