Emergency surgeries often require rapid provision of patient-specific implants. Delivery of customized dental prosthetics (Figure 1), for example, must take place within a few days to ensure successful surgery and patient care. In production companies such as MACK Dentaltechnik GmbH, which manufacture such dental prosthetics, fast throughput times must therefore be implemented. The creation of tool paths and the selection of process manipulated variables in the CAM system have a major influence on the throughput time. Automated and adaptable process planning helps to significantly accelerate the production of individual parts, which ensures high productivity.
In the TempoPlant research project, the Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover, together with MACK Dentaltechnik GmbH, has developed and tested an industry-oriented solution for fully automated adaptive path planning, using the production of dental crowns as an example. Knowledge from the manufacturing process on the relationship between process parameters and component quality is being integrated. In this way, ideal process parameters for achieving component tolerances will be selected before production in the future.
Time-consuming process planning by qualified personnel
Software tools such as CAD/CAM systems, post-processors and numerical control simulations are used to plan the necessary movements of the machine tool. CAM planning represents the bulk of the workload. Here, design features – such as holes, pockets, slots, radii and chamfers – must be assigned appropriate manufacturing operations, such as face milling, contour milling or drilling, to produce these features in the part. Figure 2 illustrates that in most manufacturing operations, the assignment of manufacturing operations to design features is done manually by skilled operators.
The time-consuming manual assignment of manufacturing operations in the CAM system reduces productivity and is associated with high costs. Therefore, a method for automated process planning was developed and tested in the TempoPlant project.
Scientists generate process knowledge from machining tests
The method described below was created in the commercially available CAD/CAM software Siemens NX 2206. With reference to the production of dental crowns, the method for automated adaptive path planning is developed and tested on a tightly toleranced fitting surface of the dental crowns (Figure 1) in the form of a cone. The dental crown is later connected to the patient’s jaw via this fitting surface.
The knowledge regarding the effects of different process parameters on quality parameters of the component to be manufactured was generated by machining tests. This knowledge will be used in future CAM planning processes to achieve the tight manufacturing tolerances of the dental crowns without run-in processes. The process knowledge was stored in Siemens NX’s own database, the so-called Machining Knowledge Editor (MKE) (Figure 3).
The design feature “taper” in the CAD system including product and manufacturing information (PMI) is first saved as a template. In a next step, the initial CAM planning is performed manually for a reference part. The reference part consists of a block with 16 conical holes of the same dimensions. The process parameters cutting speed vc and penetration depth ap are varied in the CAM planning for different conical bores. The conical bores are then machined on a 5-axis DMG Milltap 700 machining center and measured using, among other things, a MarSurf PS 10 roughness tester using the contact cutting method.
Influence of cutting speed and depth of cut on surface quality
The results of the experimental tests on the influence of the process parameters vc and ap on the surface quality in the form of the arithmetic center roughness value Ra are shown in Figure 4. The target value Ra indicates the firm fit of the tooth crown in the jaw and is therefore highly relevant.
The test results show that both increasing the cutting speed and reducing the depth of cut result in significantly lower roughness values. Accordingly, it can be stated that of the process parameters tested, the combination of ap = 0.75 mm and vc = 105 m/min provide the best results with regard to the surface finish of the implant. These results are stored in the MKE in the corresponding manufacturing operation to produce the final contour for the dental crown, as shown in Figure 3.
Process chain of automated adaptive CAM planning
With the method developed and researched in the TempoPlant project (Figure 5) for adaptive CAM planning, an NC code can now be generated automatically. With its help, the fitting surface of a dental crown can be produced directly within the manufacturing tolerance. The CAM planning software carries out several steps automatically in the background. First of all, automatic feature recognition is carried out on the basis of the CAD file. The Siemens NX feature recognition system detects the feature and saves it internally in the program.
Subsequently, a form element process is automatically applied to the design feature. The associated manufacturing operations and process variables are loaded directly from the MKE. After this, the tool paths are automatically calculated and translated into a machine-readable format by means of a postprocessor. The resulting NC code is automatically saved so that it can be transferred directly to the machine.
Producing dental crowns faster and more economically
The method developed makes it possible to constantly expand the MKE database with process knowledge and to define new features in addition to standard design features and make them available to future CAM planning scenarios. This eliminates time-consuming manual steps in the production of dental crowns and significantly increases cost-effectiveness.
It has been shown that automated CAM planning can save up to 88% of the time required for planning compared to a conventional approach. In the future, IFW will further explore the developed system by adding additional process parameters and extend it with freeform surfaces as design features.