A mat cutter is an instrument in the hands of a skilled practitioner. Mat board is a dense stack of paper, several laminates thick held together with glue. The blade used to cut it is very thin. It must be, because if the blade was too thick it would tear the paper. Yet because it's so thin it has a tendency to flex, and if the blade flexes, the line of the cut will waver. This common flaw is most evident at the beginning of the cut where the blade first penetrates the mat and is seen as a slight dip or "hook" near the corner.
To help alleviate the problem some mat cutters are designed so the blade passes through a narrow slot which keeps the blade rigid to the point of penetration. Others have a tiny screw that can be tightened against the face of the blade to stiffen it. Despite these helpful features, blade flex can still occur if the cutting head is twisted, rocked or torqued during the cut. It is ultimately up to the operator to make sure consistent directional pressure is applied to the cutting head throughout the cut. Even with that precaution, blade flex can still occur if the cut is too deep. In mat cutting, a scrap piece of mat board called a backing sheet is used under the mat when cutting.
When the cut is too deep, the blade penetrates through the subject mat board and halfway or more through the backing sheet. This puts excess drag on the blade, leading to blade flex. Adjusting the depth so it just scratches the surface of the backing sheet will alleviate the issue. Once the backing sheet has a lot of scores in it, it should be replaced. Otherwise the blade can catch in previous scores and track on them, flexing out of line. Any left over or remnant of mat board can be used as a fresh backing sheet.
A dull blade can also lead to blade flex. When the blade is new it passes through the mat board with little resistance, but as it becomes dull it drags more and more. The more it drags, the more it flexes. If blade flex occurs, three steps should be followed to eliminate it. Change the blade depth, change the backing sheet and change the blade. Rough or ragged edges are another common problem in mat cutting. Different manifestations of the problem can be detected by different feels in the cut.
If the cut feels crunchy, the blade is dented or chipped and must be changed. If the cut feels like it's pulling or tugging slightly, you are cutting repeatedly in the same score on the backing sheet and the backing sheet must be moved slightly one way or another to find a fresh place. Patchy cutting through or failing to cut through consistently over the length of the cut indicates a chipped blade (you've lost the last little bit at the tip that allows consistent penetration). Change the blade. Alternatively, the problem could be the result of a depth setting that is too shallow. Adjust the blade depth.
Proper blade depth is the key to eliminating many of the common flaws seen in mat cutting. Unbeknownst to most novice framers, mat board varies in thickness. Even within a so-called line of "standard thickness" mat board, minor variations in thickness are common due to the different thickness of the face papers. These different thicknesses are largely responsible for the most bedeviling issue in mat cutting, a seeming inability to stop precisely and consistently in the corners, leading to over cuts and under cuts.
All mat cutting blades penetrate the mat at an oblique angle, meaning that they penetrate the back of the mat at a point further back than they emerge from the face. If the mat board is thicker, it takes the blade longer to emerge, leading to under cuts. If the mat board is thinner, the blade emerges earlier, leading to over cuts.