Round dial machines were made from 1938 through 1945. Many were made for military
use (they were used extensively by the Manhattan Project), and were used on board U.S. Navy ships.
If you can find one in good condition it is worth acquiring. The 1938-1940 machines used a Sundstrand hydraulic drive; later machines used motor generators. The earlier machines tend to have more chrome and cast iron than the war-time machines. The hydraulic drive machines have an added bonus: there is a nice tool cabinet for accessories including a 5C collet rack in the space where the motor generator sits in MG machines. If the bed lubrication system has been kept up, the beds are likely to be in good shape.
There are some advantages to round dial machines: it is easier to add metric threading (you can buy a pair of gears
from Scott Logan); the MG sets are easier to
maintain than the tube sets used on most square dial machines; they tend to be cheaper, including accessories like indicator
holders, follower rests, etc., or at least that has been my experience; there are a lot more cast iron parts on a round dial machine (the end covers and side covers and bars, the light bracket, the taper attachment tray); the bezels/nameplates on the round dial machine can be found in brass, which looks really sharp.
There are also some disadvantages: it is getting harder to find a round dial machine with a nice bed and nice parts; rebuilding the gearbox and the apron (both of which I did for a 1942) calls for some unobtanium bearings or some creative adaptation, and the bearing preloads are somewhat difficult to setup; 3HP is the largest factory-supplied motor.
The center height is 1/2" lower on a round dial machine, so the redesign affected nearly
every part on the machine, which limits the parts interchangeability between round and square dial.
The range of accessories available for a round dial machine is not quite as extensive as a square dial machine. There are indicator holders that fit dovetails on both sides of the saddle; metric and SAE indicators; left and right bed stops with and without micrometers; turret stop; taper attachment; coolant pump and fittings; steady and follower rest; electric leadscrew reverse lever; metric gearing; lever-operated collet closer; long cross slide with rear toolpost; bed turret, etc., and all of them are different from the square dial version of the same. There are also some accessories for the round dial that are the same as square dial: 5C nosepiece, Jarno 12 to MT2 spindle adaptor, knock-out rod.
However, if you find a round dial lathe in nice condition, buy it. The more complete in terms of accessories, the better.
-Dave
