Why ruff?
Depends on how picky you are or need to be.
Holding in the mandrel may not hold the part properly for the rough cutting.
Ruff cut may be within 010 or 005 of final then in the op once it is spinning on the shaft or arbor one can remove or clean up any minor stuff.
We do finish operations in same operation if following operations not dependent on it.
If one is making a part that needs to run true on a shaft it can be better to affix to a mandrel to do the last cuts to insure it runs true on that shaft.
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Tis' a simple part...
Picky would be what ever the test indicator says on the OD's and faces on both sides when done (for one part)...
...if you bore an ID to size to the OD (OD's) and the face (faces) dead on, the hole (bore) will be dead on...
...more set ups including to/with an arbor = more chance of error...
...2 set ups for 1 part (besides the OD cutter, face cutter (or turn tool or holder) and change to boring bar)...one side then flip to the other side and indicate = done for one (in this case using shims (pads) or a ring so as not to mar up the boss on the 1st side)...
...for turning mutiples I would still finish the major OD, OD of the boss, face both faces and drill and bore the hole to size all in one set up in the chuck (just by ruff indicating or eyeballing the stock concentric and straight depending on how much xtra stock there is)...then face and turn a nice fit boss on an arbor to all the finished ID bores and slip the parts on one at a time, tighten the bolt (or nut on the threaded rod) with proper sized "washers) and just face them off to thickness (but the 2nd side face on the multiples won't be guaranteed as dead on parallel accurate as what a indicator tells me if I test indicate the back sides...and I still have to cut the extra of the 2nd side under the bolt/washers unless I counter bore the hole to face the whole 2nd side (unless I use an expandable arbor in the ID's (usually for a bigger ID on bigger OD parts and still with those you have to indicate)
... but yoose' could also tap multiples around to dead on and face them all to OA thickness 1st...and then yada, yada (lol)
...Add: Its just as easy to adjust an OD to .0002 concentric and knock a face around to .0002 or less out of flat/parallel as it is to .002... and maybe takes another, what, 30 seconds or a minute.
...Add: Everybody, besides a 1 or 2 inch dial indicator, needs a .0005 grad 0-15-0 (.030 total travel) test indicator with attachments and a mag base for use in set up at all Machine Tools (except the saws)...
I have a Starret "lost word" (lol), a Brown and "scrap" (lol) and a B&S .0001 all with the various atachments including mag base clamps, collet "do dads" for mills and EDMs, and a mini mag base and a "snake" mag base besides a couple of 1" and 2" dial indicators with bases
Just tightening a center drill or drill in a drill chuck in a lathe tail stock yoose' can stick the mag base on the chuck if a part is in it and sweep the center drill...or drill...or reamer on center to the spindle...same in a drill press...or a mill...
...and yada yada (dozens of other thangs")
