US2398874A

US2398874A - Electric furnace

Info

Publication number: US2398874A
Application number: US318900A
Authority: US
Inventors: August L Weyhing
Original assignee: Individual
Current assignee: Individual
Priority date: 1940-02-14
Filing date: 1940-02-14
Publication date: 1946-04-23
Anticipated expiration: 1963-04-23

Description

April 23,1946.- A. L, WEYHING ELECTRIC FURNACE Filed Feb. 14, 1940 Sheets-Sheet 1 Aprily 23, 1946.` A1., wEYl-HNG-V ELECTRIC FURNACE 2 Sheets-Sheet 2 Filed Feb. 14, 1940 W M A mm WW T. NM N E .R V n mL T TJ A w Patented Apr. 23, 1946 ELECTRIC FURNACE August L. Weyhing, Louisville, Ky., assignor to Edmund A. Steinbock, Louisville, Ky.

Application February 14, 1940, Serial No. 318,900

1 Claim.

This invention relates to improvements in furnaces, and particularly electric furnaces.

The furnace of this invention was developed primarily for use in small production laboratories such as dental laboratories and the like, which are usually located in office buildings and other locations where draft chimneys or stacks are not available and cannot be readily constructed. Another use for these furnaces is where a relatively small and portable furnace is desirable or necessary, since they may necessarily be shifted from position to position.

It is one of the chief objects of this invention to provide a furnace from which the fumes and products of elimination caused during operation of the furnace can be readily and completely disposed of without contaminating the atmosphere of the room in which the furnace is used.

Another object of this invention is the provision of an electric furnace which must generate a relatively high heat and in which deleterious gases are generated and prevented from attacking the heating elements.

Another object of this invention is the provision of means in a furnace which is used for burning out wax from molds to prevent the wax from interfering with or breaking down the heating elements.

A still further object of this invention is the provision of an electric furnace for heating molds made of investment compositions and in which a neutral or oxidizing atmosphere is maintained to avoid breaking down of the mold.

A still further object of this invention is the provision of a furnace in which gas or the like may be introduced during its operation, or in which a partial or semi-vacuum may be established, depending upon the content of the furnace and the result desired.

Other objects and advantages of the present nvention should be readily understood by reference to the following specication when considered in connection with the accompanying drawings and forming a part thereof, and it is to be understood that any modifications may be made in the exact structural details therein shown, without departing from or exceeding the spirit of the invention as defined by the appended claim.

In the drawings:

Fig. l is a front elevation of an electric furnace embodying the improvements of this invention.

Fig. 2 is a side elevation of the furnace shown in Fig. 1, as seen from the right hand side thereof, parts being broken away to clearly illustrate the interior construction of the furnace.

Fig. 3 is a rear, elevational view of the furnace as shown in Fig. 1.

Fig. 4 is an enlarged, fragmentary, vertical, sectional view taken on line 4 4 of Fig. 3.

Fig. 5 is an enlarged, vertical, sectional view taken at right angles of Fig, 4, on line v5-5 of Fig. 2.

Fig. 6 is an enlarged, fragmentary, horizontal, sectional view taken on line 6--6 of Fig. 3.

Fig. 7 is an enlarged, vertical, sectional view, taken on line 'l--l of Fig. 3. Y

Throughout the several views of the drawings, similar reference charactersare employed to denote the same or similar parts.

As was noted above, this furnace was primarily developed for use in dental laboratories and similar places for the burning out and heating of molds made from investment compositions and which laboratories are generally situated in office buildings, where access to exhaust stacks and the like are impossible. As is well known, metals, whether in the pure state or alloyed, have considerable shrinkage when cooling from the fluid state to the solid state after being cast. In dental work, and whenusing a gold alloy as bridges, fillings, plates, and the like, a linear shrinkage of 1.25% is the rule. To overcome this shrinkage, the mold investment composition is composed generally of some form of plaster, a ller, a time setting element and a thermal expanding agent, together with other ingredients to provide the mold with certain characteristics. The mold has embedded :therein a wax pattern made accurately to the size and shape of the dental part to be made. The mold is placed in a furnaceand heated rst to dissipate the wax pattern by melting, and burning, and then the mold is heated to cause the thermal expansion agent to expand the cavity left by the disappearing wax pattern an amount equal to the metal shrinkage upon cooling from the fluid state to the solid state. As can readily be appreciated, the melting wax would normally flow onto the heating element embedding surface, which is more or less porous, and would nd its way to the heating elements themselves, and in a short time damage said elements. At the same time, the constituents of the mold, during the thermal expansion thereof, give off gases which may be deleterious to the heating elements and to :the membranes of the head and throat, if not downright poisonous to life itself, would permeate the laboratory room if not properly and completely exhausted from the furnace to the atmosphere.

It has been found that a furnace, without the improvements disclosed in this application, burned out in ve runs, where the furnace had been heated up to 1300o F.'for gold alloy castings and to 1700 for stainless steel castings; that is, the

heating elements were destroyed to the extent that the furnace was useless, while, with the improvements of this application, the furnace was operated continuously for months without in videntied by, reference character 35. ,toI maintain the Vdoor in its open position, use is the front frame I is a rear frame I3, similarin all respects to the front frame I0 and having.

depending rear legs I4 and I5. A sheet`metal housing I6 extends between thefront and rear frames I0 and Il3, and comprises aV top/I1 and opposing sides I8. The sides I=8 are joined by means of a sheet metal tray I9 at a point Ajust above the end frame legs. The top, 4sides and bottom ofthe furnace are similarly constructed and'compris'e affirst layer 20 of heat-insulating materiala second layer 2| of heat insulating material and a final inner lining 22 of refractory material. Theseseveral layers are formed initially in blocks"orfslabs and supported one on 'another with the inner liners interlocking and forming the supports for the vwhole and the'proper spacing ofthe inner `cavity top, bottom and yside walls. The front wall 23 and rear wall 24 are similarly construetedand likewise made up of preformed elements, the outermost 24a being compressed asbestos material in such a manner as t'o providea slate like outer surface, and which contacts the end edges 4of the outermost insulation layer'Z. Behind the outermost layer gfla Yis alayer 2'5 vof heat refractory material,

""sruchasn'febrick, which contacts the edges vof 'the second heat insulating layer 2I and the inner refractory liner `,22. The yfront wall 23 has its layers-24a and v2`5p'r`ovided with a rectangular openingf26 whichaligns'with the inner surfaces of the inner Vliner22 and'provides anopening to thehfrnaemuiile chamber. Y

Tleope'ning 26in'the` front wall of the furnace is clcsedjb'y the usual swinging 'door 21, which, as 'ulsialjcomprises an louter sheet metal casing VY28, 'vvitl 1 Ya suitable heat resisting material-'29 llli'ng. Securedto `thelower `ends lof the 'door sides v"are arms f30 Vhaving apertures below,

thrug'h whichpas'ses'an aXlebar3I. The axle bar 3`I'is secured in Ythe door arm bearings by any suitable means -`and Athe axle bar passes 'thr'igh apertures formed in angle iron brackets '32 'andf33 vwlich'ha've their 'opposite ends respectivelysecured to the front frame I0. The axle bar'30'projec't`s to' onesde of the furnace proper and "has secured thereto or integral therewith 'a counterwe'ight` 34 employed to hold the door in the closedfposition, as illustrated in the drawings. The doorfhowever, may be opened or positioned at rightjangles to that illustrated in solid linesjin the drawings,and this position is illustrated by dot and dash lines in Fig. 2, and In order madehof an abutment plate 36 which convenientlywtakeus themform of adslhort piece of angle iron, suitably secured to vthe front of the furnace.

, ,Tlle'mief. linings 2.2 A0f the .furnace palper orV the muiile, as noted above, arehof refractory material, and they have secured or yembedded therein the heating wires or elements, and these elements have their ends projecting from the refar end of the linings. These projecting ends, indicated by reference character 31, project through -the'rear wall 24 of the furnace. By this construction, adjacentsides of the mufe liners 2 2jhave their heating elements 'exteriorly of the furnace Y adjacent 'one v:another [and fare:

connected by"'a'suitable connector. By this form sides for the rheostat compartment, while the front of this compartment is formed by a plate 39 suitably secured to an inner surface of the front leg front flanges. Projecting through the front 39 is a shaft 40 carrying exteriorly of the Yplate -39 a iknob 4I.` Within the `rheostat compartment y42,*'1-,he sh'aft 46 is journale'd'in a bracket 143 andhas secured thereto a bevel gear or `piniong44 meshing with -a gear or pinion v45 carried by Ithe rheostat'shaft 45. The shaft 46, vas is `Ausual, is journaled lin the rheostat casing V li'l and carriesamovable 'control arm 48 which is adapted to lpass over the resistance buttons A(notsh'own) Ywhich have the electrical-resistance conr'iected therewith. The rheostat'casing 41 is secured to the undersurface of the furnace bottom I9 in any suitable manner, such, for example, as by straps or arms 43.

The rheostat hasrthe usual contactfsupportving vplate 55 lfor binding posts V`5I vand Y52 which Vare respectively connected withthe resistance '58 Vof the electric source or commercial Voltage is connected by means of wire 59 with another heatingelementjconnector 38. As will be readily lseen, this vwiring makes a complete electrical Acircuit from lthe electric source or commercial voltage, by wayY o f -wrire51 to wire "54, rheostat, wires 53,V heatinglelements and their connectors toV wirey 59v ;and"wire'58 backto the electrical source orcommercial voltage.

The back Ywall 2,4 ofythe furnace is provided 'with an` aperture 50 llocated substantially centrally of the width ofthe muffle chamber and'substantially at the top of the Vertical height thereof. "Secured tothe outer surface ofthe back Wall 24o-is anfapertured `member 6I which conveniently takes the formof a standard pipe flange andhaving a central threaded aperture in axial alignment with thegaperture 60 Yin the back wall 2 4.U Threadedfintothe pipe flange 6I is a nipple vv62 whichhas itsotherend threaded into apipe T vB3. Extendiri'gjfrm thel upper side of the T 63 is a pipe 64 which,-as^wil1'later be 'made clear. 1s the exhaust pipe for the furnace. VThe lower side lof the vT63 'has threaded 'thereinto a plug '65 whichis centrally bored or apertured,

alsat

6,6. It might be notedoatthis time that the aperture 65 provides aclrainA for the moisture 'drawn Vinto the vexhaust pipe '64 and that the 'exhaust pipe 64,' which'is` nothing more than vordinary iron pipe, extends tota suitableV point in 'the wall of the building or room, `such 'as a, window, 'where it is projected into the atmosphererfor dissipated gases and '.thevlike. Letintothe T Ymember'63,

through onesi'de thereofjat the uppermost point above the aperture 68, is a tube 61 having its inner end 68 upwardly disposed into the exhaust pipe 64. In order to secure the exhaust pipe 68 in the side of the T 83, use may be conveniently made of the usual tubing connection 68. The other end of the tubing 61 is connected to one side of a T 10, the other end of which T 'el is connected by a piece of tubing 1i with an air pressure gauge 12. The third opening of the T 18 has connected therewith one end of a tube 13, the other end of which is connected to an adjustable air control valve 14. The air control valve 14 has its other side connected with a pipe 15 which extends from an air pressure generator, tank or the like.

The air control valve 14 may be cf any convenient or desirable form, one such valve being shown, in Fig. 6, and comprising a valve body 1,6 provided with a valve seat 11 with which a needle point 18 co-operates. The needle point valve 18 has a threaded stem 19 projecting therefrom and passing through a packing nut 80 carried by the valve body 15. The outer end of the valve stem 18 has secured thereto knob 8l by which the valve is adjustable.

The valve 14 is disposed behind an instrument pane1 82 which is secured to and upstands from the rear end of the furnace top I1.

Any suitable means may be employed for positioning the valve that shown including a short metal strap 83 through which the air supply pipeY 15 passes and is welded as by welds Se. One end of the strap 83 is provided with a threaded aperture through which a screw 85 passes for clamping thereto a flange 86 rearwardly projecting from one lateral Side of the instrument panel 82. It should be noted that the air pressure gauge 12 is secured to the panel 82 and is conveniently located adjacent the air regulating valve 14 so that the pressure in the tube 68 can be readily determined and adjusted.

Formed through the rear wall 24 of the furnace at one of the upper corners of the mulile chamber is a second aperture 81 in which is disposed a thermocouple 88. In order to protect the thermocouple, it is encased in a substantially non-porous refractory covering 89 which has its inner end closed as at 98 and its outer end open to permit the ends of the thermocouple to project therefrom. The thermocouple is a continuous wire member bent in the middle and having its ends outwardly of the rear of the furnace. Said rear ends of the thermocouple will bend downwardly parallel with the back of the furnace and are respectively indicated by reference numerals 9i and 82, respectively connected with

binding posts

83 and 84 carried by the furnace back wall. The binding posts 93 and 94 are respectively connected by wires 95 and 96 with binding posts at opposite sides of a pyrorneter 98 carried by the instrument panel 82.

In order that the air gauge and pyrometer may be readily read, the forward face of the instrument panel 82 carries a suitable light 99 arranged to direct its rays onto the dials of the said pyrameter and air gauge.

In operation, the exhaust pipe 64 is connected to atmosphere while the pressure pipe 15 is connected to an air pressure generator or pressure tank. The electric wires 51 and 58 are connected with an electric source for heating the heating elements surrounding the muifle chamber. The

. air valve 14 is opened to permit an air flow through the

tubes

13 and 61 into and through the exhaust pipe 64. Depending upon conditions, the air valve 14 is more or less opened and the pressure within the

tubes

13 and 61 determined by the pressure gauge '.'2,which measures the pressure in the

tubes

13 and 61. The now of pressure out of the tube 51 by way of the nozzle 63 creates a vacuum therebelow and draws the atmosphere within the muiile chamber'through the aperture 88 and nipple 82 into the exhaust pipe 64 and forces same out to the atmosphere. To prevent the creation of a vacuum in the mufile chamber, the door 35 is provided with a pair of apertures or openings H38 and these permit a continuous supply of fresh air to the furnace, thereby providing a neutralizing or oxydizing atmosphere to develop in the furnace muiile chamber. At the same time, should it be desired to pass some other gas into the muflle chamber besides fresh air, this may be done by supplying same through the openings |08 and having it drawn into and through the furnace chamber.

As is usual, it may be desired to vary the temperature within the furnace, from a point at or below 1250 F., the usual low point for gold alloy castings, to a point of 1700 or 1800" F., the usual high point of standard stainless steel alloy castings, and for which purpose the rheostat 41 is adjustable by the knob 4I. The temperature of the furnace cavity is at all times available on the pyrometer 98 through the thermocouple 88.

As was mentioned above, the furnace is used, first to melt out the wax pattern in the mold and in order to prevent this molten 'wax from contacting the heating elements in the furnace bottom inner liner 2|, there is placed within the furnace a removable metal tray I0 l.

From the foregoing, it will now be appreciated that there has been provided an electric furnace primarily for use in dental laboratories and the like, which adequately meets the objects initially set forth above.

What is claimed is:

In an electric furnace of the class described, including an inclosing metallic shell, a muilie substantially centrally of the shell, heat insulat` ing lining between the shell and mullle and filling the space therebetween, a heating element associated with the muiile, means for controlling the current flow to the heating elementI said muille chamber having its one end open and its other end insulated by the lining from the shell back wall, pressure exhaust means connected with the muflie chamber through the back wall for exhausting gases and moisture from the muiile chamber, including an opening through the back wall, a nipple carried by said back wall in line with the opening therein, an exhaust pipe connecting the nipple to the atmosphere, and a tube connecting a pressure source with the exhaust pipe at a point above the nipple for causing forced draft through the exhaust pipe, a valve carried by the shell for regulating the oW of the pressure through the tube and thereby regulating the force y of the exhaust draft and a gauge for indicating the pressure flow through the tube into and through the exhaust pipe.

AUGUST L. WEYHING.