C-Bag,
Normally we see these screw jacks used on milling machines to support long parts that must extend way beyond the mill table vise. However, I have seen some very good examples of machinist jacks in use from YouTube videos by Keith Fenner. See his YouTube video starting at about 24 minutes into the video "The Manual 3D Metal Printer Part Seven" where he uses screw jacks to support a part on a spinning lathe chuck. It looked dangerous but his screw jacks fit perfectly into the back of part so there was no chance of these flying out when the chuck was spinning. That is where I got the idea of adding the 0.250” rod attachment for wedging into holes or to use the ball bearing to wedge into a part recess. Thanks for asking, Paul