Marking gauges are small tools of the craft used for running outlines, and other lines on stock, to which cutting tools are to work.

In order to produce such lines, whether straight or curved, the edge of the stock from which they are guided must be of the same contour, and should be perfectly square with the surface scored. In an emergency a gauge can be made at home by driving a brad through a stick.

Attention is directed to Fig. 57, which gives the names of the various parts of a common marking gauge. As will be seen, such a gauge consists of four principal parts:

The bar, the head, the thumbscrew and the point. The latter is made of steel, the other parts, of both metal and wood, according to the maker’s idea.

The bar is usually scaled along one side. But inasmuch as the points often get bent inward or outward, it is best for the workman to avoid the habit of taking his sets from this scale, and, in preference, always use his rule for them. The head slips along the bar, and by means of the thumbscrew can be set immovably at any point between the ends of the bar.

On the best gauges the head is faced with inset metal to prevent wear. Where a metal thumbscrew is used with a wooden bar, a metal shoe, to protect the bar against dents,-should be fixed in the head at the end of the screw.

And where a metal thumbscrew is used with a wooden head, the latter should have inset a threaded metal bushing, to keep the threads in the head from stripping.

The ordinary curved gauge has the face of the head concaved, and this may be used on both concave and convex surfaces. Gauges may be purchased, too, specially constructed for marking mortises. These tools are supplied with two points, so that two lines may be run at one time.

As a rule, a marking gauge should not be used on rough unplaned stock; and, in finishing work the head must never be placed against a surface that has not been trued up.

Rough surfaced stock should be lined with a pencil, preferably a carpenter’s pencil, with some form of straightedge or ruler, and this also will hold good with planed coarse, open grained woods, such as red oak and chestnut as the thin cut of the gauge point will not show clearly on such stock.

But on smoothed close grained woods of all kinds the marking gauge is the proper tool for lining widths and thicknesses within its limit of reach. For bounding the other dimension – length - the try square is used.

To set a gauge by rule, the workman will do well to proceed as follows: Grasp the lower end of the bar in the left hand, and the thumbscrew between the thumb and index finger of the right hand. Hold the gauge upright, with the sharp end of the point toward the left.

Loosen the thumbscrew, and draw the head downward, at the same time lifting the thumb of the left hand up along the bar until it stops the head a trifle below the dimension desired, as shown by the scale on the bar.

Release the thumbscrew, and while the head is supported by the thumb named, set a rule end containing the first unit on the upper side or face of the head, so that the rule’s dimensions will run upward past the point, touching it.

Now advance or retreat the thumb supporting the head as required by straightening or bending it but without moving the hand, until the desired dimension on the rule exactly coincides with the point (Fig. 58).

Remove the rule, and with that hand tighten the thumb screw of the head.

Test for correctness by replacing the rule against the face of the head, holding the gauge horizontal, and noting the dimension that comes opposite the point. The method is shown in the accompanying illustration.

With practice, this entire operation can be done in less than six seconds.