Blade will not tighten star stripped on spindle

If the blade has a star pattern you have to be sure the blade mates the spindle properly. You should be able to turn the bolt in with your fingers. I usually allow the blade to slowly turn with the bolt/washer until the bolt pushes blade into the spindle mount. Then rotate and position the blade until it mates and locks onto the spindle... continue to turn bolt with fingers until it is snug. Then tighten to 50 ft/lb... which is hard to do with a short (less than a foot) wrench.

You should have a 18" breaker bar or better a torque wrench, and a scrap piece of 2x4 to use as a wedge to prevent blade from turning while tightening to spec.

There is a positioning "washer" that needs to be in place. If you don't line up the blade and position the washer it will not tighten. The blade will just spin around. Some even have a washer in front and in back. Be sure you have correct diameter of arbor that matches your blade too.

did you put the blades on right ??? they have a star pattern and it needs to be lined up correctly... if that's ok does the belt have a bad spot in it or is the engagement mechanism operating correctly ???

this does sound like a serious design problem unless you had hit something with blades like large tree root or blades not tighten enough from manufacturer. Spindles, bolts, lock or antivibration washer and blades should all be changed with new, you can add loctite to threads and torque to manufacturers specs. and check the blades every ten hours of use for being loose.

If you can't find a spindle lock you can unplug the tool and find a piece of wood and put the blade against it and loosen and also tighten the blade, but most newer saws have a spindle lock located in the gear housing somewhere.

the reason it sheard off is because you did not put the blade on right.
you have to hold blade in position untill you turn nut all the way up and make sure blade has the star part in the blade when you tighten it.
you have to buy a new shaft most come with the lower bearing on them.

Bet on it- you might try adding an additional nut of the right size/thread or several washers to utilize the remaining healthy threads.

I dont believe there is a shear pin on your blade spindles, what I have found on these machines is the the spindle nuts on the balde spindles sometimes loosen or wear and no longer squeeze the pulley on the blade spindle. To make this process easy on your self you need to tighten these with a impact wrench. However if you dont have access to a impact wrench you can use hand tools and block the blades you are trying to tighten with a wooden block. Just make sure you block the blade you are trying to tighten