s is operated on there should be no top
rake, as illustrated in Fig. 41, thus assuring a smooth job.

LAPS.--This is a tool which is very useful, particularly for grinding
and truing up the cylinders of internal combustion engines, as well as
for all kinds of bores of refractory material which cannot be handled
with the cutting tool of the lathe.

It is made up of a mandrel or rod of copper, with lead cast about it,
and then turned up true, so that it is but the merest trifle larger than
the hole it is to true up.

USING THE LAP.--The roller thus made is turned rapidly in a lathe, and
the cylinder to be trued is brought up to it and the roller supplied
freely with emery powder and oil. As rapidly as possible the cylinder is
worked over on the roller, without forcing it, and also turned, so as to
prevent even the weight from grinding it unduly on one side.

More or less of the emery will embed itself in the lead, and thus act as
an abrasive. The process is called "lapping."

SURFACE GAGES.--Frequently, in laying out, it is necessary to scribe
lines at a given distance from some part of the work; or, the conditions
are such that a rule, a caliper, or dividers will not permit accurate
measurement to be made.

For such purposes, what is called a surface gage was devised. This is
merely a heavy base, provided with a pivoted upright on which is
mounted a scribe that is held by a clamp so it may be turned to any
angle.

[Illustration: _Fig. 43.--Showing uses of the Surface Gage._]

SURFACE GAGE.--The clamp holding the scriber is vertically movable on
the pivoted upright. By resting the base of the surface gage on the line
to be measured from, and swinging one point of the scriber to the place
where the work is to be done, accuracy is assured. One end of the
scriber is bent, so it can be adapted to enter recesses, or such places
as could not be reached by the straight end.

CHAPTER IV

ON THE USE OF THE FILE

The most necessary tool in a machine shop is a file. It is one of the
neglected tools, because the ordinary boy, or workman, sees nothing in
it but a strip or a bar with a lot of cross grooves and edges, and he
concludes that the only thing necessary is to rub it across a piece of
metal until he has worn it down sufficiently for the purpose.

THE FIRST TEST.--The fact is, the file is so familiar a tool, that it
breeds contempt, like many other things closely associated in life.

Give the boy an irre