My mixer started to smoke!!

The gearbox grease has separated and is starting to move (slowly) onto the motor armature area through the front bearing. Replace the grease and since it will be apart, use a lot of cloth strips (NO Q-TIPS! TOO STRINGY - LEAVES A MESS) and alcohol to clean off the motor's armature, housing area and any place you see grease slung around. It's not going to be easy, but worth it. I will try to post pictures as I am doing this right now 12/28/17 to my wife's Kitchenaid for this exact reason.The first image is the gear side of the motor bearing, where the thinning grease is moving through. The second image is a shot of the armature of the motor where the brushes touch and will smoke if grease gets that far (it does). The third image is a cleaned out armature area (brushes removed and speed switch moved)The last is the pooled oil separated and sitting just behind the gear box (where will it go if you tilt the head back?)

When I made the bread dough the mixer little bit shaking maybe it was from the heavy dough and then the smoked come out from the head back of mixer but when I tried it still work.. but I still worry whether it dangerous or not.

If it started to smoke out the back it may just be the motor brush.There are several parts available online I have included one site of many HERE
There are a lot of parts sites available you may want to find one close to you.You can check your brushes before you order replacement ones by unscrewing the large black screws on each side of the mixer if there is not much left of the metal portion on the end of the spring that's the problem.Hope this helps you.