Surface Gage 

Most often used in layout work, a surface gage is used to scribe layout lines at a chosen distance parallel to a reference surface. Typically this tool is used with a measuring tool to transfer the dimension to or from a work piece.

The spindle can be adjusted at any position relative to the base (typically a surface plate) and is tightened with a spindle nut. A fine feed screw is used for finer adjustments.

The tool comes with a straight scribe and a curved end scribe. Between the two shapes, a wide array of angles can be achieved for marking work and the curved end makes marking near or even below the surface plate an easy task. Retractable pins may be extended and used to aid in moving the gage horizontally along the edge of the surface plate for scribing a horizontal line. The scribe can be removed and a test indicator mounted in the larger holes for checking parallelism.

In addition, the tool comes with two steel post of different lengths.

The bottom is grooved to accommodate round stock. This allows one to align two shafts with a test indicator.

If you mount a piece of round stock in a v-block, you can use a surface gage to mark true center by scribing 4 lines. Scribe the first line across the end of the stock, then rotate the piece 90 degrees and scribe a second line. Do this two more times until four lines are scribed. A box will be scribed where the lines intersect. By marking an X in the middle of this box, you will find true center of the stock. See Chris' Tips for more information on surface gage use.