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Modular Fixturing System
- Tech Articles - Modular Fixturing VS Dedicated Tooling
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TECHNICAL ARTICLE |
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Modular Fixturing VS. Dedicated Tooling
The Pros and Cons
By
Robert L. Coope, President
SME Technical Paper #TE93-388
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INTRODUCTION |
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The introduction of the modular fixturing concept for machine tool applications has met
with varying degrees of acceptance on the part of decision makers in this country. On one
hand, managers with a strong bias in favor of traditional workholding techniques are
reluctant to embrace new approaches. This reluctance translates into lost opportunities
for important gains in productivity. The opposite extreme is the manager who is totally
enamored with the modular concept and is wrongly advised by an over-zealous salesman to
dispose of all of his permanent dedicated tooling. This extreme can have unfavorable
consequences of a different nature.
The purpose of the following is to suggest a way of analyzing the pluses and minuses of
continued use of dedicated fixtures versus the all-modular approach of creating temporary
holding fixtures using modular accessories. A measured case-by-case consideration of each
approach and how it fits in with specific product mixes and production requirements can
yield good results.
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SMALL-LOT PRODUCTION |
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In discussing the relative merits of dedicated fixtures and modular fixtures, it is
assumed that the reader is involved in small-lot production runs and is consequently
interested in reducing 1) setup time, 2) tooling expense, and 3) tooling lead time.
Whether dedicated tooling or the all-modular approach is used, a proven way to reduce
machine setup time is to use an accurate grid pattern for locating fixtures, vises, and
other accessories. The ability to pin fixtures and components into an accurate grid
pattern eliminates the need to indicate or probe for alignment and position.
While an accurate grid pattern greatly reduces machine setup time, the decision must be
made whether and when to go one step further in utilizing this same grid pattern to
address the other factors in small-lot production: tooling expense and tooling lead time.
When should a custom dedicated fixture be designed and built? When should the all-modular
approach be used?
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CASE STUDY |
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Exhibit 1 is an example of a permanent holding fixture specifically designed to locate
and hold a particular workpiece. The fixture has been designed to dowel and screw to two
Stevens Narrow Profile Angle Plates which are in turn doweled and screwed to a Stevens
Subplate. This fixture plate onto which the workpiece mounts was designed and built in a
few days using a stock pre-jigbored tooling plate. Setup time is minimal because
components pin into the standard grid pattern. |
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Exhibit 1A Exhibit 1B |
| Exhibit 2 illustrates how the same workpiece shown in exhibit 1 could be set up
using all-modular setup components. This fixture was designed and built in 3-4 hours using
standard off-the-shelf setup hardware. If the modular fixture is documented with
photographs, sketches, or CAD drawings, it can later be rebuilt in about 15 minutes and
will repeat location of the workpiece from the prior production run. |
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Exhibit 2A Exhibit 2B
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| Both of these approaches offer the benefits
of a standard grid pattern, but there are advantages and disadvantages when comparing the
two approaches to each other. |
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ADVANTAGES OF DEDICATED TOOLING |
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When given a clean sheet of paper, an adequate
budget, and sufficient lead time, a skilled tool designer can design a custom
workholding fixture that is ideally suited to the application. Not limited to
standard hardware, the designer may create custom details that offer the
strength and rigidity to withstand aggressive cutting forces without sacrificing
compactness or workpiece loading ease. Locators and clamps may be sized and
shaped specifically for the part they are being used to hold. Tapped and bored
holes may be incorporated into the fixture at any location.
The all-modular approach generally makes some
sacrifices in terms of 1) rigidity and strength, 2) ease and speed in loading
the workpiece, or 3) overall compactness of the fixture.
Modular accessories are designed for maximum
versatility in a wide range of applications. At times, a clamp or locator made
from all-modular components may consist of more than one component coupled
together with some possible overall loss in rigidity versus a one-piece custom
detail used in a dedicated fixture. Rigidity in an all-modular setup often comes
at the expense of overall compactness. As a result, a larger working envelope
may be required to accommodate a modular fixture.
A custom fixture can be optimized for the
ergonomic convenience of the operator, making loading of the workpiece easy. Careful
planning of the all-modular fixture can minimize inconveniences but not always eliminate
them.
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ADVANTAGES OF THE ALL-MODULAR APPROACH |
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When given a clean sheet of paper, an adequate
budget, and sufficient lead time, a skilled tool designer can design a custom
workholding fixture that is ideally suited to the application. Not limited to
standard hardware, the designer may create custom details that offer the
strength and rigidity to withstand aggressive cutting forces without sacrificing
compactness or workpiece loading ease. Locators and clamps may be sized and
shaped specifically for the part they are being used to hold. Tapped and bored
holes may be incorporated into the fixture at any location.
The all-modular approach generally makes some
sacrifices in terms of 1) rigidity and strength, 2) ease and speed in loading
the workpiece, or 3) overall compactness of the fixture.
Modular accessories are designed for maximum
versatility in a wide range of applications. At times, a clamp or locator made
from all-modular components may consist of more than one component coupled
together with some possible overall loss in rigidity versus a one-piece custom
detail used in a dedicated fixture. Rigidity in an all-modular setup often comes
at the expense of overall compactness. As a result, a larger working envelope
may be required to accommodate a modular fixture.
A custom fixture can be optimized for the
ergonomic convenience of the operator, making loading of the workpiece easy. Careful
planning of the all-modular fixture can minimize inconveniences but not always eliminate
them.
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ECONOMIC EVALUATION |
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As indicated earlier, there are benefits to
be gained by the use of an accurate grid pattern regardless of whether dedicated fixtures
or the all-modular approach is used. There are additional and quite different benefits to
be gained by the use of all-modular setup components. It is important to understand both
types of cost savings in evaluating the potential for the use of modular tooling in widely
differing manufacturing operations.
The calculation below shows a method for
estimating the savings in setup cost by using subplates, angle plates, or columns with an
accurate grid pattern versus using conventional techniques. The probable payout time for
the investment is also shown for the setup shown in exhibit 1. Every individual making
such an evaluation will need to rely on his own experience and understanding of his
manufacturing operation to plug realistic numbers in the formula.
Payout equals the investment in table
tooling with an accurate grid
pattern divided by the savings in setup cost.
Setups like the one shown
in exhibit 1 can be done in 0.1 hours or less using Stevens modules. The same setup using
conventional techniques on a T-slotted table or pallet would require 3.8 hours.
Therefore payout in this case would be $3,776.00 divided by (3.8-0.1) x $45.00/hr or 22.67
setups.
This means that if average savings per setup
approximate $167.00, the investment in table tooling with an accurate grid pattern will be
paid for in about 23 setups. If setups are changed once a week, the investment will be
paid for in less than 6 months.
The next calculation shows a way of
determining additional savings which might result from using modular setup
components in place of dedicated fixtures. The probable payout time for the purchase of
additional setup components required for the setup in exhibit 2 is shown.
The setup shown in exhibit 1 requires a
fixture costing about $1,800. However the net savings in tooling expense will be $1,800
minus the extra costs associated with setting up modular setup components. The setup shown
in exhibit 2 would require 3.6 hours in design and build time. The actual costs of the
components used in this setup is about $5,000. However, an assortment of modular tooling
with setup components offering reasonable flexibility in setting up parts in this size
range would approximate $13,000. Accordingly, the payout is calculated as shown in the
next paragraph.
Payout equals the investment in modular
setup components divided by the net savings in tooling expense. The net savings in
tooling expense would be $1,800 minus (3.6 hours x $45/hr) or $1,674. The payout
would then be $13,000 divided by $1,674 or 7.7 fixtures.
This means that a kit of modular setup
components will pay for itself by eliminating the need for building 8 dedicated fixtures.
Payout time will depend on how many fixtures are normally made in a specific time span.
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CONCLUSIONS |
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It has been well established that
significant gains in manufacturing productivity can and have been made through use of
modular tooling in manufacturing operations. This is particularly true in factories and
shops where small batch quantities and continually changing production requirements
necessitate frequent setup changes and reduced response time. A thorough understanding of
the variety of ways that modular tooling may be applied to a broad range of manufacturing
conditions will help users to apply it properly and profitably.
This paper used with permission of the
Society of Manufacturing Engineers, Dearborn, Michigan.
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Stevens Engineering, Inc., 3946 W. Clarendon Avenue, Phoenix, AZ 85019
1-(800)635-0657 or (602)272-6766 - FAX
(602)272-0717
info@stevenseng.com
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