# Spare Parts Made by Additive Manufacturing to Improve Preventive Maintenance
## 2 Methodology
### Proposed Process for Preventive Maintenance Using AM  
In a manufacturing company, when the production process stops, it is because of a problem; when it occurs, the nature of the problem must be defined immediately [1]. If the problem is external to the spare parts, a solution is required to continue the production. If the problem is related to a spare part, the process begins by checking the inventory for the needed spare part. If it is present in the inventory, the technicians or the engineers take it directly from the inventory and replace it. However, if the spare part is not available, they print it using an adequate 3D printer for its posterior use. Figure 1 is a flowchart showing how the decision to print a spare part using AM technologies is made. In preventive maintenance, as an improvement on corrective maintenance, technicians must detect the problem as early as possible in order to prevent the problem. Then they could analyse if the spare part could be printed and know the print time (PT) and the replacing time (RT) and incorporate them in the manufacturing chain before the production stops, as shown in Figure 1. Ideally, the parts should be printed at the company in a minimum amount of time to save costs, or have a local supplier of AM spare parts.  
Figure 1: Flowchart decision for AM spare parts in Preventive Maintenance.  
Figure 2 shows the systematic replacement of printed spare parts respecting a calendar or time intervals. For example, every week, it depends on the mean of the printed part working time _(MPPWT)_ where its formula is expressed in Equation (1).  
\[MPPWT=\sum_{i=1}^{N}\frac{PPWTi}{N}. \tag{1}\]  
* _MPPWT_: Mean printed part working time;
* _PPWTi_: Print part i working time;
* _A1...An_: Schedule time to change the printed spare part.  
The scheduled time to change the printed spare part is the sum of the printing time (PT), the replacing time (RT) and the mean printed part working time (MPPWT). As can be seen in Figure 2, this time goes from the A2 point to the start A4 point. The printed part 1 life starts from end A1 when its manufacturing begins; it takes a certain time for the part to be printed and ready to use, then it is taken into the equipment to replace the old part, which takes a replacing time and, finally, the object can start functioning in the equipment. The working lifetime of the printed part 2 is larger than the working life of the printed part 1 due to the initial decision to print not being taken into account and this PT is always used in scheduled intervals of time.  
The MPPWT is the working time of the printed part or the time that it is operational on the equipment. As a condition, the 3D printer must be within the company that will use this type of preventive maintenance. One of the advantages of this type of preventive maintenance is the on-demand printing of parts, which results in good working conditions for the stock, as spare parts are released due to just-in-time on-demand printing [32]. If there is a need to replace old equipment that uses regular spare parts already in the company's stock, it will cause a loss of money that can be avoided by printing the parts just in time when the moment for replacement is near. However, many companies prefer to purchase their spare parts instead of manufacturing them because of money issues. Even for this problem, the solution may lie in providing printed parts from 3D printing manufacturers.