Cheap CNC mill, improving the Z axis

I got some 41xx steel rod that fits linear bearings perfectly, zero noticeable play!
Then I realize that means the guide rails must be perfectly parallel... @#$!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.