The Tools You Need.—To make a model of any description you need the usual tools that are the handservants of every jeweler and machinist, see Fig. 68, and you ought to have a small drill press, see Fig. 69, and a screw cutting lathe, as shown in Fig. 70, if you can afford these machines.
There are two or three tools that nearly everybody knows about and yet which very few folks know enough about to use them. One of these tools is the vernier and another is the micrometer and both are used for precision measurements.
Fig. 70. A FOOT POWER SCREW-CUTTING LATHE
The vernier is not, strictly speaking, a tool in itself but it is a device that is applied to scales which makes it possible to measure small fractions of an inch much more easily and accurately than can be obtained with the scale alone. The vernier is also used on calipers, micrometers and other tools and instruments.
The vernier is a short scale which is fitted to and slides along the edge of a fixed scale as shown in Fig. 71. The fixed scale is divided into 10ths of an inch and the vernier, which is ⁹/₁₀ inch long, is divided into 10 spaces.
Suppose now when you measure a part of your model that you move the vernier over to the right so that the first mark of the vernier and the first mark of the fixed scale are exactly in a line with each other, then the vernier will have moved just ¹/₁₀ of a scale division which is ¹/₁₀₀ of an inch.
Fig. 71. A VERNIER FOR ACCURATE MEASUREMENT
If the record marks of the vernier and of the fixed scale are exactly even then the vernier will have moved ²/₁₀ of a scale division or ²/₁₀₀ or ¹/₅₀ of an inch, and so on. The fraction of the ¹/₁₀ inch that is obtained with the vernier is added to the number of inches and the fractions of an inch of the part which is measured. The vernier gets its name from Pierre Vernier who invented it in 1631.