Miter Gauge
Tuesday, June 17th, 2008What
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:
- The angle scale is, as with most woodworking tools, amazingly crude and inaccurate.
- The bar doesn’t provide a good, play-free fit in the miter slot.
- 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
Start the alignment procedure by placing the beam of an indicating square against the face of the miter gauge as shown in the photo. An indicating square is made from a single piece of hardened tool steel between 3/16″ - 5/16″ thick (depending on size). It is ideally suited for use with dial indicators. It’s not good for sighting tiny gaps (because it’s thick) and it’s not good for marking stock (because the blade and the beam are the same thickness). If you don’t have an indicating square, you can place the blade of a normal square against the face of the miter gauge and the stylus of your dial indicator against the beam of the square. You don’t need a big square for this purpose because you don’t generally use the miter gauge to cut wide pieces. I find that a 6 to 8 inch square is more than adequate.
The square is being used for two purposes. First, it provides a reference for true 90 degrees. So, an accurate square is important. And, accuracy doesn’t come in the form of brass, rosewood, or aluminum. These are very pretty squares, they generally cost more than some of the best squares, and their makers typically claim all sorts of things about their accuracy. But, such squares are inadequate for precise alignment of machinery. Keep them in a display case, or use them for marking stock (when cutting to a line) but don’t use them for machinery alignment. When it comes to the words “precision” and “accuracy”, there’s a whole host of manufacturers who prey on naive woodworkers. They thrive on the fact that most woodworkers don’t have the capability to confirm their claims. Don’t fall victim to such nonsense.
The second purpose of the square is to translate the face of the miter gauge so that it can be measured with the dial indicator. So, it’s imperative that it be something that works well with dial indicators. The typical square has a thin blade and a thick beam. It’s difficult to run the stylus of a dial indicator along the thin edge. And, since the blade and beam are different thicknesses, it doesn’t lay flat. Nothing is more annoying than trying to balance the stylus of a dial indicator on the thin edge of a square that won’t stay still! I highly recommend getting an indicating square for this purpose.
Here you can see the Offset Bar comes in handy again. If you don’t have one you can raise the square off of the table so that it’s even with the stylus of the dial indicator. Don’t tilt the dial indicator down to the level of the square. Remember: your most accurate and reliable readings occur when the dial indicator plunger is perpendicular to the surface being measured. Tilting it will introduce a lot of error and make it very difficult to obtain repeatable results.
Here’s a closeup showing the stylus on the end of the Offset bar contacting the edge of the indicating square.
Zero the dial indicator with the stylus at the base of the square (closest to the face of the miter gauge). This is your reference point.
Move the miter gauge back (not the alignment jig) so that the stylus is at the far end of the blade. If you move the alignment jig (instead of the miter gauge), then you won’t be measuring the face of the miter gauge in relation to it’s motion. You’ll be measuring the face of the miter gauge in relation to the motion of the dial indicator jig. The change in reading that you see is the error in the miter gauge setting. Adjust the miter gauge so that the reading goes back to zero.
Adjusting the miter gauge will cause your reference point to change. So, you must move the miter gauge forward so that the stylus is near the base and re-zero the dial indicator. Now move the miter gauge back and check the result. It should be much closer than before. Make any additional adjustment as needed and check again.
Setting miter gauge angles
By substituting a good steel angle block for the square, the same procedure can be used to set precise angles on the miter gauge. Again, the key to success is the quality of the angle block. Plastic, aluminum, and brass are not good materials for this purpose. Precision ground hardened tool steel is. And, it needs to be thick enough to allow the stylus of the dial indicator to follow its edge.
As with the square, zero the dial indicator with the miter gauge pushed forward (stylus near the face). This is your reference point. Notice the block of wood clamped to the miter gauge. This will help to keep the angle block from moving during the alignment process.
Check for a change in reading when the miter gauge is pulled back. Adjust the setting to bring the dial indicator back to zero. Push forward, zero the dial indicator, pull back, adjust the miter gauge. It’s just that simple. No sighting tiny gaps, no worries about blade flatness or alignment, and no test cuts.
Ed Bennett
ejb@tablesawalignment.com