Saw spiders have 3 legs of equal length. The fourth leg is shorter than the others. This shortness makes it wobble like an uneven table or chair. And it is the wobble-distance that measures the set of your tooth. With the spider's short leg touching a tooth you can feel how much it wobbles. If there is no wobble the tooth matches the shortness of the fourth leg.
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Spiders allow you to set your saw-teeth quickly and precisely.
Here is how I use it. Viewing from underneath…
And the same thing from the top…
Here is how I calibrate with a feeler gauge to check the shortness of one leg
Here is how I shorten one leg if the calibration needs changing. If you drop it, or have just taken delivery, you may need to adjust it.
And if you have read this far, Dolly Chapman labelling convention will interest you. Each file notch indicates 5 thou of an inch. This prevents confusion and use of the wrong size spider.