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Chapter 2High Speed Machining
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Defined as the use of higher spindle
speeds and axis feed rates to achievehigh material removal rates without a
degradation of part accuracy or quality
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The most appropriate definition of high
speed machining is based on theworkpiece material grade or type
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Today the machining center market constitutes
at least 60 percent of the total market,.
Within this market are high-speed machiningcenters used in small, medium and large batch
production.
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Why HSM ?
Money: increased productivity andefficiency
To respond to customers demand forshorter lead times
Intense competition with low labor cost
Management:
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Through HSM, the machining center can
reduce the need for polishing. It can deliver EDM electrodes more
efficiently. It can even eliminate EDM in some cases.
Produce complex tooling competitively in a
single setup.
Production:
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The smoothness of the machined surface is determined in
large part by the height of the cusp between adjacentpasses with a ballnose tool.Take a small step over and cusp height goes down. In thisway, lighter depths-of-cut can contribute to drastically
reduced polishing time.
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There are two cases in which HSM can letthe machining center perform effectively in
applications where EDM would typically beused:
Hard milling
Milling at high L:D ratios
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HSM can let milling serve as an alternativeto EDM for making dies or molds from thehardest materials (50+ Rc).
Hard Milling
HSM allows a forging supplier to
produce dies like this one in asingle setup on a machiningcenter, where once a combinationof milling and EDM was required.HSM produces the die faster. HSM
is also more accurate, becausefewer steps result in reduced errorstacking.
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Milling At High L:D Ratios
The ability to take light cuts
quickly makes the machiningcenter more efficient with delicatetools that are very long relative totheir diameter. This helps in at
least two cases: Milling deep cavities or deep
slots
Milling fine details with verysmall tools.
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HSM Vs. EDM
Though HSM expands what milling can do
in die/mold machining, EDM still has arole.
Hardness, geometry, accuracy and finishall determine where HSM and EDM arethe most effective.
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Machine-Tool Structure
Materials
Machine-tool design consideration
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Gray cast iron:
good damping capacity and low cost but very heavy.
made from class 40 and class 50 cast iron.
Welded steel
Lighter than cast iron structure.
Advantages: available in wide range of sizes and
shapes, desirable mechanical properties, goodmanufacturing characteristic, low cost, high stiffness
to weight ratio, low damping capacity
MATERIAL
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Ceramics: High strength, stiffness, corrosion resistance,
good surface finish and good thermal stability,
low density. Composite:
Consist of polymer matrix, metal matrix
(MMC), ceramic matrix (CMC). Can be tailored to obtain appropriate
mechanical properties, suitable for high
accuracy and high speed machining. However, it is very expensive and limited in
use.
MATERIAL
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Granite-epoxy composites: Consisted of 93% crushed granite and 7%
epoxy binder.
Good castability (allows design versatility,high stiffness to weight ratio, thermal stability,
resistance to environment degradation, good
damping capacity.
Polymer concrete:
Mixure of crushed concrete and plastic(polymethylmethacrylate).
MATERIAL
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Can be cast into desired shape and design,
good damping capacity, can be used forsandwich construction with cast iron thus
improving the mechanical properties.
Low stiffness (about one-third of class 40 castiron), poor thermal conductivity.
MATERIAL
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A popular method of frame construction forHSM is the bridge-type construction, a
modified bridge construction or anoverhead gantry system as compared toC-frame by conventional machining
center. The bridge design provides an optimal
distribution of the moving masses and thespindle is rigidly mounted close to theguideways.
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A new type of construction so called bridge
design in polymer concrete becomingincreasingly popular for high-speed machiningcenters.
Advantages: provides excellent accessibility to the working area from thefront for an operator.
The large opening allows good access for a pallet changer orrobot.
The pyramid-shaped construction with three-point supportprovides a stiff stationary structure to absorb the highacceleration/deceleration forces of the moving slides.
provides six to 10 times better vibration dampening than castiron.
offers superior thermal stability, as the thermal conductivity ofpolymer concrete is 1/20 that of cast iron. Overall, the results are better tool life, better accuracy and
surface quality, plus noise reduction.
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Machine-Tool Structure and Guideways
An example of amachine-tool structure.
The box-type, one-piecedesign with internaldiagonal ribssignificantly improvesthe stiffness of themachine..
Steel guideways integrally-cast on top of the cast-ironbed of a machining center.Because of its higher elasticmodulus, the steel provideshigher stiffness than cast
iron.
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New monoblock bridgedesign in polymerconcrete.
Assembled machine with integratedpallet changer.
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Machine-tool design consideration
Important consideration factors:
Design, materials and constructionSpindle technology and tool holder
Thermal distortion of machine components
Error compensation and the motion control ofmoving components along slideways.
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Appropriate material selection, gooddesign and construction are essentialespecially in high precision machining andhigh speed machining.
Role of machine structure is to provide
stiffness and, accuracy, thermalstability,good damping, adequate workvolume and ease operator access.
Design, materials and construction
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Machine stiffness:
Major factor in the dimensional accuracy of a
machine tool. It is a function of:
Elastic modulus of the materials used
Geometry of the structural components includingspindle, bearing, drive train, slideways.
Can be enhanced by design improvement bu
using diagonally arrange interior ribs.
Design, materials and construction (cont)
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Static stiffness
Measured in unit of N/mm
Measured the resistance of a body to deflectionunder load.
Simple example is the embodiment of static
stiffness is a linear spring. Stiffness of machine tool structure is closely
related to machine tool capability, will affects the
accuracy and the extent to which the machinecan support high feed rates and accelerations.
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Closed loop static stiffness for the Z axis is
determined using a load cell to measure the forceapplied when a specified axis move is commanded.
Distance between the spindle nose and table top is
measured to determine overall deflection of themachine.
Load cell(measures force
DeflectionMeasurement
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Open loop static stiffness of the Z axis is
measured by applying a known force to the Zaxis spindle and measuring its deflection.
Only the stiffness of the drive train, slide and
column is measured.
DeflectionMeasurement
Known force
applied
F
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Dynamic stiffness
Lower than static stiffness and varies
infrequency. Resonant frequency is a frequency at
which vibration is not adequately damped,thereby affecting controllability of thesystem, surface finish and accuracy.
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Damping: Very critical factor in reducing or eliminating vibration
and chatter in machining operations.
Involves: Type of materials used
Type of joints
Number of joints (welded, bolted)
Greater the number of joints the more the damping.
Well damped machine tools provide better surfacefinish and support high accelerations and feed rates
because more control algorithms can be used.
Design, materials and construction (cont)
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Internal Damping of Structural Materials
The relative damping capacity of (a) gray cast iron and (b) epoxy-granite compositematerial. The vertical scale is the amplitude of vibration and the horizontal scale is
time.
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Joints in Machine-Tool StructuresThe damping of vibrations as a function of the number of components on a lathe. Jointsdissipate energy; the greater the number of joints, the higher the damping capacity of themachine.
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Major design components required:
a) Belt driven
Assembly consists of spindle shaft, held withbearing system and supported by thespindle housing.
Spindle shaft incorporates the toolingsystem including the tool taper, drawbarmechanism and tool release system.
Motor is mounted adjacent to the spindleand the torque is transmitted to the spindleby cogged or V-belt.
1) Spindle style:
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Advantages of belt driven:
Reasonable cost Wide variety of spindle characteristic
High power and torque possible
Limitations:
Max speed is limited
Belt utilize bearing load capacity
Major design components required (cont):
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Major design components required (cont):
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2) Spindle bearings
Bearing type must be fulfill these
requirements: High rotational speed
transfer torque and power to the cutting tool
capable to reasonable loading and life.
Major design components required (cont):
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Major design components required (cont):
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M j d i i d ( )
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Most common used is an AC induction motor.
Rotor is attached to the spindle shaft withthermal or adhesive clamping.
3) Spindle motor design
Major design components required (cont):
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The bearings aremounted to the frontand rear of the shaftand the shaft is thenfitted into the spindle
housing. Spindle shaft must
capable to transfer thepower from the motor to
the cutting tool avoidfrom bending.
Major design components required (cont):
4) Spindle shaft
M j d i t i d ( t)
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Spindle housing shaft and motor for HSM
is held in a cartridge type housing. Primary function:
Locate the bearings
Provide the lubrication, air seal, cooling water
or oil.
5) Spindle housing
Major design components required (cont):
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Tool holder
Common type: BT40, CAT 40 and ISO 30 CAT 40 up to 20,000 rpm
At higher speeds, spindle taper expands more thancutting tool causing: Axial position changes Tool may bind in spindle
Current trend: HSK (Hollow Shank Taper) tool holder German DIN69893 & DIN69063 Better axial positioning Will not bind in spindle Faster tool change
Shorter length Lower mass Require more attention to cleanliness
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Th l di t ti
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Thermal distortion
50% of machine-tool errors and accuracy are
due to temperature.
Methods of heat transfer: Conduction
Convection
Radiation Source:
Internal: from bearing, ballscrew, machine ways,
spindle motor, pumps etc External: cutting fluid, sunlight, ambient temperature
etc
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Solutions:
Thermal and geometric real-time error
compensating features was implemented for
accurate ballscrew positions.
Gas or fluid hydrostatic spindle bearings.
New design for traction or friction drives for
linear motion. Extremely fine feed and position controls
using microactuators
Fluid circulation channels in the machine-toolbase for the maintenance of thermal stability.
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Video presentation
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Video presentation
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Video #4