Broaching

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Broaching Sawing Processes

• General Manufacturing Processes Engr.-20.2710
• Instructor - Sam Chiappone

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Broaching

• Broaching is the process of removing metal with a
  tool which has teeth arranged in a row. Each
  tooth is successively higher than the previous
  tooth and removes more material. In broaching,
  one stroke or cycle of the machine produces a
  finished part.
• Broaching is used to produce both internal and
  external features. Production rates are high and
  tolerances of /- .0005 are possible.

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Broaching
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Broaching
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Broaching
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Broaching

• Chip Formation
• Chip formation involves three basic requirements
  
• The cutting tool must be harder than the part
  material
• There must be interference between the tool and
  the part as designated by the feed rate and cut
  per tooth
• There must be a relative motion or cutting
  velocity between the tool and workpiece with
  sufficient force to overcome the resistance of
  the part material.

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Broaching
Tool Feed Direction
Tool
Gullet
Depth of cut per tooth
Workpiece
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Broaching

• Chip Formation
• As long as these three conditions exist, the
  portion of the material being machined that
  interferes with the free passage of the tool will
  be displaced to create a chip.
• Many combinations exist that may fulfill such
  requirements.
• Variations in tool material and tool geometry,
  feed and depth of cut, cutting velocity, and part
  material have an effect not only upon the
  formation of the chip, but also upon cutting
  force, cutting horsepower, cutting temperatures,
  tool wear and tool life, dimensional stability,
  and the quality of the newly created surface.

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Broaching

• The Mechanics of Chip Formation
• Empirical metal-cutting studies reveal several
  important characteristics of the chips formed
  during the broaching process
• The cutting process generates heat
• The thickness of the chip is usually greater than
  the thickness of the layer from which it came
• The hardness of the chip is usually much greater
  than the hardness of the parent material, and
• The other three relative values are all affected
  by changes in cutting conditions and in
  properties of the material to be machined

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Broaching

• The Mechanics of Chip Formation
• These observations also indicate that the process
  of chip formation is one of deformation or
  plastic flow of the material, with the degree of
  deformation dictating the type of chip that will
  be produced.

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Broaching

• Plastic Deformation
• Originally, it was thought that chips formed in
  metal cutting were created in much the same way
  that wood chips are formed when split by an axe.
  This may be partially true for brittle materials
  such as cast iron, but it does not hold true for
  the majority of metals. The process by which
  chips are formed with metal-cutting tools is
  called plastic deformation, and was first
  described by Rosenhain at the Stratsfordshire
  Iron and Steel Institute in 1906.

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Broaching

• Plastic Deformation
• What actually happens in this shearing process is
  that the metal immediately ahead of the cutting
  edge of the tool is severely compressed resulting
  in temperatures high enough to allow plastic
  flow.
• When the resisting stresses in a material exceed
  their elastic limit, a permanent relative motion
  occurs and further deformation is withstood.
• This strengthening is called work or strain
  hardening, and is characteristic of all steels,
  but demonstrated most dramatically in stainless
  steels.

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Broaching

• How and Where Heat is Generated
• The force or energy that is put into the tool
  creates movement in a group of metal atoms in the
  workpiece. This group is a finite number of atoms
  which are forced to change their positions in
  relationship to each other.
• As the atoms in the metal ahead of the tool are
  disturbed, the friction involved in their sliding
  over one another is thought to be responsible for
  60 or more of the total heat generated.
• This internal friction, and the heat it
  generates, can be compared to the friction and
  heat caused by bending a paper clip back and
  forth until it breaks.

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Broaching

• How and Where Heat is Generated
• As the tool continues to push through the work
  piece, a chip eventually slides up the cutting
  face of the tool. This sliding creates an
  external friction which again releases heat. This
  external friction accounts for about 30 of the
  total heat generated.
• The third area of heat generation is on the land
  or flank of the tool. This area accounts for
  about 10 of the heat generated. This is assuming
  that the tools are sharp and made correctly as
  far as clearance angles and face angles are
  concerned. As the tool wears, the above
  percentages will vary, especially when there is
  excess wear on the land, or if the clearance
  angle is insufficient for the material or the
  part configuration. This contact zone will
  actually increase as the part continues to close
  in after the cut resulting in extremely high
  pressures on the land area of the tool.

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Advantages Disadvantages

• Advantages
• Rough to finish in one pass
• Production rates are high
• Cutting time is quick
• Rapid load and unload of parts
• External and internal features
• Any form that can be produced on a broaching tool
  can be produced
• Production tolerances are excellent
• Surface finishes are equal to milling
• Operator skill is low

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Advantages Disadvantages

• Disadvantages
• Tooling cost can be high
• In some cases--not suited for low production
  rates
• Parts to be broached must be strong enough to
  withstand the forces of the process
• Surface to be broached must be accessible

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Methods of Operation

• Pull broaching - broach is pulled through or
  across stationary work
• Push broaching - broach is pushed through or
  across work
• Surface broaching - either the work or the broach
  moves across the other
• Continuous broaching - the work is moved
  continuously against stationary broaches. The
  path of the movement may be straight or circular.

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Machines

• Vertical single and double slide - Table moves
  part into position for broaching, part is
  broached and the table retracts for unloading.
• Vertical push broaching - Used for internal
  features such as holes, rounds, or slots.
• Vertical pull down
• Tool is suspended above work
• Lowered into pull mechanism in the base of the
  machine
• Advantages
• Part positioning is easy
• Large parts are handled efficiently

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Machines

• Vertical single and double slide

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Machines

• Horizontal
• Versatile machine capable of producing internal
  and external features
• key-ways
• gear teeth
• riffling
• High cutting speeds in the range of 10-40ft/min
  with return speeds of 110 ft/min
• MRR of ¼ in per stroke is possible

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Methods of Operation
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Machines

• Horizontal

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Machines

• Rotary
• Parts are mounted to a rotating table and are
  moved to different stations for different
  operations
• Primarily used on small parts
• Typical operations include
• Slotting
• Holes
• Key-ways

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Broaching Tools Design Construction

• Considerations
• Material to be broached
• Size and shape of cut
• Quality of surface finish
• Part tolerance
• Productions rates
• Type of machine
• Workholding method
• Strength of the workpiece

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Sawing

• Depending on part tolerance, sawing can be a
  vital first operation or the total process.

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Machines Classifications

• Reciprocating saws
• Horizontal hacksaw and vertical sawing machines
• Light to heavy duty
• Simple and most economical to operate
• Manual to fully automatic feed mechanisms
• Uses blades similar to hacksaw blades

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Machines Classifications

• Circular saws
• Sometimes called cold sawing machines
• Saw blades are large and rotate at low Rpms
• Cutting is similar to a milling operation due to
  geometry of saw blade

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Machines Classifications

• Band saws
• Irregular shapes
• Very versatile
• Profile cuts
• Internal cuts
• external configurations
• Blades are continuous
• HSS - Carbide tipped
• Diamond impregnated
• Filing

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Machines Classifications

• Band saws

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Saw Blades

• Circular
• Straight
• Band

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Saw Blades

• Terminology
• Set of the blade - Directional offset,
  left-right, from the blades centerline.
  Sometimes referred to as the kref. The kref
  provides clearance for the blade as it cuts
  through the work.
• Straight - one tooth left one tooth right.
  Typically used for brass, copper, and plastic.
• Raker - Three tooth sequence, left, right,
  straight. Typically used for steel and cast
  iron.
• Wavy - Alternate arrangement of several teeth to
  the right and left. Used to cut tubes and light
  sheet metal.

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Saw Blades
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Saw Blades

• Tooth forms
• Variable positive
• Variable
• Standard
• Skip
• Hook