What

The miter gauge is primarily used for cross cutting.  Recent years have seen an explosion of after-market miter gauges in an effort to provide the accuracy apparently lacking in the stock model.  With a couple of quick modifications, and some precision methods for alignment, I’ve never had trouble getting extremely accurate results with my stock miter gauge.

Two primary methods for miter gauge calibration are proposed in books, magazines, and TV shows:

1. Holding a square against the face of the miter gauge and the side of the blade:

Once again we find ourselves pondering the value of what seems intuitive and comfortable - and find it wanting.  Apart from the difficulty of sighting tiny gaps with something laying on the table surface (dark background), this technique depends on the accuracy of the blade alignment, and the flatness of the blade.  It’s always bad practice to make one alignment dependent on the accuracy of another alignment.  And, as I mentioned when discussing blade alignment, blades aren’t reliably flat. 

Flat plate advocates jump on this second issue, citing their flat plate as a great solution.  Even if the flat plate was accurate (and it’s not), it would still be difficult to sight the tiny gaps against a dark background.   One maker demonstrates his competence by anodizing his flat plate product a nice deep dark black! 

Then there are those who recommend this method be performed with a plastic drafting triangle.  Well, I suppose they overcome the “dark background” problem.  It solves none of the other issues and introduces a new one: plastic drafting triangles are not accurate enough for machinery alignment.  They are designed for drawing pencil and ink lines on paper (vellum, Mylar).  Every time I’ve seen someone recommend a plastic drafting triangle, they have followed it up with a recommendation for a test cut. 

The bottom line with this method: the angle of the cut is not defined by the angle between the surface of the blade and the face of the miter gauge.  You can get an inaccurate cut even if you manage to get the miter gauge perfectly squared up with the blade body.

2. Making test cuts: 

The technique involves making a cross cut, flipping one piece over, and butting the two cut ends against eachother while the edges are against a flat surface (like the table top).  In theory, the error is doubled and therefore obvious.  What could be more intuitive and comfortable than a test cut?  Most test cut advocates neglect to mention that you can’t just grab any old stick off the scrap pile.  The wood used for test cuts needs to be prepared with the same accuracy that you expect from the alignment method.  For example, if opposite edges of the board used in this method aren’t parallel, then the results will be invalid.  They also tend to minimize the difficulty of sighting a tiny gap between two thick boards.  And, they generally fail to mention how difficult it is to estimate the amount of machine adjustment required to correct the doubled error.  So, more trials with more error are inevitable.

People who use these methods tend to believe that the bad results they obtain are evidence that the stock miter gauge is incapable of any accuracy.  In truth, the methods are flawed in many ways and good results are more a matter of luck than anything else.  Maybe the saw is properly aligned, maybe the blade is flat, maybe I don’t miss the tiny gaps (in spite of the dark background), maybe my test cut stock is properly prepared, and maybe I get lucky and make just the right amount of adjustment to obtain an accurate setting on my miter gauge.  Maybe I’ll win the lottery too!

Why

In reality, some stock miter gauges are horrible but most are deficient in three basic ways:

1. The angle scale is, as with most woodworking tools, amazingly crude and inaccurate.
2. The bar doesn’t provide a good, play-free fit in the miter slot.
3. The face is slick, allowing stock to slip during the cut.

Most of the after-market miter gauges attempt to address all three of these deficiencies.  In addition, they offer extended fences which can be adjusted from side to side and are equipped with movable stops.  So, there are still good reasons to consider such an investment.  However, they still require calibration.  Most often, their makers recommend one of the two procedures above.  It’s somewhat disconcerting that the maker of an item doesn’t understand the pitfalls of their suggested alignment procedure but it doesn’t necessarily reflect on the quality of the product.

The procedure shown here can be used to provide accurate and reliable results because it measures the geometry which defines the angle of the cut: the face of the miter gauge in relation to it’s line of travel in the miter slot. 

How 

My method for eliminating the miter bar play involves drilling four holes in the bar (#3 drill bit) and tapping them for 1/4-28 socket head set screws.  I ground the tips of the set screws flat and smooth and installed them with medium strength thread locking compound.  This allows the screws to be adjusted but they are stiff enough to hold their position.  I adjusted the set screws so that the miter gauge would slide freely with a minimum amount of play.

To prevent work from slipping, I attached some self-adhesive sandpaper to the face of my miter gauge.  If pressed out flat without any bubbles the surface will remain accurately flat.

View the video…

Squaring the miter gauge

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