Tuning the Post Drill Chuck

My recent video on my new post drill sparked quite a few comments about the wobble present in the bits. First let me say that a post drill is not a precision press like a modern drill press and no matter what you do there is always a bit of eccentricity in the quill. That being said, using modern bits with various diameter shanks will create wobble even when the quill and chuck are perfectly centered. Essentially the chuck is a cylinder with a 1/2″ hole bored in it. The bit is secured with a set screw through the side that presses the bit up against the far wall and holds it tight. Post drill bits have a 1/2″ shank with flat facet on one side that locks in place when the set screw is tightened. So when I chuck up a 3/8 bit with a 3/8″ shank the bit is pushed to the side and out of center. The result doesn’t actually cause that much of a problem as the hole is still drilled perfectly plumb and as long as you don’t clamp your work piece down it is allowed to move with the off center arc described by the bit. Where things get hairy is with much smaller bits. They are pressed against the far wall but they can also deflect from front to back thereby skewing the bit even further out of line and forcing a much wider arc. In this case I needed a fix.

I’m hesitant to remove this chuck as I can’t actually see how it is attached. If it is press fit, it is so tight I don’t think I can remove it. It may actually be machined from a solid piece as part of the quill too. I could add a modern Jacobs chuck with a 1/2″ shank and that will solve all my problems but then I am reducing my vertical capacity by 2-3″. This may still be an option but I wanted a simpler one in the meantime. I’m fine with the minimal displacement when the bit is pressed against the far wall and just needed a way to counteract any twist. This was accomplished by filing a small V groove into the set screw. I made the V just a tad wider on one end as well. This allows me a little extra wiggle room when tightening the bit for the V to self center the bit in the chuck. Even if it comes tight with the V perpendicular to the bit, I can cinch it down 1/4 turn and that wider V opening drops in place allowing me to center the bit vertically. This works really well for 1/4″ and larger bits and I have them running almost dead true now.

For the really small drill bits, I have to thank Isaac Smith of Blackburn tools for his ingenuity and machinist skills. Isaac sent me a bit “carrier” that is a 1/2″ stainless steel cylinder with a 3/16 hole drill in it and a set screw. I can slip the smaller bits into this little guy and the whole assembly slides up into the chuck dead center. This solution has me thinking about some similar options for larger bits that could be constructed from readily available materials too. In reality I don’t do that much drilling where this kind of precision is needed so these ideas may remain unrealized. If I did a lot of precision work at the drill press then I probably would not have opted for a post drill in the first place. For the work I do, this drill is a great solution.