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I was running some 1/2" plywood through the router to make the groove for the bottom of the drawer. While I was pushing the piece through, the bit raised up enough to come through the top. I was using push blocks, so nothing hurt except the drawer side. I put the wrenches on the collet and it was tight. Has anyone else had this happen? Time to replace the collet?
Most common cause is bottoming out the bit in the collet. For the collet to tighten properly there has to be a little space for the bit to be pulled in as it’s tightened
+1 to Steve's reply. I'll add that cleaning the bore of the collet and especially the bit shank with mineral spirits or the like is a good idea, especially if the bit is new. New bits often have oily shanks.
My opinion that the most common cause is a worn out collet. They will hour glass with big cuts and long bits. Replace the collet.
Assuming you were using a router table with a lift, have you ruled out the possibility that the problem is with the lift slipping? I have a shop made lift and sometimes the lift lowers the bit probably from vibration if it's a spiral bit it could also pull the bit into the workpiece. Just something to check.
I'm betting the bit did not have the 1/8th space. I've worked with guys who had many years of trade work behind them
with no incidents ...and had never left the space ,and never even heard of it. But failing to leave the space can cause
the bit to move.
An "O" ring or grommet that just fits snugly below the collet will prevent the bit from "bottoming out and prevent this bottoming out problem. It's easy to instal, and will be there for you for the rest of the life of the router. You need to clean the collet too. Highland Woodworking sells brushes for cleaning them https://www.highlandwoodworking.com/colletbrushkit.aspx
Charley
Another +1 to Steve's reply. It's a fairly common problem when the user isn't aware that bottoming out the shank is an issue.
I think it would be useful to know what router, size of the shank, and type of bit being used. Spiral bits can move more easily than straight bits. A down cut spiral bit can be pulled into the material. I have had it happen to me.