The five basic steps for fixture design By Ray Okolischan, Vice President, Carr Lane Manufacturing Co ucce cess ssfu full Suc
fixt fi xtur ure e
desi de sign gns s
begi be gin n wi with th a
logical and systematic logical systematic plan. With a comple complete te analysis of the fixture's functional requi req uirem remen ents ts,, ve very ry fe few w des desig ign n pr prob oblem lems s occu oc curr. Wh When en th they ey do do,, ch chan ance ces s ar are e so som me desi de sign gn re requ quir irem emen ents ts we were re fo forg rgot otte ten n or underestimated.
available machine tools may affect the extent of planning needed. Preliminary analysis may take from a few hours up to several days for more complicated fixture designs. Fixture Fix ture des design ign is a fiv five-st e-step ep pro problem blem-sol -solvin ving g process. The following is a detailed analysis of each step.
The workpiece, processing, tooling, and
Step 1: Define Requirements To initiate the fixture-design process, clearly state the problem to be solved or needs to be met. State these requirements as broadly as possible, but specifically enough to define the scope of the design project. The
designer
should
a sk
some
basic
accuracy accura cy,, fas faster ter cyc cycle le tim times, es, or bot both? h? Is the tooling intended for one part or an entire family of parts? The tooling designer must determine how much freedom and input there is on each project. If many
choi ch oice ces s
rega re gard rdiing
mac achi hine ne
questio ques tions: ns: Is the new too toolin ling g requ require ired d for
operations, and cutting tools have already been
first-time first -time production or to impro improve ve existi existing ng
made, the designer's role will have a relati relatively vely
production? If improving an existing job, is
narrow focus.
the goal greater
Step 2: Gather/Analyze Information Collect
all relevant data and assemble it
for ev for eva alu luat atio ion. n. The main so sou urc rces es of info in form rmat atio ion n ar are e th the e pa part rt pr prin int, t, pro proces cess s sheets, and machi machine ne specifi specification cations. s. Make sure that part documents and records are current. For example, verify that the shop prin pr intt is th the e cu curr rren entt re rev vis isiion on,, and th the e proc pr oces essi sing ng in info form rmat atio ion n is up up-t -too-da date te.. Chec Ch eck k wi with th th the e de desi sign gn de depa part rtme ment nt fo forr pending part revisions. An important part of the evaluation process is not note e tak taking ing.. Com Comple plete, te, acc accura urate te not notes es all llo ow
tool to ols, s,
desi de sign gner ers s
to
reco re cord rd
impo im port rta ant
information. With these notes, they should
ope pera rati tion ons s re requ quiire red d to make th the e pa part rt,, numberr of opera numbe operations tions performed, sequence of opera operations, tions, inspection requirements, requirements, and time restrictions. Avail Av ailab abili ility ty
of
equip eq uipme ment nt
requ re quir ired ed
to
machine, assemble, and inspect a part often determ det ermines ines whe whethe therr the fix fixture ture is des design igned ed for a single part or multiple parts. A process engineer sometimes selects the equipment to machin ma chine e par parts ts bef before ore the too tooling ling desi designe gnerr begi be gins ns th the e
desi de sign gn..
The Th e
tool to olin ing g
desi de sign gner er
should verify what equipment will be used for each operation.
be able to fill in all items on the " Checklist
A ver vertic tical al mi milli lling ng ma machin chine, e, for exa examp mple, le, is
forr fo
idea eas, s,
well suited for some drilling operations. But
thoughts, observations, and any other data
for operations that require a drill jig, a drill
about the part or fixture are then available
press is the most cost-effective machine tool.
for later reference. It is always better to
Typi ypica call lly y,
equip equ ipme ment nt
have ha ve to too o ma many ny id idea eas s ab abou outt a pa part rtic icul ular ar
followi wing ng
factors:
design than too few.
machines, inspection equipment, scheduling,
Design Des ign
Cons Co nsid ider erat atio ions ns.."
All
crite cri teri ria a types
inclu in clude de
an d
sizes
the th e of
Four categories of design considerations
cutting tools, and plant facilities.
need to be taken into account at this time: operation
Personnel considerations deal with the end
variables, availability of equipment, and
user, or operator, of the equipment. Fixture
personnel.
These
designers
separately
covered
workpiece
specifications,
categories,
while
should put
themselves
in the
actually
machine operator's shoes and consider all
interdependent. Each is an integral part of
the operational scenarios they can. Designers
the
be
should consider not only correct usage of the
thoroughly thought out before beginning
fixture, but also possible incorrect usage.
the fixture design.
They must ask, "Is there any way for me to
evaluation
here,
phase
are
and
must
hurt myself while operating this equipment?" Workpiece specifications usually are the most
important
largest
influence
factors on the
and
have
fixture's
the final
Additional factors usually considered in this category
are operator
fatigue, efficiency,
considerations
economy of motion, and the speed of the
include the size and shape of the part, the
operation. The designer also must know and
accuracy required, the properties of the
understand the general aspects of design
part material, the locating and clamping
safety and all appropriate government and
surfaces, and the size of the run.
company safety rules and codes
design.
Typically,
these
Operation variables include the type of
Step 3: Develop Several Options This
phase of the fixture-design process
requires the most creativity. A typical workpiece can be located and clamped several different ways. The natural tendency is to think of one solution, then develop and refine it while blocking out other, perhaps better solutions. A designer should brainstorm for several good tooling alternatives, not just choose one path right away. During this phase, the designer's goal should be adding options, not discarding them.
In
the
interest
of
economy,
alternative designs should be developed only far enough to make sure they are feasible and to do a cost estimate.
three options: permanent, modular, and workholding.
Similarly, there are countless ways to clamp a part, using a wide array of standard clamping devices. For example, a workpiece can be clamped from the top, or by gripping its outside edge or an internal surface. For preliminary drawings of the fixture, use several colored pencils. Often black is used to sketch the fixture, red for the part, and blue for the machine tool. Use isometric graph paper to keep the sketch proportional. The exact procedure used to construct the
The designer usually starts with at least general-purpose
internal features (such as holes and slots). The choice of standard locating devices is quite extensive.
Each
of
these options has many clamping and locating options of its own. The more
preliminary
design
sketches
is
not
as
important as the items sketched. Generally, the preliminary sketch should start should start with the part to be fixtured. The required locating and supporting elements, including a base, should be the next items
standard locating and clamping devices
added. Then sketch the clamping devices.
that a designer is familiar with, the more
Finally, add the machine tool and cutting
creative he can be.
tools. Sketching these items together helps identify any problem areas in the design of
Areas for
locating a
part include
flat
exterior
surfaces
(machined
and
unmachined),
cylindrical
and
the complete fixture.
curved
exterior surfaces, and
Step 4: Choose the Best Option The fourth phase of the tool-design process is a cost/benefit analysis of different tooling options. Some benefits, such as greater operator comfort and safety, are difficult to express in dollars but are still important. Other factors, such as tooling durability, are difficult to estimate.
In our example, run costs for the permanent and modular fixtures are the same, while power workholding lowers costs by improving cycle time and reducing scrap. Modular fixture: $4.50 Permanent fixture: $4.50 Permanent hydraulic fixture: $3.50
In analyzing fixture costs, the emphasis is on comparing one method to another, rather than
Setup Cost. This is the cost to retrieve a
finding exact costs. Estimates are acceptable.
fixture, set it up on the machine, and return it
Sometimes
to storage after use. The permanent fixture is
these
methods
compare
both
proposed and existing fixtures, so that, where
fastest to set up, the power workholding
possible, actual production data can be used
fixture is slightly slower due to hydraulic
instead of estimates.
connections, and the modular fixture is slowest due to the assembly required.
To
evaluate
the
cost
of
any
workholding
alternative, first estimate the initial cost of the
Modular fixture: $240
fixture.
Permanent fixture: $80
To
make
this
estimate,
draw
an
accurate sketch of the fixture. Number and list each
part
and
component
of
the
Permanent hydraulic fixture: $100
fixture
individually. Here it is important to have an
Lot Size. This is the average quantity
orderly method for outlining this information.
manufactured each time the fixture is set up. In this example, lot size is 100 for all three
For modular fixtures, total component cost
options.
should be amortized over the system's typical lifetime. Although somewhat arbitrary, dividing
Tooling Cost. This is the total cost of labor
total component cost by 100 (10 uses per year,
plus material to design and build a fixture. The
for 10 years) gives a fair estimate.
modular fixture is least expensive because components can be re-used.
The next step is calculating the cost of material and labor for each tooling element. Once again
Modular fixture: $341
it is important to have an orderly system for
Permanent fixture: $1632
listing the data. First list the cost of each
Permanent hydraulic fixture: $3350
component, then itemize the operations needed to
mount,
machine,
and
assemble
that
Total Quantity Over Tooling Lifetime. This
component.
Once
those
steps
are
listed,
quantity is the lesser of 1) total anticipated
estimate the time required for each operation
production quantity and 2) the quantity that
for each component, then multiply by the labor
can be produced before the fixture wears out.
rate. This amount should then be added to the
The following results are obtained by
cost of the components and of the design to
evaluating the cost-per-part formula at
find the estimated cost of the fixture.
different lifetime quantities.
The total cost to manufacture a part is the sum
For a one-time run of 100 pieces, the modular
of per-piece run cost, setup cost, and tooling
fixture is clearly the most economical choice. If
cost. Expressed as a formula:
10 runs (1000 pieces) are expected, the permanent fixture is best. For 2500 pieces and above, the power workholding fixture would be the best choice. This analysis assumes that all noneconomic factors are equal.
These
variables
are
described
below
with
Pieces Modular
Permanent
Permanent Hydraulic $38.00 7.85 5.84 5.17 4.84
sample values from three tooling options: a modular fixture, a permanent fixture, and a hydraulically powered permanent fixture. Run Cost. This is the variable cost per piece to
100 1000 2500 5000 10,000
$10.31 7.24 7.04 6.97 6.93
$21.62 6.93 5.95 5.65 5.46
produce a part, at shop labor rate (material cost does not need to be included as long as it is the same for all fixturing options).
Step 5: Implement the Design The
final
phase
of the
fixture-design
process consists of turning the chosen design approach into reality. Final details are decided, final drawings are made, and the tooling is built and tested. The
following
guidelines
should
be
misunderstandings between the designer and the machine operator. Whenever possible, a fixture's function and operation should be obvious to the operator without instructions. Once sketches and the basic fixture design have
been
completed,
final
engineering
considered during the final-design process
drawings, also called shop prints, are used in
to make
the toolroom to build the fixture.
the fixture
less costly while
improving its efficiency. These rules are a mix
of
practical
considerations,
sound
design practices, and common sense. Use
standard
time is by simplifying the drawing. Words or symbols should be used in place of drawn
The
components.
The easiest way to reduce manual drawing
details where practical. All extra or
economies of standardized parts apply to
unnecessary views, projections, and details
tooling
should be eliminated from the drawing.
components
as
well
as
to
manufactured products. Standard, readily available
components
include
clamps,
locators, supports, studs, nuts, pins, and a host of other elements. Most designers would never think of having
Drawing a complete clamp assembly, for example, adds very little to the total design. Simply showing the nose of the clamp, drawn in its proper relation to the workpiece and labeled with its part number, conveys the
the shop make cap screws, bolts, or nuts
same information in a fraction of the time.
for a fixture. Likewise, no standard tooling components should be made in-house. The
For drawings that require more detail, use
first rule of economic design is: Never build
tracing templates to reduce drawing time.
any component you can buy. Commercially
These
available
are
components in several views. If necessary,
manufactured in large quantities for much
they may be enlarged or reduced on a copier
greater economy. In most cases, the cost
to any scale needed for a drawing.
tooling
components
templates
show
most
standard
of buying a component is less than 20% of Once the proper tracing template is selected,
the cost of making it.
simply slip it under the drawing sheet and Labor is usually the greatest cost element
align it with the drawing. When the template
in the building of any fixture. Standard
is properly positioned, tape it down and trace
tooling components are one way to cut
the component on the drawing sheet. Tracing
labor costs. Browse through catalogs and
templates save drawing time and improve
magazines
the quality of the drawing.
to
find
new
products
and
application ideas to make designs simpler Computers are
and less expensive.
rapidly replacing
drawing
boards as the preferred tool for preparing Use prefinished materials. Prefinished
engineering drawings. Almost every area of
and preformed materials should be used
design
where possible to lower costs and simplify
Computers, from large mainframes to micros,
construction.
are becoming standard equipment in many
These
precision-ground
flat
materials stock,
include
drill
rod,
is
affected
by
the
computer.
design departments.
structural sections, cast tooling sections, precast tooling bodies, tooling plates, and
A standard tooling library often is used to
other
materials.
add the fixturing components and elements
Including these materials in a design both
to the drawing. Using a standard library in
reduces the design time and lowers the
designing the fixture dramatically reduces
labor cost.
drawing time. All components are drawn to
standard
preformed
full scale in a variety of views. Scaling down Eliminate finishing operations. Finishing
is best done in the final drawing, not when
operations should never be performed for
storing
cosmetic purposes. Making a fixture look
Storing a large fixture base at 1/4 scale does
better often can double its cost. Here are a
little good, because all components will have
few suggestions to keep in mind with
to be 1/4 scale to fit on it. For ease of use, all
regard to finishing operations.
components should be stored at full scale.
standard-component
drawings.
Each component can be called up from the •
Machine only the areas important to
library and placed on the drawing where it is
the function and operation of the
required.
component. For example, do not
•
•
machine the edges of a baseplate.
A CAD system also can be useful during the
Just remove the burrs.
initial phase of the workholder design as
Harden only those areas of the
numerous tooling options are
fixture subject to wear.
CAD is sometimes faster than sketching by
Grind only the areas of the fixture
hand, especially when detailed cost estimates
where necessary for operation.
developed.
Keep tolerances as liberal as possible.
are required.
The most cost-effective tooling tolerance for a locator is approximately 30% to 50%
Once
of
Tighter
checked, the next step is actually building
tolerances normally add extra cost to the
the actual fixture. During the building stage,
tooling with little benefit to the process.
the designer should make sure the toolroom
Where necessary, tighter tolerances can be
personnel know exactly what must be done
used,
when making the fixture. By periodically
the
workpiece's
but
tighter
tolerance.
tolerances
do
not
drawings
with
have
the
been
fixture
thoroughly
necessarily result in a better fixture, only a
checking
builder,
the
more expensive one.
designer can help eliminate any possible misunderstandings and speed the building Elaborate
process. If there are any difficulties with the
designs often add little or nothing to the
design, the designer and builder, working
function of the fixture. More often, a power
together, can solve the problems with a
clamp can do the same job at a fraction of
minimum of lost time.
Simplify
tooling
details.
the cost. After the fixture is completed and inspected, Keep the function and operation of a fixture
it should be tested. The fixture is set up on
as simple as possible. The likelihood of
the machine tool and several parts are run.
breakdowns and other problems increases
The designer should be on hand to help solve
with complex designs.
any problems. When the fixture proves itself
These
problems
multiply when moving parts are added to the design. Misalignment, inaccuracy, wear, and malfunctions caused by chips and debris can cause many problems in the best fixture designs. Reducing design complexity also reduces
in this phase, it is ready for production.
