Two totally different jacks for a wide range of interest.

FARM JACK
Can't go wrong with an actual name brand Hi-Lift. If you want to save
money Reese is crap in my opinion. The forged (cast?) Harbor Freight
Jacks are better. I don't like the stamped and welded from any
manufacturer or reseller.

MACHINIST JACK
I haven't used one. I do almost exclusively CNC and I want my machining
cutting the part as fast as possible. Time IS money. I've always been
afraid that a jack may slip if the machine is doing hard fast reversals.
Is pinching it between the piece of stock and the table enough to keep
it from moving? I do a lot of aluminum so flex is really an issue.
Most times if I need to cut larger pieces of stock I usually put two
vises on the table. Sometimes its not practical or extremely
inconvenient. For example if I have a setup on part of the table I
don't want to tear down because I'm half way through a multiple piece
order, or on the smaller machines if its just not practical to have two
vises on the table. Do any/many of you use a stud on the bottom of a
machinist jack to secure it to the table with a t-nut? What about a
swivel pad on a jack? Is that a good idea?