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By Harold P. Strand

THERE are many uses for an electricoven of the industrial type in small

shop and laboratories, where moderatebaking temperatures, accurately con-trolled, are necessary. Small parts, Whichhave been painted or lacquered often re-quire baking. Damp or wet pieces of equip-ment may require drying out. Windings,after dipping in insulating varnish, need acertain amount of baking. Then too, anoven may be required in laboratory workof a special nature. The oven illustratedwas built in an electrical engineeringlaboratory and has proven invaluable.

Measuring 24x18x16 in., this oven is smallenough to be used on the bench, yet it willaccommodate quite a bit of work. Heat isprovided by two 350 watt strip heaters,which are mounted to the inside surface ofthe oven, at each side and quite near thebottom. Control is provided by a FenwalThermoswitch, which is mounted on theend of the oven. This device has a brass

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tube projecting into the oven space inwhich thermal contracts are built. With anarrangement of a dial and knob, as shown,it is possible to accurately control a rangeof settings to 400 degrees Fahrenheit.

The material used for the sides, top andbottom of the oven is in. asbestos-ce-ment board, known as Transite. Angle ironis used as a framework and the cover isequipped with strap hinges and a wire pullhandle.

This insulating and heat resisting ma-terial is adequate with moderate tempera-tures of 140 to 150 degrees Fahrenheit. But

at higher temperatures there will be toomuch heat loss through the material andadded insulation will be necessary. Atemperature of 200 degrees Fahrenheit isabout the limit that can be obtained withthe wattage mentioned and the insulatingmaterial shown. To go above this, an outercasing can be made from sheet metal,about 3 to 4 inches (Continued on page 142)

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larger all around than the oven. Into thespace between, rockwool insulation shouldbe placed. With a well-insulated cover,temperatures considerably higher shouldbe possible and safe. One of the drawingsillustrates this suggestion. Strip heaters ofincreased wattage can also be obtained if

desired.To start work on the oven cut two piecesof Transite 24x18 in. and two additionalpieces measuring 18x15 in. Next, cut fourpieces of in. angle iron 21 in. long, tomake up the four corner braces. Extending3 in. below the sides, they also serve aslegs for the oven. The sides are positionedon the angle iron and C-clamps used tohold them in place. With a portable elec-tric drill, make four holes to a side throughboth the angle pieces and the side ma-

terial, to receive 3/16 in. stove bolts. Makesure the surface of the irons is exactly in. in from the edge of the Transite, asshown in the drawings, so the end sectionscan be fitted in flush. The end pieces arethen placed in position, holes drilled andbolts used to make the assembly firm.

The bottom section is fitted flush withthe bottom edges and angle irons, cut asrequired are used to secure this section tothe sides and ends, using bolts as before.Take care to fit this section tightly, to keepescapement of heat at an absolute mini-mum.

Angle iron ( in.) is used to form aframe around the top edge. The ovenwill now take the form of a large box andshould be rigid and firm. The cover is apiece of the same material and shouldmeasure 24x16 in. This cover is fitted withan angle iron frame on the inside surfaceas detailed in a drawing, which adds to itsstrength and rigidity. Two strap hinges

and a wire pull handle, fitted as shown,complete the cover. A chain, secured at oneend to the cover and at the other end tothe inside of the oven, serves to preventthe cover from going too far back.

The strip heaters, which can be pur-chased from any large electrical supplyhouse, are of 350 watts, 115 volt rating.Two are required. These should have theterminals at one end of the strips andshould be so specified when ordering. Theyare mounted to the sides, about 2 in. up

from the bottom, one on each side.The Thermoswitch is mounted in the

center of the right hand end by drilling a5/8 in. hole through the insulating materialto receive the brass tube. In addition, a holemust be drilled through the metal base ofthe switch, just to the right of the dial,which should be about 7/16 in. in diameter.This hole is [Continued on page 150]

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Electric Oven fox Shop[Continued from page 142]

carried through the Transite and serves as an

opening for the entrance of the wires from thestrip heaters. Two small holes to fit stove boltsshould also be drilled through the end, whichshould be in line with the mounting holesprovided in the base of the switch.

Wiring is carried out with No. 16 asbestos

stranded wire or cable from each heater ter-minal to one of the switch terminal screws,as shown in the drawing. The remaining wiresfrom the heater terminals, also carried intothe base of the switch, connect to one side ofthe asbestos heater cord as a soldered andtaped splice. The other side is attached tothe switch terminal. In this way the switch iscut in series with the line and the two heaters,which in themselves, are wired in parallel.

The wires inside the oven should be neatly

placed from the opening back of the switch,down to the bottom and then bent at rightangles to supply each heater. A small curvedclip and a bolt serves to secure the wires.Angle pieces, made from sheet steel or alumi-num and 2 in. wide, are placed over the ter-

minals of the strip heaters to prevent acci-dental contact with the live terminals,When mounting the hinges it will be neces-

sary to file a slight recess in the top edge ofoven to clear the raised part of the hinge atthe pin. The latter should be in a line with

the top edge of the back side, bending thestrap, as required, over the top of the cover.

The Thermoswitch has a dial numberedfrom 1 to 7. It is possible to loosen twosetscrews in the knurled knob and with ascrewdriver in the slot in the end of the shaft,the latter can be turned clockwise to reducetemperature and counterclockwise to increasetemperature. With this changed setting, thepointer will indicate an entirely new range oftemperatures. It is thus possible to adjust the

oven to about any degree desired, within thespecified limits. Once set properly, the switchwill maintain the temperature within aboutplus or minus of .1 degree Fahrenheit. Theswitch, a No. A-7700 Thermoswitch, manufac-tured by Fenwal, Inc., Ashland, Mass., should

be ordered with "Regular" contacts, whichmeans that the contacts will close on a de-crease in temperature. It will be found inadjusting this switch that one revolution ofthe sleeve adjustment will provide about 125degree change in setting.

By carefully checking temperatures it ispossible to make adjustments on the Thermo-switch so that current will be cut.off at theright point. The switch will then maintainthis temperature accurately for an indefiniteperiod.

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