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Milling

M.V.TADVILECTURERMEDSVNIT, SURAT

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Figure Typical parts and shapes produced by milling.

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Milling Processes

• Milling is one of the basic machining processes. Milling is a very versatile process capable of producing simple two dimensional flat shapes to complex three dimensional interlaced surface configurations.

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The Process

• The milling process:• Typically uses a multi-tooth cutter• Work is fed into the rotating cutter• Capable of high MRR• Well suited for mass production

applications• Cutting tools for this process are

called milling cutters

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Classifications• Milling operations are classified into two major categories:

• Peripheral (side) • Generally in a plane parallel to the axis of the cutter• Cross section of the milled surface corresponds to the contour of the

cutter• Face

• Generally at right angles to the axis of rotation of the cutter• Milled surface is flat and has no relationship to the contour of the cutter• Combined cutting action of the side and face of the milling cutter

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Peripheral Milling (horizontal milling)

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Face Milling (vertical milling)

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Up milling Milling is another basic machining process by which surface is generated progressively by the removal of chips from a work piece as it is fed a rotating cutter

Also called conventional milling,

- Wheel rotation opposite of the feed- The chip formed by each cutter tooth starts out very thin and increases its thickness- The length of the chip is relatively longer- Tool life is relatively shorter- Need more clamping force to hold the work part still.

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Down milling:

Also called climb milling,- Wheel rotation is parallel to the feed- The chip formed by each cutter tooth starts out thick and leaves out thin- The length of the chip is relatively short- Tool life is relatively longer- Need less clamping force to hold the work part still.

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Peripheral Milling

The axis of the cutter rotation is parallel to the work piece surface to be machined in peripheral milling.Slab milling:

Cutter width extends beyond the work piece on both sidesSlotting (Slot milling):

Cutter width is less than the work piece width, creating a slot. If the cutter is very thin, it can be used to cut a work part into two, called saw milling.

Side milling:

Cutter, machines the side of the work piece.Straddle milling:

Similar to side milling, but cutting takes on both sides of the work part simultaneously.

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Milling Types

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Slab Milling (peripheral milling): process where axis of cutting tool is parallel to the workpiece surface to be machined

-- used to create flat surfaces or slots

-- cutter may have either straight or helical teeth

 

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Slab Milling tools

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Milling Cutter Types

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Face Milling

The generated surface is at right angles to the cutter axis and is the combined result of actions of the portions of the teeth located on both periphery and the face of the cutter. Most of the cutting is done by the peripheral portions of the teeth, with the face portions providing some finishing actions.Conventional face milling:

Diameter of tool is larger than work part’s width.Partial face milling:

The cutter overhangs from one side of work part .End milling:

Cutters diameter is less than the work part’s width.

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Continue...

Profile milling:

Outside periphery of flat part is cut.Pocket milling:

Similar to end milling, but the shape created is a shallow pockets in flat surfacesSurface contouring:

A ball-nose cutter is fed back and forth across the work part to create a contoured surface perpendicular to the cutter.

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Face milling

Pocket milling End milling

Profile milling

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More Examples on Face Milling

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Face Milling tools

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Milling Cutters

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More Milling Cutters

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Milling Cutter Materials

• Cutter Characteristics• Harder than metal being machined• Strong enough to withstand cutting

pressures• Tough to resist shock resulting from

contact• Resist heat and abrasion of cutting• Available in various sizes and shapes

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Cutting Tool Material Choices • Carbon and medium alloy steels• High Speed Steels• Cast-cobalt alloys• Carbides (or cemented or sintered carbides)• Coated tools • Alumina based ceramics• Cubic boron nitride • Diamond

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Cutting Tool Material Choices: 

Carbon and medium alloy steels -- of historical importance; but limited to low speed operations 

High Speed Steels-- first produced in early 1900's, -- can be hardened to various depths, have toughness and good wear resistance, and are relatively inexpensive--molybdenum(M series): contains up to 10% molybdenum with chromium, vanadium, tungsten, and cobalt alloying elements.--tungsten(T series): contains 12 to 18% tungsten with chromium, vanadium, and cobalt alloying elements.--95% of these are M series because of better abrasion resistance, less heat distortion, and less expensive than T series.-- can be coated or heat treated or case hardened to improve performance.-- accounts for the largest tonnage of tool materials used today-- used for applications such as drills, reamers, taps, gear cutters -- use is limited by cutting speed and high temperatures.

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Cast-cobalt alloys-- cast alloy of cobalt, chromium, and tungsten.-- good wear resistance and hardness at higher temp. than HSS-- not very commonly used-- less tough than high speed steels but can handle faster speeds.  Carbides (or cemented or sintered carbides)--usually made of tungsten carbide in a cobalt bonding matrix or titanium carbide in a nickel-molybdenum alloy matrix.--have high elastic modulus, thermal conductivity, and low thermal expansion.--usually manufactured as inserts to be clamped or brazed to the tool shank.

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Coated tools--common coatings include titanium nitride, titanium carbide, titanium carbonitride, and aluminum oxide.-- improve characteristics such as lower friction, higher crack resistance, longer wear, chemical stability.-- applied by chemical vapor deposition methods. Alumina based ceramics-- fine-grained high purity aluminum oxide with other agents cold pressed and into insert shaped and sintered at high temperatures.-- very high abrasion resistance and hot hardness-- chemically stable and low adherence to workpiece material-- lack toughness and subject to failure by impact or thermal fatigue.-- cermets (also called black or hot-pressed ceramics), are 70% aluminum oxide and 30% titanium carbide are slightly more expensive than the alumina-based ceramics.

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Cubic boron nitride -- second hardest material presently available-- made by bonding polycrystalline cubic boron nitride to a carbide substrate by sintering under pressure.-- shock resistance with high wear and cutting edge strength-- chemically inert to nickel and iron, and resistant to oxidation.-- is sensitive to thermal and fatigue failure Diamond-- hardest known substance-- low friction, high wear resistance, good edge maintenance-- more likely used as polycrystalline diamond form which is made of very small synthetic crystals fused at high temperature and pressure.-- due to chemical affinity, not recommended for steel, titanium, nickel or cobalt based alloys, but very good with soft non-ferrous and abrasive nonmetallic materials.

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High-Speed Steel

• Iron with additives• Carbon: hardening agent• Tungsten and Molybdenum: enable steel to

retain hardness up to red heat• Chromium: increases toughness and wear

resistance• Vanadium: increases tensile strength

• Used for most solid milling cutters

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Cemented-Carbide

• Higher rates of production (3-10 times faster)

• Must select proper type of carbide• Straight tungsten carbide: cast iron, plastics• Tantalum carbide: low/medium-carbon steel• Tungsten-titanium

carbide: high-carbon steel

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Milling Cutters

The tool used in milling is known as a milling cutter, the cutting edges called teeth. Types of milling cutters are related to the milling operations can be classified as:Plain milling cutters:

- Used in peripheral milling operations- Cylindrical or disk shaped- Have several straight or helical teeth on periphery - Used to mill flat surfacesSide milling cutters:

- Similar to plain milling cutters- Teeth extend radial part way across one or both ends of cylinder toward the center- Relatively narrow

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Plain Milling Cutters

• Once widely used • Cylinder of high-speed steel with teeth

cut on periphery• Used to produce flat surface• Several types

• Light-duty• Light-duty helical• Heavy-duty• High-helix

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Light-Duty Plain Milling Cutter

• Less than ¾ in. wide, straight teeth

• Used for light milling operations

• Those over ¾ in have helix angle of 25º• Too many teeth to permit chip clearance

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Heavy-Duty Plain Milling Cutters

• Have fewer teeth than light-duty type• Provide for better chip clearance

• Helix angle varies up to 45º• Produces smoother surface because of

shearing action and reduced chatter

• Less power required

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High-Helix Plain Milling Cutters

• Have helix angles from 45º to over 60º

• Suited to milling of wide and intermittent surfaces on contour and profile milling

• Sometimes shank-mountedwith pilot on end and usedfor milling elongated slots

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Standard Shank-Type Helical Milling Cutters

• Called arbor-type cutters• Used for

• Milling forms from solid metal• Removing inner sections from solids

• Inserted through previously drilled hole and supported at outer end with type A arbor support

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Side Milling Cutters

• Comparatively narrow cylindrical milling cutters with teeth on each side and on periphery

• Used for cutting slots andfor face and straddle millingoperations

• Free cutting action at highspeeds and feeds

• Suited for milling deep, narrow slots

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Half-Side Milling Cutters• Used when only one side of cutter

required

• Also make with interlocking faces so two cutter may be placed side by side for slot milling

• Have considerable

rake

• Able to take heavy

cuts

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Face Milling Cutters

• Generally over 6 in. in diameter• Have inserted teeth made of high-speed

steel held in place by wedging device

• Most cutting action occursat beveled corners andperiphery of cutter

• Makes roughing and finishing cuts in one pass

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Face milling

Pocket milling End milling

Profile milling

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More Examples on Face Milling

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Angular Cutters

• Teeth neither parallel nor perpendicular to cutting axis

• Used for milling angular surfaces• Grooves, serrations, chamfers and

reamer teeth

• Divided into two groups• Single-angle milling cutters• Double-angle milling cutters

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Angular Cutters

• Single-angle• Teeth on angular surface• May or may not have teeth on flat• 45º or 60º

• Double-angle• Two intersecting angular surfaces

with cutting teeth on both• Equal angles on both side of line

at right angle to axis

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Form milling cutters:

- Another peripheral milling cutter- Teeth ground to a special shape to produce a surface having a desired transverse contour, convex, concave shape.End milling cutters:

- Looks like a drill bit, but it cuts with peripheral teeth instead of it’s end. - Have multiple teeth- Used in milling slots, profiling and facing narrow surfaces.

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Formed Cutters

• Incorporate exact shape of part to be produced

• Useful for production of small parts• Each tooth identical in shape• Sharpened by grinding tooth face (may

have positive, zero or negative rake)• Important to maintain original rake• Difficult to sharpen

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Milling Types

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Types of Formed Cutters

Concave Convex Gear Tooth

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Milling Cutter Types

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Metal-Slitting Saws

• Basically thin plain milling cutters with sides relieved or "dished" to prevent rubbing or binding when used

• Widths from 1/32 to 3/16 in.• Operated at approximately 1/4 to 1/8 of feed per

tooth used for other cutters• Not advisable to key saw to milling arbor• Backlash eliminator should be engaged

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Metal-Slitting Saws

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End Mills

• Cutting teeth on end as well as periphery

• Fitted to spindle by suitable adapter

• Two types• Solid end mill: shank and cutter

integral• Smaller with either straight or helical flutes• Two flute or four flute

• Shell end mill: separate shank

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End mills

(left to right) roughing end mill, center-cut end mill, ball mill.

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Examples of end milling

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End Milling Cutters

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Fig : A typical part that can be produced on a milling machine equipped with computer controls.Such parts can be made efficiently and respectively on computer numerical control (CNC) machines, without the need for refixturing or reclamping the part

Example of a part produces in a CNC milling machine

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Freeform Surfaces

Example of a surface that can be milled with a computer-controlled ball mill

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T-slot cutters:

- Have teeth on periphery and both sides- Used for milling the wide groove of a T-slot- In order to use them, a vertical groove must first be made with a slotting mill or an end mill to provide a clearance for the shank- T-slot cutter must be fed carefully, because it cuts in 5 surfaces

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T-Slot Cutter

• Used to cut wide horizontal groove at bottom of T-slot

• After narrow vertical groove machined with end mill or side milling cutter

• Consists of small side milling cutter with teeth on both sides and integral shank for mounting

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Figure (a) T-slot cutting with a milling cutter. (b) A shell mill.

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T-Slot Milling Cutters

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Dovetail Cutter

• Similar to single-angle milling cutter with integral shank

• Used to form sides of dovetail after tongue or groove machined

• Obtained with 45º, 50º, 55º, or 60º angles

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Woodruff Keyseat Cutter• Similar in design to plain and side milling

cutters• Small (up to 2 in) solid shank, straight teeth• Large mounted on arbor with staggered

teeth

• Used for milling semicylindrical keyseats in shafts

• Designated by number system

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Vertical-Spindle Milling Machine Tools

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Cutting force in milling• Estimation of cutting power consumption, which also enables

selection of the power source(s) during design of the machine tools • Structural design of the machine – fixture – tool system • Evaluation of role of the various machining parameters ( process – VC,

so, t, tool – material and geometry, environment – cutting fluid) on cutting forces

• Study of behaviour and machinability characterisation of the work materials

• Condition monitoring of the cutting tools and machine tools

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Singnificance of cutting force• where, PZ = tangential

component taken in the direction of Zm axis

• PX = axial component taken in the direction of longitudinal feed or Xm axis

• PY = radial or transverse component taken along Ym axis.

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pt

pr

pz

py

Cutting forces developed in plain milling(with single tooth engagement

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Cutting force in milling

• Tangential force PTi (equivalent to PZ in turning) • Radial or transverse force, PRi (equivalent to

PXY in turning) • R is the resultant of PT and PR • R is again resolved into PZ and PY as indicated

in Fig. 8.4 when Z and Y are the major axes of the milling machine.

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Gear: Definition

• A gear is a wheel with teeth along its rim. • It is used to transmit effort from one shaft to

another.

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Gear Train

• Combination of two or more meshed gears. Used to change the rate of rotation, the direction of rotation, and the amount of torque.

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Gear Ratio

• Gear ratio is also called “Mechanical Advantage”

Gear ratio Number of Driver Teeth

Number of Driven Teeth

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Gear Ratios

The gear ratio is the ratio of the number of teeth on one gear to the number of teeth on the other gear.

Gear ratio = 40 to 8 or, simplifying, 5 to 1.

That means it takes 5 revolutions of the smaller gear to get 1 revolution of the larger gear. Try it!

40 teeth

8 teeth

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Types of Gears

• Spur gears

• Worm gear

• Rack and pinion

• Bevel gears

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Spur Gears

• Simplest• Most widely used in Instrumentation and Control

Systems• Used to drive parallel shafts

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Spur Gear Terminology

• Pitch circle diameters• Size of teeth (Diametral pitch)• Center distance• Number of teeth• Pinion• Gear