The blade on my Delta Model 36-075 miter saw spins freely - motor works fine - i disassembled down to the gear drive - the gear appears to be fine no missing teeth or obvious damage. Does the inner shaft have a key that has broken? Where can i find replacement? Not shown on Delta drawing.

CAN YOU SPIN THE BLADE WHEN THE SAW IS NOT RUNNING , CHECK TO SEE IF THE MOTOR TURNS WHEN YOU DO THIS . IF IT SPINS YOU NEED TO RETIGHTEN THE BOLTS HOLDING THE BLADE IN PLACE CHECK THE DRIVE BELT IF IT HAS ONE FITTED.

no they are brand/model specific

Not familiar with your model; but when mine did that, it was a loose nut on the blade. It acts as a slip clutch.

look for a shear pin in one of the gears that has done it's job and has prevented mass destruction of a gear by shearing off. Find the gear that is attached to the shaft that drives the blade and see if you can turn I while holding onto the shaft

YOU PROBLEPLY WORE OUT THE WORM GEAR.ALWAYS KEEP YOUR BLADES SHARP. I HAVE THE SAME SAW AND THE ONLY PROBLEM I HAD WAS I BROKE THE DUST BAG OFF, MY FAULT.

Assuming that this model has a belt drive from the motor to the saw arbor. Remove the belt and turn the motor on--same problem? If motor is a capacitor start unit, the capacitor may have failed. Be sure to check motor rotation by hand--it should spin freely. If ok, check the turning action of the blade itself--it should spin freely and coast for a little while. If not, check the arbor bearings for failure. Hope this helps!

Exploded diagram for your SM100M miter saw is here.
User manual is here.

Page 20 covers blade change operation.

Make sure you unplug your saw! You need to remove a screw from the cover and rotate the cover to the rear. Use a hex wrench in the motor shaft to keep the saw blade from turning, and loosen the arbor nut with a 1/2" wrench. The arbor is reverse threaded, so you'll turn it clockwise to loosen, then counterclockwise to tighten it up after you have swapped the blade.
Rotate the cover back after that's done, and replace the screw.

If this was useful, please vote. Thanks.

Un-plug the saw. Remove the screws holding the side saw guard in place. Look at the center of the saw blade for the nut holding it in place. Take a socket wrench that fits the nut and loosen it. You'll need to take a scrap piece of wood 2x4, and jamb the teeth of the blade into it while loosening the nut. The nut will come off first and then a washer/spacer.

Reverse the order for installing the new blade. Make sure you use the proper size diameter blade (10 inch) and mounting hole in the replacement blade. Also, use the provided washer/spacers that were on the original blade and secure the nut. Spin by hand to see if everything clears. Mount the protective guard back on. Plug in the saw and test cut .

Good luck. If you are still unsure - ask a handy neighbor to help. It takes about 5 minutes to complete this task.

Maybe the bolt which holds the blade on is loose. If the motor is on the RIGHT side of the blade, then the bolt has 'left-hand' thread. Turn it counter-clockwise to tighten.

It sounds as though one or both of the items are stripped. They mesh to drive the blade so I would not take a chance on just replacing just one. Also when you look at this you might also want to look at replacing the bearings and brushes. Make sure you clean out the gear housing or you will have immediate problems again. Also check where you buy these parts for their recommendation on the grease that needs to be used.