Spiders allow you to set your saw-teeth quickly and precisely. With one leg shorter than the others, the spider ‘limps’ along the tooth tips. The shortness of the leg measures the set of the tooth. I prefer the spider and not some dial gauge because I can hear when a spider works.
My knowledge is only about long saws. I imagine the spider will work fine for small saws too and I have not tried this.
Here is how I use them. (There will be other ways too – if you come to sawfest, I’d love to see yours and what length of saw you use it on e.g. some folks set teeth with the saw vertical, and others with saw on an anvil. )
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.
You can make them fairly easily. Here is a selection ( some home-made) from the sawcourse taught by Dolly Chapman at shedtherapy.com course in May 2017.
If you don’t want to make your own, go to my shedtherapy website to buy a ready-made spider.