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Chapter 24 Milling Processes (Review) EIN 3390 Manufacturing Processes Summer A, 2012

Chapter 24 Milling Processes (Review) EIN 3390 Manufacturing Processes Summer A, 2012. Milling is the basic process of progressive chip removal to produce a surface. Mill cutters have single or multiple teeth that rotate about an axis, removing material.

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Chapter 24 Milling Processes (Review) EIN 3390 Manufacturing Processes Summer A, 2012

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  1. Chapter 24 Milling Processes (Review)EIN 3390 Manufacturing ProcessesSummer A, 2012

  2. Milling is the basic process of progressive chip removal to produce a surface. • Mill cutters have single or multiple teeth that rotate about an axis, removing material. • Often the desired surface in obtained in a single pass of cutter or workpiece with very good surface finish. • Milling is particularly well suited and widely used for mass production. • More flat surfaces are produced by milling than by any other machining processes. 24.1 Introduction

  3. Milling is classified in two categories: • Peripheral milling (also called Slab milling) - the surface is generated by teeth located on the periphery of the cutter body. The surface is parallel with the axis of rotation of the cutter. • End milling: also called facing milling, the surface generated is at a right angle to the cutter axis. Material is removed by the peripheral teeth and the face portion providing finishing action. 24.2 Fundamentals of Milling Processes

  4. FIGURE 24-1 Peripheral milling can be performed on a horizontal-spindle milling machine. The cutter rotates at rpm Ns , removing metal at cutting speed V. The allowance for starting and finishing the cut depends on the cutter diameter and depth of cut, d. The feed per tooth, ft and cutting speed are selected by the operator or process planner. Peripheral Mills

  5. The milling variables, such as cutting speed V and feed per tooth depend upon the work material, the tool material, and the specific process. The rpm of the spindle is determined from the surface cutting speed V, the cutter diameter D (in inch) as below: Ns = (12V)/(p D) The feed of table fm, in inch per minute, is calculated: fm = ft Ns n Where ft, feed per tooth, and n is the number of teeth in the cutter. The cutting time is: Tm = (L + LA)/fm Peripheral Milling

  6. The length of approach is: LA = SQRT[D2/4 – (D/2-DOC)2 ]= SQRT(d(D-d)) The MRR is: MRR = Volume/Tm = (LWd)/Tm = Wfmd in3/min where W is width of the cut in inch, d is the depth of cut in inch. If ignoring LA, the values for ft are given in Table 24-1, along with recommended cutting speeds in feet per minute. Peripheral Milling

  7. Suggested Starting Feeds and Speeds using HSS and Carbide Cutters

  8. FIGURE 24-2 Face milling is often performed on a spindle milling machine using a multiple-tooth cutter (n = 6 teeth) rotating Ns at rpm to produce cutting speed V. The workpiece feeds at rate fm in inches per minute past the tool. The allowance depends on the tool diameter and the width of cut. Face Mills

  9. The rpm of the spindle is determined from the surface cutting speed V, the cutter diameter D (in inch) as below: Ns = (12V)/(p D) The feed of table fm, in inch per minute, is calculated: fm = ft Ns n Where ft, feed per tooth, and n is the number of teeth in the cutter. The cutting time is: Tm = (L + LA+ L0)/fm The MRR is: MRR = Volume/Tm = (LWd)/Tm = Wfmd in3/min For a setup where the tool doesn’t completely pass over the workpiece, L0 = LA = SQRT(W(D – W)) for W < D/2 L0 = LA = D/2 for W>=D/2 Face Milling

  10. For a 4” diameter, six-tooth end mill, using carbide inserts (Fig 24-3), the work material is low-alloy steel, annealed (BHN = 200). Please determine rpm at the spindle and the feed rate of the table. Face Milling Example

  11. Using cutting data recommendations, select V = 400 sfpm with a ft = 0.008”/tooth. Ns = (12V)/(p D) = (12 x 400)/ (3.14 x 4) = 392 rpm The feed rate of table is: fm = ft Ns n = 0.008 x 6 x 392 = 19”/min If slab milling were being performed (see Fig. 24-4) with the same parameters being selected, the cutting time for face cutting is more than for slab milling because the allowances A0 for face milling are greater than for slab milling. In milling, power consumption is usually the limiting factor. Face Milling Example

  12. FIGURE 24-3 Face milling viewed from above with vertical spindle-machine. FIGURE 24-4 Slab or side milling being done as a up milling process with horizontal spindle-machine. Vertical and Horizontal Cutters

  13. End Milling FIGURE 24-5 End milling a step feature in a block using a flat-bottomed, end mill cutter in a vertical spindle-milling machine. On left, photo. In middle, end view, table moving the block into the cutter. On right, side view, workpiece feeding right to left into tool.

  14. In Fig 24-5, an end mill with 6 teeth on a 2” diameter (carbide cutter) is used to cut a step in 430F stainless. d = 0.375” and the depth of immersion is 1.25”. The vertical milling machine tool has a 5-hp motorwith an 80% efficiency. The specific horsepower for 430F stainless(BHN = 300) is 1.3hp/in3/min. Can the step be cut in one pass or in multiple passes? End Milling Example

  15. The maximum amount of material that can be removed per pass is usually limited by the available power. Hp = HPs x MRR = HPS x fmW DOCmax = HPs fm x DOI x DOCmax From Table 24-1, select ft = 0.005 ipt, V = 250 fpm. Ns = (12 x 250)/(3.14 x 2) = 477 rpm fm = ft x n x Ns = 0.005 x 6 x 477 = 14.31”/min The actual table feed rates for selected machine are 11”/min. or 16”/min. Select fm = 11”/min. End Milling Example

  16. DOCmax = (0.8hp)/(HPs fm DOI) = (0.8x 5)/(1.3 x 11 x 1.25 ) = 0.225” So two cutting passes are needed because 0.375/0.225 = 1.6. The first pass: DOC1 = 0.225” rough cut The second pass: DOC2 = 0.15” For DOC2 = 0.15” the ft would be only slightly increased to 0.0057ipt Ft = (0.8hp)/(HPs n Ns DOC DOI) = (0.8 x 5)/(1.3 x 6 x 477 x 0.15 x 1.25) = 0.0057ipt End Milling Example

  17. Up milling or Conventional milling • The cutter rotates against the direction of feed of the workpeice. • The Chip is very thin at the beginning and increased along its length. • The cutter tends to push the work along and lift it upwards from the table. The action tends to loosen the workpiece from the fixture. • In the up milling, chips can be carried into the newly machined surface, causing the surface finish to be poorer than in down milling. Up Versus Down Milling

  18. Down milling or Climb milling the cutter rotates in the same direction as the direction of feed • Advantage: • The work piece is pulled into the cutter, eliminating any effects from looseness of the work table feed screw. • There is less tendency for the machined surface to show toothmarks, and the cutting process is smoother, with less chatter. • The cutting force tends to hold the workpiece against the machine table, permitting lower clamping force. • Disadvantage: • The maximum chip thickness is at the point of tooth contact with the work piece. Dulling the teeth more quickly, especially for workpiece with a hard surface. Up Versus Down Milling

  19. FIGURE 24-6 Climb cut or down milling versus conventional cut or up milling for slab or face or end milling. Climbing versus Conventional Mills

  20. Milling is an interrupted cutting process . • Impact loading • Cyclic heating • Cycle cutting forces As show in Fig. 24-7, the cutting force, Fc, builds rapidly as the tool enters the work at A and progresses to B, peaks as the blade crosses the direction of feed at C, decreases to D, and then drops to zero abruptly upon exit. The interrupted-cut phenomenon explain in large part why milling cutter teeth are designed to have small positive or negative rake angles, particularly when the tool material is carbide or ceramic. Cutters made from HSS are with positive rakes, in the main, but must be run at lower speeds. Milling Surface Finish

  21. Facing Mill FIGURE 24-7 Conventional face milling (left) with cutting force diagram for Fc (right) showing the interrupted nature of the process. (From Metal Cutting Principles, 2nd ed., Ingersoll Cutting Tool Company.)

  22. There are a variety of mills used, the most common being face mills and end mills • End mills are either HSS or have indexable inserts (Figure 24-8) • End Mills come in a variety of geometries • Plain End Mills • Shell End Mills • Hollow End Mills 24.3 Milling Tools and Cutters

  23. FIGURE 24-11 Arbor (two views) used on a horizontal-spindle milling machine on left. On right, a gangmilling setup showing three side-milling cutters mounted on an arbor (A) with an outboard flywheel (B). Arbor Milling

  24. FIGURE 24-12 The chips are formed progressively by the teeth of a plain helical-tooth milling cutter during up milling. Helical Mills

  25. The four most common types of manually controlled milling machines are listed below in order of increasing power (and therefore metal removal capability): • 1. Ram-type milling machines • 2. Column-and-knee-type milling machines • a. Horizontal spindle • b. Vertical spindle • 3. Fixed-bed-type milling machines • 4. Planer-type milling machines 24.4 Machines for Milling

  26. Milling machines whose motions are electronically controlled are listed in order of increasing production capacity and decreasing flexibility: • 1. Manual data input milling machines • 2. Programmable CNC (Computer Numerical Controlled) milling machines • 3. Machining centers (tool changer and pallet exchange capability) • 4. Flexible Manufacturing Cell and Flexible Manufacturing System • 5. Transfer lines Machines for Milling

  27. Most mills consist of column-and-knee designs • The column is mounted on a base and the spindle mounted on a knee extending from the column. • The knee has vertical movement • The material in mounted on a table with longitudinal movement, and the table is mounted on a saddle with transverse movement • Most common of this type mill is the Ram mill which has a motor and pulley system mounted on the top of the column. Basic Mill Construction

  28. When purchasing or using a milling machine, consider the following issues: • 1. Spindle orientation and rpm • 2. Machine capability (accuracy and precision) • 3. Machine capacity (size of workpieces) • 4. Horsepower available at spindle (usually 70% of machine horsepower) • 5. Automatic tool changing Milling Machine Selection

  29. Review Questions: 2, and 9 (pages 674) Problems: 1, 2, 6. (page 675) Note: For HW 1: Number of teeth n = 8. For HW 6: Material of workpiece: Cast iron, medium hardness, d = DOC = 0.214” HW for Chapter 24

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