I have shop-made drilling jigs to deepen 8mm and 10mm mortises, but not for 6mm ones because I didn't expect to build bigger projects using 6mm dominoes. In a recent project, I did need to make a couple mortises deeper. I tried drilling freehand with a 6mm dia. bit and it worked!
The trick is to stand at the end of the long side when drilling to make sure the bit is perpendicular to the mortise walls. After drilling to the desired depth (25mm in my case), rock the bit back and forth to remove most of the waste.
The trick is to stand at the end of the long side when drilling to make sure the bit is perpendicular to the mortise walls. After drilling to the desired depth (25mm in my case), rock the bit back and forth to remove most of the waste.
