U.S. patent number 3,585,980 [Application Number 04/805,960] was granted by the patent office on 1971-06-22 for method and device for removing mortar or the like from between superimposed rows of bricks or the like.
Invention is credited to Fred Mellor.
United States Patent |
3,585,980 |
Mellor |
June 22, 1971 |
METHOD AND DEVICE FOR REMOVING MORTAR OR THE LIKE FROM BETWEEN
SUPERIMPOSED ROWS OF BRICKS OR THE LIKE
Abstract
A method of removing mortar from between the rows of
superimposed bricks while collecting the dust generated thereby and
a blade guard for a grinding disc with a tangential dust
outlet.
Inventors: |
Mellor; Fred (Mossley,
EN) |
Family
ID: |
32329945 |
Appl.
No.: |
04/805,960 |
Filed: |
February 20, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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484878 |
Sep 3, 1965 |
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Current U.S.
Class: |
125/13.01;
451/41; 451/456; 451/451 |
Current CPC
Class: |
B24B
55/102 (20130101) |
Current International
Class: |
B24B
55/00 (20060101); B24B 55/10 (20060101); B28d
001/04 (); B24b 055/04 (); B24b 001/00 () |
Field of
Search: |
;51/170,268,273,283
;125/12,13 ;143/159 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,280,025 |
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Nov 1961 |
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FR |
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1,231,379 |
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Apr 1960 |
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FR |
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1,121,968 |
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Jan 1962 |
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DT |
|
848,564 |
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Sep 1960 |
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GB |
|
Primary Examiner: Whitehead; Harold D.
Parent Case Text
This application is a continuation of application Ser. No. 484,878,
filed Sept. 3, 1965 and now abandoned.
Claims
What I claim is:
1. A dust free method of removing mortar or the like from between
rows of superimposed bricks or the like by the use of a guard
having a planar open edge and a spinning cutting disc substantially
enclosed and pivotally carried by said guard and operating through
said open edge, and a vacuum discharge passageway in said guard
extending outwardly from one end of said planar open edge at an
angle greater than 90.degree. to the plane of said open edge
comprising, turning on the vacuum apparatus, and the spinning
cutting disc to maximum bringing the guard into substantial angular
contact with the wall with the said one end of the planar open edge
into initial substantial point contact with the wall, pivoting the
guard about the point of wall contact until the disc enters the
mortar of the wall, a predetermined depth and simultaneously
bringing the open edge of the guard in full contact with the wall
and then moving the guard and cutting disc while in full contact
along the mortar line of the wall whereby the path of the spoils
leaving the rim of the cutting disc is maintained substantially in
line with the discharge passageway in the guard at all times
including the initial cutting period.
2. The method of claim 1 wherein the outlet is shaped to form a
venturi to thereby enchance the extraction force.
3. The method of claim 1, wherein a shallow space is formed between
the disc and guard both above and below the disc to cut down the
volume of air close to the disc.
4. The method of claim 1, wherein the axis of the outlet path for
the spoil is at all times approximately tangential to the point on
the rim of the cutting disc where the rim emerges from the mortar
and the guard is swung relative to the cutting disc to maintain
said path at all times substantially in line with the discharge
passageway in the guard.
5. A dust free method of removing mortar or the like from between
rows of superimposed bricks or the like at the intersection of a
pair of walls by the use of a guard having two straight open edges
forming a V-shaped opening at one side and a vacuum discharge
passageway at the other side and enclosing a spinning cutting disc
operating operating through the apex of said V-shaped opening
comprising, turning on the vacuum apparatus, and the spinning
cutting disc to maximum, bringing the guard into contact with each
wall with the straight open edges of the V-shaped opening in
contact with the respective walls, then pivoting the cutting disc
through each of the open edges and simultaneously into contact with
the mortar of each wall and cutting a slot in each wall and sucking
off the spoil through the vacuum discharge passageway.
6. The method of claim 5, wherein the outlet is shaped to form a
venturi to thereby enhance the extraction force.
7. The method of claim 5, wherein a shallow space is formed between
the disc and guard wall on each side of the disc to cut down the
volume of air close to the disc.
8. A dust free method of removing mortar or the like from between
rows of superimposed bricks to a predetermined depth in a wall or
the like by the use of a guard substantially enclosed pivoted
spinning cutting disc operating through a planar open edge of the
guard, said guard having a vacuum discharge passageway therein
leading from one end of said planar open edge and at an obtuse and
an acute angle thereto comprising, turning on the vacuum apparatus
an the spinning cutting disc to maximum, bringing the guard against
the wall with the planar open end at an arcuate angle to the face
of the wall, contacting the cutter disc with the wall substantially
aligning the path of the spoils leaving the rim of the cutter disc
up with the discharge passageway in the guard and swing the planar
open edge of the guard toward the face of the wall to maintain said
substantial alignment as the disc cuts to the predetermined depth,
then upon contact of the planar open edge with the face of the wall
moving the guard and cutter along said wall.
9. The method as defined in claim 8, wherein said acute angle made
by the planar open edge with the wall is substantially equal to the
first-named acute angle made by the passageway with the planar open
edge of the guard.
10. The method as defined in claim 8, wherein the fixed obtuse
angle made by the passageway with the planar open edge is
substantially the same as the angle made by the path of the spoils
with the planar open edge when the disc is at its maximum depth in
the wall.
11. A spoil guard for use with a cutting tool having a rotatable
grinding or cutting disc, said guard comprising a housing for
enclosing at least a part of the disc and defining a straight edged
opening through which said disc can extend and a spoil outlet
opening, means for mounting said disc in the housing and defining
the rotational axis of the disc, said outlet opening being spaced
to one side of said rotational axis and defining the entrance to an
outlet duct, and the outlet duct having one wall extending from the
end of said straight edged opening on said one side of the axis and
the spoil outlet duct defining a longitudinal axis making
approximately a right angle with the line joining the said
rotational axis of the disc to the point of intersection of said
longitudinal axis and said edge.
Description
This invention is concerned with devices for attachment to portable
rotary grinding tools using grinding discs, to remove the spoil,
waste and dust particularly when the spoil and dust of such a size
as to be harmful to human respiration, or prevents continuous
operations of the tool.
Numerous proposals have been made for extracting dust and waste
particles from permanently installed cutting, grinding and
polishing tools in factories and workshops but when the grinding
and cutting operations are to be carried out in different places by
the same toll for example in civil engineering and building
projects, dust prevention and disposal is a serious and ever
present problem. In certain circumstances the materials being
ground away may form pyrophoric dust which introduces the danger of
fire as well as the danger of lung damage.
Portable grinding tools are well known in civil engineering and
related fields but when used to cut brick, concrete, stone and the
like they produce enormous quantities of spoil and dust which
spreads from the area surrounding the cut so quickly that the tool
becomes invisible to the operative after a few seconds of cutting.
This is pronounced when cutting inside a building.
The application of water to the cutting area is messy, troublesome,
sometimes inconvenient and usually unsuccessful and often not
suited to the type of grinding discs which are required for
masonry.
The present invention is designed to overcome the limitations
outlined above and proposes an arrangement whereby the use of
grinding tools as described may be made more acceptable,
particularly for indoor use.
According to this invention there is provided for use with a rotary
tool of the type using one or more thin grinding or cutting
members, an extraction device comprising a guard shaped to enclose
part at least of the said member or members, having an open end to
engage with the surface to be worked and a spoil outlet located
substantially tangentially to the emergent working edge of the
member so as to collect spoil thrown from the wheel, and means to
connect the device in an operative position relative to the
tool.
Addition air may be supplied by means of vents located near the
open edge of the guard.
The total area of the inlet or inlets is preferably at least as
large as the area of the outlet, but smaller inlets work almost as
well.
The outlet usually subtends between itself and the open edge, an
angle of between 90.degree. and 130.degree. depending upon the
depth of penetration intended or permitted by the guard.
The guard is preferably connected to the tool such that the
selected outlet angle does not change as the wheel moves further
into the surface to be cut.
The guard is preferably a one-piece housing the tool being provided
with an extension arm and the guard being pivoted to the end of
said arm at a point opposite to the location of the outlet.
The guard is normally urged by spring means into a position
enclosing the major part of the cutting wheel, which latter may be
swung into the surface to be cut thereby allowing a greater depth
of cut while maintaining the edge of the guard in contact with the
surface to be cut so preventing the escape of spoil.
The purpose of providing a central aperture to the guard is to
admit extraneous air into a zone of low-pressure air within the
guard caused by the air being spun from the center by centrifugal
force to the peripheral zone of the guard by the rotating disc,
creating a zone of higher pressure air such that the air mixes with
the spoil generated at the cutting edge of the wheel and carries it
to the outlet. The uneven abrasive surface of the cutting disc is
sufficient, if the speed of rotations fast enough, to impart
considerable momentum to the air within the guard and therefore,
the necessarily airflow speed to remove the spoil is ensured.
Usually the peripheral speeds required for cutting, that is in the
region of 6,000 r.p.m. combined with the abrasive surface of a
sintered wheel are sufficient to create adequate air movement to
send all the spoil through the guard outlet.
We have found that the extraction may be enchanced in various ways.
Firstly the outlet may be shaped such that a venturi is formed,
that is by giving the outlet a conical shape. This has the effect
of collecting both light and heavy particles thrown off the disc
along diverging paths. The outlet is preferably smoothed interiorly
so as to promote smooth airflow. SEcondly the space above the disc
and below the disc is preferably kept shallow of the order of half
an inch deep. The existence of only a shallow space above and below
the disc ensures that only a small volume of air may be in the
vicinity of the disc, and this enables the wheel to raise the speed
of the airstream to such a degree that both light and heavy spoil
particles spun off the surface of the cutting disc are forced from
the outlet.
Thirdly the space or pocket within the guard nearest the entering
edge of the disc suffers from a pileup of air at the surface to be
cut and this space is preferably closed off by a short diagonally
situated wall so that the interior contour of the guard more
closely follows that of the disc when in the cutting position. This
prevents the air being blown into the cut being made.
The spoil may be collected in a filter bag of loose weave connected
to the guard outlet. The bag is selected to retain the spoil but
must not set up too great a back pressure to the airflow emanating
from the guard. In use the airflow set up by the disc alone is
usually not great enough to inflate a filter bag.
The guard may however be in two or more parts with spring means
expanding them to enclose the major part of the cutting wheel, the
edge of one such part being adapted to engage the surface to be cut
and to be telescoped into the other part or parts as the depth of
the cut increases.
However, the efficiency of spoil extraction may also be improved by
providing a zone of reduced pressure at the outlet.
The reduced pressure may be set up in two ways. Firstly by the
provision of a fan driven by the cutting tool, and ducting and a
nozzle through which the fan may discharge the air into the venturi
part of the outlet so as to create a partial vacuum in the outlet
zone. The nozzle preferably projects as little as possible into the
outlet zone thereby causing least turbulence to the airflow. The
extra airflow contributed by the fan is usually sufficient to
inflate a filter bag easily and to overcome the back pressure of
the filtering action. Secondly a zone of reduced pressure may be
provided by a vacuum pump drawing the spoil and air through a
filter pad. In this instance the provision of an inlet in the guard
is not necessary, although preferable, as the purpose of the pump
is to create a partial vacuum inside the guard which is destroyed
only by extraneous air entering the slot or kerf which is cut by
the wheel.
The edge of the part of the guard which contacts the surface to be
cut may be provided with an outwardly directed flange. The greater
area afforded by the flange enables the tool to be moved more
steadily over the surface, being less susceptible to diversion by
upstanding mortar or surface blemishes than would be the case with
a plain rim. The flange may also be extended and modified so that
the plane of the flange lies at 45.degree. to the plane of the
surface to be cut and therefore, also the plane of the cutting
wheel. The flange will be arranged to clear the wheel as the latter
moves past it into the surface making one side of a V-cut. It is
proposed to form the latter type of flange as a fitment to
removably engage the permanent flange of the guard.
Certain embodiments of the present invention will now be described
with reference to the accompanying drawings in which:
FIG. 1 shows a perspective view of apparatus for cutting continuous
slots in mortar,
FIG. 2 shows a similar view of apparatus for cutting slots in two
intersecting surfaces,
FIG. 3 shows a similar view of apparatus for cutting, grooving and
chasing masonry, and
FIG. 4 shows a similar view of another embodiment for the same
purpose, and
FIG. 5 is a vertical cross-sectional view of FIG. 1.
EQUIPMENT FOR CONTINUOUS CUTTING OF MORTAR
Referring to FIGS. 1 and 5 the equipment for cutting mortar such as
would be needed to insert a dampproof course in an existing
building comprises a portable hand tool employing a thin flexible
grinding disc 2, a guard 4 for the disc 2, a power unit (not shown)
to drive the disc through a flexible power cable 6 and a power unit
to drive a vacuum pump which is connected to the guard via a
cyclone separator (not shown).
The portable tool consists of an angle grinding head 8, an inlet 10
for the flexible power drive cable 6 which acts as a handle for
manipulations, and an outlet 10 for a drive shaft. The disc 2 is
driven in a clockwise direction, see Arrow A, FIG. 1, or
anticlockwise movement of the disc in FIG. 5, relative to the drive
side of the disc at 3800--6000 r.p.m. depending on the safe running
speed of the disc, by a 2 b.h.p. electric motor now shown. A
fiberglass and resin grinding wheel disc 2 14" in diameter and
one-eighth inch thick made by FLEXIVIT is clamped to the drive
shaft.
The guard 4 encloses about one-half of the grinding disc and is
substantially semicircular in shape having a straight open edge 12
and terminating at the side downstream of the emergent cutting edge
of the wheel in a straight closed side edge 14 situated at a little
more than 90.degree. to the said open edge 12. The straight closed
edge forms one side of a triangular outlet extension 16 to the
guard. The open edge of the guard is one-half inch longer than the
diameter of the disc and the opening itself is 1 1/2inch deep.
There is a clearance between the curved parts 18 of the guard and
the rim of the disc of about one-fourth inch but this clearance
increases as the disc wears. The guard itself is formed from sheet
steel pressings welded at the seams, and is clamped by a ring
flange 20 to the angle head 8.
FIG. 5 discloses the guard 4 with its open straight edge 12 beyond
which about half of the grinding disc 2 projects and a spoil outlet
including side edge 14 forming one side of the spoil outlet
extension 16 and the spoil outlet 22. A clearance 18 for the disc 2
with respect to the interior of the curved portion of guard 4 is
provided. The guard 4 is clamped by a ring flange 20 to the angle
head 8, FIG. 1. The outlet is actually spaced to one side of the
denoted rotational axis R and defines the entrance to the outlet
duct D which has one side edge 14 extending from the end of said
straight edge 12 opening on one side of the axis and the spoil
outlet duct D and defines a longitudinal axis making approximately
the critical right angle with the line joining the rotational axis
of the disc to the point of intersection of the longitudinal axis
and said edge 12 as shown in FIG. 5 by the broken lines A - B and
the 90.degree. relationship of these two lines. This 90.degree.
relationship has proven essential and critical with respect to the
proper functioning of the device with respect to the removal of
substantially all the spoil removal particularly when the device is
used inside an enclosure.
The outlet 22 is connected by a flexible hose 24 to the inlet of a
cyclone separator through which spoil-laden air is drawn by means
of a vacuum pump at the rate 140 c.f.m. Such apparatus is supplied
by the British Vacuum Company.
In use the vacuum apparatus is started up and the disc revolutions
brought up to maximum by a clutch provided on the power unit. The
guard is held by the cable drive inlet and the hose outlet with the
outlet end of the straight open edge contacting the wall, the
spinning disc being as yet spaced from the wall. The tool is then
pivoted upon the point of wall contact until the disc enters the
mortar of the wall. Even though the open edge of the guard is still
spaced from the wall by a wedge-shaped gap the spoil which leaves
the wheel at an angle slightly greater than 90.degree. passes down
the outlet also lying at that angle.
Some air revolves within the guard but most of it is drawn off by
the vacuum pump. Very little dust escapes during the initial entry
of the disc and none escapes when the disc is fully in the
wall.
The open edge of the guard is then moved along the mortar line
abutting the wall all the time, thereby cutting a 51/2inch deep
slot in the mortar and the vacuum set up within the guard is
partially destroyed by extraneous air drawn in through the slot
already cut.
The outlet is arranged at 90.degree. to the open edge of the guard
because the spoil follows a path which is approximately tangential
to the point on the rim of the wheel where the rim emerges from the
mortar.
Although the disc is not insertable into the wall up to its
diameter when the tangent would be at 90.degree. to the wall, the
spoil tends to move in the direction of the disc rotation and this
corrected path is made the axis upon which the outlet is
situated.
The equipment enables dust free cutting 51/2inch deep through most
mortars at the rate of 1 foot per 10 seconds.
EQUIPMENT FOR CUTTING SLOTS IN THE MORTAR OF TWO WALLS INTERSECTING
AT RIGHT ANGLES
Referring next to FIG. 2, a special problem is presented when it is
necessary to cut a slot at the intersection of a pair of walls
because the enhanced thickness of the walls prevents penetration
sufficient to cut entirely through the intersection.
For such work the guard for an angle head grinder disc is a
flattened substantially quadrant-shaped casing 204 which encloses
the grinding disc 202.
The V-shaped front part 212 is open along both edges the latter
being protected by rubber strips, and is bisected along the upper
surface of the guard by a slot 226 extending rearwardly to abut the
center of the surface of the guard.
The slot 226 is wide enough to receive the shaft 228 of the angle
head 208 and is bordered by two lengths of angle iron 230 welded to
the said surface and extending in rearwardly diverging paths to the
edge of the arcuate part 218 of the guard.
The ends of the two angle iron braces 230 overlap the upper face of
the casing and have tapped holes in the overlapping parts 232. The
lower face of the guard is braced by a flat bar 234 which is welded
thereto so that the ends of the bar overlap the casing directly
beneath the overlapping parts of the angle members. The bar has
similar tapped holes and two levelling bolts 236 are carried in the
two pairs of tapped holes. In this way the device may be slid over
the floor keeping equidistant therefrom.
The rear part of the guard is shaped to form a smoothly contoured
outlet nozzle 216 for connection to vacuum equipment as described
in the previous example. The extreme end of the V-shape is cut away
to form a blunt end 238 which if left sharp might prevent the open
ends mating closely with the walls.
The angle grinder 208 and disc assembly is free to slide from a
position within the casing to a position where the angle grinder is
close up to the intersection of the walls, permitting the slot cut
by the disc to meet other slots cut by the equipment previously
described.
GUARD FOR CHASING TOOL EQUIPPED WITH 10 INCH DIAMETER DISC
Chasing in this specification refers to the cutting of slots
1--11/2inch deep in walls in pairs so that the margin of wall
between the slots can be easily chipped out to form a recess for
pipes or wires or the like.
Equipment comprises a portable PERLES HSW 56/6 power tool which has
a 1200 -watt, 1.6 -h.p. electric motor. The motor turns a 10 inch
flexible abrasive disc made of silicon carbide and resin (by RASTA,
Switzerland) at 6,000 r.p.m. in a clockwise direction through an
angle head attachment which is adapted to reverse the normal
anticlockwise direction of the motor shaft.
Referring now to FIG. 3 that shaft of the angle head 308 projects
through a protective sheet steel guard 304. The guard is of
substantially semicircular shape terminating in a flat open edge
312, 15 inch wide, which is surrounded by an outwardly turned
flange 342 which mates with the surface to be cut. The guard forms
a flattened casing 13/8inch deep around the cutting disc 302 and
the casing has along the side which is downstream of the emergent
cutting edge of the disc a shaped outlet 316 11/2inch in diameter,
the midaxis of which lies at 125.degree. to the flat open edge 312.
The outer edge 314 of the outlet 316 is curved outwardly so as to
cause the particles formed during cutting to flow without undue
turbulance over the walls of the outlet. The edge 314 may be
straight if desired but in practice it is found to cause a layer of
packed spoil to accumulate on the edge. A curved edge is therefore
preferred. The the drive side of the guard is provided with an
elongated arcuate slot 346 11/4inch wide extending from the center
of the guard to near the flat open edge.
The opposite side of the guard (not shown) is provided with a
central aperture 2 inch in diameter which is coverable by a
pivoting cover plate mounted on the guard.
The guard 304 is attached to the tool by means of a ring flange
320. The ring flange is part of an arm 348 which is of flattened
triangular shape the apex 350 being provided with a hole which
overlies corresponding holes in the top corner of the guard. A bolt
352 passes through the casing of the guard and through the arm 18
the assembly being clamped together by a pair of nuts and washers
screwed on to both threaded ends of the bolt 352.
The area surrounding the bolt 352 is enclosed in a sealed
compartment 354 which is packed with grease to ensure an easy
pivoting action. The bolt 352 is shaped to prevent the casing
collapsing when the nuts are tightened.
The triangular arm 350 has two arms which support the base part 356
which is deep enough to cover the slot 346 in the drive side of the
guard. The corner of the base part nearer the said flat open edge
312 is provided with an aperture 11/4inch in diameter to receive
the output shaft of the angle grinder head.
The ring flange 320 which connects the arm to the angle head 308 is
surrounded by a circlip 358 so as to grip the periphery of the
angle grinder. The circlip can be tightened by means of a screw in
the usual way.
The base or cover part 356 of the bracket has a central projection
360 which engages the guideway defined by an arcuate strip of metal
362 bolted to the drive side of the guard. The arcuate metal strip
has a slot therein such that a threaded pin projecting upwards from
the drive side of the guard beneath the strip may move to and fro
in the slot and may be locked in any desired position by a wing
nut. The operator is able by clamping the wing nut to limit the
travel of the disc in the arcuate slot and hence into the wall. In
practice the discs wear quite rapidly and a quick-adjustment screw
is provided to take up the wear.
Since the guard has a radius of 5 5/16 inches over its semicircular
part it is capable of pivoting rearwardly to enclose all the disc,
uncovering at the same time the arcuate slot 346. The flange 342
surrounding the flat open edge may be placed up to the wall and
when the disc is turning at full speed the power tool is gripped at
the handle 362 and the outlet 322 and pivoted about the top corner
bolt 352 against the pressure of spring means (not shown) which
tends to resist the emergence of the disc. As the disc emerges it
cuts the wall and at the same time the arcuate slot 346 is
gradually covered up by the base part 356. The aperture on the
opposite side may be then opened if desired.
In practice the necessary air ingress takes place through two sets
of louvres (not shown) the first lying on the said opposite side of
the guard located between the aperture 346 and the said flat open
edge 312, the second laying between the aperture 346 and the top
corner bolt 342.
TELESCOPING GUARD FOR CHASING TOOL EQUIPPED WITH 10 INCH DISC
Referring now to FIG. 4 the guard comprises a main body portion 404
which is secured to the angle head 408 of the same power unit as
used with the previous embodiment, by a V-shaped bracket 464. The
wall of the main body portion has on outwardly curved wall on the
outlet side. The open edge of the main body portion has an arcuate
recess 446 in the upper surface of the said portion.
There is a triangular-shaped extension 466 adapted to telescope
within the main body portion 404 and pivotally mounted thereto by a
bolt 452 located at the outlet side of the main body portion. The
extension 466 has an arcuate recess 468 in its upper face
corresponding to the recess 446 in the main portion so that the two
together form an arcuate slot of variable length in which the
output shaft 440 of the angle head can move. The extension is
biased into a fully extended position enclosing the disc 402 by
spring means not shown. The extension has an open end and a flange
442 similar to the guard shown in FIG. 3.
In use the vacuum equipment and motor are switched on so that air
passes down the outlet and when the disc is running at full speed
within the guard the open end of the extension is placed against
the wall to be worked and a tilting pressure applied to the main
body portion, about the bolt. The extension then telescopes into
the main body portion against spring pressure allowing the disc to
enter the wall.
The purpose of both latter guards is to direct the spoil created at
the cutting edge of the disc away from the said edge and toward a
spoil collection area. I have tried to collect the spoil in a
filter bag of woven material connected to the outlet but the rate
of increase of weight due to accumulating spoil was of great that
only short chases could be done before the bag grew too heavy to be
comfortable, but for short chases the filter bag was quite
acceptable.
Hence I prefer to use a flexible outlet pipe leading to a cyclone
separator through which air is drawn by a vacuum pump at the rate
of 40 cu. ft./min.
In order to improve the shape of the chases an adjustable guide may
be mounted on an arm extending from the guard such guide being
insertable into and slidable in the first slot of the chase,
thereby enabling the disc to cut the second slot parallel to the
first.
In another embodiment not illustrated the guard is made slightly
deeper and adapted to enclose two discs side by side so as to cut
two parallel cuts simultaneously leaving masonry between to be
chipped out at a speed with a power chisel. The increased torque
for this embodiment is preferably supplied by flexible cable driven
to the discs rather than by a hand tool which would tend to burn
out if used continually under the extra load.
* * * * *