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2 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Assembly Design Guidelines Issue XIX, Aug 2016
3 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
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4 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Welcome to another issue of the DFM Guidebook. This issue of DFM Guidebook will focus on design for assembly guidelines.
This time we will not be talking about DFM. On the other hand we will cover some aspects of DFA or Design for (ease of) Assembly. Consideration of assembly issues during early design stage shortens product development time, minimizes development cost, and ensures a smooth transition into production with minimal rework. If a product contains fewer parts it will take less time to assemble, thereby reducing assembly costs. With each part there is an opportunity for a defective part and an assembly error. As the number of parts increases, the probability of a perfect product goes down exponentially. In addition, if the parts are provided with features which make it easier to grasp, move, orient and insert them, this will also reduce assembly time and assembly costs. Additionally, there are many assembly related issues which occur on the shop floor and can be avoided during the design stage. Read the guidebook to understand rules for: Hole Alignment, Hole to Slot Alignment, Slot parameters based on thread size, Interference Detection, Fastener Clearance, Fastener Accessibility, Fastener Engagement Length, Minimum Clearance between Components and Check for Preferred Components. If you have any feedback or questions on DFM guidebook, please write to us at [email protected] Happy Reading…. Regards DFMPro Marketing.
5 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Contents
Hole alignment .............................................................................................................. 6
Hole and Slot pairs......................................................................................................... 7
Hole to Slot Alignment ............................................................................................................ 7
Slot Parameters based on Hole Thread Size ........................................................................... 8
Interference Detection .................................................................................................. 9
Fastener Guidelines ..................................................................................................... 10
Fastener Accessibility ............................................................................................................ 10
Fastener Clearance ............................................................................................................... 11
Fastener Engagement ........................................................................................................... 12
Recommended Thread Connection .............................................................................. 13
Check Contact Material................................................................................................ 14
Check Preferred Components ...................................................................................... 15
6 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Hole alignment
Hole alignment plays a major role in assembly design. Holes are used for insertion of fasteners, dowels,
cables, etc.
It is recommended that, holes in connecting parts should be collinear to avoid difficulties in assembly.
Blocked holes and unused holes which lead to increase in manufacturing cost should be avoided.
7 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Hole and Slot pairs
Slot holes are used in single-direction accurate joints. This is typically used where the alignment
tolerance along the length of the slot is not critical.
Hole to Slot Alignment
Correct hole to slot alignment id provided for proper position of the slot with respect to the threaded
hole in the adjacent part. This should be measured by slot position center to the threaded hole axis.
It is recommended that, hole should collinear and at the center of a slot. If the hole is not at the center
of the slot, then it may leads to misalignment in future adjustment.
8 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Slot Parameters based on Hole Thread Size
Slot parameters (Slot width and length) should be maintained as per the corresponding threaded hole
sizes. These slots will act as clearance holes.
It is recommended that, for ease of manufacturing, slot parameters should be in standard sizes.
9 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Interference Detection
To achieve desired functionality and ease of assembly interference between assembly components
must be avoided. Interference detection is most useful in complex assemblies, where it can be difficult
to visually determine whether components interfere with each other.
Failure to avid interference may result in rework and engineering changes.
10 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Fastener Guidelines
A fastener is a mechanical component which is used to join two or more parts/components together.
Fasteners play a major role in assemblies.
Fastener Accessibility
Fasteners should be easily accessible for assembly and disassembly.
It is necessary to ensure sufficient space around the fastener head for spanners, wrenches, bits for ease
of fastening or extraction. Also, a designer should provide sufficient space axially for insertion and
removal of fastener into the holes.
11 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Fastener Clearance
For the ease of insertion of fasteners there should be sufficient clearance between non-threaded parts
and the fasteners (radial clearance).
Also, it is advised to provide sufficient gap between bolt and bottom of the hole or the next component
(axial clearance) to absorb excessive tightening load if any.
A = Axial Clearance , B = Radial Clearance
12 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Fastener Engagement
The axial distance through which the fully formed threads of both the nut and bolt are in contact is
called the fastener (thread) engagement length.
It is recommended that, engagement between the internal and external thread should be long enough
to provide adequate cross-sectional thread area. Insufficient thread engagement results in a tensile
failure of the bolt rather than thread stripping. To achieve the required strength, there should be
sufficient engagement between fastener and the component.
13 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Recommended Thread Connection
The threads on male (external) and female (internal) sides of a threaded connection should be of
compatible types/forms. It is recommended to use only the compatible thread types/forms on male
and female sides of a threaded connection to avoid issues like improper connection, cracks, stress
ruptures etc. during functioning.
Designer should select right threads (standard and size) in both male and female parts. For example, If
the male part thread is metric thread (M8 X 1.0) female part should have metric thread (M8 X 1.0).
In some cases, even though thread standard and sizes are same but male thread is fine thread and
female part thread may coarse thread, designer has to avoid such mismatches.
14 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Check Contact Material
Compatible contact material is one of the critical criteria in assembly design. Material properties
determine, how the parts react to the structural or thermal loads applied. While selecting material,
designer need to consider density, thermal conductivity, Specific heat, Coefficient of thermal expansion
and so on.
Metal oxidizes in presence of oxygen. Atmospheric effects and thermal cycling accelerates this process,
when moisture is present. To avoid chemical reactions like oxidization or corrosion etc. contact
materials should be compatible.
SURFACE CONTACT Examples LOWER UPPER SURFACE
SURFACE GAL ZINCALUME COLORBOND S/STEEL ALUMINIUM COPPER ZINC LEAD
GAL YES NO NO NO NO NO YES Y/NC
ZINCALUME YES YES YES Y/NC YES NO YES NO
COLORBOND YES YES YES Y/NC YES NO YES NO
S/STEEL Y/NC Y/NC Y/NC Y/NC Y/NC Y/NC Y/NC Y/NC
ALUMINIUM YES YES YES Y/NC YES NO YES Y/NC
COPPER Y/NC Y/NC Y/NC YES YES NO YES Y/NC
ZINC YES NO NO NO NO NO YES Y/NC
LEAD Y/NC Y/NC Y/NC Y/NC Y/NC Y/NC Y/NC YES
NOTE : YES CONTACT ACCEPTABLE
Y/NC CONTACT ACCEPTABLE BUT NOT DIRECT METAL CONTACT
NO NO DIRECT METAL CONTACT
15 A DEFINITIVE GUIDE TO DESIGN FOR MANUFACTURING SUCCESS
Check Preferred Components
In most organizations, similar products with different versions or series are produced by way of
modification or replacement of components. This is done with a purpose of making the product
available with various features at different costs based on market demand.
In such cases, it is important to ensure the presence of preferred and allowed components in the final
assembly based on the product or series. Goal is to exclude disallowed components from the final
assembly of a particular product version or series. Also, sometimes, materials assigned for disallowed
component are deemed obsolete by organizations. Hence, it needs to be ensured that such
components are not be shipped with the final assembly.