Chapter 20 Fundamentals of Machining/Orthogonal Machining (Part I Review) EIN 3390 Manufacturing Processes Fall, 2010

20.2 Fundamentals Variables in Processes of Metal Cutting: Machine tool selected to perform the processes Cutting tool (geometry and material) Properties and parameters of workpiece Cutting parameters (speed, feed, depth of cut) Workpiece holding devices (fixture or jigs)

FIGURE 20-1 The fundamental inputs and outputs to machining processes.

20.2 Fundamentals 7 basic chip formation processes: shaping, turning, milling, drilling, sawing, broaching, grinding (abrasive)

FIGURE 20-2 The seven basic machining processes used in chip formation.

FIGURE 20-2 The seven basic machining processes used in chip formation.

20.2 Fundamentals Responsibilities of Engineers Design (with Material) engineer: determine geometry and materials of products to meet functional requirements Manufacturing engineer based on material decision: select machine tool select cutting-tool materials select workholder parameters, select cutting parameters

20.2 Fundamentals Cutting Parameters Speed (V): the primary cutting motion, which relates the velocity of the cutting tool relative to the workpiece. For turning:V =  (D 1 N s ) / 12 where, V – feet per min, N s – revolution per min (rpm), D 1 diameter of surface of workpiece, in. Feed (f r ): amount of material removed per revolution or per pass of the tool over the workpiece. In turning, feed is in inches per revolution, and the tool feeds parallel to the rotational axis of the workpiece. Depth of Cut (DOC): in turning, it is the distance that the tool is plunged into the surface. DOC = 0.5(D 1 – D 2 ) = d

FIGURE 20-3 Turning a cylindrical workpiece on a lathe requires you to select the cutting speed, feed, and depth of cut.

20.2 Fundamentals Cutting Tool is a most critical component used to cut the work piece selected before actual values for speed and feeds are determined. Figure 20-4 gives starting values of cutting speed, feed for a given depth of cut, a given work material, and a given process (turning). Speed decreases as DOC or feed increase Cutting speed increases with carbide and coated- carbide tool material.

FIGURE 20-4 Examples of a table for selection of speed and feed for turning. (Source: Metcut’s Machinability Data Handbook.)

20.2 Fundamentals To process different metals, the input parameters to the machine tools must be determined. For the lathe, the input parameters are DOC, feed, and the rpm value of the spindle. N s = 12V / (  D 1 ) = ~ 3.8 V/ D 1 Most tables are arranged according to the process being used, the material being machined, the hardness, and the cutting-tool material. The table in Figure 20-4 is used only for solving turning problems in the book.

20.2 Fundamentals DOC is determined by the amount of metal removed per pass. Roughing cuts are heavier than finishing cuts in terms of DOC and feed and are run at a lower surface speed. Once cutting speed V has been selected, the next step is to determine the spindle rpm, N s. Use V, f r and DOC to estimate the metal removal rate for the process, or MRR. MRR = ~ 12V f r d where d is DOC (depth of cutt). MRR value is ranged from 0.1 to 600 in 3 /min.

20.2 Fundamentals MRR can be used to estimate horsepower needed to perform cut. Another form of MRR is the ratio between the volume of metal removed and the time needed to remove it. MRR = (volume of cut)/T m Where T m – cutting time in min. For turning, T m = (L + allowance)/ f r N s where L – length of the cut. An allowance is usually added to L to allow the tool to enter and exit the cut. MRR and T m are commonly referred to as shop equations and are fundamental as the processes.

20.2 Fundamentals One of the most common is turning: workpiece is rotated and cutting tool removes material as it moves to the left after setting a depth of cut. A chip is produced which moves up the face of the tool.

FIGURE 20-5 Relationship of speed, feed, and depth of cut in turning, boring, facing, and cutoff operations typically done on a lathe.

20.2 Fundamentals Milling: A multiple-tooth process. Two feeds: the amount of metal an individual tooth removes, called the feed per tooth ft, and the rate at which the table translates pass the rotating tool, called the table feed rate f m in inch per min. f m = f t n N s where n – the number of teeth in a cutter, N s – the rpm value of the cutter. Standard tables of speeds and feeds for milling provide values for the recommended cutting speeds and feeds and feeds per tooth, f r.

FIGURE 20-6 Basics of milling processes (slab, face, and end milling) including equations for cutting time and metal removal rate (MRR).

FIGURE 20-7 Basics of the drilling (hole-making) processes, including equations for cutting time and metal removal rate (MRR).

FIGURE 20-9 (a) Basics of the shaping process, including equations for cutting time (Tm ) and metal removal rate (MRR). (b) The relationship of the crank rpm Ns to the cutting velocity V.

FIGURE Operations and machines used for machining cylindrical surfaces.

FIGURE Operations and machines used to generate flat surfaces.

Should consider allowance