Closing the Gap in Flexible Machining
John Lenz and John Lenz III
This statement by David R. Brousell "Despite the fact that these ideas have been around
for some time and, at least in case of the digital factory, have been expressed in different
terms previously, big gaps exist between the vision of what they can do and what
manufacturers are actually accomplishing with them today" is appropriate for the state of
flexible machining. This difference between vision and reality (referred to as Gap
Theory) is prevalent in most flexible machining projects.
We have spent the past two years helping several manufacturers close this gap in their
flexible manufacturing projects. As we discover problems, we develop solutions in this
iterative method of problem solving. Following are the observations from these
experiences. This paper is intended to explain why gaps exist in flexible manufacturing
and what to do to close them.
Following is a general definition of flexible machining and what benefits it offers to
manufacturing.
Definition of Flexible Machining
Flexibility in a machine process provides for multiple paths. A path is a combination of a
part number, pallet number, fixture number, and machine number. For example, a four
(4) machine cell with 20 pallets and 15 part numbers has a maximum of (4 x 20 x 15) or
400 paths. This would be the number of paths if any part number could use any
fixture/pallets and be machined at any one of the machines. However, most systems have
fixtures/pallets that are dedicated to only a few of the part numbers. In this case,
assuming each part number is dedicated to one fixture/pallet the number of paths is (4 x
20) or 60.
This is the vision and most assume that flexibility offers a minimum of 60 paths with a
potential of 400 paths. With this many optional ways to machine a part, there should be
no reason why the machines are not fully burdened when available. In actuality, the
flexible machining system usually has less than 25 available paths. The reasons (or
roadblocks) for the difference between vision of 100 paths and reality of 20 paths are
described below.
Benefits of Flexible Machining
Before getting excited about the number of potential paths and actual paths, we should
address the question of why is flexibility important to machining. The reason flexibility
is important to machining is best described using Little's Law that states Production is
equal to Inventory divided by Flow Time (P=I/F). This law states that a high level of
inventory will yield a high level of production. So as inventory is reduced through the
implementation of Lean manufacturing techniques, this creates a downward pull on
production. For production to remain constant when inventory is reduced, flow time
must decrease at the same percentage rate as inventory. Flexibility through its offering of
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alternative paths, allows flow time to decrease as inventory or competition for paths
decreases. With lower levels of inventory, the process balance due to product mix and
movements of bottleneck machines has much greater impact on production. Alternative
paths reduce the sensitivity among product mix, process balance, and bottlenecks. Flow
time is more predictable with alternative paths and production is positively impacted.
Flexibility is the substitute for inventory. It is the flexibility (or multiple paths) that allow
for high machine utilization in the Lean - Just-in-Time - low inventory operation.
Roadblocks
Many companies have implemented machine flexibility as part of their Lean
transformation. However most have not been able to effectively take advantage of its
benefits. Most flexible machine systems operate at 50% of capacity. Here are the
reasons why.
#1. Quality control plan requires one job for each path
Example: A four (4) machine cell has 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. This method of having one job for each path satisfies quality
plan of root cause detection. The burden is now placed on scheduling to activate jobs
according to part demand and machine availability. This limitation of one job for each
path costs the operation about 20% of the machine capacity.
Solution:
Tracking of paths - Maintain a log to record of where each part was processed.
One logged record for each load, unload, machine. Track parts through multiple
operations even when changing fixtures.
Serial numbering all parts - The unique serial number is the best method to link a
part to its recorded process events. This serial number also provides a link across
multiple operations.
Root cause using paths - The process log data contains the recorded path of each
part and its operations. When a quality issue is detected, this log will provide the list of
serial numbers and parts that used the same path since the last accepted audit.
Types of inspection - Each inspection type defines a specific set of audit tasks to
perform on the part. These range from dimensional checks using manual gauges to full
CMM programs. Critical features need to be audited more frequently than non-critical
features. The audit of a part is defined through a collection of inspection types.
Inspection signal and frequency - Each path in the machine system must be
tracked and counted for each inspection type. 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 percent. One
advantage of using a count is that when a quality issue arises, only a fixed number of
parts will need to be quarantined.
Adjust frequency to accumulated tool wear - Tool condition is the most
significant cause of quality issues. Once a tool is replaced, the first occurrence of each
path must be signaled for audit. Then the frequency can be decreased (fewer number of
signals) until the accumulated wear of the tool reaches some threshold level such as 75%
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of life. Then the frequency can be increased (higher number of signals) as the tool
reaches its life limit.
#2. Prove-out process is cut short by production
Example: During the sales, justification and acquisition phase of most flexible
machine 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 will
take. For the machine cell with 4 machines and 15 part numbers, this will yield more
than 60 paths. Each of these paths must be qualified one at a time and requires from 2-4
days each. To quality 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. Not having the correct set of qualified paths costs the operation
about 10% of machine capacity.
Solution:
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
machine 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 - Pallets offsets are the adjustments in X, Y, Z, and B
coordinates to bring the pallet into 0(zero) reference to the machine spindle. A
referencing pallet-fixture is used to 0 reference each machine. Then each pallet is loaded
on a machine to determine its deviations in all coordinates and 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.
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. Long term 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. The four types
of allocation are: Pallet/Fixture Allocation, Machine Allocation, Part Mix Allocation, and
Process Sourcing Allocation.
Use of Pre-set Tool Offsets - 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 id is then assigned to a tool pocket when it is loaded into a
machine tool magazine. With this tool is its offset so part programs are independent of
tool sizes. Any tool can then be loaded into any machine and its offsets are loaded
accordingly.
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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. The benefits of probing are that 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. In order to maximize the flexibility of the
machine requires tool planning and strategic placement of tool in the magazine. One
standard method of tool organization is to require that all magazine positions from 1 to 60
contain the exact same set of tools for each machine. This set of common tools and
configuration provides a base line. 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) need to be setup due to tool magazine capacity, the setup
process involves replacing one tool group with another in the appropriate machines.
#3. Use of work orders and batch type scheduling
Example: 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 is observed with an imbalance of work load across
the flexible machines. It is common to observe a 4 machine cell with only 3 machines
operating. The reason the 4th machine is idle is due to not having any work orders. It is
also common to observe overtime operation running only 1 or 2 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. The batch type of
scheduling costs about 10% of machine capacity.
Solution:
Active pallets - Active pallet allocation is the process of limiting a subset of the
pallets that have production demand to route 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 workload
hour pallet to cycle less frequently at the beginning of the schedule period and result 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
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. This process of
determining the number of active pallets and 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. There are four methods for allocation of paths:
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Pallet/Fixture, Machine, Part Mix, and Process Source. Pallet/Fixture allocation
determines the number of pallets assigned to a part number. For example, 3
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 machine system.
#4. Operator control of pallets
Example: Operators are located at the load stations to unload and load parts into
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 machine system. This specific sequence is limited to
what the operator can manage, understand and influence wages. This preference of
limiting work in or out of the system costs about 10% of machine capacity.
Solution:
Data flow requirements are essential to reduce operator influence - The primary
role of the operator in flexible machine operation is to load parts into pallets. This
assumes or implies that the data needed to operate is available in the control computers.
Most flexible machine projects under estimate the data flow needs and these needs are
just as important as the physical flow (part handling) needs. When the data flow needs
are deficient in the control computers, the operator is required to fill in this need.
Gradually, the operator role in the flexible operation expands from part load to supplying
the data needed into the control computers. The best way to reduce this expanded role of
the operator 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 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 tendency to
manually control portions of the flexible machines cause interruption in the operation.
Robotic equipment and hydraulic fixtures can provide pallet loading process. However,
this equipment will only remove the need for the operator when the data flow
requirements are automatic as well.
Summary
These four roadblocks provide a root cause for the gap between vision of flexible
machining and its application. From our experiences, once these roadblocks are
removed, utilization of flexible capacity exceeds 90%. Flexibility becomes a real
substitute for inventory and provides benefits to manufacturing.
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