Hello again AL,
As I said, it is possible to create a fixture to do what you want (on the jointer at least) but the task is not easy and accurate results aren't going to come cheap.
Not bad. So, as you can see already, the fixturing (the angle plate in this case) will be the most expensive element of your design. For the most part, this is always true. The first logical question involves choosing the "right size" for your angle plate. Since the indicators will be attached to the sides, they are mounted in fixed locations which are determined by the size of the angle plate. One size won't fit all jointers. So, several models would be needed. It's not a very attractive solution for someone with more than one jointer (or someone planning to upgrade to a larger model someday).
It's also a "one function" solution. It is designed for jointers only (just imagine trying to calibrate this for planers). Expensive, single function products are not a very attractive to prospective customers.
Unfortunately, this doesn't guarantee that the two indicators provide the same measurements. If you are sure that your outfeed table is sufficiently flat then this works. If you are not sure, then it's equivalent to calibrating your instrument using an unknown standard. A good calibration instrument needs to be immune to imperfections in the machinery. It needs to work for everyone who follows the instructions properly. If good alignment is possible, then the instrument should facilitate it. If good alignment is not possible, then the instrument should not fool you into thinking you have good alignment.
Just give it a try and let us know how you make out. It's not a matter of the effect that tilt will have on one indicator's ability to provide absolute linear measurements. Remember, you're trying to get two indicators to read the same thing when they are measuring the same thing. Exaggerate the possibilities in your mind or just give it a try and see what happens. If the difference in readings is less than
0.001", can you be sure that the difference in the objects being measured is less than 0.001"? You can always check the accuracy of your results using the single indicator setup (which is, in fact, quite immune to tilt so long as you use a spherical indicator point).I also think you're going to have a lot of difficulty ensuring that the dial indicators are aligned (have the same tilt).
The fixturing is definitely rigid enough and quite heavy (though not very adjustable). While two indicators may respond differently to different measurement pressures you probably don't have to worry about this too much.
Actually, the stylus points make a huge difference - especially if you plan to use flat rather than spherical indicator points (a very common mistake). Imagine the effects of indicator tilt when using a flat stylus. If there is any tilt in either direction, then the reading will vary significantly depending on where the knife contacts the surface of the stylus. How will you determine if a difference in readings is the result of misalignment or tilt related positioning of the knife edge? Can you really ensure that the indicator point touches the knife in the same spot that it contacted the table? Try it and let us know how it works out. You can always use the single indicator solution (with spherical indicator point) to check the accuracy of the results.
Yes, I know what you are talking about. It's called "bi-lateral symmetry". You are assuming that there is an orientation of the blade such that every point on one side of a centerline has a corresponding point on the other side. You can probably find blades like this. You might even own some. But, there is no guarantee that all blades will exhibit bi-lateral symmetry.
It could still be shaped like a cup (bi-lateral) but not be symmetrical. Don't make the mistake of thinking that some small sample of blades (yours for example) are representative of all blades.
Sure, I understand. And, I'm sure you understand why it's important for me to defend the hard earned reputation of my products against untested ideas which violate basic Metrological principles.
Ed Bennett snipped-for-privacy@ts-aligner.com