FIELD OF INVENTION
The present invention deals with the making of blade attachment slots at
compound angle on the periphery of tapered compressor wheels of gas turbines.
BACKGROUND OF THE INVENTION
The blade attachment slots on the periphery of gas turbine tapered compressor
wheels (or disks) are inclined at two angles in three dimensions. The slot profiles
are usually of dovetail shape. These blade attachment slots are used for
attaching the axial compressor blades at the required location and the
orientation, maintaining the required radiality, aXiality and drop values.
These dovetail blade attachment slots are usually produces by broaching by
using very costly large capacity special-purpose computer-controlled broaching
machines fitted with a special fixture arrangement to tilt the work-piece (i.e. the
tapered compressor wheel) accurately through two angles (helix angle and
inclination angle). (By another practice such dovetail blade grooves are machined
by a grinding process.) The prior art process of broaching also requires very high
cost broach-sharpening machines, expensive broaches, etc. This prior art process
is usually followed by some reputed gas turbine manufacturers of the world for
machining the high volume of tapered compressor wheels, i.e., for
manufacturing a large number of gas turbines per annum.
Thus, the prior art manufacturing process is associated with very high costs of
machine tool, fixturing arrangement, cutting tools and the sharpening facilities
for cutting tools.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide an economical and
cost-effective method for producing blade attachment slots at compound angle
on the periphery of gas turbine tapered compressor wheels which eliminates the
limitations of prior art, especially when the number of gas turbines to be
manufactured per annum is relatively small.
Another object of the present invention is to provide a method which uses a
general-purpose standard CNC machine tool with standard accessories with a
view to increase the versatility and optimize the utilization of the general purpose
standard CNCmachine tool.
A further object of the present invention is to provide a method which allows
greater fleXibility in production.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1 - shows the setup for milling of blade attachment slots at compound angle
on the periphery of tapered compressor wheels on a standard CNC horizontal
boring machine using a standard universal head.
DETAILED DESCRIPTION OF THE INVENTION
The invented method involves producing the blade attachment slots at
compound angle on the periphery of tapered compressor wheels (or disks) by a
milling process on a general-purpose standard CNC horizontal boring machine
using a general-purpose standard universal head.
The setup for the invented method is shown in Fig.!. The workpiece (2) (tapered
compressorwheel) is clamped on a locating fixture (6) at the centre of the rotary
table (12) of a CNC horizontal boring machine.The milling cutter (5) is tilted
through two angles (helix angle and inclination angle) in three dimensions by
using a standard universal head (1). As the milling cutter is tilted through two
angles and not the workpiece, the workpiece setup is very simple and rigid.
A standard universal head has a least count of 1 degree on both of its circular
scales-Scale#1 (3) and Scale#2 (4). In order to meet the stringent quality
requirements of the blade attachment slots on the periphery of tapered
compressor wheels, the milling cutter (5) needs to be accurately titled through
two angles (helix angle and inclination angle) in three dimensions before
commencing machining at compound angle.
A methodology has been invented for accurate setting of two angles on Scale#l
and Scale#2 of a standard universal head, to an accuracy of better than 1
minute, for accurate machining at compound angle, involving any general
compound angle machining application. In the invented methodology, a test
mandrel, a dial indicator and mathematical calculations are used for accurate
setting of the compound angle on Scale#l (3) and Scale#2 (4) of the universal
head (1).
This invented methodology has been used for accurately setting the helix angle
and the inclination angle on Scale#l (3) and Scale#2 (4) respectively on a
standard universal head, to an accuracy of better than 1 minute, in the invented
method for making blade attachment slots.
A reference tool is mounted on the tilted (i.e., tilted through two angles on
Scale#l and Scale#2) spindle of the universal head (1) and work co-ordinate
system is set on the work piece (tapered compressor wheel) using the reference
tool.
The blade attachment slots are then milled at compound angle on the periphery
of tapered compressor wheels, with the milling cutter (5) mounted on the tilted
(Le., tilted through two angles on Scale#l and Scale#2) spindle (7) of the
universal head (1), in three steps: (a) Rough straight slot milling of all the slots
using a straight sided roughing end mill, (b) Rough dovetail profile milling of all
the slots, leaving about 0.4 mm allowance above the finish profile, using a
rough-form milling cutter, and (c) Finish dovetail profile milling of all the slots
using a finish-form milling cutter. The tool path for all the above three steps (a),
(b) and (c) involves three-dimensional point-to-point movement of milling cutter
and simultaneous movement in X,V and Z axes during machining.
Milling cutters of different lengths are usually used in the above three steps (a),
(b) and (c). The variation in tool lengths of different milling cutters needs to be
accurately accounted for to achieve dimensional control of slot geometry. As
milling is carried out with the milling cutter (5) mounted on the tilted (i.e., tilted
through two angles on Scale#l and SCale#2) spindle (7) of the universal head
(1), standard tool length compensation can not be applied.
A methodology has been invented for effecting three-dimensional tool length
compensation while machining with a milling or drilling or reaming or boring or
facing tool, mounted on tilted (i.e., tilted through two angles on Scale#l and
Scale#2) spindle of a standard universal head, to accurately account for the
variation in tool length of different tools used during machining, involving any
general compound angle machining application. Before using a cutting tool for
machining, the initial Work Coordinate System set earlier using the reference tool
is additively translated by calculated amounts to account for the variation in tool
length (along the tool axis direction) of the particular tool with respect to the
reference tool.
This invented methodology for effecting three-dimensional tool length
compensation has been used to accurately account for the variation in tool
lengths of different milling cutters (end mills, rough form cutters and finish form
cutters), with the milling cutter mounted on the tilted (tilted through two angles
on Scale#l and Scale#2) spindle of a universal head, in the invented method for
making blade attachment slots.
It may be noted that the above methodology invented for effecting threedimensional
tool length compensation is not limited to milling tools alone and can
also be applied for all other types of tools that could be mounted on a universal
heat spindle, such as drills, reamers, boring tools, facing tools, etc., while
machining at compound angles.
KEY FEATURES
Reference
Numerals
Characterised
feature
Figure
(1) Universal Head 1
(2) Compressor Wheel 1
(3) Scale#l 1
(4) Scale#2 1
(5) Milling Cutter Axis 1
(6) Clamping Fixture 1
(7) Spindle Head 1
(8) Cutter end face 1
(9) Setting Block 1
(10) Compound Face 1
(11) Head Stock of CNC
Horizontal Boring Machine 1
(12) Machine Rotary Table 1
(13) Axis of Rotary Table of
CNC Horizontal Boring
Machine 1
WE CLAIM
1. A method for producing blade attachment slots at compound angle on the
periphery of gas turbine tapered compressor wheels on a standard CNC
horizontal boring machine using a standard universal head comprising the
steps of:
- clamping the tapered compressor wheel on a locating fixture at the center
of the rotary table of a CNChorizontal boring machine;
- mounting the universal head on to the machine ram;
- setting the two angles (helix angle and inclination angle) accurately on
Scale#l and Scale#2 of the standard universal head to an accuracy of
better than 1 minute;
- setting work co-ordinate system on the work piece (tapered compressor
wheel) using a reference tool mounted on the tilted (through two angles
on Scale#l and Scale#2) spindle of the universal head;
- rough straight slot milling of all the slots on the periphery of the tapered
compressor wheel using a straight sided roughing end mill mounted on
the tilted (through two angles on Scale#l and Scale#2) spindle of the
universal head;
- rough dovetail profile milling of all the slots on the periphery of the
tapered compressor wheel, leaving a finishing allowance above the finish
profile, using a rough-form milling cutter mounted on the tilted (through
two angles on Scale#l and Scale#2) spindle of the universal head;
- finish dovetail profile milling of all the slots on the periphery of the
tapered compressor wheel using a finish-form milling cutter mounted on
the tilted (through two angles on Scale#l and Scale#2) spindle of the
universal head.
2. A method as claimed in claim 1, wherein for effecting three-dimensional
tool length compensation while machining with a milling or drilling or
reaming or boring or facing tool, mounted on tilted spindle of a standard
universal head, to accurately account for the variation in tool lengths of
different tools used during machining, involving any general compound
angle machining application.
3. The method as claimed in claim 1, wherein three-dimensional tool length
compensation is carried out to account for the variation in tool lengths of
different milling cutters used in the machining process, with the milling
cutter mounted on the tilted spindle of the universal head.