I am having a little bit of trouble with sinking the thumb slugs lately.  I used to use the drill press quill to wedge the slug into the hole, but after the whole motor assembly lifted up and I had to recalibrate everything, I stopped doing that.  I used a rubber mallet for a while, but that doesn't work very well.  Lots of banging, and very little movement.  I have tried starting the slug in the whole and using body weight to sink it in, off the floor, but the last ball I did this with, developed a few small splinter cracks around the thumb hole. 

I have thought out trying to widen the whole slightly with a sander, but don't want to generate a big gap.

Anyone have any suggestions, other than those mentioned? 

I have had that drill bit sharpened in the past, but I don't think they mess with the width.  I can try the cool down method, maybe that would help.

Maybe a new drill bit ? Has yours been re sharpened correctly? Also, you might try drilling the hole, waiting a little while for everything to cool down and then redrilling slowly. drilling up/down slowly for a while to get a better/cleaner cut. You might be drilling too fast.

Used to use the quill as an arbor?  I guess you learned the hard way on that method...

What I do is take a piece of sand paper, form a circular tube and turn the slug a few times until it will slip into the hole.  Not so much that it falls in but with a light tap goes in easily.  Been doing it that way for years and never a problem.

It may sound silly, but be sure you are using slow zip super glue.  The faster drying may be locking it up before it goes all the way in.

I had a similar issue of the slug needing to be hammered in. It worked fine but took a little work.

Replaced the drill bit and noticed that now the slug could just barely be tapped into the hole.

Sometimes depending on the core of the ball you will notice that the hole isn't perfectly  round going all the way in. There are dips and peaks from the difference in densities from material. This can also be an issue as to why it requires a little more work then usual.

Slow drying glue and the chuck of the drill with an 1 1/2" slug as a buffer is how I do it. Sand the slug around and it goes in extra easy. Just be sure that you first ease the slug in the hole first by hand to account for pitch then run the chuck down. A lot of people mash down on the handle and the slug locks up and they mash it down anyway. That can lead to damage. Everyday I doublecheck the pitches and depth of the press and haven't had any issues with damaging it.