FIELD OF THE INVENTION
The present invention relates to a method for eliminating the blow-out blemish
during hole machining using abrasive water jet cutting technique in nonferrous
materials.
BACKGROUND OF THE INVENTION
The abrasive water jet cutting is a non-traditional machining method that utilizes
an extremely thin and coherent jet of water-abrasive mixture to carry out
machining of materials. Initially water is pressurized using a high pressure pump
capable of delivering water at pressures as high as 415 Mpa. Then, this
pressurized water is forced through an orifice at a very high speed of equal to
Mach 2, so as to form a thin coherent jet of water. The orifice is typically made
of sapphire or ruby. After exiting through the orifice, the jet is focussed at the
work material by using a number of tungsten carbide nozzles. The water jet
exiting through the nozzle can cut materials by initially piercing through the full
depth and subsequently, the water jet can be guided under the control of a CNC
machine or a programmed computer so as to configure a required profile on the
work material. A pure water jet can also be used for the cutting application
which is however, enabled to cut soft materials only, for example plastics and
wood. An addition of abrasive particles in the pure water jet makes the water jet
highly suitable for machining of a wide variety of ferrous and non-ferrous
materials. Any profile can be cut on such materials by guiding the jet through
CNC or computer control. The abrasive particles are entrained in the water jet,
when the jet passes through a mixing chamber after exiting from the sapphire or
the ruby orifice. The abrasive particles are stored in a hopper at atmospheric
conditions and are allowed to impinge on the high velocity jet and entrained in
the stream of water jet in the mixing chamber by means of the known venturi
effect. The resulting abrasive laden water jet is then discharged on a work
material through a focussing nozzle or cutting nozzle and machining of the
material is carried out.
The abrasive water jet cutting technique can be used for.machining different
profiles on a wide variety of materials. During machining of the materials
adopting an abrasive water jet cutting process, the jet of abrasive water mixture
is allowed to. initially pierce the job through its full depth or full thickness of the
material and then a profile machining is carried out by guiding the jet along the
required profile by way of computer or CNC controls. This initial piercing of the
jet normally takes a substantial period of time depending on the hardness and
thickness of the material that is required to be machined. For machining holes or
other profiled cavities, the initial piercing is done inside the required profile and
cut out slug material is generated as a result of the cut. For starting the
cutting of holes of required profile, the high velocity abrasive laden water jet
exiting through the focusing nozzle is allowed to strike the material surface at a
point inside the profile that is desired to be machined out. The jet is allowed to
stay at this point for a period of time sufficient to fully pierce through the depth
or thickness of the material. This initial static piercing is first done, because the
material inside the profile of the hole is cut out as a slug which doesn't form a
part of the component that is being machined. Following this static piercing of
the jet to the full thickness of the material, the jet is moved to the point where
the required profile of the hole begins. The path travelled by the jet from the
point of piercing to the point where the actual profile of the hole begins is known
as 'jet entry profile path' or 'jet-entry profile'. After entering the required jet-
entry profile, the full profile machining of the hole is carried out by guiding the
jet with a speed of travel, referred to as 'travel speed' or 'traverse speed', along
the required profile by way of computer or CNC controls. The jet comes back to
the initial point where it began tracing the profile of the hole. At this instant, the
cut out slug begins to detach from the hole. Then, the jet is made to travel some
extra distance along a specified path, from this initial point to a point inside the
so produced hole, and allowed to remain there for some time more, after the cut
out material detaches from the parent material. This path travelled by the jet
beyond the point of detachment of cut material from the produced hole is known
as the 'jet exit profile path'. If the cutting jet of abrasive water mixture is
stopped exactly at the instant when the cut out slug of material detaches from
the parent material, it causes a left over bit or nib of material still clinging on to
the work material. This leftover material that remains clung onto the work
material after completion of the full profile machining, is known as'blow-out'
blemish. The presence of the blow-out blemish seriously affects the tolerance
specifications of the machined hole which has to be machined subsequently by
an additional finish machining process like grinding, honing or lapping etc., which
inter alia increases the cost of machining and time of machining as well.
Therefore, it becomes necessary to eliminate the blow-out blemish by adapting
jet entry and exit profile paths including a technique to maintain a close
tolerance for the holes machined by abrasive water jet cutting.
US Patent Document US 2011/0076405 A1 describes a method of drilling of holes
through a coating on a substrate with a close proximity back wall using water jet
cutting.
US Patent Document US 7469620 B2 describes a method of eliminating multiple
external pierces in NC cutting of nested parts i.e. parts that are programmed to
be cut out from a single plate of raw material.
US Patent Document US 2011/0086579 A1 describes a water jet cutting system
that includes an A1ignment device using visible light for properly locating the
point of piercing of the water jet in the material.
US Patent Document US 4540320 describes a method and an apparatus involving
the use of a cutting tool and a pivoted link mechanism for cutting holes.
US Patent Document US 4955164 describes a method and apparatus for drilling
small diameter holes in fragile material that tend to crack when impacted with
high velocity jets.
The present inventors noticed that elimination of blow-out blemish largely
depends on the type of entry and exit profiles selected for cutting the hole using
abrasive water jet cutting technique. The conventional practices use one of
straight lines; inclined lines, and arcs for the jet entry and exit profiles. A random
selection of these profiles although results in minimizing the blow-out blemish
but fails to completely eliminate the blow-out blemish. A blow-out blemish,
however small it is, if present, either causes the hole dimensions to exceed
allowable tolerance or results in out-of-roundness of the machined holes.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for eliminating in
abrasive water jet cutting technique, the blow-out blemish during hole machining
on non ferrous materials, which eliminates the disadvantages of prior art.
Another object of the invention is to propose a method for eliminating, in
abrasive water jet cutting technique the blow-out blemish during hole machining
on non ferrous materials, which is enabled to machine smooth holes of several
profiles maintaining close tolerance.
A still another object of the invention is to propose a method for eliminating in
abrasive water jet cutting technique the blow-out blemish during hole machining
on non ferrous materials by increasing the dwell time of the cutting jet at a
location where the cut-out slug materials detaches from the parent material.
A further object of the invention is to propose a method for eliminating in
abrasive water jet cutting technique the blow-out blemish during hole machining
on non ferrous materials, which allows slowing down of the travel speed of the
cutting jet mixture after travelling a defined distance along the perimeter of a
selected hole profile.
SUMMARY OF THE INVENTION
According to the invention, there is provided a method for eliminating in abrasive
water jet cutting technique the blow-out blemish during hole machining on non
ferrous materials. According to the invention, a modified jet entry and exit profile
paths with defined parameters for machining smooth holes of profiles, that are
fully circular or containing at least one circular arc, is selected and used with a
view to completely eliminate the blow-out blemish occurring at the end of the
abrasive water jet cutting of holes in non-ferrous materials.
As per the invention, a desired work material is loaded onto a supporting table of
a water jet cutting system. The focusing nozzle of the water jet cutting system is
positioned just above a point inside the required profile of the hole that is
desired to be machined on the material. A distance between the nozzle and the
work piece is maintained within 5 mm. A high velocity abrasive laden jet exiting
from the focusing nozzle of the system is allowed to pierce this point inside the
required profile of the hole. Such a static piercing of the jet is allowed a time
sufficient for the jet to completely pierce the full thickness of the material. After
the cutting jet completely pierces through the material, the jet is guided from
this point to a starting point of the required profile of the hole, along a modified
arc type of jet entry profile path by using CNC or computer controls. After the
cutting jet enters at this starting point of the profile of the hole, it is guided by
CNC or computer controls along the perimeter of the required profile of the hole
until a specified distance is travelled along the perimeter of the hole. Thereafter,
the travel speed of the jet is slowed down as described hereinabove, and further
at the instant, when the cut out part of the slug material from inside of the hole
begins to detach from the parent material, the cutting jet is given a dwell time in
order to completely remove the leftover bit of material, which otherwise would
remain in the parent material in case, the cutting jet is abruptly stopped at that
instant. After the dwell time elapses, the cutting jet is allowed to travel along the
modified arc type of the jet exit profile path in order to completely eliminate the
blow-out blemish in machining holes in non-ferrous materials. The different
parameters pertaining to the aspects mentioned hitherto including the arcs
defining jet entry and exit profiles, the specified distance of travel along the
perimeter of the desired profile of the hole and the dwell time to be given for the
jet at the instant when the cut out slug of materials detaches from the parent
material, are described in further details mentioned below.
The present invention makes use of a modified jet entry and exit profile paths
with a defined set of parameters, obtained as result of the extensive research
work carried out by the present inventors, which when used, completely
eliminates the blow-out blemish, so as to avoid defects like, out-of-roundness to
the holes machined by the abrasive water jet cutting technique. Further, this
invention discloses a method of retarding the travel speed of the cutting jet upon
travelling a defined distance along the perimeter of the intended profile of the
hole. Thereafter the cutting jet is allowed to maintain said travel speed till such
time the cut materials are detached and fall off from the parent material, which
enables a complete elimination of the blow-out blemish and hence results in a
smooth hole with close tolerance.
The present invention selects a pre-determined jet entry and exit profiles which
are used for machining of holes in non-ferrous materials by using an abrasive
water jet cutting method in order to eliminate the formation of blow-out blemish
at the end of the abrasive water jet cutting process.
The present invention thus discloses a method of piercing the material internal to
the profile of the hole that is desired to be machined and by using a pre-
determined jet entry and exit profiles for cutting precision holes in non-ferrous
materials, the blow-out blemish at the end of cutting is totally eliminated. The
present invention eliminates use of a light alignment device of prior art.
Accordingly, the disclosed method is enabled to produce smooth holes in non-
ferrous materials.
The present invention allows machining of any diameters of holes, by using the
abrasive water jet cutting technique and by employing pre-determined jet entry
and exit profile paths without the formation of blow-out blemish.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The proposed invention will be better understood by the following description with
reference to the accompanying drawings:
Figure 1 Shows a hole of circular profile with proposed jet entry profile path
according to the invention.
Figure 2 Shows a circular profiled hole with jet entry profile path and direction of
machining of hole according to the invention.
Figure 3 Shows a circular hole with jet entry and exit profile paths according to the
invention.
Figure 4 Shows an oblong hole with jet entry and exit profile paths according to
the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, a non-ferrous material in which one or more holes of a
profile containing at least one circular arc segment, is required to be machined,
is loaded onto a worktable of a water jet cutting machine. A focusing nozzle of
the abrasive water jet cutting machine is oriented to a first point (2) inside a
desired profile (1) of a hole to be machined. The focusing nozzle is kept at a
standoff distance of not more than 5 mm from the surface of the work material
in which the hole with the desired profile (1) is to be machined. The cutting or
focusing nozzle is positioned above the point of initial pierce (2) in a plane that is
perpendicular to a plane containing the figure 1. The abrasive and water mixture
is turned on, with the cutting nozzle positioned as stated above. The abrasive
laden water jet is allowed to strike the material at the first point (2), so that the
abrasive water jet can pierce through the entire thickness of the material. After
this static piercing of the jet at the first point (2), the jet is made to cut the
material along an arc type of jet entry profile path (3) from the first point (2) to
a second point (4) by guiding the jet along the circular arc (3), with a speed of
traverse of the cutting jet not exceeding 10 mm/minute. The second point (4)
represents the start point of the profile (1) of the hole, required to be machined
in said non-ferrous material. The circular arc (3) defining the jet entry profile
path (3) from the first point known as point of initial pierce (2) of the cutting jet
to the second starting point (4) of the hole is defined as below

In the above equation, 'H' represents the diameter (in mm) of the circular arc
present in the profile (1) of the hole that is required to be machined in said
material and 'L' represents the diameter of the circular arc (in mm) that defines
the jet entry profile path from the first point (2) to the second point (4), being
the starting point of the profile (1) of the hole. In case of simple circular holes,
'H' is the diameter of the hole, whereas in the case of the profile of hole
containing at least a circular arc segment, 'H' represents the diameter of this
circular arc segment. The jet entry profile path (3) connecting the first point (2)
to the second point (4) is a circular arc of diameter 'L' and with a sector angle of
90°.
After the cutting jet traverses the jet entry profile path (3) from the initial pierce
point (2) to the start point (4) of the hole as defined by the circular arc (3) with
parameters as said above, it is made to travel along the profile (1) of the hole.
The cutting jet is guided by CNC or computer controls and it traverses the exact
profile (1) of the hole from the start point (4) of the hole in a counter clockwise
direction (5) and cuts the material All along as it traverses along the profile (1) of
the hole, as guided by CNC or computer controls. The speed of traverse of the
cutting jet shall not be more than 20 mm/minute during the time the jet
traverses along the profile (1) of the hole from the start point (4) of the hole.
After the cutting jet traverses 'R%' of the total perimeter of the profile (1) of the
hole that is required to be machined, the speed of traverse of the jet is slowed
down gradually to not more 5 mm/minute. The calculation for determining 'R' is
detailed below

In the above equations, 'H' and 'L' represent the same parameters as described
above. The parameter 'T' represents the total perimeter (in mm) of the profile
(1) of the hole that is required to be machined, 'd' represents the diameter (in
mm) of the outlet of the cutting nozzle through which the abrasive water mixture
exits said nozzle. After the jet is slowed down gradually as stated above, the
cutting jet further traverses to cut the material All along the profile (1) of the
hole, in the clockwise direction and a point is reached, where the cut out slug of
material from the inside of the hole slowly begins to detach from the parent
material. At this instant, speed of traverse of the cutting jet is reduced to zero
and the jet is made to dwell at that point where the cut but slug of material
begins to detach from the inside of the hole for a length of time of not less than
1 minute. After the cut out slug of material completely falls off from parent
material thus leaving behind the hole, the speed of traverse of the cutting jet is
slowly raised to not more than 5 mm/minute and the jet is made to travel to the
start point (4), of the profile, and travels stilt further along the jet exit profile
path, which is a circular arc (6), in order to completely avoid formation of the
blow-out blemish and out-of-roundness of holes, which would otherwise form at
the end of machining of the said hole. The circular arc (6) is of diameter 'L' and
of a sector angle 60°. After traversing the jet exit profile path as defined by the
arc (6), the abrasive water jet mixture is switched off and can be used for further
machining of the said material.
The same method described hitherto can be extended to holes of any profile that
contain at least one circular arc segment in non-ferrous materials so as to
eliminate the blow-out blemish occurring at the end of the abrasive water jet
cutting process. For instance, to machine an oblong hole (7), the abrasive water
jet mixture exiting through the cutting nozzle is allowed to pierce through the full
thickness of the material at a third point (8) inside the oblong profile (7) of the
hole. After static pierce of the cutting jet, it is made to traverse along a new jet
entry profile path (9) connecting the third point (8) to a fourth point (10) of the
oblong hole profile. The diameter 'L' of the circular arc of sector angle of 90°
defining the new jet entry profile path (9) is calculated using the formulae
mentioned above. After reaching the fourth point (10), the jet cuts the material
as it traverses all along the oblong profile (7) of the oblong hole with a traverse
speed of not more than 20 mm/minute, in a clockwise direction (12). After
traversing 'R%' of the total perimeter of the oblong profile, the cutting jet is
slowed down gradually to a traverse speed as described in case of profile (1) of
the hole. Further, at the instant when the cut out slug of material detaches from
the parent material, the dwell time of not less than 1 minute is given and the jet
further traverses along a new jet exit profile path (11), a circular arc of diameter
'L' and sector angle 60°, with a speed of traverse not more than 5 mm/minute, in
order to completely avoid the formation of blow-out blemish and any possible
out-of-roundness, which would otherwise happen at the end of machining of
holes using the abrasive water jet cutting process.
The proposed invention as narrated herein above should not be read and
construed in a restrictive manner, as some modifications, adaptations and
alterations are possible within the scope and limit of the invention as defined in
the encompassed appended claims.
WE CLAIM
1. A method for eliminating blow-out blemish during hole machining of a
work piece formed of non-ferrous materials in a water jet cutting machine,
the machine comprising at least a work table, an orientable and movable
focusing nozzle, a source for supplying pressurized water jet, a mixing
chamber with abrasive particles, and a programmed control means to
implement water jet cutting technique on the non-ferrous material, the
method comprising the steps of:
- disposing the focusing nozzle at a vertically upward distance about 5 mm
from the surface of the work-piece;
- selecting a first point of initial pierce and a second point representing start
point of the hole-profile;
- allowing the abrasive-laden water to strike via the nozzle at said first point
so as to pierce through the entire thickness of the job;
- selecting an arc type jet entry profile and cutting the non-ferrous material
by the water jet along said profile starting from said first point to reach
said second point, the water-jet profile being controlled under a
programmed apparatus while the speed of traverse of the jet is restricted
to a first predetermined value;
- subsequently making the water jet to travel along the profile of the hole
from the second point at a counter-clock direction at a second pre-
determined speed till such time the water jet travels R% of the total
perimeter of the hole profile;
- gradually reducing the speed of travel of the water jet to zero and
allowing the jet to dwell at a point till such time cut material from the cut-
hole commence detaching from the parent material;
- increasing the travel speed of the jet profile to a third pre-determined
speed so as to travel from the second point along a jet exit profile path;
and
- deactivating the water-jet supply source after completion of jet traverse
along the jet exit profile path.
2. The method as claimed in claim 1, wherein the first predetermined speed
of travel of the abrasive laden water jet is not exceeding 10 mm/minute.
3. The method as claimed in claim 1, wherein the circular arc representing
jet entry profile is defined by the relationship of:

Wherein 'H' is the diameter of the circular arc present (in mm) within the
hole profile to be configured, 'L' representing the diameter (in mm) of the
jet entry profile path between the first and second point.
4. The method as claimed in claim 1, wherein the second predetermined
speed of travel of the abrasive-laden water jet is not more than 20
mm/minute.
5. The method as claimed in claim 1, wherein said R% of the total perimeter
of hole profile is determined from the relationship of:

the parameters of 'H' and 'L' being the same
defined on claim 5.
6. The method as claimed in claim 1, wherein the third predetermined speed
of travel of the abrasive-laden water jet is not more than 5 mm/minute.

ABSTRACT

A method for eliminating blow-out blemish during hole machining of a work piece
formed of non-ferrous materials in a water jet cutting machine, the machine
comprising at least a work table, an orientable and movable focusing nozzle, a
source for supplying pressurized water jet, a mixing chamber with abrasive
particles, and a programmed control means to implement water jet cutting
technique on the non-ferrous material, the method comprising the steps of,
disposing the focusing nozzle at a vertically upward distance about 5 mm from
the surface of the work-piece; selecting a first point of initial pierce and a second
point representing start point of the hole-profile; activating the water jet and
allowing the jet to pass through the mixing chamber; allowing the abrasive-laden
water to strike via the nozzle at said first point so as to pierce through the entire
thickness of the job; selecting an arc type jet entry profile and cutting the non-
ferrous material by the water jet along said profile starting from said first point
to reach said second point, the water-jet profile being controlled under a
programmed apparatus while the speed of traverse of the jet is restricted to a
first predetermined value; subsequently making the water jet to travel along the
profile of the hole from the second point at a clockwise direction at a second pre-
determinedspeed till such time the water jet travels R% of the total perimeter of
the hole profile; gradually reducing the speed of travel of the water jet to zero
and allowing the jet to dwell at a point till such time cut material from the cut-
hole commence detaching from the parent material; increasing the travel speed
of the jet profile to a third pre-determined speed so as to travel from the second
point along a jet exit profile path; and deactivating the water-jet supply source.