Having a different approach to the Boothroyd and Dewhurst method, it has the advantage of taking into account the manufacturing complexity of our part in addition to its assembly complexity.
This method was developed in the 1980s by Lucas Corp in England, in cooperation with the University of Hull. Very different from the approach Boothroyd and Dewhurst.
Note that all the values of the various criteria evaluated are tabulated in the download document opposite.
Step 1: Functional Analysis
At first, we will separate in 2 all the parts of our assembly. Groups will be defined as follows :
- Le groupe A : These are the “functional” parts and therefore necessary for the system.
- Le groupe B : These are the secondary parts such as fasteners, connection cables … that could potentially be designed differently to avoid the need.
For example for a lighthouse, the bulb, the hulls … are parts of group A. The connection cable and the fasteners are of group B.
From this analysis, we will deduce the efficiency index :
The target is the same as in the Boothroyd and Dewhurst method. We are looking for an IE greater than 60%.
Step 2: Analysis of the manipulations
Then, for each piece, we identify a difficulty index to manipulate it by evaluating the following criteria :
- A : Size and weight of the part
- B : Difficulty in handling
- C : Part Orientation
- D : Rotating the part
The final score of the manipulations being the addition of the criteria scores:
Total Handling Score = A + B + C + D
Once the scoring is done, we deduce the performance of the manipulations according to the following equation :
If the score is above 2.5, a design review is considered necessary.
We analyze each of the fastenings of our assembly according to the following criteria :
- A : Positioning and Fixation
- B : Process Direction
- C : Insert
- D : Accessibility and visibility
- E : Alignment
- F : Insertion Force
The final assembly score being the addition of the criteria scores :
Total Score = A + B + C + D + E + F
Once scoring done for each part, we calculate the Assembly Ratio for the whole system :
We will consider that if the value is greater than 2.5, we will have to review the design of our parts.
Step 4: Manufacturing evaluation
It is this step that is the advantage of the Lucas method. Indeed, the Boothroyd and Dewhurst method will mainly rely on reducing the number of pieces. But will have the effect of complicating the design of other rooms that it does not manage.
In the Lucas method, this level of manufacturing complexity will be highlighted. It will determine the cost of each component of our assembly. Without giving an exact “exact” cost value, it will help us identify areas for improvement. The calculation formula per part is as follows :
IP = Cc * Cmp * Cs * (Ct ou Cf) * Pc + V * Cmt * Wc
- IP : Production Index
- Cc : Complexity factor
- Cmp: Matter factor
- Cs : Minimum thickness
- Ct : Tolerance Factor
- Cf : Finishing factor
- Pc : Quantity factor
- V : Volume of material
- Cmt : Cost of the material per unit volume
- Wc : Coefficient of waste
4.1 Complexity Cc
First, we identify the complexity factor in the tables to download. It depends on its general shape (cylinder …), its characteristics (threading …) and the manufacturing process. (molding…).
4.2 Material factor Cmp
Depending on the type of material and the type of process, a material factor is identified. It depends on its material and the type of process.
4.3 Minimum thickness Cs
Then we search for the minimum thickness of our part and we identify a coefficient that will depend on the thickness and the process used.
4.4 Choice of factor Ct or Cf
We will have to choose between the tolerance factor or the finishing factor, taking the highest value of the two. Thus, if the tolerance factor is at 6.1 and the finishing factor at 3, then we will choose a tolerance factor in our formula.
4.5 Quantity factor: Cost per quantity depending on the process Pc
Of course, the price will be impacted by the amount we will produce. It depends on the number of parts and the process used.
4.6 Cost of the material per unit volume : Cmt
Another coefficient directly affecting the price of parts, the volume of material..
4.7 Coefficient of waste Wc
Last coefficient of the formula, that of waste. Depending on the type of process and the forms we need to achieve, we will waste more or less material.