**Abstract**

 2.5D pocket machining is extensively used in aerospace, shipyard, automobile, dies and mold industries. In machining of 2.5D pockets, conventional toolpath strategies, such as directional parallel tool path and contour parallel tool path, are generally used. However, these tool paths greatly limit the machining performance in terms of machining time, surface finish and tool wear because of repeated machining direction alteration, stop-and-go motion, sharp velocity discontinuity, and frequent repositioning, retraction, acceleration and deceleration of the tool. To overcome the above-mentioned problems of conventional tool path strategies, many tool path strategies are proposed of which spiral tool path is most widely used. The spiral tool path provides an efficient way to produce a pocket, but it suffers from some problem such as bottleneck in a narrow region, variation in step over and cutter engagement angle (hence, change in cutting load), etc. This change in cutting load affects surface roughness. Hence, a review for feed rate optimization of spiral tool path using cutter engagement angle and step over is discussed.

**Keywords:** pocket machining, cutter engagement angle, step over, feed rate optimization, spiral tool path, high speed machining (HSM)

#### **1. Introduction**

 Generally, the material inside a closed region on a flat surface of a workpiece is removed up to a fixed depth using flat bottom end mills in pocket machining. Roughing operation is done to cut the volume of material at the initial stage and after that the pocket is finished by a finish end mill. Most of the industrial milling operations (up to 80%) can be machined by 2.5D CNC milling [1]. Pocket machining (**Figure 1**) refers to making an inner empty volume starting from a workpiece face. The created empty volume is known as a pocket. A cutter refers to a milling cutter that is used to create a pocket on the workpiece. It also can be seen that the material is removed from stock, layer by layer until pockets are formed and a manufactured part emerges [2].

 Feed or feed rate is the "relative velocity at which the cutter advances into the workpiece and in a given direction", which is measured in millimeters per minute (mm/min), or inches per minute (in/min). Step over as shown in **Figure 2** is a

*A Review for Feed Rate Optimization of Spiral Tool Path Using Cutter Engagement Angle… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

**Figure 1.**  *Representation of pocket machining [3].* 

**Figure 2.**  *The term step over [4].* 

 milling parameter that is defined as "the distance between two neighboring passes over the workpiece". Step over is also used to calculate the number of tools being passed and cut again on a finished surface [4]. Cutter engagement angle (α) is defined as the "angle which spans the part of the tool surface that performs cutting" as shown in **Figures 3** and **4** [5]. **Figure 4** shows that the cutter engagement angle varies depending on the availability of tool path and shape of pocket required.

The spiral tool path starts as a spiral from the center of pocket geometry and takes the shape of the pocket boundary. The spiral tool path strategy is expected to machine pocket with lower tool path length, lower machining time, less tool wear, capacity to extract the material at corners, the capability of a tool path to reduce variation in step over, etc. The machining parameters such as feed, speed, depth of cut, tool path strategy, step over, engagement angle, etc., affect machining time, surface roughness, tool wear, accuracy and efficiency of the tool path so the selection of these parameters is important and it should be precisely selected and controlled.

 High speed pocket machining (HSM) is described as end milling with small diameter tools (less than 10 mm) at a high rotational speed (greater than 10,000 rpm) and power is greater than 10 HP compared to conventional techniques.

#### **Figure 3.**

*(a) Straight segment of the tool path (b) Curved segment of the tool path with cutter engagement angle (α) [5].* 

#### **Figure 4.**

*Changes of cutter engagement angle (α) at different tool motions [6].* 

#### **1.1 Advantages of high speed pocket machining**


#### **1.2 Limitations of high speed pocket machining**


*A Review for Feed Rate Optimization of Spiral Tool Path Using Cutter Engagement Angle… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

### **2. Literature review**

 Bieterman and Sandstrom have described a novel curvilinear tool path generation method but it is not suitable for pockets that are too concave. They showed experimentally that morphing leads to reductions in tool wear when cutting hard metals and machining time up to 30% in cutting all metals [2]. Topal and Eyüp Sabri have showed the role of the step over ratio in surface roughness prediction studies. It was found that surface roughness values are influenced by step over and it could not be possible to predict the surface roughness precisely without considering step over [4]. Held, Martin and Christian Spielberger proposed a geometric heuristic to disintegrate an arbitrarily complex pocket with or without an island into uncomplicated sub-pockets by considering cutter engagement angle and step over for acceptable tool path for efficient spiral high-speed machining. They showed that pocket disintegration improved quality parameters, improvement in step over variation, the maximum slope of the engagement angle and the tool path length [5]. Kloypayan, Jirawan and Yuan-Shin Lee have presented a technique for analyzing the material removal rate when a tool moves in linear, circular or parametric curved motions. They showed that optimal material removal rate is obtained by increasing the feed rate in the ball end milling process [6].

 Gupta et al. suggested a geometric algorithm described for computing piecewise stable closed-form cutter engagement functions for 2.5D milling operations to determine an efficient cutter path and to improve it. Further results are compared with discrete simulations which showed that cutter engagement angle increases in circular cuts and linear half-spaces of the same [7]. Dharmendra and Lalwani created a spiral tool path of elliptical pockets for CNC machine using PDE. It was found that machining time depends on the ratio of the major to minor axis of the elliptical pocket and tool path length also increased [8]. Further, they carried out an investigation on AISI SS material using a spiral tool path for elliptical pockets. They reported that tool path length, tool utilization, machining time and surface roughness etc. was affected by variation in the shape of pockets and different tool path strategies [3].

 Stori and Wright proposed a generative spiral-in algorithm for 2.5D material removal of convex geometries. It was found that there were decreased changes in cutting forces during cornering and pocketing application [9]. Kaymakci et al. found that the cutting forces for complicated sculptured surfaces can be forecast precisely using new forced based feed rate scheduling strategy. It was validated experimentally that the cycle time of sculptured surface machining is reduced up to 38.1% greatly with forced based feed scheduling strategy [10]. Bae et al. proposed an automatic feed rate adjustment method for cutting force model, they showed that cutting load and feed rate can be easily determined without any computations [11]. Chen et al. proposed the feed rate optimization of the high-speed ball end milling process. It was found that the feed-interval scallop height limits the feed rate for high efficient machining with use of advanced tool technology and if the specifications of the notch cut profile can be optimized then the feed rate can be increased more [12].

 Dong et al. investigated the chances of scheduling or changing the feed rate by considering the geometry of the contour. By regulating the magnitude of cornering errors, the different jerk constraints were fixed [13]. Gong and Feng invented a new method to resolve the cutter workpiece engagement for general milling processes which were validated by a series of case studies of increasing machining complexity to indicate its usability to general milling processes [14]. Ko and Cho invented an analytical model of off-line feed rate scheduling to resolve desired feed rates for 3D ball-end milling. It was found that the presented feed rate scheduling model

 decreased machining time with more accuracy [15]. Merdol and Altintas presented the modeling of process simulation in a potential environmental and computationally effective algorithm for optimal selection of machining specifications while considering process constraints and changes in the part geometry along the tool path. Results showed a maximum material removal rate and productivity without affecting torque, power, and tool deflection limits [16].

Uddin et al. invented a new tool path modification plan for constant engagement with a workpiece in 2.5D end milling. Machining results were analyzed and results showed that this plan can increase the machining accuracy related to feeding rate control strategies [17]. Tounsi and Elbestawi suggested an optimized feed scheduling approach increasing the metal removal rate in 3-axis machining while assuring the machining accuracy. It had been concluded that this approach gives great adjustment of the cutting force and develops the reference tool locations for a given tool path [18]. Desai et al. suggested a technique to minimize surface error variation due to engagement changed in pocket end milling by adjusting machining approach for the curved components. It was observed that the suggested technique is quite effective for reducing the variation in surface error [19]. Pateloup et al. suggested a method to minimize machining time and tool path advancement by adjusting the corner radius using computer-aided manufacturing system. It was concluded that the use of B-spline for the tool path calculation is a significant advancement related to lines and arcs [20].
