Chapter 24Chapter 24
Milling Processes Milling Processes (Review) (Review)
EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSummer A, 2012Summer A, 2012
24.1 Introduction24.1 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 24.2 Fundamentals of Milling ProcessesProcessesMilling 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 MillsPeripheral Mills
FIGURE 24-1 Peripheral milling can be performed on a horizontal-spindle milling machine. Thecutter 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 MillingPeripheral MillingThe 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)/( D)
The feed of table fm, in inch per minute, is calculated:
fm = ft Ns nWhere ft, feed per tooth, and n is the number of teeth
in the cutter.The cutting time is:
Tm = (L + LA)/fm
Peripheral MillingPeripheral MillingThe 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.
Suggested Starting Feeds and Speeds using HSS and Suggested Starting Feeds and Speeds using HSS and Carbide CuttersCarbide Cutters
Face Mills 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 pastthe tool. The allowance depends on the tool diameter and the width of cut.
Face MillingFace MillingThe rpm of the spindle is determined from the surface
cutting speed V, the cutter diameter D (in inch) as below:
Ns = (12V)/( D)The feed of table fm, in inch per minute, is calculated:
fm = ft Ns nWhere 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/2L0 = LA = D/2 for W>=D/2
Face Milling ExampleFace 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 ExampleFace Milling Example Using cutting data recommendations, select V = 400 sfpm
with a ft = 0.008”/tooth.
Ns = (12V)/( 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.
Vertical and Horizontal Vertical and Horizontal CuttersCutters
FIGURE 24-3 Face millingviewed from above with verticalspindle-machine.
FIGURE 24-4 Slab or sidemilling being done as a up milling process with horizontalspindle-machine.
End MillingEnd 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 ExampleEnd 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/in3/min. Can the step be cut in one pass or in multiple passes?
End Milling ExampleEnd Milling Example
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 ExampleEnd Milling Example 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 cutThe 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
Up Versus Down MillingUp 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 MillingUp Versus Down MillingDown 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 Climbing versus Conventional MillsMills
FIGURE 24-6 Climb cut ordown milling versusconventional cut or up millingfor slab or face or end milling.
Milling Surface FinishMilling Surface FinishMilling 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 MillFacing 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 Cutters24.3 Milling Tools and CuttersThere 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
Arbor MillingArbor Milling
FIGURE 24-11 Arbor (two views) used on a horizontal-spindle milling machine on left. On right, a gangmillingsetup showing three side-milling cutters mounted on an arbor (A) with an outboard flywheel (B).
Helical MillsHelical Mills
FIGURE 24-12 The chips areformed progressively by theteeth of a plain helical-toothmilling cutter during up milling.
24.4 Machines for Milling24.4 Machines for MillingThe 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 MillingMachines for MillingMilling 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 ConstructionBasic Mill ConstructionMost 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.
Milling Machine SelectionMilling Machine SelectionWhen 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 24HW 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”