Millstar: Tech Articles
Horizontal Machining Of Molds
Machining a mold in the upright position departs from tradition, but the advantages are considerable–especially if a shop takes a systems approach.
Automated horizontal machining offers a practical solution for North American mold makers who find themselves trapped in a squeeze between pressures for shorter leadtimes, lower costs and higher quality. Technology advances make automated horizontal machining and "one machine does all" capability viable today for even the largest molds and hardest materials. Dedicated horizontal machining centers (HMCs), created specifically for automated mold processing, provide pallet capacities up to 130 by 70 inches and up to 40,000 pounds. Any mold produced by traditional vertical machining can now be machined on a horizontal machining center.
These new horizontal machining centers provide capabilities for automated machining, untended machining and high speed finishing.
They can reduce total mold processing time by 20 to 25 percent through:
- Performing rough and finish machining–as well as peripheral operation such as boring, tapping and gun drilling–all on one machine, often in one continuous operation;
- Eliminating much of the manual effort and wasted time inherent in traditional mold making–the multiple setups, changing of tools, hen cling and moving of parts from ma chine to machine and department to department, and the waits and delay in getting on the next machine;
- Machining to such high levels of accuracy and finish that electrical discharge machining (EDM) and benching operations can be greatly reduced; and
- Enabling greater automation, systemization and management control over the mold-making process.
Forces Of Change
North American mold makers have been slow to accept automation. Even worldwide, horizontal machining of molds to date has made significant market penetration only in Japan. However, global competition is bringing to North American mold makers the same market forces that drove the development and acceptance of horizontal machining in Japan. Industry trends to fast-track product design and development, shorter product life cycles, continuous improvement and vendor partnering all demand greater agility responsiveness and cooperation from manufacturers, suppliers and subcontractors.
Such pressures hit particularly hard on mold makers, who traditionally have required some of the longest leadtimes in manufacturing. Labor-intensive operations make it extremely difficult to reduce processing time further or raise productivity. Mold shop owners and managers are coming to the realization that when the process has been perfected to the degree possible, it may be time to re-think the process. More and more of them are turning to their machinery suppliers for a process solution instead of a specific kind of machine.
Driving Out Downtime
Adopting new machining methods, to realize the advantages of automation more fully, holds great productivity potential. The industry has traditionally relied on vertical machining centers for their load capacities in accommodating large, heavy mold bases, rigidity against high cutting forces, and easy workpiece loading of large mold bases. However, computer numerical control (CNC) capability and the productivity gains it provides have been limited mainly to tool path control. Many vertical CNC machines found in mold shops do not have automated tool change, so manual tool change drives up the labor burden and machine idle time.
Even when fitted with automated tool change, vertical machines require frequent operator attention, particularly for:
- Chip removal. Traditional machining techniques with the hardened steels used for molds tend to allow build-up of chips, particularly in cavity molds. Recutting of the hardened chips creates machining problems–accelerated tool wear, compromised surface quality and accuracy. The standard approach to these problems is to have the operator intervene to remove the chips manually. These frequent interruptions of the machining operation for chip removal can add significantly to machining cycle time and labor burden.
- Setup. Squaring and shimming a mold workpiece on a vertical machine can take as long as six to eight hours for a large mold. Meanwhile, the machining center sits idle for lost productivity. In fact, the lost productivity can be multiplied several times over in traditional mold processing. Rough machining, boring, tapping, gundrilling of cooling lines, and finish milling are typically done on different machines, each requiring separate setup and downtime.
Even finish machining tends to be done on fairly massive verticals with limited spindle speeds, slow servo response, and a significant amount of spindle growth/thermal instability. All limit machining precision. Slower speeds and feeds mean pick feed values and tolerance must be relatively broad to avoid still longer machining times and program files. This leaves excessive cusp height, requiring extensive EDM and handwork to achieve the desired finish. Part quality is dependent on costly, lengthy hand finishing, tryout and rework–typically 25 to 35 percent of total mold processing time for medium to large molds.
The high level of equipment/process specialization in traditional mold shops limits flexibility and efforts to improve scheduling and machine utilization. A walk through a typical mold shop will find a significant number of machines idle at any time and lots of workpieces either waiting to get on a machine or being transported to the next processing station.
The extent of the problem can be surprising–and revealing. One mold shop which recently did a flow chart of operations found that it can handle a mold up to 78 times from beginning to end. The problem afflicts shops of all sizes in trying to cut leadtimes and costs and optimize use of resources.
New Direction Needed
CNC horizontal machining has the potential to eliminate much of this machining downtime and lost productivity–with greatly reduced finishing time and handwork as a bonus.
Effective horizontal machining requires an entirely different approach to mold processing. Maximizing machining productivity–by minimizing both cut and non-cut time–requires a synergy of technologies: high speed spindles that do not compromise horsepower, fast cutting feeds, highly accurate and dynamic machine control, innovative tooling, creative fixturing, and automatic tool and pallet changes. The following examines how the various factors work synergistically to achieve this mold processing efficiency:
- Chip disposal. If asked the benefits of horizontal machining, most people would cite chip removal first. Vertical mounting of workpieces allows HMCs to use gravity, supplemented by high-pressure coolant (through-spindle and external), to rapidly remove chips from the cutting zone into a chip removal system. The operator does not have to stop the machine during the cycle to wipe away the chips. Superior chip removal promotes better surface finish, longer tool life and reduced thermal distortion. It is especially critical on deep cavities, a particular problem with vertical machines. Besides avoiding the chip recutting, HMCs can permit higher metal removal rates and shorter cycle times on deep cavities. Highly effective and reliable chip removal, combined with auto tool change and tool monitoring, makes possible untended and even lights-out machining and around-the-clock machine utilization.
- Automatic workchanger. The horizontal machining center can be fitted with a dual-pallet, automatic workchanger. This permits off-line setup of a workpiece while another part is running. When one workpiece is completed, pallets are exchanged to start the next one. A touch probe squares the new workpiece to the spindle and machining resumes. Total machine downtime for change-over is typically only minutes. Smaller molds can be fixtured on tombstone pallets for batch processing, as in production machining, to further optimize machine utilization and opportunities for untended machining.
- Automatic toolchanger. An automatic toolchanger and tool magazine allow continuous machining without operator intervention after startup. Tool monitoring systems sense tool wear and allow automatic replacement of worn tools to assure accurate surface geometry. Advanced control technology permits tool change even midstream in the machining of a critical feature. The control assures that the new tool resumes the cut at precisely the same depth to achieve a consistent, seamless surface geometry without the "steps" or "dips" typical of manual tool change. Automatic tool and pallet changing are essential to realizing process systemization, high machine utilization and higher margins. Heavy levels of operator attention and machine downtime result in typical vertical machining centers achieving only about 40 percent utilization–actual in-cut time out of total available shop hours. Automation capabilities let horizontal machining centers consistently deliver utilization rates of 80 percent or more.
- Multiface, single-setup machining. In effect, a standard horizontal machining center with a rotary table provides the capabilities of a specialized vertical machining center with a swivel head or articulating head. The workpiece can be repositioned throughout 360 degrees in a single setup to allow machining of multiple faces, optimize spindle orientation for the feature being machined, and permit four-axis contour machining. In many cases, the horizontal machining center provides advantages in accuracy over the specialized vertical machines. By having the part/pallet move, cutting forces are transmitted straight through the spindle into the column mass, rather than being absorbed by the moving spindle through its pivoting/articulating (often weakest) member. The horizontal machining center also provides greater spindle access to permit shorter, more rigid tools for slant machining. Rotating the workpiece allows peripheral machining–drilling, boring, tapping, gundrilling–on multiple sides of a mold for single-setup efficiencies. For example, rotating the part/pallet 90 degrees allows machining of features that would be in a horizontal plane relative to a vertical machining center, hence inaccessible. This ability to rotate the part and present a surface normal to the spindle eliminates separate operations and setups on specialized equipment. It also reduces downstream processing and rework, since each new setup will introduce more accumulation or "stack up" of error in the work. This is one cause of expensive rework with traditional mold processing.
- Thermal control. Mold shops may easily have $250,000 or more invested in computer-aided design and manufacturing systems to generate tool paths for complex surfaces, yet run those programs on machines unable to hold the tool path. Finish accuracy is compromised. Thermal change and spindle growth may cause tool-path deflection of 0.005 inch or more. Thermal stability can be achieved effectively on a horizontal spindle, particularly at higher rpm. Horizontal orientation permits advanced lubrication and sealing technologies to be used for spindle cooling. This helps assure that the tool is following the programmed toolpath for a high-precision finish. These technologies also enable horizontal machining centers for mold work to deliver up to 18,000 rpm with No. 50 taper spindles for "one-stop" processing from roughing to finishing.
- Agile roughing. The new horizontal machining centers take advantage of developments in cutter and control technology to provide comparable metal removal to large verticals in roughing operations, while using less horsepower (30-40 hp vs. 50-60 hp). In roughing operations, HMCs can use advanced positive-rake tools, at shallow depths of cut but at higher spindle speeds and feeds. This produces favorable metal removal rates, hut with lower cutting forces. The high horsepower, slow speeds and large tools traditionally used for rough machining of molds often impart stresses that need to be relieved, adding further delays, costs and steps to the process.
- High speed finishing. A combination of factors–broad power bands, high spindle speeds, dynamic response, and effective thermal and geometric control–allows HMCs to perform high speed, "high-definition" finish machining. This capability can significantly reduce the EDM component in finishing operations, while cutting benching time and costs. Advances in control technology enable high speed HMCs to achieve cutting precision at high feed rates through the 3D tool path changes required for complex mold geometries. Machine tool builders have responded with special software for this purpose. For example, Makino has developed a proprietary software package called Geometric Intelligence that predicts and compensates "on the fly" for axis reversal backlash, machine inertia, and other machine dynamics to hold high feed rates otherwise affected by droop, dwell gouging, undercutting, or overshooting. Because these operations involve data-intensive interpolations for rapid, three-axis changes to the tool path, features such as dual processors and high rates of data flow across a direct numerical control (DNC) network, or a hard-drive data server, must be considered.
All this permits extremely fine picks and high feed rates in machining molds to very high definition. Finishes and features can be achieved through hard machining that formerly were possible only through EDMing or benching to simplify and shorten the mold making process.
Mold makers are beginning to discover the benefits of high speed finishing. However, many high speed machining centers continue to follow the pattern of vertical configurations with smaller spindles and very low torque. As specialized designs, they continue the paradigm of separate machines for roughing and finishing.
HMCs that perform a full range of machining operations, often in one setup, mark a clear departure. They make available high torque at low rpm for roughing operations, then can shift to high rpm for finishing operations. These machines are typically characterized by direct coupling of spindle and motor and electronic gearing, to avoid power losses from shafts and gears, for fast startup and high spindle acceleration.
Challenging Perceptions
Some entrenched perceptions can inhibit consideration of horizontal machining centers. Many of these perceptions are no longer valid for the new, dedicated, mold machines. But other misperceptions may remain:
- Cost. Horizontal machining centers can cost up to twice as much as vertical units offering the same strokes/machining cube. On the flip side, one HMC may perform the work of two verticals, one for roughing and one for finishing. This applies both to function and to actual in-cut machining time (80 percent utilization for HMCs vs. 40 to 45 percent for vertical machining centers). Moreover, an HMC dedicated to mold work is likely to be equipped with an automated toolchanger, an integral fourth axis, and high speed machining capabilities, features which add to the cost of vertical machines and shrink the price differentials. In addition, an HMC may also perform the work of a boring mill and substantially reduce EDM requirements. On a capital basis in setting up a new shop, horizontal machining can be extremely competitive. In terms of labor costs, HMCs offer a high potential for savings on operator staffing, finishing handwork, and work movement through the shop. Properly supported, HMCs can provide around-the-clock productivity. Cost justifications can shift radically to HMCs' favor when calculated on that basis.
- Machine rigidity. Experienced machinists may have concerns that high cutting forces could cause deflection of the pallet. The dedicated HMCs, created specifically for mold machining treat the issue from two directions. First, different tooling and cutting strategies use high spindle speeds and feeds to achieve comparable metal removal with less power and reduced cutting forces. Second, certain structural design resist deflection. For example: large-diameter bearing surface equal to width of index table provides full bearing support to eliminate overhang and resist tipping forces; mechanical brake prevents table rotation under cutting forces; massive angular buttressing of fixtures transmits forces into table, resisting deflection in vertical plane; four-point, at-the-corners pallet clamping in place of conventional center clamping, prevents tipping or canting; massive base and column of heavily ribbed, cast iron construction; and large, hardened and ground ways provide broad, rigid bearing support against deflection forces.
- Setup. Mold makers initially expect that horizontal machining center will be more difficult to set up with large mold workpieces. Once convinced that the machine structure and fixtures will securely support the workpiece, they find that horizontal setup has ergonomic advantages.Workpieces can be easier to maneuver when transported in vertical position. Two eye bolts are required, in stead of four. There is less overhead interference to contend with. Operators can work upright and use the full body, especially leg muscles, to apply leverage to positioning and mounting the workpiece. Fixturing methods permit adjustment screws on the back of the fixture for squaring the workpiece in the Y axis after mounting. Squaring in the X axis can be handled automatically through probing and table indexing.
Commitment To Change
Horizontal machining of molds is so much a departure from tradition–in technology and methods–that most remain skeptical until they see the equipment at work and witness the results. The key is recognizing that horizontal mold machining requires a reorientation of mold processing strategies, not merely a change of spindle orientation.
Automated horizontal machining holds solutions to shorter leadtimes, higher machine utilization and margins, better control over costs, reduced dependence on handwork and craft skills, and more effective production management. However, effective, profitable operation demands a systems approach. The horizontal machining center must be supported by CAD/CAM tool path and program generation, high speed data transfer with DNC or a data server, fixture design, automated tool and pallet change, and tool monitoring. The investments involved require courage and commitment to change, but are justified by the rewards. MMS
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