These are used when setting up awkwardly shaped objects on the mill. The height can be adjusted up to suit. Lock nuts may be required for coarser threads, but fine threads (like wheel nuts) are, in most cases, ok without them.
Edge Finder: Cost $0
I found out very quickly that one of the hardest parts of milling is setting up the work accurately, ready for machining. Of course, the first thing is to use engineers-square, protractor, combination-square or dial indicator (plunger & finger types) with magnetic stand (I found you can't have too many of these around), to make the milling direction parallel to the x, y or z axis as appropriate.
The next necessity is to be able to 'pick-up' where the edges of the work are & zero the dials. This allows us to then make accurate movements of the table/work relative to known reference points or edges.
I have a wiggler to do this but I thought I would try my hand at making an edge finder to augment it. I now use the wiggler for some jobs, the centre finder for others & the edge finder for others - it all depends on what is the most suitable under different circumstances.
I decided to make the 'business end' 5mm in diameter. The reason for selecting 5mm is that, one complete revolution of my feed screw is 2.5mm, hence, when the edge finder picks up the edge, its centre is 2.5mm away from the work edge - I can just zero the dial & one full turn would put the machines centre directly over the work edge. If your milling machine has a different amount per revolution of the dial, then just multiply this figure by 2 (or 4 etc). For example: English machines might have a screw based on 8t.p.i. (1/8" movement per turn), then the finished diameter would be 2 x 1/8" or 1/4". The length is just made to be the minimum convenient length - the shorter it is the better it works. Mine ended up being about 65mm long in the body & 33mm long for the end piece including a 15mm tip.
The only materials I used were a 1/2" shaft from an old printer, a long slim spring & a small (<1/8") piece of shaft to make the pins, both from an old VCR.
This edge finder works well, better than I expected it to. It probably doesn't equal the Starrett model, but theirs does cost a bit more.
Addendum: I decided that the double ended one was too long to be held properly in my chuck, as the chuck only has a short grip length. So I have since cut it in half. Now it is two shorter units (of course I had to drill a new hole in each & use shorter springs as well).