I will hardly address single participants in a forum, we all sit at a really big table.
Some commenters feel 'late' input is a lower tier of involvement. Not so. Check the home page, there are how many posts - not merely original threads? Discount likelihood your interests expand, to the point interest and searches take you into different territory. Barring some calamity, this information isn't volatile, 'newbies' are born everyday, methods expand because someone experiments, even when some of those fail.
Point is, machinist jacks are invaluable. Depending on workpiece size, many unaware just how large those get, capacity in 1000's of pounds.
- Personally, use of jacks is more of an inspection necessity, on a surface plate; leveling a casting or compound angular features to pull measurements. Sorry to point out, that means 3 jacks are a set.
- 2 suffice but require a 3rd element (even if not adjustable) somewhat higher than minimum the adjustable pair can do. Note; few commercial sets offer that option, seeing only potential as setup components.
- Adaptable (different heads, feet & bodies) jacks should be the standard. That makes logical argument for a case containing all the bits, easily transported to work area. Have a minimalist trait? Use an empty soup can.
- Any that cannot be set lower than your vise bed are worth avoiding. Of variety being castings, they will lose thread depth (and load bearing capacity) if you consider shortening them. Cutting foot bottoms out the screw.
- Very few (commercial) have good proportions of decent thread diameter, and fine thread pitch. Shop made versions never seem to follow that mistaken intuition. Try leveling a 20" long workpiece .002 with 16 TPI - 1 revolution @ 0.0625th inch - 0.002 is a serious feat of dexterity (0.0003472 increments of a turn).
- Well done, close fitting threads deserve complement. But without holes for tommy bar or wrench flats, very hard to turn under moderate loading.
- The concentric ring pattern is more practical than cosmetic. Unlike gauge blocks, rarely a workpiece and jack are equally clean, those are recesses for dirt, and irregularities on castings. The user, while scooting jack, around detects stability.
- Sometimes that enlarged surface shifts part laterally as it turns; if pointed, averts that. Alternately, make cap independent of screw.