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Reducing Cost and Material Usage for Large Parts
This one-day workshop applies DFM principles to large, heavy,
or complex parts for major reduction in cost and material usage. These could be
retrofitted onto current products or used as a basis for new designs.
The workshop will show how to develop backward-compatible substitutes that will
replace expensive weldments, casting, or unnecessarily heavy machined parts with
more steel-efficient parts or assemblies of CNC-machined parts that are
accurately assembled by various DFM techniques.
Cost Reduction for Large Machined Parts
For fabricated parts now made of excessively thick steel, the workshop group would start by
identifying the loads and load paths. Then we conceive of structural shapes that
match the load, which is will probably not be a constant cross-section slab that has a
lot of understressed material that leads to unnecessary cost and weight. Then we
brainstorm on optimal assemblies of CNC-machined parts whose cross-sections are
matched to the load, thus approaching constant-stress parts, which have the
highest strength per weight. This would be accurately and rigidly assembled by
various DFM techniques in which the precision is entirely determined
automatically by the CNC
machines.
Cost Reduction for Welded Assemblies
The most applicable large welded asssemblies should be analyzed for opportunities
at the conceptual level with the goal of avoiding the following costs:
high steel usage and cost at a time when steel prices are rising and will
continue to rise as raw material and as will transportation costs both for
incoming materials and outgoing products.
high-skill labor cost to weld plus other labor to position, fixture,
straighten warpage, and grind.
the cost and delays for annealing the weldments
or the risk of fractures from
residual stresses.
the imprecise and labor intensive practice of
mounting parts in slots or
large holes and then aligning them manually.
machining large parts after welding, which may require large machine tools and
furnaces to anneal them, which:
are very expensive to buy or have high hourly charges to outsource
usually involve labor-intensive on-line setups which adds more
time to expensive machine charges and delays the flow
may involve transportation and queuing delays.
loss of strength in the heat affected zone from welding and annealing, thus
requiring more steel compared to steel used at its full cold-rolled strength.
The Strategy
The strategy would be to
commercialize
proven parts with backwards
compatible replacements with the same functionality and strength (possibly
enhanced) with much less total cost. This would provide cost reduction now on
existing products. This would also encourage a leap-frog strategy where
these low-cost parts could then become the basis for new generation products.
The specific strategy to eliminate the abovementioned costs would be to create
an optimized concept/architecture for constant-stress trusses and
structures (which, by definition, use the least material) with the following
steps.
The Approach
The approach would be based on the following premises:
Fabrication. All machined parts would be small enough to be set up and made
quickly on readily-available CNC machine tools in a single setup (Guideline P14 in the
DFM book, which states that all operations should be
done in one setup on versatile CNC machine tools).
Welded parts would be limited to those that are small enough to be annealed and
machined after welding by the typical in-house machine tools and furnaces. This
may be appropriate for bearing blocks and other junction parts if not feasible
to machine from a single block.
Assembly. Precise alignment of these assembled pieces would be assured by DFM
principles, such as DFM Guideline A3, in which mating parts would be aligned to
sub-mill tolerances by inexpensive pairs of round diamond down pins in reamed
holes. Parts could be designed to be self-fixturing or simple fixtures could be
concurrently engineered to hold parts during alignment and clamp together for
fastening.
Aligned parts would then be bolted/riveted together with appropriate bolt
strength, torque settings, and retention strategies.
The above techniques
could replace many hard-to-make large parts and improve multi-part assemblies.
Steps for Reducing Cost on Complex Large Parts
and Assemblies:
Identifying existing loads, directions, and attachment points, which would
then be graphically represented. In a workshop setting, these could be printed
on several sheets of large paper with dark lines for the next step.
Brainstorming on various ways to support these loads, with many ideas sketched
on many printouts.
Then optimize design of these parts for manufacturability and
currently engineered manufacturing
strategies for trusses or space frames:
Struts. Purely tension members could be made of rods; compression struts must be
wide and axisymmetric with the load path to resist buckling. This favors tubing
with threaded holes at the end to bolt to the node blocks. Very thick wall
tubing could be plugged with threaded disks that could be joined to the tubing
with a low-heat axisymmetric weld. For smaller wall thickness, a clean
inexpensive strut could be made by swaging down the ends to just past the tap
diameter for tapping threads that could then be bolted to the node blocks.
Node Blocks. Each node block would be designed and dimensioned so that all
operations for node attachments would be made in one setup (Guideline P14) on a
versatile CNC machine tool. Families of constituent parts would be machined on flexible fixtures that would
be able to make all parts in the family without setup delays and cost.
The Results
The results would be much lower cost from:
quick machining on readily-available CNC machine tools
quick setup (maybe autofeed) concurrently engineered for whole part families
to further reduce machine time
quick assembly with accuracy assured by
automatically machined features
higher strength per weight (meaning higher strength per material cost) because
of:
more structurally efficient designs (lower stresses to support a given
applied load)
all material would remain at cold-rolled strength and heat-treated strengths
could be preserved
Workshop Format
The group would explore some of the most promising opportunities in the workshop
to the point where they look feasible and it is clear how to proceed at which
point responsibility could be assigned to pursue each opportunity.
The group would also identify future opportunities to be explored later based on
pre-workshop research that will have identified some opportunities.
Opportunities will be summarized and then the workshop group will vote on them
for a baseline prioritization of opportunities.
Pre-workshop research would also plot steel cost from the time current products
were designed and extrapolate price trends into the future.
Audience. Product development team with all designated and potential members,
with at least one person representing each function and one person knowledgeable
about each proposed candidate structure. The workshop would benefit from close
proximity to the physical structures being analyzed.
Alternative: Consulting Studies
An alternative would be ask Dr. Anderson to do the above as a
design study
on a consulting basis, which would present promising approaches for the
company to evaluate, select, and implement. Dr. Anderson is particularly
effective for complex parts that could benefit from very manufacturable design
concepts and concurrently engineered low-cost processing.
To discuss this further, contact:
Dr. David M. Anderson, P.E.; CMC; Fellow, ASME
HalfCostProducts.com
www.design4manufacturability.com
www.build-to-order-consulting.com
1-805-924-0100;
anderson@build-to-order-consulting.com
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