## 1 Introduction  
The nuts and bolts industry is industrially significant for Taiwan, which was the third largest global exporter in 2020 [1]. The prominence of nut manufacturing is supported by machines kept in good operating condition and production to a high standard. Research to improve fasteners and their machine performance has focused on wear simulation or improving die life through material improvement. Studies on material improvement included the combination of different materials to create a stronger mould and die [2; 3] and the use of heat treatments [4; 5; 6]. The simulation of die wear for a nut manufacturing machine was done through a finite element (FE) analysis that forecasted the parts of the die which experienced wear more rapidly [7]. [PERSON] et al. [8] performed FE numerical wear analysis to calculate tool wear and also improve the design of the mould to extend its manufacturing life. [PERSON] et al. [9] also used FE analysis to evaluate the stresses in the forging tools used to deform the material for a cold heading mould which constantly failed. The study was able to optimize new geometry for the mould. A study was done on the die impact in sheet cold forming. It discovered that the die damage can be controlled by the back-up plate, and die rupture results from the thermo mechanical fatigue at the die head [10]. Another study focused on improving the geometry of moulds to increase their lifespan by using finite element analysis to identify the source of premature breakage in order to modify the die profile to increase productivity [11]. [PERSON] et al. [12] monitored screws to identify defective products by integrating an automated optical inspection unit to check screw dimensions after they were produced. [PERSON] et al. [13] focused on how to target the misalignment of moulds, which affects the quality of screw production. To facilitate this, an optical measuring instrument and vector decomposition--for dimensional accuracy--along with other parameters, were input into a system which allowed the mould case to adjust its position to create better quality products. Most recently, [PERSON] [14] developed a real-time monitoring system which measured the forging force of a screw-forming machine and established upper and lower limits of the mean forging force of the first three moulds. When the limits were exceeded, the machine would halt manufacturing. Although recent studies have been helpful in the fastener industry, there are still drawbacks, as the studies have only identified damage after the product has been manufactured, leading to sub-optimal quality control. Moreover, much of the focus was on the moulds of the particular machines. In contrast, this research focused on condition monitoring using die force data from a six-die nut manufacturing machine during commercial operation to detect anomalies that are indicative of not only mould damage, but also die damage, quality degradation, and damage to other machine components, in order to facilitate maintenance when necessary, thereby optimising machine operation and maintaining product quality integrity. Maintenance is key to the proper functioning of the nut manufacturing machine. Generally, this industry practices either run-to-failure (RTF) maintenance [15; 16] or condition maintenance [17; 18]. Run-to-failure maintenance addresses die or non-die component damage after the parts fail due to tribological causes, cracking, breakage, other deformations and misalignment, all of which impede production and proper quality control. This not only results in several hours or days of machine downtime based on the damage severity, but also substandard products. In condition maintenance, a product sample is examined routinely to check dimensional accuracy. Adjustments may be made to correct for tolerance, and the dies may be checked for wear.