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Vishwadeep Khatri

Design for Assembly

Design for Assembly (DFA)

 

Design for Assembly (DFA) is a design philosophy where products are designed keeping in mind the ease of assembling the product. Factors which influence the assembly are 
1. Number of parts in the product - lesser the number of parts, faster and easier is the assembly
2. Assembly / Disassembly Efficiency - how easy is it to join the parts together
Design for Assembly Index (DFA - Index) is the measure of how easy is it to assemble a component / system. It is denoted by 
DFA = 100 * Nm * tm / ta 
Nm - theoretical minimum number of parts
tm - minimum assembly time per part
ta - estimated total assembly time
Higher the DFA index, easier is the assembly operation

 

An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Vastupal Vashisth and Natwar Lal.

 

Applause for the respondents-  Natwar Lal,  Mohamed Asif, Vastupal Vashisth & Manjula Pujar

 

 

Question

Q. 178  Explain the meaning and application of DFA index for a component.

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

 

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DFA - Design for Assembly

 

Design for Assembly is one of the approaches in Design for Excellence (DFX). The X here can take many forms like Manufacturing, Safety, Cost, Service, Reliability etc. So how is DFA different from others and when should one go for it

 

DFA should be the preferred if the product that we are designing needs to be assembled and disassembled often. Because in such situations more than anything else, it is more important that the assembly should be

1. easy

2. efficient

3. effective

 

My top of the mind items that usually require to be assembled and disassembled are military guns and toys (especially track toys and Lego).

 

Elements that you need to consider in DFA

1. Number of parts - product with lesser number of parts is easier to assemble. Therefore the number of parts should be kept to a bare minimum. Parameters to check if a part can be removed or not are

a. Is it absolutely necessary to have the part made of a different material?

b. Does the part has a movement relative to the other parts of the product?

c. Is the part used as a fastener or for securing other parts?

2. Time taken to assemble or ease of assembly - there are quiet a few things that are considered here

a. Easy to handle parts - neither too small nor too big

b. Symmetry of the parts - symmetrical parts are easy to handle

c. Remove flexible, slippery, sticky parts along with parts that have sharp edges

d. Easy to insert - unidirectional, self inserting and easy to align

 

For a given design (after considering the above parameters), one could also compare the options using DFA-index i.e. Design for Assembly Index. It is given by the below formula

 

DFA = 100 Nm tm / ta

 

Nm - theoretical minimum number of parts

tm - minimum assembly time per part

ta - estimated total assembly time

 

Higher the DFA, better is the design for assembly.

 

Taking a hypothetical example below to explain

 

Soldiers are frequently required to disassemble and re-assemble their guns.

Soldiers will not be using revolvers, but typically their guns are also without too many screws and fasteners. Most of parts are easily assembled using uni-direction motion and fit into one another.

 

 Considering two revolvers here

 

Gun 1 - revolver with a rotating chamber for each bullet

Gun 2 - revolver with a magazine holder for bullets

 

Theoretical minimum number of parts in both the guns are same. Therefore Nm = 4

1. Barrel

2. Firing Pin

3. Ammunition Chamber (rotating or magazine)

4. Holder

 

tm - minimum assembly time per part remains 4 seconds.

 

ta - total estimated assembly time varies for each gun. In gun 1, it is 90 seconds because before closing you need to match the chamber with the barrel. In gun 2, it is 60 seconds.

 

DFA for gun 1 = 100*4*4/90 = 17.78 

 

DFA for gun 2 =100*4*4/60 = 26.67

 

DFA index for gun 2 is better, therefore as a manufacturer you should go for the design of 2nd gun.

 

Gun 1 - Rotating Chamber Revolver

Rotating Chamber revolver.JPG

Magazine Type Revolver

magazine type revolver.JPG

P.S. - Images only for illustration

 

 

 

 

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Design for Excellence ( DFX):

 

Before starting this let us discuss some basic concepts that any design in this world will have variability and randomness and you can question whether it is deterministic or not . example of washer, it has its own ID of 10 mm , suppose a OEM needs it and they are going for it to manufacture from a supplier and it snot necessary that every time its ID will come 10 mm , may be possible that its ID can come 9.9 mm, 10.1 mm so this variation will always be there and in engineering we used to call it as tolerance. now if OEM is asking to manufacturer  it the next question of supplier is what is your tolerance because every process has its own limitations. 

For example a car manufacturing is producing 1000 cars / day , all these car are looking same but there are some difference which normally a common man cant find out. similarly  lets say that 1000 engines are producing from same machining, robotic process because there are more chances to have more errors if followed manually all process, but still engine no 1 is different from engine no 50th because of some variation of machine or environment condition or tool condition. visually it will look same but difference is there. idea is to minimize the variation thats why they followed robotic line.

Another example of mileage of a vehicle say it is Xkm/L but we have seen it is written with asterisk mark which means that under test condition so now in actual condition mileage will be different because the condition in which you are driving is different form test condition, so it always comes lower what has written while purchasing the vehicle. so we need to design things accordingly and for customer prospective so that tehy can use it in better way and companies also not loose any cost. 

 

Design for excellence or DFX is a systematic design approach that includes wide range of guidelines and standards focused on optimizing the product realization lifecycle. in reality, the term DFX is better to be called as Design for "X" where the "X" is variable and can be interchangeable  with one of many values depending on any particular objective.

DFX has some guidelines which ensures the issues related to manufacturing, cost , quality , assembly and serviceability are addressed at the design stage itself. if any of these guidelines not adhered during the design change it can lead to critical engineering change at later stage which will be very costly and can be cause for product delays.

Below are some common substitutes for X:

  1. Design for Manufacturing
  2. Design for assembly
  3. Design for Production
  4. Design for cost
  5. design for service
  6. Design for Safety
  7. Design for manufacturing and assembly (DFMA)
  8. Design for Reliability

Before going to design for assembly lets first have a look on design of manufacturing and product design.

In design industry, normally designer follow a principle called "OVER THE WALL DESIGN" in which there will be ground level research by marketing team about the needs and wants of customer and then they convey these requirements to designers and from there they start to work on it to finalize the product and gives to manufacturing after doing engineering or designing and then for sales or service. Here method is followed like Marketing then engineering/designing then manufacturing then sales or servicing. 

Marketing team gives a raw information about potential buyer of your product about which segment, how much, when , where  and then work starts by engineering team or designing team for example car segment marketing team do research on wants and needs of customer like they are going for hatchpack, sedan, SUV, compact SUV. 

 

Designer need to think from all prospective like manufacturability, , servicability, sales, transportation . for example he has designed a very beautiful product as per requirement but there is no manufacturing technique to produce it then there will be no meaning for that design. Nowadays 3D Printing are opened up in the manufacturing but it also have its limitations in real life performance. You can make several things with 3 D Printing but you cant make a part from it which is used in engine which is going to assemble in the final vehicle so designer need to think all those critical issues so that product can be easily produces, transported form one place to other and can perform better in market. 

for example a designer is designing a washing machine or refrigerator then he needs to think all prospective including transportation also , they have given a height of such type that they can accommodate two over one another and can transfer around 50 in one container and lets suppose a designer increase height by one inch then it is not possible to accommodate two washing machine over one another and transportation capacity becomes straight away half and  its a loss to the organization so a designer need to consider all factors while designing any product. 

Now comes on design for manufacturing which is to be taken care by designer only. it should not be like they any product is ok from design prospective but not ok form manufacturing prospective, because it has no meaning of fine drawing but have implementation problem 

Design for manufacturing (DFM) is the method of design for ease of manufacturing of the total parts that will form the final product after assembly . We should focus on OPTIMIZATION OF THE MANUFACTURING PROCESSES. DFM is the developing practice and emphasizing the manufacturing issues while product development process. if we are having successful DFM then it leads to lower production cost without compromising the product quality. Manufacturing cost includes Component COst, Assembly Cost & Overhaed Cost which can be fixed cost or variable cost 

Methods of DFM:

  1. Estimate the manufacturing cost
  2. reduce the cost of components 
  3. reduce cost of assembly 
  4. reduce cost of supporting factors
  5. includes the impact of DFM final decision on other factors    

After having a brief discussion over product design and design of manufacturing lets understand Design of Assembly. 

 

Design for Assembly: 

 

by reading this first question comes in mind is What is assembly and why we need design for assembly . 

 

Assembly is handling of two or more components together and make a single output by forming it as per guidelines . we need design for assembly because we used ot handle so many parts/components and if there is no ease to do assembly then it may lead to wastage of time, less production, more rejection and more cost to the organization. 

for example cloth drying clip is very interesting example of this. it has three components only , the left side, right side, and a spring . the manufacture of it done a very good thing for its manufacturing and assembly. Manufacture has designed symmetrically both L & R side  and a spring so now for assembly it can be possible that all are in one bin and spring in other, whatever comes in left hand is left side and whatever comes in right hand is right side and by assembly of one spring into it it can produce one clip. similarly there can be another method of producing it by having Left and right hand side different, in this case manufacture need to have separate bin, more space to store and a guideline to assemble left and right together and most importantly it will slow down production and more cost  so first method is suitable having symmetry for booth leads to cost saving and more production, so we need to consider assembly point while designing of any product.

 

So we need DFA because  Design of Assembly is the method of design of the product for ease of its assembly in other words "Optimization of parts/system assembly ". It is the tool which is used to assist the design teams in designing of the product that will transition to production at a minimum cost by focusing on the number of parts and their handling and ease of assembly.

Another example you can take of side mirror assembled in car, car manufacturing companies have same size of mirror for both side L & R , only difference is that box of both sides, now on assembly line they can take same mirror for both sides and it will lead to save time otherwise need to identify which is left and which is right and need extra space and make a simple process complex one. ts 

there are some principles for Design for Assembly  which are given below:

  1. Minimize the part count
  2. Always design parts with self locating features
  3. Always design parts with self fastening features
  4. there should be minimum re-orientation of parts while assembly
  5. design parts fr retrieval, handling and insertion
  6. emphasize top down assembly
  7. do standardization of parts and minimize the use of fasteners
  8. design for a base part to locate other components
  9. design for component symmetry for insertion

We have two major activities while assembly for any product which are given below:

1. Part handling

2. Insertion or fastening

below is the guidelines for these two activities while assembly:

1. Design with end to end symmetry or rotational symmetry

2. If we can not achieve symmetry , then make it obviously asymmetric

3. Prevent jamming when parts are stored

4. avoid those features which might allow tangling of parts

5. avoid parts which stick together, slippery, delicate, flexible, very small or very large( condition applied)

6. Very little or no resistance during insertion.

 

These are just set of rules that need to be followed. A designer can design better with this background because it is an approach that provides a designer with organized method to design a product for easy assembly, evaluate competing designs. for example nail and screw assembly are two different job , nail can go with just hammering , no need to match anything axis but in case of screw you have to match with axis , example of plastic chair they can go over one another without jamming and can store at one place , similarly example of disposal glass they can go in each other an can be stored . so need to  think all prospective. in point no 5, condition applied unless until there is requirement of that type or part for example diamond ring, in that you cant have a big diamond obviously it will be of small size in these type of cases there will be different techniques to handle them.

 

there are two different assemble for the same product, how will you identify which one is good or which one is not, how will you quantify the features?

we can quantify following features during design of assembly for a systematic assembly :

1. Symmetry

2. Size

3. Weight

4. Thickness

5. Flexibility

6. chamfers

7. Insertion time

8. Restricted access

 

for example assembly of metro lines, basically it is assembly of only concrete structures, handling with crane and put them above in line by inserting into each other. so here also we are doing two thing only, Part Handling and Insertion its a different thing that size is very big so need to take care properly and need of special equipment. so concrete structure will have some kind of hollow space or long road which will be inserted into previous one on the top of the pillars, so there will be some marking to define which is front side and which one is back so that crane opertor know exactly from where he is going to pick it up and insert it otherwise suppose he lifts it up and after lifting they know that side is wrong then there will be very difficulty to do again . so it will be waste of time . so need to specify, quantify all factors in design of assembly.

 

Assembly Efficiency:

Ema = Nmin* Ta/Tma

Where Ema is eficiency 

Ta is basic assembly time.( Time averaged over parts with no handling, insertion and fastening difficulties)

Tma is estimated time to complete the assembly of the actual product

Nmin is  theoretical minimum number of parts that in a product satisfying one or following criteria:

1. The part moves relative to all other parts which are already assembled during the normal operating mode of final product

2. Proper functioning of the product requires the part to be of a different material than the adjoining parts for example electrical isolation

3. these criteria need to be applied without accounting for general design requirement. fasteners generally dont meet the above criteria so eliminated form this. 

 

During design of assembly we need to consider part symmetry , for example a component is given then how would you quantify it symmetry ?

basically there are two types of symmetry :

1. Alpha Symmetry: it is defined as the angle through which a part must be rotated  about an axis perpendicular to its axis of insertion to return to the original orientation.

2. Beta Symmetry: it is defined as the angle through which a part must be rotated about an axis parallel to its axis of insertion to return to the original position.

 

While measuring the above we consider only one rotation at a time and higher the value of alpha and beta means less symmetric part is..

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Would like to give an overview of DFX and DFMA before we get into DFA-index.

DFX is Design for Excellence; DFMA is Design for Assembly and Manufacturing.

DFMA is predominantly used in Manufacturing industry to keep the product cost minimal through various improvement through design changes and process optimization.

DFX.jpg.081deee613928a7f4572ab249ee73637.jpg

 

While Developing Product: We could use dFMEA, DFR (Reliability)

While High level design: We could use DFT (Testability)

During Physical design: DFA (Assembly), DFF (Fabrication)

During Prototype: DFM (Manufacturing)  

 

With this high level overview, we shall narrow down to DFA versus DFM, which are commonly compared in Manufacturing terms

Diffs.jpg.4f7bf3ab99732211066c77861cbed2d4.jpg

DFA Index:

This indicates level of easiness to assemble a component.

It is used to Measure Assembly Efficiency  

 

DFA Index is an Integral metric in DFA Method. 

Formula:485236874_DFAIndex.png.84ba9f5eef18b7c20e9d8e45e9a9cf2c.png

Where, Nm = Minimum number of slides (Theoretical)

tm = Minimum Assembly time (per part)

ts = Total Assembly time (Estimated)

 

For Example:
If Total time to assemble is 500s, with 9 parts and if Minimum assembly time is 50s.
Then DFA Index = 90

 

Larger the Value of DFA Index, more efficient is the Process

 

This is mainly used to Analyze the data, which could give clarity to take further actions,

"Part Elimination", "Redesign of Specific Parts"...

Quote

"The best design is the simplest one that works" - Albert Einstein

 

There are various software tools and applications available for performing DFA Analysis.

DFMA, DFA10 are few software's.

 

Applications of DFA Index:

 

DFA indicator is a good indicator of Assembly efficiency and this index could be used to analyze the data. Decision based on data analysis could lead to Fewer Parts or Simplified Assembly which could have the following benefits.

 

2016569889_DFA3.jpg.60c8270a225423db5846430ed0654ddf.jpg

 

Explaining DFA from DMAIC methodology preceptive:

Define: Collecting product information and identifying opportunities for improvement

Measure: Measuring current Assembly time and cost. Calculating DFA Index (Assembly efficiency)

Analyze: Analyzing results to determine various complexities of product assembly

Improve: Improve efficiency of assembly by reducing number of parts or by reducing/changing part types

Control: Continue with best practices and follow DFA process improvement Cycle

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MEANING AND APPLICATIONS OF DFA INDEX FOR COMPONENT.

 

Design for assembly is process by which products are designed with ease of assembly in mind. If a product contains fewer parts it will take less time to assemble, there by reducing assembly costs. In addition if parts are provided with feature it makes it easier to grasp, move and assemble.

 

Design for assembly index indicates how easy it to assemble component. DFA can be expressed as

 

DFA=100Nm* (Tm/Ta)

 

DFA ----> Design for assembly index

Nm ----> Theoretical minimum number of parts

Tm -----> Minimum assembly time per part

Ta -----> Estimated total assembly time

 

Example : A component requires 400 seconds in total to assemble. There are 12 parts and minimum assembly time per part is 6 sec.

 

DFA=100Nm*(Tm/Ta)

DFA=100*12*(6/400)

DFA= 18

 

In general DFA is a tool to assist design teams in design of products that will transition to production at minimum cost focusing on number of parts.

 

APPLICATION OF DFA

* Minimizes part count

* Design parts with self fastening features

* Minimize re-orientation of parts

* Allows standardization of parts

* Follows Top down count

* Minimize the cost

 

 

 

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The joint winners for today's question are Natwar and Vastupal. Read through the rest of the answers to get additional inputs and detailed understanding.

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