On 19/07/2020 23:34, John Doe wrote:
Looks like DeWalt's mud mixer drill spindle is 5/8" 16 TPI.

It measures .59 inches outer diameter and it's shaved down a bit more
than most bolts. It measures precisely 16 TPI.

https://www.flickr.com/photos/27532210@N04/?

Need a nut for it.

Thanks.

They are available just not common. Taps are available in that size so
you could make your own nut but why not use the back end of the chuck
you've already removed.