I recently made a donut chuck from MDF so I can reverse mount a bowl and detail and sand the bottom. I made it with sections of threaded rod so I can adjust a wide range of bowl heights. My first one was rather short so I had about 4 inches of threaded rod exposed out the backside of the chuck (I figured there would not be a safety issue on the backside) On my first run the thing went great, the bottom cleaned up beautifully. I was so proud, my head grew two sizes, proportionally my safety awareness shrank two sizes and I reached to turn of the machine......passing my hand right through the spinning threaded rods that I couldn't see spinning at
1000rpm. I got one of those nasty THUMP...OUCH....grab your hand and squeeze while your mind wonders if your fingers are still going to be attached and in one piece when you look.. Luckily it only caused some minor bruising and small cuts. Once bitten, twice shy.....