Then go over the surface with rosin and a half and half solder. It may be necessary to dip into the acid several times to get the piece thoroughly tinned. Rubbing the surface of the iron with a piece of zinc while the acid is still on it will facilitate the tinning.

Another method of soldering cast iron is to clean the surface as in the previous operation and then brush over with chloride of zinc solution and sprinkle powdered sal ammoniac on it; then heat until the sal ammoniac smokes. Dip into melted tin and remove the surplus; repeat if not thoroughly tinned. Half tin and half lead works well as a solder for this.

Commutator wires and electrical connections should never be soldered by using an acid solution, owing to the corrosive action afterward. A good flux is an alcoholic solution of rosin.

Cold Soldering for Metals, Glass, Porcelain, etc.

Precipitate the copper from a solution of the sulphate by putting in strips of zinc. Place the copper powder in a porcelain or wedgewood mortar and mix it with from 20 to 30 parts of sulphuric acid of 1.85 degrees Baumé. Then add 70 parts of mercury when well mixed, wash well with water to remove the excess of acid and allow it to cool. To use it, heat it and pound it well in an iron mortar until it becomes plastic. It can then be used and adheres very firmly when cold. No flux is needed, but the surfaces must be clean. This is used where heat cannot be used, or to join metal parts to glass or porcelain.

A solution of copper for copperplating steel or cast iron before soldering will work by simply immersing the work in it. This is also useful to copper the surface of dies and tools to enable the mechanic to "lay out" or scribe the work so that the lines can be readily seen. Take copper sulphate 3 ounces, sulphuric acid 3 ounces, water 1 to 2 gallons. Dissolve the copper in the water and add the acid.

Cycloidal or Epicycloidal. — A curved tooth generated by the point of a circle rolling away from the gear wheel or rack.

Involute. A curved tooth generated by unwinding a tape or string from a cylinder. The rack tooth has straight sides. Involute Standard.

The standard gear tooth has a 14 degree pressure angle. which means that the teeth of a standard rack have straight sides 14 degrees from the vertical.

Involute - Stubbed. — A tooth shorter than the standard and usually with a 20-degree pressure angle.

Addendum.

Chordal Pitch.

straight line.

Circular Pitch.

Length from pitch line to outside.

Distance from center to center of teeth in a

Distance from center to center of teeth meas

ured on the pitch circle. Clearance. Extra depth of space between teeth. Dedendum. Length from pitch line to base of tooth. Diametral Pitch. Number of teeth divided by the pitch diameter or the teeth to each inch of diameter.

Face. Working surface of tooth outside of pitch line.
Flank. - Working surface of tooth below pitch line.
Outside Diameter. Total diameter over teeth.

Pitch Diameter.

Pitch Line.

Diameter at the pitch line.

Line of contact of two cylinders which would have the same speed ratios as the gears.

Linear Pitch. - Sometimes used in rack measurement. circular pitch of a gear.

Same as

No. I table shows the diametral pitches with the corresponding circular pitches.

No. 2 table shows the circular pitches with the corresponding diametral pitches.

It is most natural to think of gears in circular or linear pitch and we soon get to know the size of any pitch, as 12, as being a little over inch from center to center. But the diametral system has many advantages in figuring gear blanks, center distances, etc.