Fig. 1633.
In [Fig. 1633] is shown at a a male gauge for testing the V-guideways in the bed, and at b a female gauge for testing those on the table. These two gauges are accurately made to the correct angle and width, and fitted together as true as they can be made, being corrected as long as any error can be found, either by testing one with the other or by the application of a surface plate to each separate face of the guides and guideways. The surfaces c and d of the respective gauges are made parallel with the V-surfaces, a point that is of importance, as will be seen hereafter. It is obvious that the female gauge b is turned upside down when tried upon the table.
Fig. 1634.
Suppose it is required to test the sides e, f, of the bed guideways in [Fig. 1634], and the gauge must be pulled over in the direction of the arrow so that it touches those two sides only; a spirit-level laid upon the top of the gauge will then show whether the two faces e, f, are of equal height. It is obvious that to test the other two faces the gauge must be pulled over in the opposite direction.
This test must be applied while fitting the Vs to the gauge. Suppose, for example, that when the gauge is applied and allowed to seat itself in the ways, the two outside angles e, g, are found to bear while the two inside ones do not touch the gauge at all, then by this test it can be found whether the correction should be made by taking a cut off e or off g, for if the spirit-level stood level when the gauge was pulled in either direction, then both faces would require to be operated upon equally, but suppose that the gauge and spirit-level applied as shown proved end e to be high, then it would be the one to be operated on, or if when the gauge was pulled over in the opposite direction end g was shown (by the spirit-level) to be high, then it would be the one to be operated upon.
By careful operation the table and bed may thus be made to fit more perfectly than is possible by any other method. To test the fit of the gauge to the Vs it is a good plan to make a light chalk mark down each V and to then apply the gauge, letting it seat itself and moving it back and forth endways, when if it is a proper fit it will rub the chalk mark entirely out. It may be noted, however, that a light touch of red marking is probably better than chalk for this purpose.