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Патент USA US3064406

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Nov. 20, 1962
L. con-:s. JR
CONTROL MEANS FOR USE WITH GRINDING WHEELS
"
3,064,396
HAVING MAGNETIC PROPERTIES
Original Filed Jan. 16, 1959
Hg. 2'
AMI’.
INVENTO
Loewe C055 E.
W
WM” @ MM
ATTORNEY
United Etates Patent 0
3,064,396
or
EQQ
Patented Nov. 20, 1962
1
2
3,064,396
Instead of magnetite Fe3O4 I can use cobaltous co~
baltic cobalt oxide, C0304 or cobalt ferrite, CoO-Fe2O3,
Loring (1025, In, Princeton, Mass, assignor to Norton
Company, Worcester, Mass., a corporation of Massa
or nickel ferrite NiO-Fe2O3. Nickel doesn’t have any
magnetic oxide of its own. Mixtures of the two metals
with the three oxides in any combinations can be both
CONTROL MEANS FOR USE WITH GRINDENG
WHEELS HAVING MAGNETIC PROPERTEES
produced and used.
To produce a grinding wheel according to the in
chusetts
Original application Jan. 16, 1959, Ser. No. 787,216. Di
vided and this application Jan. 27, 1961, Ser. No.
85,249
6 Claims.
vention or to produce any other type of abrasive wheel
according to the invention, I ?rst procure a vitri?ed
grinding Wheel manufactured in accordance with any
(or. 51—165)
known technique.
The invention relates to machine tools using magnetic
Nearly all vitri?ed abrasive wheels
are porous to some extent, enough for the present pur
poses.
abrasive wheels.
This application is a division of my copending appli
As the greater part of precision grinding and lapping
cation Serial Number 787,216, ?led January 16, 1959.
One object of the invention is to provide machine tools 15 is done with vitri?ed wheels having fused alumina
abrasive which means that the abrasive is alumina A1203
and electronic components with a magnetically suscepti
of the alpha variety which has been fused in an electric
ble abrasive wheel for controlling grinding operations.
furnace, I will ?rst describe the invention in connection
Another object is to provide a combination of instru
with such a wheel.
mentalities including a magnetically susceptible abrasive
product for controlling an abrasive operation with pre 20
EXAMPLE I
I procured a grinding wheel which was made of white
cision.
Other objects will be in part obvious or in part pointed
out hereinafter.
aluminum oxide abrasive grains of a purity of better
than 98%, half of it being 280 grit size and the other'
half (by weight) being 320 grit size, and having a
In the accompanying drawings,
FIGURE 1 is a magni?ed sectional View of an abrasive
wheel according to the invention,
FIGURE 2 is a block and wiring diagram of a thread
grinding machine to illustrate one application of the
volume percentage of abrasive of 47.3%, a volume per
centage of bond of 15.0%, and a volume percentage of
pores of 37.7%. These pores were interconnected. The
abrasive wheel of the invention,
bond formula is given in the following table.
FIGURE 3 is a cross sectional view of a furnace 30
which can be used to produce the abrasive wheel of the
invention.
7
TABLE 1
-
Parts by weight
Although abrasive grains are bonded with many dif
ferent kinds of bond to produce grinding wheels and
lapping wheels, for use in machine tools for precision
grinding and lapping, the vitri?ed bond has for many
years been preferred. Abrasive grains are bonded with
tion of clays, ?uxes such as feldspar, frits (crushed glass
of known composition according to formula for the
production of bond of speci?c composition). Vitri?ed
bond is a combination of oxides, with usually a la: ge pro
portion of silica so that most vitri?ed bonds are sili~
cates. They are analyzed as a combination of oxides
frequently some TiO2. Iron oxide is usually kept at a
low ?gure because of its excessive ?uxing properties
and because it discolors the wheels irregularly. It also
gives a bond somewhat unpredictable characteristics in
?ring.
The invention provides a thoroughly practical vitri
?ed bonded abrasive wheel, a grinding wheel or a lap
ping wheel or according to any other designation, of free.
65.13
Alumina A1203
16.96
Iron oxide Fe2O3 _________________________ __
vitri?ed bond by mixing them with any of a combina
of which, besides silica SiO2, alumina A1203 is usually
prominent along with MgO, CaO, NaQO, K20, B203 and
Silica SiOZ _
1.72
Titanium oxide TiO2 ______________________ __
Zirconium oxide ZrO2 _____________________ __
1.15
0.06
Phosphorous oxide P205 ___________________ __
0.07
Calcium oxide CaO _______________________ _._
5.08
Magnesium oxide MgO ____________________ .__
2.73
1.03
2.70
Sodium oxide NazO _______________________ __
Potassium oxide K20 ______________________ .__
45
3.49
Iron oxide Fe()
This wheel was 12" in diameter, 1A” thick and had
a 3” central hole. It was made in the usual way, by
coating the abrasive grains with a water solution of
dextrine and then mixing therewith the bond in dry
powdered form, molding, ?ring.
.
*I placed this Wheel in a controlled atmosphere furnace
heated to 400° C. A stream of nitrogen gas saturated
with iron carbonyl vapor was passed through the furnace
at the rate of two litres per minute for two hours. Then
the furnace was flushed out with nitrogen and steam was
cutting precision quality which has at the same time 55 passed through it at the rate of two litres per minute
magnetic properties, that is, it is permeable and will
a?ect a magnetic ?eld. It is easy to manufacture as‘ the
following description will show. The magnetic material
does not deleteriously affect the grinding, lapping or
other abrasive properties.
for two more hours, the temperature saill being held
at 400° C. The furnace was then allowed to cool and
when the wheel was cool it was complete. Naturally
60 more than one wheel will be made at one time.
FIGURE 1 shows the condition of this wheel. The
fused aluminum oxide abrasive grains of 28-0 and 320
grit size are indicated by the numeral 1. The bond is
indicated by the numeral 2, the pores by the numeral
but is usually written Fe3O4. This oxide is magnetic.
I reject iron because it too readily oxidizes to a non 65 3. The coating of magnetite Fe3O4 on the pore walls
is indicated by the numeral 4 and the wheel as a whole
magnetic oxide and furthermore its presence in the pores
is indicated by the numeral 5. This wheel is sufficiently
of a grinding wheel would cause smearing of the work
magnetically permeable for the purposes of this invention.
piece being ground which would be undesirable. The
The furnace used in making the wheel of Example I
magnetic metals cobalt and nickel, singly or in com
bination, can, however, be used as they do not have bad 70 is shown in FIGURE 3. It consisted of a chamber com
According to the invention the magnetic material may
be magnetite, ‘5e30,. This is sometimes said to be a
combination of one mol of FeO plus one mol of Fe2O3,
smearing properties and furthermore don’t readily oxi
dize to non-magnetic oxides.
'
prising a bottom 6 and a top 7 connected by a sand seal
8 ‘with an inlet pipe 9 an outlet pipe 10 and a bar 11 to
hold the wheels 5. The inside diameter of the bottom
6'was 24" and the overall height of the furnace was 44"
and the other dimensions were about in proportion there
to as shown in FIGURE 3. All parts except the sand
were made of stainless steel. The furnace was cylindrical.
Wheels. according to the invention are useful ‘for con
However, the machine should have a circuit to cause
the slide 24 to withdraw at the end of a grinding operation.
Referring to the lower right of FIGURE 2, a push button
switch 45 is connected by lines 46 to the lower‘lines 2t)
trolling‘grinding' operations as illustrated in the diagram
of FIGURE 2. In FIGURE 2 the work piece 12 is be
ing ground by a grinding ‘wheel 5 made in accordance
with the invention. In Example I the manufacture of
the ‘double rel-ay switch 21. The double relay switch 4-9
is connected by lines 50 to the power lines 20 and by
lines 51 to the lines 25, and it will be seen that the direc
and when closed, through lines 47 energizes a relay sole
noid 48 which closes a double relay switch 49 and opens .
a straight wheel was described, but this can be trued to
a shape like that shown in FIGURE ;2 and this is the
way to make such a shape. The wheel 5 is mounted on
a spindle 13 which is rotated as by means of belts 14-.
As ‘FIGURE 2 is simply a diagram, a feed nut 15 is 15
tion of the current is reversed through the double relay
switch 49 as compared with through the double relay
switch 21. The motor 18 is a reversible motor, since
the current through its ?eld coils is not reversed as they
spindle 13 forward and back in the direction of the work,
are
the
the
the
the spindle 13 being journalled on the slide 16. The
thus stopping the withdrawal of the slide. Referring to
nut 15 is moved by a screw shaft 17 driven by an electric
the bottom middle left of FIGURE 2, to start the ma~
motor 18. Referring now to the lower right of FIGURE
chine up again the operator momentarily presses a push
button switch '55 which connects lines 56 to lines 57, the‘
represented which is connected to a slide 16 to move the
2 and working to the left and up, the armature of the
motor 13 is energized by power lines 20 through a double
connected by lines 20a to the lines 20 not through
switch 4.9, so when the push button switch ‘45 is closed
slide 16 retreats, and when it gets to a certain position
switch 24 opens the circuit and the motor 18 stops
relay switch 21, another double relay switch 22, lines
former being connected to lines 23 and the latter to lines
25, Later the circuit is reestablished through the slide‘
23, a slide controlled switch 24 and lines 25.
switch 24.
The slide
Similarly, my combination. of a magnetic‘
controlled switch 24 has an insulated element attached 25 abrasive product and a magnetic sensing device operating
relatively to feed the abrasive member and work piece
to the slide itself which, when it Withdraws to a certain
point, opens the vcircuit as diagrammatically indicated. In
this description the abrasive product is moved relative to
the "workpiece as a result of the action of the magnetic
toward and away from each other may be readily applied 7
to abrasive operations, involving segments, discs, ‘sticks
and other shapes in surface grinding and other types of
V sensing'device', but since it is only the relative motion 30 abrading operation as well ‘as cylindrical grinding.
between the abrasive'and the work that is necessary, it
is obviously possible operatively to position the sensing
EXAMPLE '11
device to move‘the work piece" relative to the abrasive,
or to move both, in order to achieve the desired result.
' Referring now to the upper right hand part of FIGURE
I coated the pores of the same kind of a grinding wheel
as speci?ed in Example I with magnetite, FeaOgin the
2 and working‘ to the left, the stopping and starting of
following manner:
The wheel, the same size and composition and volume
percentage of abrasive bond and pores as in the case of
thei'motor 18. is controlled bypower lines 28 to an off
and on control 29. energized by an ampli?er 30 which
Example I, was placed in a tightly ?tting can, open at
receives its signal from a bridge. 31. energized by an
oscillator 32.. The bridge 31 is connected by wires 34'
and‘35. to a magnetic sensing device 36 which is sup
the top, and was ‘covered with a 3Q% water solution of
‘ferric 'nit-rate, -Fe('NO3)3, although other water soluble
salts of iron could be'used. This ean was then placed
ported from any stationary portion of the machine. ' This
in a closed container having an atmosphere of ammonia
and the can with the wheel'in its immersed in the ferric
magnetic sensing device carries more current with a given
E.l\/I.F..when the wheel 5. comes nearer to it on account
nitrate solution was kept in this atmosphere of ammonia
for three days. The ammonia NH3 reacted with the
water to produce ammonium hydroxide, NH§OH, which
of the magnetic permeability of the wheel '5. When the
device 3.6 carries more current, the bridge 31 has a greater
output. The input to the bridge 31 is from the‘oscil
lator 32. The circuit of the bridge 31 is from the wire
. S4 to‘thedevice‘ 36 to‘the wire 35 through a variable
resistance 40 through a resistance 41 and a'variable con
denser 42 through a variable inductance 43 and back to
reacted. with the ferric. .t?trate to Produce ammonium
nitrate NH4N03 and iron hydroxide M01113; This
50
the‘ wire 34. a As controlled by the setting of the bridge
31, when the wheel 5 has moved'a certain distance into
the work piece 12, the output from the bridge 31 is ampli
?'ed by'the amplifier 30 to a high enough value to cause
the'otf and on control 29' tola'ctuatev the relay solenoid
precipitated the ammonium nitrate into the pores.
55
The when. was. then?red in. a mlt?k furnace at 550” C
for four hours during which, time an atmosphere of
steam was maintained in the furnace. The steam removed
~ the ammonium nitrate and converted the ferric hydroxide
44 which'opens the switch 22 and stops the infeed. ' But '
when'the wheel 5_'has been worn away some thus becom
ingof less diameter, the spindle 13 has to advance farther
toactuate the 0E and on control 29. ‘The face of the
caused the precipitation of the iron hydroxide in the pores
Qf the wheel leaving a Water solution of ammonium ni
ttate also in the cafe with. rsrylittle. else.- This, Wheel
was the?! dried at ‘39° C- i917 twenty-four hours which
to magnetite Fe3O4. , ‘This wheel weighed about 6% more
60
than it did originally and had a magnetic susceptibility ,7
of 40X i076 c.g.s_. units. This wheel is also fully illus
trated in FIGURE 1 and, is usable as already described
wheel 5 will ‘be at the same spot. In thread grinding
machines, the grinding wheels are trued at frequent in
tervals, but this mechanism in combination with a mag
for the wheel of FIGURE 1. The furnace of FIGURE 3
nleticavlly susceptible wheel brings the wheel every time
to a position to cut the thread in the work piece 12 to
the same depth. The magnetic sensing device 36 is one
is a muffle furnace which is used to ?re this wheel of
65 Example II. ‘ For purposes of complying with the patent
statute, this example is selected. as representing the best
or-rnore coils or the equivalent as a loop.
' The arrangement shown in FIGURE 2 is particularly
useful for thread grinding of all kinds and can also be
used for-surface grinding and cylindrical traverse grind: 70
ing. Since this invention is in a grinding wheel struc
ture and a method of producing it I don’t need com
pletely to describe, the machine, since the above descrip
tion is merely‘ to show the utility of the wheel and such
machines actually grist
rank for vitri?ed bonded whsels
EXAMPLE I11
Vitri?ed grinding Wheels have their Peres. coated with,
cobalt ferrite CoO-Fe2O3 by proceeding as in Example II
substituting a solution containing 10% cobaltic nitrate
and 20% ferric nitrate for. the 30% ferric nitrate of‘
75 Example II. All other steps are identical.
5
6
...
not be much above 500° C. However, the grinding wheel
should not be deformed by the heat. The non-oxidizing
atmosphere mentioned throughout is nitrogen gas. How
ever, any other non-oxidizing atmosphere can be used
including the inert gases, argon, helium, etc. The oxi
EXAMPLE IV
In order to coat the pores of a vitri?ed wheel with
nickel ferrite, NiO-Fezoa, I proceed as in Example II
substituting a water solution containing 10% of nickel
nitrate and 20% of ferric nitrate.
EXAMPLE V
dizing atmosphere speci?ed herein is steam. However,
other oxidizing atmospheres such as oxygen itself can be
used. Ordinary air can be used but more time would be
needed to elfect the conversion.
.
In order to coat the pores of a vitri?ed wheel with
cobaltous cobaltic oxide, C0304, I proceed as in Example 10
11 but use a 30% water solution of cobaltic nitrate,
For coating the pores of vitri?ed wheels using nitrate
solutions, I have speci?ed 550° C. for ?ring the wheels
in a mu?le furnace to convert the hydroxide to mag
netic oxide or metal or combination. Ordinarily a tem
All of these wheels are magnetic, but I have only
perature above 400° C. would be used to decompose the
measured the magnetic susceptibility of the wheel made
according to Example II. This had a magnetic suscepti 15 hydroxide and temperatures above about 1000” C. are
undesirable because of possible loss of magnetic proper
bility of 40><l0-6 c.g.s. units. For use in accordance
ties through the formation of ferrous oxides.
with this invention a magnetic susceptibility of at least
It has recently been found that for precision grinding
10-6 c.g.s. units may be satisfactory for operating some
if high wheel speeds are used as measured in surface feet
equipment of this type. While there is no upper limit,
it would be extremely di?icult to coat the pores of a grind 20 per minute (s.f.p.m.), organic bonded grinding wheels
1 give superior performance in many cases, such as in the
ing wheel to give it a magnetic susceptibility of more than
CO(NO3)3‘
600><l0~6 c.g.s. units.
thread grinding described. Organic bonded grinding
wheels, especially phenolic resin bonded grinding
The examples herein produce
wheels having a susceptibility of more than l0><l0—6
c.g.s. units which is desirable‘.
wheels, can ‘be operated at higher speeds measured in
25 s.f.p.m. than vitri?ed bonded wheels because the former
are stronger. This more than overcomes the superior
EXAMPLE VI
cutting rate of vitri?ed bonded wheels at a given s.f.p.m.
For the manufacture of a grinding Wheel having in the
This
has caused the development of new grinders capable
pores thereof cobalt metal, I may proceed in accordance
of operating at higher wheel speeds, but these are now
with Example I using cobalt carbonyl vapor instead of
.
iron carbonyl vapor, stopping the process after passing 30 available in some quantities.
So
far
as
the
machine
tool
combination
of the present
the cobalt carbonyl vapor through the furnace in the
invention is concerned, or other abrasive member, the
stream of nitrogen gas. This causes the deposit of cobalt
grinding wheel can be made with various bonds.- Of'the
in the pores of the wheel.
EXAMPLE VII
35
organic bonds, phenolic resin is usually preferred for
grinding operations of thetype herein disclosed. This
wheel is made by incorporating into the wheel mixture
For the manufacture of a grinding wheel having in
any one or combination of the magnetically susceptible
the pores thereof nickel metal, I may proceed in accord
materials previously listed in su?icient quantities to give
ance with Example I using nickel carbonyl vapor instead
the wheel a magnetic susceptibility of at least 10—6 c.g.s.
of iron carbonyl vapor, stopping the process after passing 40 units.
the nickel carbonyl vapor through the furnace in the
EXAMPLE X
stream of nitrogen gas. This causes the deposit of nickel
I A wheel 8" in diameter, 1/2" thick with a V2" central
in the pores of the wheel.
hole Was made in the following manner. The ingredi
EXAMPLES VIII AND IX
ents
were:
'
.
Table II
45
In order to coat the pores of a wheel with cobalt, I
may proceed as in Example II using a 30% water solu
I v01.
the wheel in the muf?e furnace at 550° C. for four hours
in an atmosphere of hydrogen without any steam. The
cobalt hydroxide is converted to cobalt metal and the
ammonium nitrate is driven off. In the same way the
pores of a wheel can be coated with nickel, substituting
nickel nitrate for the ferric nitrate of Example II.
Bond:
FURTHER EXAMPLES
.
'
Phenolic resin ___________________________________ __
62
30. 2
Magnetite 320 grit and then.
30
59.0
8
10. 8
-_
Calcium oxide ___________________________________ _.
heel:
55
Mixtures and complexes of the oxides and the metals
Wt.'
Percent Percent
tion of cobalt nitrate (instead of ferric nitrate) ?ring
Sp. G. 2.63
_
150 mesh fused alumina _________________________ __
50
67. 6
Bond.__
36
32.4
Porosity _________________________________________ __
14
WtJVIOI. 2.92
The fused alumina of 150 mesh (grit size) was ?rst
wet with 20 cc. of resin per pound of bond, then phenolic
ing wheel by proceeding in accordance with the principles
resin which was in powder form and of the brand BR2417
of Example II and using a single nitrate mentioned herein 60 together with the magnetite and the calcium oxide (to
or mixtures of these nitrates and ?ring in the muf?e
take up water during curing) were added and thoroughly
furnace for a short time in steam, the rest of the time
mixed whereupon 10 cc. of anthracene oil per pound of
in an atmosphere of nitrogen. Mixtures of metals can
bond was added to settle the dust. This mixture was cold
be deposited using mixtures of nitrates and ?ring in
65 molded in a closed mold and baked for twenty-four hours
nitrogen without steam.
at 175° C. The wheel had a magnetic susceptibility of
I have given 400° C. as the temperature for heating
about 60><10-6 c.g.s.
the abrasive wheel in carbonyl vapor. This temperature
For purposes of complying with the patent statute, this
must be above about 150° C., which is the approximate
Example X is selected as the best mode for organic
decomposition point of the carbonyl vapor to form metal. 70 bonded wheels.
Other magnetic material besides those already men
The upper limit for use in my process is inde?nite be
tioned can be used in this phase of the invention. Among
cause other factors come into consideration such as the
these are various magnetic ferrites, iron and its alloys,
initial temperature at which the wheel was ?red, the ease
aluminum-nickel-cobalt alloys, copper-manganese alloys
of apparatus operation, etc. ordinarily to avoid premature
decomposition of the carbonyl, the temperature should 75 and rare earth garnets can be used. They are preferably
can be deposited on the surfaces of the pores of a grind
progided in‘?nely powdered form .to smooth outmixing
procedures and avoid di?iculties of poor distribution.
The reason why many other; magnetically susceptible ma
terials can be used is thatthc temperature of baking is
very low in comparison With'the temperature of vitrify
ing. ;Since wheels are usable in the invention if they have
‘the minimum magnetic susceptibility of at least 10-5
c.g.s. units, it is unnecessary ‘to de?ne a complete group
of the magnetic materials which do not come under any
8
the wheel on the magnetic sensing device connected to
control said feeding means.
2. Apparatusaccording to claim 1 in which the" abra-'
sive wheel comprises abrasive grains bonded with vitri?ed
‘bond.
'
3. Apparatus for controlling a grinding operation com
prising an abrasive wheel comprising abrasive grains
bonded with organic bond containing‘ magnetically sus
ceptible material, the wheelhaving a magnetic suscep
recognized‘ classi?cation with respect to reactivity proper 1.0 tibility of at least 10“6 c.g.s. units, a magnetic sensing
ties.
device, means to feed said grinding wheel ‘in the direction
of a work piece, means to position said magnetic sensing
device in the path of movement of the wheel toward the
work piece, and electronic means responsive to the e?ect
bonds include shellac and natural or synthetic rubber. 15 of the magnetic susceptibility of the Wheel on the mag
Also such inorganic bonds for grinding wheels as mag
netic‘ sensing device connected to control said feeding
Other commonly used organic bonds can be -;employed
to make vgrinding wheels or other abrasive products as
components of the apparatus of the invention. Such
nesium oxychloride and “silicate,” by which is meant
means.
'
4. Apparatus according to claim 3 in which the organic
bonding ingredient in the mixture, may be employed,
bond is selected ‘from phenolic resin, rubber, butadiene
and the wheels are made magnetic by the same procedure 20 styrene and butadiene acrylic nitrile.
used for organic wheels. These wheels are made by
5. Apparatus for controlling a grinding operation com
methods known to the art with the addition of the mag
prising an abrasive wheel comprising abrasive grains
bonded with magnesium oxychloride bond containing
neticaily susceptible material as a ?ller. Metal bonded
wheels are readily made‘ magnetic by using magnetic
magnetically susceptible material, the wheel having a
25 magnetic susceptibility of at least ,10—6 c.g.s. units, a mag
metals as a constituent of the bond.
It will thus be seen that there has been provided by
netic sensing device, means to feed said grinding wheel
this invention magnetic abrasive wheels and other abra
in the direction of a Work piece, means to position said
sive products, methods of_ making'them and machine tools
magnetic sensing device in the path of movementof the
for using them in which the various objects hereinabove
wheel toward the work piece, and electronic means re
set forth together with many thoroughly practical ad 30 sponsive to the effect of the magnetic susceptibility of the
vantages are successfully achieved. As many possible
wheel on the magnetic sensing device connected to con
embodiments may be made of the above invention and
trol said ,feeding means.
,
as many changes might be made in the embodiments
6. Apparatus for controlling a grinding operation com
prising, an abrasive wheel comprising abrasive grainsr'
above set “forth, it is to be understood that all matter
hereinbefore set forth, or shown in ‘the accompanying 35 bonded with. silicate bond containing magnetically sus
drawings, is to be interpreted as illustrative and not inv a
ceptible material, the wheel having a magnetic. SllSC?P',
tibility of at least 10-6v c.g.s. units, a magnetic sensing Q
bonds based on the use of alkali silicate'as a primary
limiting sense-
‘
-
'
‘
I claim;
1. Apparatus for controlling a grinding operation coma
device, means to ‘feed‘said grinding wheel ‘in the direction
netic FCQQQ .cobaltous-cobalitic oxide C0304, cobalt fer
of the magnetic susceptibility of the wheel on the mag:
netic sensing device connected to control said feeding
of a Work piece, means to position said magnetic sensing
prising an abrasive wheel containing magnetically suscep 40 device in the path of movement of the wheel toward the
tible material selected from the group consisting of mag
work piece, and electronic means responsive to the effect '7
rite Goa-R2203, nickel ferrite -Ni_O-Fe2O3, cobalt and
nickel and mixtures thereof, the wheel having a magnetic
susceptibility of at least- 19*? c.g.s. units, a magnetic
sensing device, means to teed said grinding wheel toward
and away from a work piece, means to position said
magnetic sensing device in the path of movement of the
means.
7
7
References Cited in the ?le of this patent '
wheel ‘toward, the work piece, and electronic means re
UNITED STATES PATENTS
2,116,232
Blood ____,____, _____ _____,_, May '3, 1,938
sponsive to the effect of the magnetic susceptibility of
2,377,995
Coes ___,_____,____,_____ June 12, 1945
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