Offered by Boothroyd and Dewhurst in the 1980s, this predetermination method is specific to the Design For Assembly method..
This method was developed by Boothroyd and Dewhurst, American researchers at the University of Massachusetts, in the 1980s. This predetermination method is specific to the Design For Assembly method. In their logic, the main factors that influence assembly costs are :
- The number of parts
- The simplicity of handling, screwing…
It is based on the identification of each manipulation time : screwing, insertion …. For each of these movements, this method gives average times which are compiled in tables.
STep 1 – Identify the number of parts needed
1.1 Is the part useful?
We will initially collect the needs in terms of functionality. Then we go, via the following decision diagram, decide whether the part is an “essential” part or not. This diagram will push to ask 3 questions :
- Is the part mobile in relation to others ?
- Are the properties of the part necessary ?
- Is it separated from others to allow smooth operation ?
For ” essential ” parts, they will be kept as necessary. We will simply look at whether there is any design improvement. In DFA terminology, this part is part of the “Critical Path”.
For non-essential parts, we will have 2 choices:
- Either they are really useless, they will be deleted.
- Either they are actually combinable with one another, allowing us to combine them.
1.2 : Is the part standardizable ?
This question arises on different perimeter. So, is it standardizable :
- At the assembly station ?
- At the workshop level ?
- At the factory level ?
- At the group level ?
- At the level of the entire industry ?
1.3 : Calculate the theoretical efficiency
We will calculate a first ratio. This will highlight our level of efficiency in our analysis. It is thus calculated :
GIn general, 60% is targeted. Theoretically, we are looking to have 40% fewer pieces between the basic design and the design obtained after this first series of questions.
1.4 : Determine the relative cost of parts
Finally, for each of the pieces, we will estimate the relative cost of these compared to the total cost of the other parts
Step 2 : Perform a DFA Analysis
From this analysis and the resulting actions, we will determine the number of parts actually needed Nm.
STep 3 : Estimate the handling time
This estimate is made according to the difficulty of entering or handling the parts, the tools used … The data are tabulated in the Excel file to download.
Step 4 : Estimate the assembly time
In the same way, we estimate assembly times according to criteria sum the simplicity of alignment, the resistance of the assembly … The data are tabulated in the Excel file to download.
STep 5 : Estimate the time of operations
From the time estimates we made during steps 3 and 4, we will deduce the total time of the operation. This one calculates as follows :
Operation time = NbOp * (Tm + Ti)
- NbOp : number of times we do the operation consecutively
- TM : Movement time
- Ti : Assembly time
STep 6 : Calculate the coefficient of efficiency
The authors define the coefficient of efficiency as the comparison between the estimated assembly time if we had only the necessary pieces “necessary“, with the real time that we estimated. The formula is the following :
CE = Nm * 3 / TT
WithEfficiency coefficient :
- CE : Coefficient d’Efficacité
- Nm : Minimum theoretical number of parts (see step 2)
- 3 : This is the time, 3 sec, basic estimated for a part not presenting any particular difficulties of handling and assembly.
- TT : estimated average time to perform a complete assembly equal to the sum of our NbOp calculated in step 5.
- CE ≥ 1 : We have a powerful design. We can stop there unless we have specific performance constraints to achieve.
- CE < 1 : Our Total Assembly Time is greater than the desired target. We must start again from the step 1.
STep 7 : Re-design our parts
We will work on the parts estimated as useless. This is the main axis to reduce assembly times. In general, if we have a number of useful pieces smaller than the number of consecutive editing operations, there is probably food for thought. Clearly, if we have several times the same room, maybe there is the possibility of one.
Then we will reflect on our parts generating the most assembly time. Without doubt that by reviewing the design, we can reduce the difficulty of editing and reduce the time.