Slide 1© Department of Mechanical Engineering
ME-321 Manufacturing Process I
Cutting conditions in milling
The cutting speed is determined at the outside diameter of a milling cutter
This can be converted to spindle rotation speed using the formulae
The feed f in milling is usually given as a feed per cutter tooth; called the chip load, it represents the size of the chip formed by each cutting edge
This can be converted to feed rate by taking into account the spindle speed and the number of teeth on the cutter as follows:
Where fr = feed rate, mm/min; N = spindle speed, rev/min; nt = number of teeth on the cutter; and f = feed per cutter tooth in mm/tooth
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
Cutting conditions in milling
Material removal rate in milling is determined using the product of the cross sectional area of the cut and the feed rate
Accordingly, if a slab-milling operation is cutting a workpiece with width w at a depth d, the material removal rate is
MRR = w d fr
This neglects the initial entry of the cutter before full engagement
This equation can be applied to end milling, side milling, face milling, and other milling operations, making the proper adjustment in the computation of the cross sectional area of cut
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
Time in milling
The time required to mill a workpiece of length L must account for the approach distance required to fully engage the cutter
First, consider the case of slab milling
To determine the time to perform a slab milling operation, the approach distance A to reach full cutter depth is given by
Where d=depth of cut, mm; and D = diameter of the milling cuter, mm. The time to mill the workpiece Tm is therefore
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ME-321 Manufacturing Process I
Time in milling
For face milling, it is customary to allow for the approach distance plus an over travel distance O
There are two possible cases
In both cases A = O
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ME-321 Manufacturing Process I
Time in Milling
The first case is when the cutter is centred over the rectangular workpiece
It is clear from figure that A = O = Half the diameter of the cutter
The second case is when the cutter is offset to one side of the work
In this case the approach and over travel distances are given by
Where w = width of the cut, mm. Machining time in either case is therefore
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ME-321 Manufacturing Process I
Horizontal knee-and-column milling machine
Vertical Milling Machine
Universal milling machine
That has a table that can be swivelled in a horizontal plane to any specified angle
This facilitates the cutting of angular shapes and helixes on workparts
Ram mill
The tool head containing the spindle is located on the end of a horizontal ram; the ram can be adjusted in and out over the worktable to locate the cutter relative to the work
The toolhead can also be swivelled to achieve an angular orientation of the cutter with respect to the work
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ME-321 Manufacturing Process I
Universal-overarm, arbor, and cutter omitted for clarity,
Ram type
Bed type milling machines
Tracer mill
X-Y tracer
X-Y-Z tracer
Highly automated machine tool can perform multiple machining operations under CNC control in one setup with minimal human attention
Typical operations are milling and drilling
Three, four, or five axes
Other features:
Pallet shuttles
Operator can change the work, while machining is performed on the other
Automatic workpart positioning
Universal machining center; highly automated, capable of multiple machining operations under computer control in one setup with minimal human attention (photo courtesy of Cincinnati Milacron).
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ME-321 Manufacturing Process I
CNC 4axis turning center (photo courtesy of Cincinnati Milacron); capable of turning and related operations, contour turning, and automatic tool indexing, all under computer control.
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ME-321 Manufacturing Process I
Tool monitoring
Mill-Turn Centers
Highly automated machine tool that can perform turning, milling, and drilling operations
General configuration of a turning center
Can position a cylindrical workpart at a specified angle so a rotating cutting tool (e.g., milling cutter) can machine features into outside surface of part
Conventional turning center cannot stop workpart at a defined angular position and does not include rotating tool spindles
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ME-321 Manufacturing Process I
Operation of a millturn center: (a) example part with turned, milled, and drilled surfaces; and (b) sequence of operations on a millturn center: (1) turn second diameter, (2) mill flat with part in programmed angular position, (3) drill hole with part in same programmed position, and (4) cutoff
Operation of Mill-Turn Center
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ME-321 Manufacturing Process I
Similar operations
Both use a single point cutting tool moved linearly relative to the workpart
(a) Shaping, and (b) planing.
Shaping and Planing
Interrupted cutting
Low cutting speeds due to startandstop motion
Typical tooling: single point high speed steel tools
Difference is that in planning, workpart is moved, while in shaping, the workpart is fixed while the cutting tool is moved
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ME-321 Manufacturing Process I
Components of a shaper
Shaper
Return stroke (non-cutting stroke) is relatively faster than the forward stroke
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
Moves a multiple tooth cutting tool linearly relative to work in direction of tool axis
Broaching operation
Variety of work shapes possible
Cutting tool called a broach
Owing to complicated and often customshaped geometry, tooling is expensive
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ME-321 Manufacturing Process I
Performed on internal surface of a hole
A starting hole must be present in the part to insert broach at beginning of stroke
Work shapes that can be cut by internal broaching; crosshatching indicates the surfaces broached.
Internal Broaching
External Broaching
Performed on the outside surface of the work to create a certain cross-sectional shape on the surface
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ME-321 Manufacturing Process I
Sawing
Cuts narrow slit in work by a tool consisting of a series of narrowly spaced teeth
Tool called a saw blade
Typical functions:
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ME-321 Manufacturing Process I
power hacksaw –linear reciprocating motion of hacksaw blade against work.
Power Hacksaw
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ME-321 Manufacturing Process I
bandsaw (vertical) – linear continuous motion of bandsaw blade, which is in the form of an endless flexible loop with teeth on one edge.
Band Saw
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ME-321 Manufacturing Process I
circular saw – rotating saw blade provides continuous motion of tool past workpart.
Circular Saw
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ME-321 Manufacturing Process I
High Speed Machining (HSM)
Cutting at speeds significantly higher than those used in conventional machining operations
Persistent trend throughout history of machining is higher and higher cutting speeds
At present there is a renewed interest in HSM due to potential for faster production rates, shorter lead times, and reduced costs
© Department of Mechanical Engineering
ME-321 Manufacturing Process I
Indexable tools (face mills)
hp/rpm ratio = ratio of horsepower to maximum spindle speed
Conventional machine tools usually have a higher hp/rpm ratio than those equipped for HSM
Dividing line between conventional machining and HSM is around 0.005 hp/rpm
Thus, HSM includes 15 hp spindles that can rotate at 30,000 rpm (0.0005 hp/rpm)
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ME-321 Manufacturing Process I
Special bearings designed for high rpm
High feed rate capability (e.g., 50 m/min)
CNC motion controls with “look-ahead” features to avoid “undershooting” or “overshooting” of the desired tool path
Balanced cutting tools, toolholders, and spindles to minimize vibration
Coolant delivery systems that provide higher pressures than conventional machining
Chip control and removal systems to cope with much larger metal removal rates
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ME-321 Manufacturing Process I
High Speed Machining Applications
Aircraft industry, machining of large airframe components from large aluminum blocks
Much metal removal, mostly by milling
Multiple machining operations on aluminum to produce automotive, computer, and medical components
Quick tool changes and tool path control important
Die and mold industry
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