Closing the Gap in Flexible Machining
How to make the reality match the vision
By John Lenz
CMS Research Company
Closing the gap between the promise of flexible machining systems and the real output they deliver involves removing roadblocks that prevent the higher utilization of system capacity.
There are four roadblocks that interfere with the successful implementation of flexible machining systems. Once they are removed, flexible machining can become an effective source of Lean Manufacturing, raising capacity utilization to more than 90%, compared with the 50% most common today.
Here's a point-by-point approach to attacking the most common roadblocks to efficient performance. Let's begin by defining the concept.
Flexible machining provides multiple paths throughout the manufacturing process. A path is a combination of a part number, pallet number, fixture number, and machine number. For example, a four-machine cell with 20 pallets and 15 part numbers has a maximum of 1200 paths (calculated by multiplying 4 x 20 x 15). This would be the number of paths if any part number could use any fixture/pallet and be machined at any one of the machines. With this much flexibility, management expects that machines should always be busy while producing daily part demands.
However, most systems have fixtures/pallets that are dedicated to only a single part number, and each pallet is dedicated to only one machine. In this case, the number of actual paths is reduced to 20 (20 x 1 x 1).
The vision that flexibility offers a potential of 1200 paths and the reality of something less, as little as 20 paths that are actually used in the operation, illustrates the problem that arises when you seek to implement flexible machining. The true number of available paths lies somewhere in between. Here are the ways that utilization rates can be increased by removing the four roadblocks to more flexible machining:
Roadblock: Quality control plan requires one job for each path.
Situation: Consider a four (4) machine cell with 20 pallets for 15 part numbers. Each job in the system is restricted to one part number on one pallet to one machine. This restrictive routing provides an easy method of tracing parts to specific pallets and machines that were used to process them. Having one job for each path satisfies the quality plan requirement for root cause detection. The burden is now placed on scheduling to activate jobs according to part demand and machine availability. The limitation of one job for each path costs the operation about 20% of the machine capacity:
Solution: Use flexible jobs for each part that lists multiple pallets and alternative machines.
To implement this solution, several operating characteristics must first be in place. These operating characteristics are listed below. Once these are in place, then it is possible to use flexible jobs for each part.
Tracking of paths: Maintain a data log to record where each part was processed. One event is logged for each load, unload, and machine, and parts are tracked through multiple operations even when changing fixtures and pallets. This data log of process events contains the recorded path of each part for all of its operations. When a quality issue is detected, this log will provide the list of parts that used the same path since the last accepted audit.
Serial numbering of all parts: The unique serial number is the best method to link a part to its data log of process events. The serial number can be attached to the part in several ways, among them attaching tags, scribing in the machine tool, or hand marking at the load station. In all cases, the data log must link the serial number to the data log for each part. This linking method can include bar code readers, part marking machines, or manual data entry. Once the serial numbers are linked to data logs, serial number queries can provide lists of parts by part number, by pallet, by machine, and any combination of paths.
Inspection signal and frequency: Each path in the machine system must be tracked and counted for each type of inspection. A type of inspection defines a specific set of audit tasks to perform on the part. These range from dimensional checks using manual gages to full CMM programs. Critical features need to be audited more frequently than noncritical features. When multiple inspection types apply to the same parts, each type must have its own count for each path. The frequency for audit (or signal) of each inspection type can be based on count or random percentage. One advantage of using a count is that when a quality issue arises only a fixed number of parts will need to be quarantined. The operator must be signaled when a part and its path has been triggered for audit.
Roadblock: Prove-out process is cut short by production.
Situation: During the sales, justification, and acquisition phases of most flexible machining projects, all parts and pallets are assumed to be routed to all machines. But no one relates this characteristic to the prove-out process and the amount of time this process will take. The machine cell with four (4) machines and 15 part numbers will yield more than 60 paths. Each of these paths must be qualified—one at a time—requiring from two to four days each. To qualify 60 paths, the prove-out time will be at least 120 days. Due to production demands, the installation time is cut short, and only an essential number of paths become qualified.
Solution: Determine the minimum number of paths and continually progress until these paths are qualified.
Each path in the flexible machining cell must be identified as: Not Needed, Not Ready, Prove Out, or Production Ready. Following are characteristics that are essential in the path qualification process.
Capacity planning is used to determine the number of paths needed: Determining the minimum number of paths needed to support a product-mix demand is one of the most important tasks in the management of flexible machining. Longterm demand (one month forecast) is entered into a computer model of the flexible capacity. Paths are added, subtracted, and shared as necessary in the computer model until the simulated output matches the long-term demand. This set of paths establishes the minimum number of paths. These must be proved out and available for the production process. Daily allocation uses a subset of these paths to meet daily or short-term demand.
Part programs must be independent of machines and pallets: Most part programs on stand-alone machines are customized for the fixture, pallet, tool, and machine. These customizations are necessary to meet tolerance specifications for the part. In a flexible machining process, the fixtures, pallets, and tools are routed to multiple machines, and the practice of customizing part programs is not allowed.
Use of pallet offsets and preset tool offsets: Pallet offsets are the adjustments in X, Y, Z, B coordinates to bring the pallet into zero (0) reference to the machine spindle. A referencing pallet-fixture is used to zero-reference each machine. Then each pallet is loaded on a machine to determine its deviations in all coordinates. These deviations become the pallet offsets. Each pallet has its own set of offsets for each machine. These offsets are then used in the machine control to reference the pallet, fixture, and machine to a home location. The part program is now independent of pallets, fixtures, tooling, and machines. Machines that share pallets and fixtures must also share tools. The best method to provide tool-sharing is to assign each tool with unique tool identification. This identification is then assigned to a tool pocket when the tool is loaded into a machine-tool magazine. With this tool and its offset, part programs are independent of tool sizes. Any tool can then be loaded into any machine, and its offsets are loaded accordingly.
Use of probing: Probing is the process of using a spindle probe to locate the fixture and reference it to the machine. When probing is used, some of the pallet offsets will not be needed. So the benefits of probing are that (some) pallet offsets are not needed, and a check can be performed that fixture-matches the part program that is selected. The downside is that probing takes machine hours, and programming is complex.
Tool planning and shared tool organization: The size of the tool magazine will always limit the flexibility of the machine. Maximizing the flexibility of the machine requires tool planning and strategic placement of tools in the magazine. One standard method of tool organization is to require that magazine positions from 1 to 60 contain the exact same set of tools for each machine. Establishing a set of common tools and a common configuration provides a baseline. The remaining tools should be organized into tool groups. A tool group is a set of special tools required for a product family. Allocation will determine the number of active paths for the combined demand of the product family. When the path(s) needs to be set up due to tool-magazine capacity, the setup process involves replacing one tool group with another in the appropriate machines.
Roadblock: Use of work orders and batch-type scheduling.
Situation: Flexible machining is designed to produce many different part numbers simultaneously. However, the work-order system is a batch process that requires large quantities for a few parts numbers at a time. Flexible machining and work-order scheduling are not compatible. This situation results in an imbalance of workload across the flexible machines. It is common to observe a four-machine cell with only three machines operating. The fourth machine is idle because it does not have any work orders. It is also common to observe overtime operation running only one or two of the machines to satisfy work-order demand. The work-order system is a batch-scheduling process, and with limited pallets and proven paths, work shortages are common. Batch-type scheduling costs about 10% of machine capacity.
Solution: Schedule using daily production demand, and machine every part every day.
Lean manufacturing requires that parts are machined to match customer demand. When a customer uses a part daily, it must be machined daily. Flexible machining provides this capability. Following are characteristics necessary to support daily demand scheduling.
Active pallet scheduling is the process of limiting a subset of the pallets that have production demand to be active at a time. In the beginning of a scheduling period, all pallets have some production demand at varying levels of hours. When all of these pallets are allowed to activate routing, the pallets with high workload hours compete directly with pallets with low workload hours. This causes the high workloadhour pallet to cycle less frequently at the beginning of the schedule period, and results in these pallets being strung out to complete the production demand. Active pallet allocation restricts the competition by limiting the number of activated pallets. This limitation allows the high-workload pallets to cycle more frequently, and pushes some machine hours for low-workload pallets to the end of the scheduling period. Determining the number of active pallets and deciding the timing of when to activate pallets requires use of a computer model.
Dynamic allocation of paths: Scheduling a flexible machine system is the allocation of capacity to demand. Capacity is defined through the number of qualified paths. Therefore, scheduling a flexible machining system becomes the dynamic allocation of paths. Paths can be added to increase production rates; paths can be removed to decrease production rates, and paths can share components for small production-rate adjustments.
Roadblock: Operator control of pallets.
Situation: Operators are located at the load stations to unload and load parts onto pallets. In most cases, their role is expanded to include job activation based on work-order demand, counting parts, determining inspection signals, and tracking parts between consecutive operations. With all of these tasks, they control the specific sequence of how parts enter and leave the flexible machining system. This specific sequence is limited to what the operator can manage and understand, and influences wages. This preference for allowing the operator to limit work in or out of the system costs about 10% of machine capacity.
Solution: Limit operators to load/ unload by automating data flows.
Most flexible machining systems underestimate the data-flow needs, which are just as important as the material flow needs. When the data flows are not automatic between orders, daily demand, shipments, factory computers, cell-control computers, machine-control computers, and material-handling computers, manual intervention is required to establish this link. The following are characteristics that automate data flow within flexible machining.
Data-flow requirements are essential to reduce operator influence: The primary role of the operator in flexible machining operation is to load parts into pallets. This assumes or implies that the data needed to operate is available in the control computers. When the data flow needs are deficient, the operator is required to compensate for the lack of information. Gradually, the operator's role in the flexible operation expands from part load to supplying the data needed by the control computers. The best way to reduce the operator's expanded role is to establish a data link between control computers and factory systems. This link requires management software that can import factory orders, allocate paths using a computer model, and roll out work instructions as the scheduling period progresses. Once this data flow is in place, the operator role returns to pallet-load tasks.
Automation of the part unload/load task: Operator attendance and the tendency to manually control portions of the flexible machines can interrupt operations. Robotic equipment and hydraulic fixtures can execute the pallet-loading process. However, this equipment will only remove the need for the operator when the data-flow requirements are automatically satisfied as well.
Four Ways To Allocate Paths
There are four methods for allocation of paths: Pallet/Fixture, Machine, Part Mix, and Process Source.
- Pallet/Fixture allocation determines the number of pallets assigned to a part number. For example, three pallets/fixtures are available to process a part number, and the pallet/fixture allocation determines when and if each pallet is activated for this part number.
- Machine allocation determines the specific subset of qualified machines for a part number and pallets. For example, three machines are qualified for a set of pallets and part numbers, and current demand only requires two machines to meet demand.
- Part Mix allocation determines the specific sequence of part release. Active pallet allocation will optimize part release among high and low workloads. Tool groups are also used in the part-mix allocation.
- Process Source allocation involves the capacity decision to add or subtract machine hours in the flexible machining system.
This article was first published in the March 2008 edition of Manufacturing Engineering magazine.