A few things came to mind as I read down through all these comments. First, if you are going to have only one extinguisher, then the only way to go is with the A B C extinguisher. The chemical in this type of extinguisher is Monoammonium Phosphate. It is hell on unprotected metals as it becomes acetic when exposed to moisture or humidity, so things will rust or oxidize after you operate the extinguisher. Vacuum it up as soon as possible and then wipe it off with a damp cloth that you rinse frequently. Any area with banks of electronics and relays (your CNC machine and computer) will be destroyed by using an ABC type extinguisher. Purple K was mentioned. It is Potassium bicarbonate and works on flammable liquids by interrupting the flame chain reaction (between the vapors coming off the liquid and the actual flames) by bonding to the active ion sites in the chemical reaction. It grabs the fuel so that it isn't available to the oxygen thus stopping burning. You have just combined two dry chemical extinguishers that are completely different in their effectiveness on different types of fire. Purple K won't be effective on wood or sawdust, so when you are faced with a fire - will you remember which extinguisher to grab? Nothing burns as a solid. All fuels must be vaporized before they take part in the oxidation process. Water works by cooling the fuel below its "flash point" (that temperature where enough vapors are coming off the material to flash into fire) Pine or fir typically begin to burn at about 400 degrees, so 60 degree water will be effective in putting out the fire (converts to steam at 212 deg. F.) if you can bring the water in contact with the hot fuel. Sawdust and shavings don't like water (pitch) but you can improve the contact by adding a surfactant (make the water like to stick to the fuel) like some dishwashing detergent. A plastic garden pump sprayer filled with water and a shot of Dawn dish detergent will be a good thing to grab for a fire in your dust collector bag. Having said this, agitating the fine dust by stirring of forceful application of the water can create a dust cloud that could detonate. Dust explosions are nasty, and it isn't the first little "Puff" that does the damage. That first little explosion will shake all the loose dust into the air and the second one will be the killer. A good estimator for the amount of water needed by the fire department to put our "ordinary" combustibles in a structure is 1 gallon per minute for every 100 cubic feet of involvement. So a 10 x 10 shop with a 10 ft high ceiling will need 10 GPM for extinguishment, but the water must be applied to the burning fuel. Squirting the wall when the fire is under the floor won't put the fire out. Lastly, since carbon dioxide was mentioned.. it puts out fires by displacing the oxygen. It is heavier than air, so works best on flammable liquids by blanketing the liquid. For an electrical motor, you will need enough CO-2 to push the oxygen out of the motor frame. Putting water on a burning, energized motor exposes the operator to possible electrocution. Surprisingly not with the electricity following the stream from the extinguisher, but by following floor water to the operators feet and then to a ground point. Hope some of this helps to generate more discussion.