Presentation on theme: "Chapter 24 Milling EIN 3390 Manufacturing Processes Spring, 2012."— Presentation transcript:
Chapter 24 Milling EIN 3390 Manufacturing Processes Spring, 2012
24.1 Introduction 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.
24.2 Fundamentals of Milling Processes 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.
Peripheral Mills 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 Milling 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: N s = (12V)/( D) The feed of table fm, in inch per minute, is calculated: f m = f t N s n Where ft, feed per tooth, and n is the number of teeth in the cutter. The cutting time is: T m = (L + L A )/f m
Peripheral Milling The length of approach is: L A = SQRT[D 2 /4 – (D/2-DOC) 2 ]= SQRT(d(D-d)) The MRR is: MRR = Volume/T m = (LWd)/T m = Wf m d in 3 /min where W is width of the cut in inch, d is the depth of cut in inch. If ignoring L A, the values for f t are given in Table 24-1, along with recommended cutting speeds in feet per minute.
Suggested Starting Feeds and Speeds using HSS and Carbide Cutters
Face Mills 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 Milling The rpm of the spindle is determined from the surface cutting speed V, the cutter diameter D (in inch) as below: N s = (12V)/( D) The feed of table fm, in inch per minute, is calculated: f m = f t N s n Where f t, feed per tooth, and n is the number of teeth in the cutter. The cutting time is: Tm = (L + L A + L 0 )/f m The MRR is: MRR = Volume/T m = (LWd)/T m = Wf m d in 3 /min For a setup where the tool doesn’t completely pass over the workpiece, L 0 = L A = SQRT(W(D – W)) for W < D/2 L 0 = L A = D/2 for W>=D/2
Face Milling Example 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 Using cutting data recommendations, select V = 400 sfpm with a ft = 0.008”/tooth. N s = (12V)/( D) = (12 x 400)/ (3.14 x 4) = 392 rpm The feed rate of table is: f m = f t N s n = 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 A 0 for face milling are greater than for slab milling. In milling, power consumption is usually the limiting factor.
Vertical and Horizontal Cutters 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.
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.
End Milling Example 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 motor with an 80% efficiency. The specific horsepower for 430F stainless(BHN = 300) is 1.3hp/in 3 /min. Can the step be cut in one pass or in multiple passes?
End Milling Example The maximum amount of material that can be removed per pass is usually limited by the available power. H p = HP s x MRR = HP S x f m W DOC max = HP s f m x DOI x DOC max From Table 24-1, select f t = ipt, V = 250 fpm. N s = (12 x 250)/(3.14 x 2) = 477 rpm f m = f t x n x N s = x 6 x 477 = 14.31”/min The actual table feed rates for selected machine are 11”/min. or 16”/min. Select f m = 11”/min.
End Milling Example DOC max = (0.8hp)/(HP s f m 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: DOC 1 = 0.225” rough cut The second pass: DOC 2 = 0.15” For DOC 2 = 0.15” the f t would be only slightly increased to ipt F t = (0.8hp)/(HP s n N s DOC DOI) = (0.8 x 5)/(1.3 x 6 x 477 x 0.15 x 1.25) = ipt
Up Versus Down Milling 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 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.
Climbing versus Conventional Mills FIGURE 24-6 Climb cut or down milling versus conventional cut or up milling for slab or face or end milling.
Milling Surface Finish 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.
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.)
24.3 Milling Tools and Cutters 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
Other Mill Cutter Types Face mills have indexable inserts along the periphery Face Mills come in a variety of geometry (Figure 24-9) ◦Center hole for arbor mounting ◦Side mill (Figure 24-10) ◦Staggered-tooth ◦Straddle milling ◦Interlocking slot cutters ◦Slitting cutters
Typical Cutter Problems
End Mill Geometry FIGURE 24-8 Solid end mills are often coated. Insert tooling end mills come in a variety of sizes and are mounted on taper shanks.
Facing Mill Geometry FIGURE 24-9 Face mills come in many different designs using many different insert geometries and different mounting arbors.
Side Milling FIGURE The side-milling cutter can cut on sides and ends of the teeth, so it makes slots or grooves. However, only a few teeth are engaged at any one point in time, causing heavy torsional vibrations. The average chip thickness, hi, will be less than the feed per tooth, ft. The actual feed per tooth fa will be less than feed per tooth selected, Ft.
Arbor Milling FIGURE 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).
Helical Mills FIGURE The chips are formed progressively by the teeth of a plain helical-tooth milling cutter during up milling.
Shaped Cutters Form Relieved Cutters are used when intricate shapes are needed. T-slot cutters are used to produce slots in material. An end mill is use first to produce the initial groove A woodruff keyseat cutter is used to produce a slot in a shaft and come in standard sizes Fly cutters are single toothed face mill cutters, with adjustable radii.
Relieved Cutter FIGURE Solid form relieved milling cutter, would be mounted on an arbor in a horizontal milling machine..
24.4 Machines for Milling 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
Machines for Milling 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
Basic Mill Construction 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.
Major Components of a Plain Column-and- Knee-Type Milling Machine FIGURE Major components of a plain column-and-knee-type milling machine, which can have horizontal spindle shown on the left, or a turret type machine with a vertical spindle, shown on the right. The workpiece and workholder on the table can be translated in X, Y, and Z directions with respect to the tool.
Ram-type Knee-and Column Machine FIGURE The ram-type knee-and- column milling machine is one of the most versatile and popular milling machine tools ever designed.
Bed Type Milling Machine Made for deep cuts and heavy material removal, the bed only had horizontal movement Once the bed is set up, the spindle height is not changed during operation. These machines are very common due to their ease of use.
Bed Type Mill FIGURE Bed-type vertical-spindle heavy-duty production machine tools for milling usually have three axes of motion.
Planer Type Mill Planer type mills can have several heads to remove large amounts of material while the material is fed slowly into the machine. Systems are setup typically for single pass operations. These are advantageous for large work pieces requiring heavy material removal.
Large Planer-type Milling Machine FIGURE Large planertype milling machine. Inset shows 90° head being used. (Courtesy of Cosa Corporation.)
Milling Machine Selection 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
HW for Chapter 24 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 Problems: 1 (page 675) (Calculate the table speed f m )