Myths and folklore in machinery alignment
There are a lot of ideas which people have about machine alignment which just don't survive Metrological scrutiny. Some are so popular that you see them coming up in books and magazines all the time. You can also find them in the manuals of several competitive alignment products and even some popular machines. Here's a few of the most common myths that I encounter on nearly a daily basis.
Use of a dial indicator jig eliminates "craftsmanship"
For some reason, a number of people resist adopting this very versatile and accurate device for alignment and adjustment of woodworking machinery. The hours of tedious trial and error that they choose to endure in this defiance borders on a genuine phobia. And the critics don't miss a chance to blame dial indicators and other "modern gadgets" with the ruination of craftsmanship among woodworkers. In addition to having invented the modern steam engine, many credit James Watt with the invention of the dial indicator in 1772. Here's a quick history of Engineering and Metrology:
Year Achievement 1772 Watt’s Dial Indicator 1775 Wilkinson’s Boring Mill 1776 Watt’s Steam Engine 1800 Moudslay’s Screw Cutting Lathe 1805 Moudslay’s 0.001 measuring instrument 1807 Fulton’s Steamboat 1851 Brown’s Vernier Caliper 1885 Daimler’s Gasoline Engine 1893 Michelson’s Interferometer 1896 Johnson’s Gauge Blocks 1903 Ford’s Model T Automobile 1903 Wright’s Airplane So, by pre-dating virtually all modern woodworking machinery, the dial doesn't exactly qualify as a "modern gadget". This Badger Pond Article by Dave Wright (on the Wood Central web site) deals with many of the more common misconceptions.
Squaring up a miter gauge against the blade
It's not the angle between the blade and the face of the miter gauge which determines the angle of the cut. The angle of the cut is determined by the the angle between the face of the miter gauge and the line it follows as it travels down the miter slot. If you follow this myth, and your blade isn't aligned parallel to the miter slot, then you will not get square cuts. Adherents to this myth rightly claim that it's "the same thing" so long as your blade is properly aligned. But, it's not a good practice to make one alignment dependent on the accuracy of another alignment. And, are you really going to check your blade/slot alignment each and every time you want to square up your miter gauge? Here's the proper method which guarantees square cuts irregardless of blade alignment.
Miter Gauge Video
Place the beam (or blade) of the square on the face of the miter gauge as shown in the photo. Position a dial indicator as shown and adjust its position so that the pointer reads mid-scale. Slide the miter gauge and square down the miter slot together. Make sure the square doesn't slip on the face of the miter gauge. You will be able to determine very quickly if the miter gauge is square to the miter slot. Watch for a change in reading and adjust the angle of the miter gauge to obtain the minimum amount of change. Always move the miter gauge and leave dial indicator stationary. If you move the dial indicator and leave the miter gauge stationary, you will be aligning the miter gauge so that it is square with the motion of the dial indicator.
Using a flat indicator tip to set jointer knives
I realize that many woodworking books and magazines recommend it, and most other products promote it, but using a flat tip on a flat surface (a jointer knife is flat in one dimension) is a major violation of Metrological basics. Flat stylus tips are for round surfaces. Round stylus tips are for flat surfaces.
When a principal as basic as this is violated, then the measurement (alignment) procedure is guaranteed to be error prone in a way that is extremely difficult to detect. In other words, you will think your jointer knife alignment is good but the results you get prove that it really isn't.
The problem here is the alignment of the dial indicator itself (the measurement device). When you use a flat tip on a flat surface, then the plunger must be precisely aligned so that it is perpendicular to the surface. If it is not, then you won't get full contact between the stylus tip and the surface. You end up with point contact along the edge of the flat stylus tip. The location of the point contact will change as you move over an edge. The error becomes a function of the radius of the contact surface. Stated mathematically, if the misalignment is l degree, the angle of the contact face to the surface is 1 degree and the diameter of the contact is 0.250 inch (radius 0.125 inch), then:
Error = (radius) x (sin 1[degree])
= (0.125") x (0.01745)
= 0.0022"As the location of the contact changes, the reading on the dial indicator will change even when there is no change in the height of the object being measured. A jointer knife is all edge. A person can't possibly slide a flat stylus tip down the edge of a jointer knife and maintain the exact same point of contact. And aligning the dial indicator to eliminate the errors requires prohibitively expensive fixturing (and some pretty extensive Metrological skills). So, the best practice is to use a round stylus tip on jointer knives just like any competent Machinist or Metrologist would do.
The rounded stylus should be removed from the Stylus Offset Bar and used for setting jointer knives. Instead of trying to slide the stylus along the edge of a knife, you should move it across the knife (at right angles) and note the highest reading. This is the technique demonstrated in the video.
Do this at several locations along the edge of the knife to confirm proper alignment. Your alignment will be more accurate and a lot less frustrating when you follow this technique using a rounded stylus.
Squaring up a machine spindle with a square
Coming soon...
Use of a blade replacement plate
As you guessed, the flat blade replacement plate isn't necessary. In fact, it's not even a good idea, Metrologically speaking. Unless you are certain that it's truly flat then it could actually lead you to misalign your saw. It takes a good deal of skill and knowledge to produce a plate which is flat to less than 0.001" that will stay flat over time. No 1/4" or 3/8" thick piece of aluminum plate could every qualify no matter how much skill and expertise is applied to it's manufacture. Both the material and thickness are completely inadequate. Buying such a product is a waste of good money no matter what the price.
A few years ago I attempted to resolve this question with a magazine editor. I made a flat blade substitute plate for him to use in comparison with the technique I proposed as the proper method. It took me most of a day to grind the 3/4" slab of 1018HR steel flat to less than 0.001". I think he saw the point but his allegiance to an advertiser prevented him from acknowledging it.
The geometry is pretty simple. If you mark a spot on your blade with a felt tip pen, and then rotate the arbor on your saw, that spot will travel in a circular path. The circle traced out by the spot will lie in a plane. That plane will be perfectly perpendicular to the axis of rotation of the arbor. It doesn't matter if the blade is flat or not. It could be bent like a potato chip and the spot will still trace a circle which lies in a plane perpendicular to the arbor's axis of rotation. Arbor run out (from being bent) will not affect the path of the spot either. If you always place the stylus of your dial indicator on the spot when you make a measurement, rotating the blade as necessary, then your results will be more reliable than any flat plate can provide (even the one I made). All measurements (alignment and blade tilt) can be done in this manner if blade flatness is suspect and the situation calls for extreme accuracy.
If I sold a certified flat blade replacement plate as a product, it would cost about $200. Since I know that it's completely unnecessary, I wouldn't feel good about how I was conducting business. And, I would never be able to live with myself if I sold a low-cost substandard blade replacement plate and claimed that it was accurate. Instead, I do my best to educate people on the best practices for table saw alignment.Aligning blade to the fence
Coming soon...
The best material for a precision square
Coming soon...
A good precision alignment tool will solve all accuracy problems
Coming soon...
A dial indicator jig is absolutely necessary for precision woodworking
Coming soon...
"Dead-on accurate"
Coming soon...
Last revised: February 18, 2008.
