The cam-follower mechanisms have wide applications in different kinds of machines such as alternative combustion engines, sewing machines, machine tools, etc., due to their advantages. For instance, they are simple and compact mechanisms, they are cheap, they have few mobile parts, and the follower displacement law is easy to design. The cam mechanisms have three parts: a cam, a follower, and a fixed frame [
1]. The most common cam-follower mechanism that is employed in combustion engines consists of a disk cam and a follower with translating or oscillating movement (
Figure 1a). The cams for combustion engines are usually symmetric, while the ones that are used in high speed applications or aeronautics are asymmetric [
2]. The cams for combustion engines can be classified into three categories: convex flank cams, tangential flank cams, and concave flank cams. In the present work, a tangential flank cam has been used (
Figure 1b).
In the past, the available technologies made it difficult to obtain cams with precise geometries and surface finish. At present, thanks to the numerical control (NC) machines, this phase has been successfully overcome. Today the tedious calculations of the cam design process are assisted by software that allows to obtain the profiles and the graphics in a faster and more precise way [
2]. In the 70s of the last century, Grant has analyzed the different methods to manufacture cams [
3]. He classified them into three categories: methods for original cams (using conventional machines and manual finish operations), methods for copied cams (using milling machines with indexing heads and/or copying mechanisms, as well as grinding machines), and methods that used numerical control machines. These machines have evolved considerably over time until today, and its use has become widespread in the field of cam manufacturing. On the other hand, for large batches, other manufacturing processes are employed, like forging [
4], sintering [
5], etc. Wire Electric Discharge Machining (WEDM) consists of using a thin wire of a conductive material like brass, which is wound between two spools, in order to cut a conductive material using electrical discharges. A dielectric fluid such as water allows isolating the wire from the workpiece to achieve a high current density, it cools the heated surface of the wire and it removes the material particles. Due to the high temperatures reached, the surface layer melts and then it quickly cools because of the fluid, leading to the so-called recast layer [
6]. Heat can also induce tensile residual stresses, cracks, or microporosity on the parts’ surface [
7,
8]. Some advantages of WEDM processes are that they allow machining different conductive materials regardless of their hardness [
9], for example, titanium alloys used in biomedical applications [
10], and that it is appropriate for delicate or fragile materials, because it eliminates the mechanical stresses that are present in traditional machining processes [
11]. Besides, complex shapes and thin-walled parts can be obtained, such as noncircular gears, including elliptical and oval gears [
12] or inclined surfaces that are used in tooling that requires draft angle [
13]. Moreover, WEDM avoids the need to manufacture the pre-shaped electrodes that are necessary for sink electrical discharge machining (SEDM) [
14]. In recent years, WEDM has started to replace conventional machining in some rough operations, for example, for the manufacture of blades, with the advantage that the amount of material to be removed in subsequent operations is considerably reduced [
15]. The main parameters of the WEDM process are: discharge current, servo voltage, pulse on and off time, and wire feed, among others. Several authors have studied the effect of WEDM parameters on surface roughness. For example, Hegab et al. [
16] optimized material removal rate, wear electrode ratio, and average surface roughness
Ra as a function of machining on-time, discharge current, voltage, total depth of cut and percentage in weight of carbon nanotube composites (CNT) added to an aluminum base. Sonawane et al. [
17] considered WEDM parameters such as pulse-on time, servo voltage, pulse-off time, peak current, feed rate, and cable tension and responses like surface roughness, overcut and material removal rate, in Nimonic-75 alloy. Magabe et al. [
18] investigated the effect of spark gap voltage, pulse on-time, pulse off-time, and wire feed on the material removal rate and surface roughness of a Ni-Ti shape memory alloy. Chaudhary et al. [
19] considered pulse-on time, pulse-off time, and current as process parameters, whereas material removal rate (MRR), surface roughness, and micro-hardness as responses, in superelastic Nitinol shape memory alloy.
Measurement of the shape error of cams is important because better accuracy of the cam profile obtained will favor the compliance of the follower with the motion-law and, consequently, a better operation of the mechanism. Most authors specify a range of tolerances for the manufacture of a cam by means of geometric tolerances, like the shape error, that is, they define a band along the entire contour that validates the functionality of the piece if the measured points on the contour are within the marked band. For example, Norton stated that shape error of ±25 μm or lower (total error of 50 μm) is appropriate for cams [
2]. Kang and Han [
20] measured form deviation errors of a disk cam on a profile-measuring machine. They found a total form deviation error of 0.1874 mm. Hsieh and Lin [
21] proposed and validated a method to carry out the automatic measurement of a cylindrical cam on a coordinate measuring machine (CMM); the method uses a homogenous transformation matrix to derive the CMM ability function matrix and the measuring probe location matrix. The cam was machined on a four-axis vertical machine tool. The maximum deviation between the experimental and theoretical profiles was 21 μm. Other authors, such as Chang and Wu, specify measurements based on dimensional tolerances [
22]. The maximum radial dimension of a cam was set to 19.05 mm in this case. Taking into account IT9 for WEDM and IT7 for CNC machining processes, tolerance amplitude values of 52 μm and 21 μm, respectively, were defined. In another example, Le and Nguyen Tank [
23] obtained dimensional tolerances for a milled planar cam (with a maximum dimension of 150 mm) of at least 0.018 mm, corresponding to IT5. Regarding the shape error of WEDM curved surfaces. Werner [
24] manufactured internal contours and measured shape errors of the profile with a coordinate measuring machine (CMM). He found deviation values between +0.008 mm and −0.010 mm with respect to the theoretical curves. In a comparative study regarding miniature brass gears manufactured using either WEDM and hobbing [
25], it was observed that WEDM gears had higher quality than hobbed gears. Form error found was 5.4 μm for the WEDM gears [
26].
As for surface roughness of cams, Islam et al. [
27] made a numerical approach based on mixed lubrication concept considering roller sliding to predict the friction, taking into account the friction from cam/roller contact, cam bearings, valve/guide, needle roller bearing, and the effect of asperity interaction and roller sliding. A reduction in friction was obtained with a rise in the camshaft speed. Furthermore, they observed that an increased asperity interaction at high operating temperature yielded to relatively higher friction magnitude, especially in the nose of the cam profile, due to a lower curvature radius. Torabi et al. [
28] presented a thermo-elasto-hydrodynamics analysis to study the behavior of the cam-and- follower contact, particularly during the running-in period. Their results have shown that the rate of flattening of surface roughness is a crucial factor influencing thermal effects during the running-in period. Therefore, according to the literature cited here, it is important to control the asperity (surface roughness) in the cam profile. Surface finish also influences the lubrication and aesthetic appearance of the cam. In contacts such as those between cam and follower, high surface roughness can lead to high local pressure and a reduction in the real contact area between cam and follower [
29]. For this reason, low roughness is required in this case. Agulló and Cardona [
30] stated that, when the unit cost is important, an appropriate surface finish for steel cams would be
Ra < 0.4 μm. Traditionally, a grinding operation was necessary in order to achieve such roughness values [
2]. In a comparative study for cams,
Ra = 1.09 μm were obtained in the outer surface of a milled cam, and
Ra = 1.42 μm in a WEDM cam, both made of AA6063 aluminum [
31]. In another comparative study between hobbing and WEDM for the manufacture of gears, better surface finish was found for WEDM [
26]. The average surface roughness was 1 μm, and maximum surface roughness was 6.4 μm in this case. In another work about gear prototypes,
Ra = 0.42 μm was reported for the WEDM parts [
32].
Machining processes have traditionally been used to obtain cams. However, other technologies like electric discharge machining could replace conventional cutting processes with important advantages, such as the possibility to use hard materials, the possibility to design more complicated shapes, and the reduction of mechanical stresses. Until now, few works are known about the manufacture of cams using WEDM [
31]. The main objective of the present paper is to consider the possibility to manufacture tangential flank cams by means of WEDM, and to compare them with milled cams, in terms of surface finish and shape error of their external contours. The geometry of a cam profile has been designed, taking as reference a real cam from a camshaft of a motorcycle engine. The structure of the paper is as follows:
Section 2 describes the methods that are used to manufacture the cams, by means of either milling or WEDM, as well as the way the roughness and shape error measurements were performed.
Section 3 shows the results of visual inspection, surface roughness, and shape error of the cams;
Section 4 and
Section 5 contain the discussion and conclusions, respectively.