Die and mould
Simulation Optimizes the Die & Mold Sector
Many die & mold makers are looking for specific ways to increase machining speed, which is not as simple as it may sound. They are looking for ways to reduce the overall time to produce a finished mold by having the mold come off the machine tool with little or no post-machining finishing work.
That means dramatically larger NC program files with very small step-over to produce extremely smooth finish machined surfaces. With a modern 5-axis machine a mold surface that would typically be machined with a 3-axis machine tool can be cut with the milling tool at specific orientations to the cutting direction. The tool can be oriented so cutting is done with the side of the ball end mill, rather than the tip. This produces much better cutting conditions where the tool contacts material.
Many die & mold makers are also seeking to reduce the number of set-ups involved with machining a part. This in turn also reduces the amount of operator involvement, and means that one operator can oversee many machines. Reducing set-ups generally requires sophisticated multi-axis CNC machinery and automation tools such as robots and pallet shuttle systems. All this automation adds complication to the prove-out process.
As machine tools and mold designs become more complex, part program verification using CNC machine simulation becomes an essential tool for ensuring that the moldmaker’s NC programs machine the mold correctly the first time. Skipping the verification step creates costly production delays, which cause the machine tool to wait idle while the NC program errors are corrected.
To be truly effective, CNC machine simulation software must interactively simulate and display the material removal process of a NC program. NC programmers use software to verify the quality and accuracy of their NC programs while 3D simulation of the CNC machine checks for collisions. However, the goal of simulation is not simply a collision-free and efficient NC program. The first and most important goal is a NC program that produces the correct workpiece.
The right solution
Reducing the time required for moldmakers to easily develop, analyze, inspect and document the CNC programming process is an additional simulation software objective. Moldmakers should look for software that creates an in-process model that can help them determine whether or not the NC program will make a correct mold. For example, many NC programs use circular interpolation. In order to measure the cylinder as an as-machined feature, the software must emulate the circle motion. Most internal simulations—the simulation included with most CAM packages—do not emulate circle motion, but instead divide the circle motion into a series of linear motions approximating the cylinder. These segments are not measurable as a cylinder.
Simulation software development is often driven by the evolutionary changes in manufacturing technology: new CAD/CAM software features; new machines and tooling; new machining techniques and processes; and a moldmakers need to implement and improve on these changes.
Trends and challenges
More and more moldmakers need to simulate specialized processes and complex machines. When a specialized process reduces production time or increases reliability, it becomes adopted by more companies. If a software developer supports simulating these special processes early, it instantly supports the next customers who adopt them. For example, years ago it was rare to see a NC program utilizing local part coordinate transformations and tool axis vector programming. Now it is fairly common. By supporting these features when they were initially adopted, others have benefited.
Adoption of complex machines is similar. Few years ago, 5-axis machines were rarely used by moldmakers. Today, more multi-axis milling machining centers are being used for moldmaking that was previously done using simple 3-axis milling machines. This trend has been fueled by a significant decrease in the price of multi-axis machining centers over the past several years. Even small and mid-sized mold shops that previously would not have considered buying a 5-axis milling machine, now have to learn how to setup and program these machines; accurate 5-axis NC program verification and machine simulation becomes a mandatory tool.
While moldmakers benefit from significantly fewer required setups, the chance for collisions among machine components, tools and parts is very high. Complex machine tools typically involve many moving parts—often moving simultaneously, in a small space, at very high speeds, very close to an expensive workpiece. Additionally, the machine parts themselves are expensive and sometimes have a long lead time if they need to be replaced owing to a collision. Wrecking a high-speed spindle can ruin more than just a moldmakers day, but the effort required to manually validate a NC program that drives a highly complex machine is impractical. Significant software enhancements are necessary to support these complex machines during their early introduction into the industry.
Simulation saves the day
No one wants to spend time simulating a machining process. However, simulation is a necessary engineering step because of today’s complex and fast-paced manufacturing environment. The faster the simulation can return results, the better. The overall simulation time is an important consideration in every software purchase. Simulation software must be constantly evaluated, and the software developer must regularly invent and implement new algorithms to improve speed.
Additionally, no one wants to spend time setting up a simulation session. To further simplify setting up a session, tool libraries should be created by importing CAD solid models of inserts and holders. This makes it simple to specify which parts of the CAD model file correspond with which holders, cutters or inserts.
Developers have spent thousands of hours optimizing simulation methods, thereby creating the fastest, most efficient motion simulation to date. As a result, the moldmaker can spend less time setting up and running simulations, and more time making molds.
With advances in cutting tools, tool materials and CAM software, it is increasingly critical to use the right feed rate for each and every cut. However, many CAM systems use machining strategies and cut patterns that are not efficient for common operations such as open boundaries, roughing cuts on complex shapes, complex pocketing or planar roughing.
Some software packages can detect and machine excess material, un-machined areas and surface blend areas, but for this useful automation, efficiency is usually sacrificed. Therefore, a lot of time is wasted cutting air and feeding slowly across the part’s surface. Additionally, the tool path may plunge the cutter into material at an incorrect or inefficient angle. Poor feed rates contribute to excessive cycle times, bad workpiece finishes, increased cutter wear and broken tools.
Optimization at the design stage
Software that enables programmers to automatically optimize NC toolpath programs is essential. This software can read the NC program file (G-code program or direct CAM system output) and automatically divide the tool motion into a number of smaller segments (determined by user-defined settings in the software). The software will then assign the best feed rate based on the amount of material removed in each segment.
Light cuts at extremely high feed rates and spindle speeds are common in today’s high-speed machining centers. Under such conditions, the manner in which each cutter tooth contacts the material is critical. Too low a feed rate produces chatter, vibration and work hardening, leading to poor surface finishes and premature cutter failure. Too high a feed rate causes excessive cutter loads and unsafe conditions that can lead to catastrophic failure of the tool, spindle, fixture and machine. Feed rate optimization solves the problem by ensuring that all cutting operations maintain a constant chip thickness. This technique works especially well in high-speed finishing operations.
Manufacturing software has made tasks achievable that were practically impossible or extremely difficult and time-consuming a decade ago. These software innovations change the way NC machining is performed while saving time, money and resources, and each year, advancements in the technology make optimization software an increasingly powerful and intuitive productivity tool.
This article was first published by MM International.
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