Gib Adjustment Bolt

 
Most mills come with some sort of hand adjustable bolt for the gibs.  For improved performance you might want to replace them with a regular cap head bolt and a nut.  This provides you with a gib adjustment that will not loosen over time.

 
 
Find a cap bolt that will fit in the existing hole and tighten down.

Once it is tight hold the cap bolt in place with the Allen wrench and tighten the hex jam nut.