Chapter 24Chapter 24

Milling Milling(Review)(Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring, 2012Spring, 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:

1) 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.

2) End milling: also called facing milling, the surface is 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 downmilling 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.

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

End Mill GeometryEnd 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 GeometryFacing Mill Geometry

FIGURE 24-9 Face mills comein many different designs usingmany different insert geometriesand different mounting arbors.

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.

Shaped CuttersShaped CuttersForm 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 wooddruf 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.

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.

FIGURE 24-14 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.

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 24Review 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: Problem 6HW for Chapter 24: Problem 6Material of workpiece: Cast iron, medium

hardness, d = DOC = 0.214”