Since the late nineties there has been a push to eliminate the mid-process heat treatment/hardening of the steel block, which has reaped several benefits: (1) regaining the time and cost of shipment to the heat treatment facility; (2) regaining the cost of tearing down the setup, re-squaring the block after return from the furnace and resetting up the block for finishing; and, (3) removal of rescheduling complications when fitting the block back into the master schedule.
Another target for process elimination has been EDM. The invention of EDM was a huge advantage to mold/die shops when it was first developed because it allowed different shapes and sizes to be machined into steel that were unheard of previously. What eventually became obvious was that EDM also was an additional process. The time to create the graphite electrodes, the cost to create CAM programs to machine them, the additional cost of setting up and resetting up the job, plus the burn time itself had to be accounted for in the cost and scheduling of the job.
Although not all situations or block sizes allow for the elimination of in process heat treatment or the need for EDM in certain situations, there is a continued need to push the envelope to compete globally, as any elimination of time or cost is effective in improving competitiveness. Hard milling is one such process that improves competitiveness if employed properly. Following is a brief overview of some of the requirements, challenges and solutions for successful hard milling.
Hard Milling Requirements
The secret of hard milling has been around for awhile. The problem is that people often don’t want to accept the requirements because of the necessary investment. You can’t take any shortcuts and expect to get results.
Hard milling is placing the CNC machine and the cutter in the worst type of cutting condition, so you must have all of the best solutions in place to counteract this. You must have a CNC machine with a rigid enough spindle to minimize vibration; a high quality end mill with the proper coating for the application; the expertise and assistance of an applications expert from the cutter company to ensure that you use the correct speeds, feeds, step-over and step-downs required for the material/hardness: too small of a step-over or step-down can be just as detrimental to a good result as too high a value; a highly accurate and rigid toolholder; and, proper CAM software.
Much of the software for CAM started out as general-purpose software. Often times the company was a CAD company first that decided to add CAM software in an attempt to supply a one-stop shopping environment. The toolpath that is output is very inconsistent and inaccurate, which causes the CNC machine to run on a rough or ragged path as opposed to a smooth path, resulting in poor surface finish and premature tool wear.
The Human Aspect of Technology
One aspect of hard milling that is often ignored is human resources. The new information that the CAM department must now understand and apply with regard to cutting tools, lasers, probes, machine tool capabilities, CAM software options, etc. has grown tremendously, so a fresh, open-minded internal review of procedures and personnel in the CAM/machining departments is required to be sure that you are not stuck in the “this is the way that we have always done it” routine. Every employee must be educated to understand that global competition is real, and will require personal sacrifices for the good of the company’s profitability to retain the jobs that we have.
Cutting tools are now more specifically application-designed and coated. All employees involved in the process must understand the specific differences between the various types and coatings. Using a tool for an incorrect application easily negates having the other variables satisfied.
Lasers being added to the hard milling process have allowed the elimination of the human in the accuracy of picking up each tool. It also has allowed for in-process checking and replacement of the tool as required caused by chipping or wear. Understanding of how the laser functions—including the macros that are used to control the laser—is now necessary as well as its required maintenance.
The probe now used in CNC machines for measurement introduces another piece of hardware/software necessary to understand. It is used for in-process checking, and in some cases alteration of the setup or offsets to bring the part into specifications.
The CNC machine has continued to develop over the years, including a more user-friendly, computer-oriented control. As the capability of the control has increased, the users learning requirements have grown. It is difficult to add more abilities to any product without requiring more involvement and understanding by the user. If there is an option available, there may be a normal default that doesn’t need to be changed very often, but someone must understand what the option is for and when it needs to be changed.
CAM software follows the same lead as the aforementioned items. Software has been developed and improved over the years based on user requests and complaints. Many of the new options are added, but the original variation is left available because many users prefer the original output. At this point a user must make a conscious decision as to what he wants the setting to be in this circumstance. This forces the user to be aware of the second or third choices—what they are designed for—to make the appropriate setting before starting the processing. This is not a one item situation. It grows as new things are added and will continue as software upgrades are released. To be effective the user must be given time to study the new capabilities of new releases and realize in his own application where they fit.
CAM Strategies for Hard Milling
In the hard milling process there are a number of concerns. For example, a build-up of heat during the cutting process can cause premature tool wear, trapped/welded chips, spindle growth, spindle/cutter vibration and part piece rigidity.
Limiting the length of tool being used is a high priority, so your CAM software must have a method of verifying holder and tool shank safety relative to the geometry at the stage that it is being machined. Some CAM software monitors the tool and holder relative to the finished model, but it needs to be monitored relative to the in-process model.
In order to keep this tool length to a minimum, more mold and die companies are investing in 4- and 5-axis machining centers. This brings in a new set of variables to deal with relative to the rigidity of the setup of the piece, as well as the rigidity of the rotating part of the machine. The ability to shorten the tool as much as physically possible is the main gain in this strategy.
The toolpaths themselves need to have certain capabilities designed into them. Trochoidal motion during roughing passes is of great value in certain circumstances because it keeps a clearance between the tool and side walls during any type of slotting and allows the chips to be evacuated while maintaining a constant load on the cutter. All roughing toolpaths also must be designed to keep a constant chip load during the course of machining.
Roughing and finishing toolpaths should include the ability to output a radial motion in corners to eliminate dual contact of the cutter as it changes directions. Re-machining toolpaths that go back in to finish the corners should be able to analyze any excess stock areas and remove the excess stock, eliminating cutter overload.
The way that the toolpath approaches the workpiece also is a crucial part of the equation of a high quality toolpath. An approach and escape that are normal to the surface being machined allows for a transitional entry from not cutting to entering the full cut.
CAM systems must have the ability to create and edit standard tool databases. This allows for recordkeeping that often gets lost. The tradition of having the data in your head and calling it experience really benefits no one, as that information and the investment is lost if the person moves on or retires. Setting standards as a company and maintaining that information for all to use shortens the learning curve for new employees and eliminates the secrets that go missing when the employee leaves. CNC machining is an exact science and much more manageable than perceived in the past, if we find a way to document our successes.
Templating—or the ability to recreate an existing process to be applied to the next job and the next job after that—has been recognized as an important feature in CAM software whether hard milling or high-speed machining. Even though products don’t always have similar geometries, there is a very similar base level process that is used from the start of cutting to the end.
The need to re-invent that process at the start of each new job is a waste of time and effort. In the mold manufacturing there is more of a standard process than people realize.
The toolpaths created by many of the off-the-shelf, general-purpose CAM systems are not smooth enough or accurate enough to be used for hard milling effectively. The requirements for hard milling cannot be accomplished with toolpath algorithms created 10 to 20 years ago, as their quality is at a lower level. The older algorithms are created on an approximation of the geometry, commonly referred to as triangulation mesh.
That method was designed and implemented in the early nineties to expedite toolpath calculation with less concern about accuracy and purity. The more effective modern algorithm uses the information from the original CAD surfaces/solids to create the toolpaths with no approximation.
Today’s CNC machines require more information to process more effectively whether it’s hard milling or high-speed machining. The output of these points must be mathematically correct, not just randomly added. You must ask that the results be demonstrated for you—not just something on a computer screen.
It has become obvious that CAM/machining is the most important part of the manufacturing process whether it is within a mold/die shop or another manufacturing/machining facility. The money spent on technology, people and training is a significant percentage of a shop’s investment as it tries to become more efficient and productive to counteract offshore competition. The choice of machine has often been what the shop down the street purchased. The CAM software has been what the new employee used in his last job. Today the investment is too high to continue making decisions that way.
A shop owner or management needs to determine their goals and expectations. For example, if we hard mill this block in five hours we can eliminate $3,000 worth of graphite, graphite machining and EDM time. They need to take those expectations to the machine and CAM vendors and have them prove that the goals can be accomplished consistently.