FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
1. TITLE OF THE INVENTION:
METHOD, SYSTEM AND COMPUTER PROGRAM FOR ASSISTING THE DECISION FOR CHOOSING PARTS TO BE ASSEMBLED
2. APPLICANT:
Name: SAFRAN AIRCRAFT ENGINES
Nationality: France
Address: 2 boulevard du Général Martial Valin, 75015 Paris, France.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which it is to be performed:

DESCRIPTION
TECHNICAL FIELD
The field of the invention is that of mechanical part production. The invention relates more specifically to a step for assembling parts together which may require a prior machining operation, for example linishing, to make good any production defects which might make it difficult or even impossible to assemble them. It finds application in particular in the assembly of aerospace parts, for example the assembly of a blade and its leading-edge reinforcement.
THE STATE OF THE PRIOR ART
The assembly of a blade made of a composite material with its titanium leading-edge reinforcement is a time-consuming step. There may be difficulties of varying degree associated with assembling a blade and its leading-edge reinforcement, depending on their respective shapes. Sometimes assembly is only possible after manual linishing of the blade. Such linishing operations are time-consuming steps to be avoided as much as possible because they are likely to degrade the mechanical performance of the final part. Sometimes even linishing is insufficient to fully make up for production defects and render assembly possible.
DESCRIPTION OF THE INVENTION
One objective of the invention is to obtain a good quality assembly by minimizing the risk of having to perform a machining operation in order to allow two mechanical parts to be assembled together, such as, for example, a linishing operation with the aim of allowing a blade and a leading-edge reinforcement to be assembled together.

In order to achieve this it proposes a method of assembling parts chosen from a plurality of first parts and a plurality of second parts respectively. The method comprises the following steps.
For each first part and each second part from the plurality of first parts and the plurality of second parts, a step for estimating a fault indicator of the assembly of the first part with the second part.
For a batch of N assemblies to be made, a step for assigning N first parts from the plurality of first parts to N second parts from the plurality of second parts, in order to form N pairs of a first part and of a second part. Said assignment is performed in such a way that said N pairs minimise a total fault which corresponds to the sum of the estimated fault indicators of each of the N pairs.
Some preferred, but not restrictive, aspects of this method are as follows:
- it comprises a reduction in the estimated fault indicator of the assembly of a first part with a second part when the first part has been available for assembly for a period which is greater than a first limit;
- it comprises a reduction in the estimated fault indicator of the assembly of a first part with a second part when the second part has been available for assembly for a period which is greater than a second limit.
- it comprises a reduction in the estimated fault indicator of the assembly of a first part and of a second part available for assembly when no second part, which will become available for assembly with the first part during the availability of the first part for assembly, can be assembled with the first part;
- it moreover comprises a reduction in the estimated fault indicator of the assembly a first part and of a second part available for assembly, where said reduction is weighted by a factor which represents a probability that no first part, which will become available for assembly during the availability of the second part for assembly, can be assembled with the second part;

- it comprises the formation of a sub-set of said plurality of first parts constituted of the oldest first parts available for assembly, where the N first parts assigned to the N second parts belong to said sub-set of said plurality of first parts;
- it comprises the formation of a sub-set of said plurality of second parts constituted of the oldest second parts available for assembly, the N second parts assigned to the N first parts belong to said sub-set of said plurality of second parts;
- the fault indicator of the assembly of a first part with a second part is estimated from geometric characteristics of the first and of the second part;
- the estimation of the fault indicator of the assembly of the first part with the second part comprises an attempt to align point clouds which represent the surfaces of the first and of the second part.
The invention also relates to a system and a computer programme product capable of implementing this method.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects, aims, advantages and characteristics of the invention will become more apparent on reading the following detailed description of its preferred embodiments, given as non-restrictive examples, and undertaken with reference to the appended drawings, wherein figure 1 shows the various steps of one possible embodiment of the method according to the invention.
DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS
The invention relates to a method of assembling parts chosen from a plurality of first parts and a plurality of second parts. One embodiment example which will be described in detail hereafter relates to the assembly of a blade (first part) and of a leading-edge reinforcement (second part).
This method of assembling comprises a first step of choosing two parts to be assembled by means of a relevant logic. During a second step these two parts are assembled without glue to check their good geometrical suitability. In a third step, if necessary, sanding is performed to remove a surplus material identified on one of the

parts. In a fourth step, it is checked whether the sanding performed is sufficient. If the verification is conclusive, in a fifth step, the bonding of the two elements is achieved. The first step is therefore decisive on the following steps and determines the duration of the operation and the quality of the final assembly.
The method according to the invention is used to optimise the choice of parts to be assembled by specifying parts to be assembled which will maximise the quality of the assembly. With the parts to be assembled thus specified, the need to machine one and/or the other of the parts to allow them to be assembled is avoided to the greatest extent possible. For example, the need to linish a blade made of composite material to allow it to be assembled with a leading-edge reinforcement is avoided to the greatest extent possible.
In the field of optimization, we generally speak of minimizing a cost function, this minimization possibly including in the context of the invention the following technical objectives:
- to minimize the number of sanded parts and the mass of sanded material (when sanding is performed);
- to minimize the dispersion of the assembled part and to have an assembled part as close as possible to specifications of the nominal part,

- to minimize the scrap rate;
- to maintain the rate of production (no break in production);
- maintain a production order of blades to facilitate production
monitoring.
An assembly station is considered where, at different moments in time, a batch of N assemblies must be made using the parts available for assembly. By way of an illustrative example, the aim may be to make one batch of 8 assemblies per day from 16 blades and from 40 leading-edge reinforcements available.
The first plurality of parts and the second plurality of parts are not restricted to the parts available for assembly, but may include parts already present in the facility but not yet within the assembly station because, for example, they are undergoing inspection operations. By way of an example, 80 leading-edge reinforcements are

delivered to the facility each week, but only enter the assembly station in batches of 8 to replace the available reinforcements in the assembly station which have been used each day to make the batch of 8 assemblies.
With reference to figure 1, for each first part A and each second part B of the first plurality of first parts and the plurality of second parts, the method comprises a CST step for estimating a fault indicator of the assembly of the first part with the second part. Thus a matrix of indicators Mc is formed, which indicates, for each first part and each second part, a prediction of the fault of the assembly of this first part and of this second part. Using the terminology of optimization, this indicator can be likened to a cost function (here an assembly cost) that we will try to minimize.
In one embodiment example the fault indicator of a pair formed by a first part and a second part may be made up of two elements:
- a first element associated with the quantity of material to be removed by linishing in order to carry out assembly;
- a second element associated with the discrepancy between the relative position of the assembled parts and a nominal relative position.
By seeking to minimise such a fault indicator (likened to an assembly cost), it is thus sought to minimise the quantity of material removed by linishing and to form pairs which are as close as possible to the nominal. This fault indicator of the assembly of a first part j and of a second part I can be expressed as Cij = Gij + ||Pij — P0||, where Gij represents the cost of quantity of linished material and ||Pij- — P0|| represents a relative position of the parts in relation to a nominal position.
The fault indicator of the assembly of a first part and a second part may be estimated from geometric characteristics of said parts measured, for example, using tracer probes.
In one embodiment, simple geometric characteristics of the parts and a predictive model capable of learning are used to predict a probability that the assembly can be made without linishing. The greater the probability then the smaller the fault indicator of the assembly of the parts is. This model is the result of a learning process

using experience acquired on a certain number of pairs for which assembly has already been attempted.
By way of an example relating to the assembly of a blade and of a leading-edge reinforcement, a target distance go between the apex of the blade and the leading-edge reinforcement may be calculated using measured geometric characteristics from a simple formula: target distance = nominal distance + measured geometric characteristics.
In parallel the measured geometric characteristics are also used to predict a distance gp between the apex of the blade and the leading-edge reinforcement using a predictive model where the learning process uses pairs for which assembly has already been attempted.
The distance gp may in particular be expressed as a linear function of certain geometric characteristics of the blade and of the leading-edge. The parameters of the linear function are learned using linear regression from data obtained on pairs for which assembly has already been attempted.
The target distance go and the predicted distance gp are then compared. The closer they are, the greater the possibility that the pair can be assembled without linishing. On the other hand, if the difference between these distances exceeds a threshold glim, it may be considered that the pair cannot be assembled.
Knowing go-gp, the probability p that the pair can be assembled without linishing is calculated. This probability p is in effect a function of go-gp and this function may have been learned using pairs for which assembly has already been attempted.
When the pair can be assembled (|g0 — gp| ≤ glim), its fault indicator may be expressed as |gp — |gnominal| + (1 — P) * Coutponcage, where gnominal corresponds to the nominal distance, p is the probability that the pair can be assembled without linishing and Coutponcage corresponds to the cost of linishing (i.e. the quantity of linished material) when |g0 — gp| = glim.
In another embodiment, point clouds which represent the 3D surfaces of the first and of the second parts are used. An attempt to align these point clouds is used to predict the areas which are to be linished (in the case where linishing must be

performed) and therefore the amount of material to be removed. This alignment may be performed according to the techniques described in the thesis by Simon Flöry entitled "Constrained Matching of Point Clouds and Surfaces", 2009. In this embodiment, the linishing cost is estimated more precisely, since the amount of material that will have to be removed can be estimated.
The fault indicators matrix is continuously calculated, that is, it is updated each time the geometry of a new part is known. It is also updated each time a part becomes unavailable (for example because the part has been assembled or scrapped): the line or column corresponding to the part that has become unavailable is then removed from the matrix.
It is moreover possible to take into account the fact that the geometry of certain parts is sometimes known long before the latter become available to be assembled. By way of an example, the leading-edge reinforcements are produced in a facility other than the facility where the station for assembly with the blades is located. The geometry of the leading-edge reinforcements may be measured in the original production facility and therefore known before these reinforcements are delivered to the facility where assembly is carried out. This makes a good overview of "future" reinforcements possible and therefore, as will be described in more detail below, improves prediction of the consequences of the choice of pairs made at a time t on choices which must be made at a later time.
With reference to figure 1, once the fault indicators matrix has been determined, the method according to the invention comprises, for a batch of N assemblies to be made (where N is a positive integer equal to or greater than two), the implementation of an assignment step AFF of N first parts Ai from the plurality of first parts to N second parts Bj from the plurality of second parts to form N pairs {Ai, Bj} of a first part and of a second part. The assignment is performed in such a way that said N pairs minimise a total fault which corresponds to the sum of the estimated fault indicator of each of the N pairs. This assignment is preferably performed so as to minimise the fault in the long term, by performing optimisation which involves anticipating the future.

This assignment step may be performed whenever necessary when an operator wishes to know which pairs must be assembled. This step is typically implemented every day in order to identify the N pairs which have to be made during the day.
This assignment step implements the solution of a linear assignment problem, that of determining N pairs which meet the quantitative objective of minimisation of the total fault of assembly whilst maintaining the production rate (i.e. making N assemblies, for example making 8 assemblies per day) in order not to slow down production.
A linear assignment problem is a problem in which resources must be assigned to tasks. A cost is associated with each resource-task pair, and the pairs must be chosen so as to minimise the overall cost of the assignment. This problem can be written in the form argmin ∑NBi=1 ∑NAj=i xi,j . ci,j,

where ci,j is the cost of assignment of the resource i to the task j, and xi,j decision variables where xi,j = 1 if the resource i is assigned to the task j, and 0 otherwise;
and under constraints according to which:
- each resource is only assigned once at most: ∀ i, ∑j xi,j ≤ 1;
- each task is only assigned once at most: ∀ j, ∑i xi,j ≤ 1 ;
- N assignments must be made: ∑i∑jxij = N.
This assignment problem may be solved using the Hungarian algorithm or in accordance with linear optimisation techniques (simplex algorithm or interior point algorithm, for example).
The assignment problem may take the constraint of compliance with production rates into account in the form of a cost referred to as R which corresponds to the inability to make one of the N assemblies required (there are not enough "assemblable" parts). Thus if a first part and a second part cannot be assembled, their fault indicator is R. A prohibitive indicator may be regarded as making parts incapable of

being assembled, and any indicator estimated as being greater than a threshold is set to R.
The assignment problem may take other constraints into account such as, for example, giving priority to the oldest parts available for assembly. A surface treatment applied to leading-edge reinforcements to improve bond quality means that these reinforcements cannot remain at the assembly station for too long a period. If they remain there more than ΔtBlim days, for example 50 days, they must be scrapped and are therefore no longer available for assembly. The cost of such scrapping is referred to as SB. On the other hand, in order to help monitor production it is preferable that the order of production for the blades is maintained for as long as possible at assembly. Thus a cost is associated with keeping a blade available for assembly (i.e. assembly takes place too late) for a time greater than ΔtBlim, for example greater than 5 days, which is referred to as DA.
This set of costs may be summarised in an objective function to be optimised. Between t = 0 and t = T:
∑TT=0 ∑N(T)P=1 C(B,(t)pA(t)p) + NbB_rebut × SB + NbA_retard × DA
where:
N(t) is the number of assemblies in the batch which must be made at the time t.
B(t)v,A(t)v is one of the pairs formed at the time t by solving the assignment
problem and C(B(t)v, Ae(t)p) is the estimated fault indicator for this pair:
R if assembly is not feasible
C(B(t)p, A(t)p) = {Gij + ||pij - Po|| otherwise
The parts in a pair formed at the instant t are chosen from the parts made available for an assembly (i.e. present at the assembly station); NbB_rebut is the number of reinforcements scrapped between 0 and T; NbA_retard is the number of blades which have been delayed at the assembly station (remaining there more than 5 days, for example) between 0 and T.
Once the N pairs are formed after the assignment step, the operator assembles these pairs. The actual fault of the assembly of these N pairs is recorded.

This information on actual faults of assembly may be used continuously and automatically in the event of the operator becoming aware that a pair that has been formed cannot in fact be assembled. In this case the parts remain at the assembly station and a record is made in memory that they cannot, in fact, be assembled, so as to prevent them being proposed once again. In order for this to occur, the estimated fault indicator of the assembly this pair is corrected in order to associate the cost R with it.
This information about the real pairing costs may also be used discontinuously and non-automatically to re-calibrate the fault indicators prediction model. The estimated indicators predicted by this model are compared with the real costs and this comparison is used to detect any deviation of the prediction model and to re¬calibrate it if necessary by repeating the learning process using new gathered data.
Still with reference to figure 1, in order to give priority to the assembly of certain parts, in one alternative embodiment the assignment step AFF is preceded by one and/or the other of a step SUB for selecting a sub-set of parts and a step CRT for correction of the estimated fault indicators.
The step CRT for correction of the estimated fault indicators reduces an estimated fault indicator for a first part and for a second part in order to favour the selection of this assembly during the assignment step. It provides a matrix of corrected costs Mc* .
The aim of this reduction is, for example, to favour assembly of parts which have been available for assembly for the longest time.
Thus a reduction in the estimated fault indicator of the assembly of a first part with a second part can be performed when the first part has been available for assembly for a time which is greater than a first limit. Thus a time for which a blade has been present in the assembly station for too long may be anticipated, for example of more than 3 days. The corrected indicator for this blade j, for any reinforcement in which can be assembled with the blade j, is stated as Cij = Cij - DA. In other terms, if this blade does not undergo assembly at a time t, then this will cost DA since this blade will be considered to be subject to a delay during the days to come.

Alternatively, and/or in a complementary manner, a reduction is made in the estimated fault indicator of the assembly of a first part with a second part when the second part has been available for assembly for a period which is greater than a second limit. Scrapping of a leading-edge reinforcement may be anticipated when it has been available for assembly for more than, for example, 43 days. The corrected indicator for this reinforcement i, for any blade j which can be assembled with the reinforcement i, is stated as Cij = Cij -SB. In other terms, if this reinforcement does not undergo assembly at a time t, then this will cost SB since this reinforcement will be scrapped during the days to come.
According to other strategies for reducing the estimated fault indicator, knowledge of future parts coming to the assembly station is used to good purpose to favour assembly of parts which are difficult to assemble in comparison with easy-to-assemble parts.
For example, a reduction of the estimated fault indicator of the assembly of a first part with a second part is carried out where said reduction is weighted by a factor which represents a probability that no first part, which becomes available for assembly during availability of the second part for assembly, can be assembled with the second part (i.e. the estimated fault indicator of the assembly of the second part with each of the future first parts is equal to R). The greater the possibility of not finding, in future first parts, a first part which can be assembled with the second part, then the greater the reduction.
Considering a leading-edge reinforcement i available for assembly from agei(t) to the time t, there are Nit blades which will be available for assembly before the reinforcement i is scrapped. These are blades which would enter the assembly station during the next ΔtBlim — agei(t) days. The term pt is the probability that this reinforcement i can be assembled with a blade (this probability may be estimated on the basis of the ability of this reinforcement to be assembled with blades which have previously been available for assembly).
The probability that no blade, which will become available for assembly during the availability of the reinforcement i for assembly, can be assembled with the

reinforcement i is Pti = (1 — pi)Nti. The estimated fault indicator of the assembly of this reinforcement i, for any blade j which can assembled with the reinforcement i, is then corrected in accordance with CiJ = Cij -SB x Pti. This reduction may be accompanied by the reduction favouring the oldest blades as described above.
According to another example of strategy for reducing the estimated fault indicator which makes good use of knowledge of future parts coming to the assembly station, the estimated fault indicator of the assembly of a first part with a second part is reduced when no second part, which is available for assembly with the first part during the availability of the first part for assembly, can be assembled with the first part (i.e. the estimated fault indicator of the assembly of the first part with each of the future second parts is equal to R). This other example may be implemented jointly with the above example including the probability Pit.
Considering a blade j available for assembly from agej(t) to the time t, there is a set of reinforcements Lj which will become available for assembly with the blade j before the blade j is considered to be too old (i.e. within the next ΔtAlim — agej(t) days). If, in the set Lj, no reinforcement can be assembled with the blade j, the fault indicator for any reinforcement i that can be assembled with the blade j is corrected according to Cij = Cij -DA. Otherwise no correction is made.
The following description relates to the step SUB for selecting a sub-set of parts in order to form a sub-matrix of indicators Mc. This selection may in particular involve forming a sub-set of said plurality of first parts made up of the oldest first parts available for assembly. During the assignment step AFF, the N first parts assigned to the N second parts belong to said sub-assembly of said plurality of first parts.
For example, each time that a batch of N assemblies is to be made, the smallest sub-assembly of blades which can be used to make N assemblies with the set of reinforcements is selected. This selection is made by including the blades in the sub-set depending on the length of time they have been in the assembly station. The resulting sub-set is referred to as S(t).

This selection requires knowledge of the maximum number of possible assemblies starting from a given set of blades and a given set of reinforcements. This is possible by representing the sets of blades and of reinforcements in a binary bipartite graph linking the set of blades to the set of reinforcements with an edge i,j if the pair i,j can assembled. An algorithm for maximum cardinality matching in a bipartite graph, such as, for example, the Ford-Fulkerson algorithm, is used to determine the number of possible assemblies.
The step SUB for selecting a sub-set of parts may also involve forming a sub-set of said plurality of second parts made up of the oldest second parts available for assembly. During the assignment step AFF, the N second parts assigned to the N first parts then belong to said sub-set of said plurality of second parts.
For example once the sub-set of blades S(t) is selected, the smallest sub-set, referred to as L(t), of reinforcements which can be used to make the N assemblies with the blades of sub-set S(t) is selected. This selection is made by including the reinforcements in the sub-set L(t) depending on the length of time they have been in the assembly station.
Alternatively, the formation of a sub-set L'(t) of said plurality of second parts is performed in such a way that the second parts of said sub-set L'(t) are used, with the first parts of said sub-set B(t) of first parts, to make M assemblies without machining, where M corresponds to the number of assemblies without machining which can be made with the second parts of said plurality of second parts and the first parts of said sub-set B(t) of said plurality of first parts. During the assignment step AFF, the N second parts assigned to the N first parts then belong to said sub-set L(t) of said plurality of second parts.
In this alternative, once the sub-set of blades B(t) is selected, the smallest sub-set L'(t) of reinforcements which can be used to make N assemblies with the blades of sub-set B(t) and to make M assemblies without linishing is selected. M corresponds to the number of assemblies without linishing that it is possible to make with the blades of the sub-set B(t) and the set of reinforcements. In such a manner the

assignment is carried out with the oldest reinforcements without adversely affecting the number of assemblies that can be made without linishing.
In one practical application of the algorithm, the number M is not in actual fact precisely known, since it is simply possible to estimate a probability that a pair can be made without linishing or not. Thus there is not a number M of assembles that can be made without linishing, but an estimate m of the expectancy of the number of assemblies that can be made without linishing if the pairs are made with the blades of the sub-set B(t) and the set of reinforcements. Thus the sub-set of reinforcements L'(t) is chosen which can be used to make N assemblies with the blades of the sub-set B(t) and give an expectancy m' of the number of assemblies that can be made without linishing (from amongst the N chosen pairs) which is close to the expectancy m : Thus m — m' < ∈ is sought where ∈ is a threshold defined depending on the applications.
The invention is not limited to the method as described above, but also extends to a system for assembling parts chosen from a plurality of first parts and a plurality of second parts, which comprises a calculation unit configured to implement this method, and in particular:
to estimate, for each first part and each second part in the plurality of first parts and the plurality of second parts, a fault indicator of the assembly of the first part with the second part;
to assign, when a batch of N assemblies is to be made, N first parts from the plurality of first parts to N second parts from the plurality of second parts by forming N pairs of a first part and of a second part, said assignment being performed in such a way that said N pairs minimise a total fault which corresponds to the sum of the estimated fault indicators of the assembly of each of the N pairs.
The invention also relates to a computer programme product which comprises programme code instructions for executing the steps of estimating and assigning of the method according to the invention when said programme is executed on a computer.

WE CLAIM:
1. Method of assembling parts chosen from a plurality of first parts and a
plurality of second parts, comprising the following steps implemented by a calculator:
- for each first part and each second part from the plurality of first parts and the plurality of second parts, the estimation (CST) of a fault indicator of the assembly of the first part with the second part;
- for a batch of N assemblies to be made, assignment (AFF) of N first parts from the plurality of first parts to N second parts from the plurality of second parts to form N pairs ({Ai,Bj}) of a first part and of a second part, said assignment being performed in such a way that said N pairs minimise a total fault which corresponds to the sum of the estimated fault indicators of each of the N pairs.

2. Method according to claim 1 wherein the first part is a blade and the second part is a leading-edge reinforcement.
3. Method according to one of claims 1 and 2, wherein the estimation of the fault indicator of the assembly of a first part and a second part comprises the estimation of a quantity of material to be linished to allow the assembly to be made and the estimation of a difference between the relative position of the assembled parts and a nominal relative position.
4. Method according to any one of claims 1 to 3, comprising a step implemented by the calculator of reducing (CRT) the estimated fault indicator of the assembly of a first part with a second part when the first part has been available for assembly for a period which is greater than a first limit.
5. Method according to any one of claims 1 to 4, comprising a step implemented by the calculator of reducing (CRT) the estimated fault indicator of the

assembly of a first part with a second part when the second part has been available for assembly for a period which is greater than a second limit.
6. Method according to claim 1, comprising a step implemented by the calculator of reducing (CRT) the estimated fault indicator of the assembly of a first part and of a second part available for assembly when no second part, which will become available for assembly with the first part during the availability of the first part for assembly, can be assembled with the first part;
7. Method according to one of claims 1, 4 or 7, moreover comprising a step implemented by the calculator of reducing the estimated fault indicator of the assembly of a first part and of a second part available for assembly, said reduction being weighted by a factor which represents a probability that no first part, which will become available for assembly during the availability of the second part for assembly, can be assembled with the second part.
8. Method according to one of claims 1 to 7, comprising a step implemented by the calculator of forming (SUB) a sub-set of said plurality of first parts made up of the first parts available for assembly for the longest time, and wherein the N first parts assigned to the N second parts belong to said sub-set of said plurality of first parts.
9. Method according to claim 8, comprising a step implemented by the calculator of forming (SUB) a sub-set of said plurality of second parts made up of second parts available for assembly for the longest time and wherein the N second parts assigned to the N first parts belong to said sub-set of said plurality of second parts.
10. Method according to claim 9, wherein the formation (SUB) of a sub-set of said plurality of second parts is performed in such a way that the second parts of said sub-set of said plurality of second parts can be used with the first parts of said sub-set of first parts, to make M assemblies without machining, where M corresponds to the number of assemblies without machining which can be made with the second parts of

said plurality of second parts and the first parts of said sub-set of said plurality of first parts.
11. Method according to one of claims 1 to 10wherein the fault indicator of the assembly of a first part with a second part is estimated from geometric characteristics of the first and of the second part.
12. Method according to claim 11, wherein the estimation of the fault indicator of the assembly of the first part with the second part comprises an attempt to align point clouds which represent the surfaces of the first and of the second part.
13. System to assemble parts chosen from a plurality of first parts and a plurality of second parts, comprising a calculation unit, configured:

- to estimate, for each first part and each second part in the plurality of first parts and the plurality of second parts, a fault indicator of the assembly of the first part with the second part;
- to assign, when a batch of N assemblies is to be made, N first parts from the plurality of first parts to N second parts from the plurality of second parts by forming N pairs of a first part and of a second part, said assignment being performed in such a way that said N pairs minimise a total fault which corresponds to the sum of the estimated fault indicators of each of the N pairs,
14. Computer programme product comprising programme code instructions for
executing the steps to estimate and to assign of the method according to one of claims
1 to 12 when said programme is executed on a computer.