Table Saw Alignment

What

There is a mechanism on most table saws that allows the arbor to be tilted for making angled (bevel) cuts.

As shown here, a crude scale and pointer are usually included to give the user an approximate indication of the blade tilt angle.  You can see the scale is divided into one degree increments from 0° (blade square to the table surface) to 45° with a line for 22.5° at mid scale.  The pointer is just a thin piece of sheet metal attached to the carriage assembly inside the saw.

Why

For some woodworking tasks the crude blade tilt scale on the saw is sufficient.  However, the accuracy needed for high quality projects will far exceed what this simple setup can provide.  Traditionally, woodworkers have resorted to trial and error methods to achieve the proper setting.  But, since angled joinery often requires multiple pieces to fit together the error is compounded.  So, extremely small error in blade tilt is multiplied by the number of cuts needed for the assembly.  For example, if a four sided frame is being assembled, then eight cuts are required (two cuts for each side of the frame).  So, the accumulated error is eight times the error in each cut.  The problem only becomes worse as the number of cuts increases.

In the example above, it would seem like the gap is pretty big and should be pretty easy to eliminate. But, that 1/8″ gap is the result of each cut having only a very small error. When all the pieces are put together, the error of each cut accumulates. In an octagon, there are a total of 16 cuts to be made (two cuts on each of eight pieces). So, the error in each cut is 1/16 the size of the gap. 

Accuracy Required

If the accuracy of angle cuts is measured in terms of degrees, then the amount of acceptable error depends on the length and width of each member. Longer joints (from wider boards) will require higher angular accuracy than shorter joints. For example, if each joint is 2″ long, then a 1/64″ gap results from a total error of about 0.45° (arc sin (1/64) / 2). For the octagon, the error in each cut will need to be 0.028° or less (16 cuts in all). For a 3″ joint the total error goes to about 0.30° and each cut will have to be accurate to 0.019°.

Accuracy Measurements

Rather than determine how long each joint is and convert the gap width to degrees, it’s much easier to measure angular accuracy in terms of how much gap (or deviation) exists between the perfect (ideal) angle and the sample being measured. This is exactly how manufacturers specify the accuracy of squares and angle blocks. Then it’s easy to predict and measure how accurately each cut must be. For the example shown in the photo above, the 1/8″ gap is produced by having 1/128″ error in each cut (1/8 divided by 16). If 1/64″ gap is needed for the octagon, then each cut must be accurate to 1/1024″ (1/64 divided by 16). In other words, there must be slightly less than one thousandth of an inch error for each cut in order to produce an octagonal frame with a total error of less than 1/64″.

No wonder it’s so difficult to obtain accurate angles! The old adage that says working wood to within 1/64″ is “good enough” just doesn’t apply to cutting angles. Such an error would produce a 1/4″ gap in the example above! Even a simple four sided frame would end up with a 1/8″ gap.  Without a good method for measuring blade tilt, many woodworkers dread the thought of changing the setting and avoid projects that require angles.

How

There are basically three methods you can use to achieve accurate blade tilt.

1. Simple Trial and Error

Just like it sounds, you give it a try and see what happens. For the octagon example, it means cutting all eight pieces and seeing how they fit. You make an adjustment to your machine based on examination of the results and try again. Given enough time, patience, and scrap wood, it’s actually possible to obtain good results with this method. But the word “simple” is a bit misleading. This is definitely a very difficult task. You’ll be called upon to judge the gap and determine how much adjustment is necessary. In the end, you’ll be trying to make adjustments to within thousandths of an inch and these are extremely difficult to do by eye. And, every cycle of the process involves cutting out all the parts to see how they fit. Most people who use this method generally settle for a larger gap than they originally wanted and resort to methods for hiding that error (like trying to even it out among all the joints).

As I’ve stated in other articles, I’m not going to entertain any trial and error methods in this blog.  They are the hallmark of a woodworker with no machinery skills.  I’m not writing this to glorify ignorance but to help people to learn better ways of working with their machinery. 

2. Trial and Error with Accurate Measurement

If you could accurately measure the results of a test cut, then you can save yourself a lot of time and scrap wood. Precise adjustments to your machine will still require significant judgment. And, devices that can measure angles this accurately are fairly expensive. The common protractors available from woodworking dealers are completely inadequate. The typical machinists protractor graduated to five minutes of arc generally starts at about $140. The big advantage of this method is that you avoid the need to cut out all the parts for every trial. Another advantage comes from knowing exactly what your machine setting produces. When you measure the result directly, there’s no question about what to expect.  The big disadvantage of this method is that you spend money on accurate instruments but still resort to ignorant machinery adjustment methods.  So, in the interest of learning new machinery skills, I refuse to entertain this method as well.

3. Accurate Machine Setup

In order to avoid test cuts altogether, you have to know what your machine setting will produce before the first cut is made. To do this your machine must be carefully aligned. You must also be able to make accurate measurements of the machine settings. This is the approach that machinists use to produce reliable and repeatable results. Generally their machines come equipped with accurate measurement devices. Unfortunately, the adjustments on most woodworking machines are very crude by comparison. So, an accurate alignment/setup tool is needed so that machine settings can be accurately measured and adjusted. Such a tool generally starts at about $40. A growing number of woodworkers are coming to appreciate this approach and many manufacturers are now producing aftermarket accessories that help facilitate it. This is the method that I advocate.

In this article I’m going to show the use of three different instruments and compare their results.  The Industrial TS-Aligner is the most accurate instrument - able to measure angles to within one minute of arc (0.014°).  TS-Aligner Jr. is capable of of discerning angles to within four minutes of arc (0.057°).  And, the Wixey Digital Angle Gauge is capable of measurements to 6 minutes of arc (0.1°).

The first step is to ensure that the blade is at zero degrees and the pointer is accurately adjusted.  The best instrument for this job is the Industrial TS-Aligner.  It has been calibrated on a Lab grade granite surface plate using a Lab grade granite square so that the vertical motion is accurate.

To obtain the most accurate results which won’t be influenced by blade warp or arbor/flange runout, all measurements will be made with the stylus at a specific location on the surface of the blade (marked by a dot).  First, a reference point is established by zeroing the dial indicator with the blade rotated so that the dot is close to the table surface.

Then the blade is rotated so that the dot is at the highest point in its travel (the center).  The reading on the dial indicator will not change if the blade is square with the table surface.  This method will only work if the saw is properly aligned.  Any blade/slot misalignment will introduce error into the measurement.

The Angle Attachment Gage is attached to the dial indicator on the TS-Aligner Jr.  The squared up blade is used to locate its proper position on the dial indicator stem.

The Wixey Digital Angle Gauge is first calibrated on the table surface.  Since it depends on gravity, a square is used to ensure that the Wixey is oriented properly.

Then the Wixey is placed on the blade.  Again, a square is used to ensure that it is oriented properly.  If it reads 90°, then the Wixey is zeroed in this new position.  This makes it easy to read the blade tilt directly.

Blade tilt angles are measured with the Industrial TS-Aligner using precision ground and hardened steel angle blocks.  The angle block is placed against the tilted blade as shown in the photo.  When the blade is tilted to the proper angle, the opposite edge of the angle block will be square with the table surface.  The first step is to zero the indicator with the stylus at the base of the angle block.

Then the stylus is raised to the top of the angle block.  Any change in reading indicates that the blade is not tilted to the proper angle.

The Angle Attachment Gage on TS-Aligner Jr. relies on simple High School Trigonometry for all it’s work.  In a right triangle, one of the angles is 90° and the other two angles can be calculated if the length of the two sides (A and B) are known. The photo above shows how the Angle Attachment Gage is precisely machined to construct this right triangle geometry so that side A is measured by the dial indicator and side B is fixed at a precise distance. When the length of side A is measured to be exactly the same as side B then the angle is 45°. When the length of side A is measured to be exactly zero, then the angle is 90°. Angles in between are easily calculated using the following formula:

Angle = Arc Tan(length of side A)

A quick reference table is included on the back cover of the User’s Guide so that angles can be easily determined without the need to do any calculations. Since it’s easier to measure how far the dial indicator plunger is pushed in (rather than the length of side A), the table uses the following formula:

Reading on the indicator = 1 - Tan(angle desired)

So, the reading is 0.000″ for 45° (plunger barely pushed in) and 1.000″ (plunger pushed in almost all the way) for 90° (actually zero degrees).

Here it is measuring the blade tilt.  If you haven’t noticed already, the Angle Attachment Gage breaks one of the most important rules about the use of a dial indicator: the plunger should always be perpendicular to the surface being measured. In this case it’s OK because the measurements are made in reference to the fixed point, which is also subject to the exact same measurement errors as the plunger.  The results are surprisingly accurate.

Like the Industrial TS-Aligner, TS-Aligner Jr. can be used with angle blocks to measure angles.  In this case, no tables or calculations are necessary.  The blade is tilted to the correct angle when the opposite side of the angle block is square with the table surface.

The Wixey Digital Angle Gauge reads the blade tilt directly as shown here.  Care must be taken to ensure that it is oriented properly.  And, the most accurate results come from making very slow adjustments to the blade tilt.  Essentially, you don’t want the pendulum inside to be swinging back and forth.

The Test Procedure

I measured blade tilt at six different angles: 5°, 10°, 20°, 30°, and 45°.  For each angle, I tilted the blade using the scale on the machine as my guide.  Each instrument was used to measure the angle and the results were recorded.  Then the blade tilt was corrected using the reading on the Wixey Digital Angle Gauge.  Finally, the corrected blade tilt was measured again with the TS-Aligner Jr. and the Industrial TS-Aligner.

The Results

Numbers for the Wixey Digital Angle Gauge are expressed in degrees.  The numbers for TS-Aligner Jr. and the Industrial TS-Aligner are expressed in inches.  Values were rounded up or down to the nearest thousandth of an inch.  For TS-Aligner Jr., the results in parenthesis is the proper reading for the angle.

5°

Wixey	TS-Aligner Jr.	TS-Aligner Jr. (angle block)	Industrial TS-Aligner
4.8°	0.916″ (0.913″)	+0.003″	+0.006″
5.0°	0.911″	-0.001″	-0.006″

10°

Wixey	TS-Aligner Jr.	TS-Aligner Jr. (angle block)	Industrial TS-Aligner
9.8°	0.827″ (0.824″)	+0.004″	+0.010″
10.0°	0.824″	-0.001″	-0.003″

20°

Wixey	TS-Aligner Jr.	TS-Aligner Jr. (angle block)	Industrial TS-Aligner
19.7°	0.643″ (0.636″)	+0.006″	+0.020″
20.0°	0.637″	+0.001″	-0.002″

30°

Wixey	TS-Aligner Jr.	TS-Aligner Jr. (angle block)	Industrial TS-Aligner
29.7°	0.429″ (0.423″)	+0.006″	+0.015″
30.0°	0.422″	-0.001″	-0.003″

45°

Wixey	TS-Aligner Jr.	TS-Aligner Jr. (angle block)	Industrial TS-Aligner
44.8°	0.007″ (0.000″)	+0.003″	+0.009″
45.0°	-0.001″	-0.001″	-0.005″

Summary

Obviously there’s some benefit to using a precise instrument to measure blade tilt!  All three of these instruments provide a significant improvement over the blade tilt scale on the saw (which was off by nearly a full degree). 

The Wixey Digital Angle Gauge is pretty much made for measuring blade tilt.  And, as I mentioned in its review, this is pretty much all it can do on a table saw.  To obtain maximum accuracy, you need to pay close attention to its orientation.  It provides enough accuracy for making a square frame with butt joints (four cuts).  It can do a fairly decent job for a four sided frame with mitered joints (8 cuts).  However, some woodworkers will find the need for slightly more accuracy.  It’s not going to satisfy many people making frames with more joints (hexagon, octagon, etc.).  If you expect to be doing any other precise alignments and adjustments on your saw, then this probably isn’t the tool of choice.

With nearly twice the angular measurement resolution, TS-Aligner Jr. is going to give you the accuracy needed to reliably produce that four sided frame with mitered joints.  The addition of angle blocks will make the task easier and faster.  But, it’s not going to satisfy the needs of those looking to make frames with more joints.   Its short measuring distance (one inch) limits the accuracy making it impossible to discern finer measurements.  It can be used to make accurate alignments and adjustments on all other aspects of the table saw so it is quite versatile.  It is also designed to work well on most “contractor” sized machinery.

The Industrial TS-Aligner (the “Sr.”) provides the highest angular measurement resolution of any device commercially made for woodworking machinery.  Six times better than the Wixey and four times better than the Jr.  It can easily accommodate the needs of those wishing to create objects (frames, segmented turnings, etc.) with dozens of joints.  Where the Wixey saw no error in the blade tilt setting, the Sr. saw as much as 0.006″ error.  Like the Jr. it also facilitates the accurate alignment and adjustment of all aspects of the table saw.  It will perform fine on a 10″ table saw (contractor’s or cabinet) but it’s bigger size makes it difficult to use on other “contractor” sized machinery (jointers, planers, miter saws, etc.).

Questions and comments are welcome!

Ed Bennett
ejb@tablesawalignment.com