# Spare Parts Made by Additive Manufacturing to Improve Preventive Maintenance
## 3 Results: Case Study
### Selected Lists  
Due to the high number of parts in the database, and in order to address the problem of applying AM to spare parts, it is necessary to first select the parts that allow us to parameterize the problem (Figure 5). For this purpose, the following six lists were defined and selected:  
* The first list (1): this family of parts is chosen because all the parts present in this family are owned by the car manufacturing company, so the copyright issue will be avoided, as the study requires the reproduction of some spare parts with 3D printing technology. The method of selection of parts from this list was as follows:
* Unit price equal to or higher than 80 \(\upmu\). Knowing that the 3D printing manufacturing process is more expensive than conventional manufacturing processes, a price limit was set to select all parts that are equal or cost more than 80 \(\upmu\);
* High consumption. The company showed a high interest in parts with high consumption, especially in the expensive spare parts for cost-saving purposes. The 95% of the parts selected in the first list was made up of mechanical construction spare parts, such as gears, cylinders, shafts, pilots, transmission wheels, welding electrodes, etc. Many of them are metallic, which are difficult and expensive to print using AM technologies. All the mentioned parts were manufactured with high precision and require a very smooth surface due to their functionality, which consists of being used in tight assemblies. The tolerance levels of these assemblies are highly respected, there is no margin for error when it comes to mechanical construction assemblies, as they have a high and critical impact on production machines. These spare parts are heavy, which means they require a considerable amount of material that can be expensive for AM metal parts and that can take a long time to manufacture depending on the 3D printing process used. This situation can contribute negatively to the maintenance time by delaying the repair time. Other assemblies have been found, such as wheels, which are made by assembling two types of materials, metal and polymer, for example, steel and polyurethane. Some of them are a mixture of other assemblies that are, at the current time, impossible to 3D print, such as the hydraulic cylinder, which is an assembly containing more than five parts. Another example, such as air conditioning tubes containing metal fibres immersed in polymer, cannot be printed, as it is still very difficult to print two different types of materials at the same time, especially when it comes to metal and polymer. The 5%, which is the remainder of the first list, represents consumables, such as tools that are hardly used. These parts are not good candidates for 3D printing because they are more expensive to print than regular parts;
* The second list (2): it is a list of parts that do not contain any technical information in the database (no drawings). Their 3D printability was studied by directly and physically checking them in the inventory to know if they are good candidates for 3 Dprinting. Most of the parts in this list have some characteristics like the ones in the first list. This list contains many mechanical construction parts such as shafts, gears, etc. ;
* Third list (3): the printability of the parts in this list was studied and some of them were selected as good candidates for 3D printing. They are different from the previous lists and a few of them present some similarities with the previous ones. This last selected list contains parts that have these characteristics:
* High Hardness: the parts that were found in the list are very hard and were post-processed by being heat-treated to be used in difficult situations, for example, parts such as electrode sharpeners which are made of very hard material used for sharpening welding electrodes.