A marking out gauge is used to mark out lines against a straight edge, mostly in woodworking. It is used for applications such as joinery, where accurate cuts need to be made so that pieces fit together exactly.
By using a pin or scribe to mark the wood, the line provided is much more accurate and beneficial to the user over a pencil marking.
Why use a marking out gauge over a pencil mark?
Accuracy
The main reason for using a marking out gauge is the accuracy it gives the user. When marking with a pin, the line made may be only a couple of thousandths of an inch wide instead of a chunky pencil line which could allow for variation in the cut.
Fence
Another reason why a marking out gauge is beneficial over a pencil mark is that the gauge has a part called a fence, which holds the tool’s position and measurement. This means that the tool can keep a consistent measurement over the whole piece of work. This includes going over the multiple sides or faces of the workpiece without having to re-measure the distance or adjust the tool.
A pencil and ruler, however, would need to re-measure the distance across the work and this allows room for errors and inaccuracies.
No tear-out
Creating a marked line also removes the risk of tear-out. Tear-out is caused when a machine saw first hits the wood without a pre-made line to enter, causing the edge of the wood to splinter and flake a little.
A cutting gauge removes the risk of tear-out even further. When sawing by hand, the indent scribed by the line of the cutting gauge provides a place to rest the saw’s teeth as you begin the sawing motion. The indent helps to prevent the saw blade jumping around in the cut and creating mess.
When using a power saw, the cutting gauge is also beneficial as it perforates the surface of the material cleanly. The power saw blade has a very aggressive cutting action, which can sometimes cause splintering when initially entering the material. With the cutting gauge, the power saw is simply carrying on the initial cut made by the cutting gauge.
Finally, marking out gauges are beneficial over pencil marks as they provide a guide for the user. For example, when chiselling, the user has a place to actually insert the chisel so knows exactly where the line is and removes the risk of slipping. This once again makes the work more accurate.