Machining Eccentrics

Machining Eccentrics

The North American Live Steamer, Volume 1 Number 5, 1956

W. I. Treadway

352 Brewster Road

Bristol, Connecticut

Nearly every locomotive has at least one eccentric in it somewhere, and an engine with a double ram feed pump, eccentric driven lubricator, plus Stephenson valve gear would sport no less than seven eccentrics. There are probably as many ways of turning eccentrics as there are locomotives on the road, so the following method may not be new. It is simple, however, highly accurate and involves no special fixtures. It is especially useful if only one eccentric is needed.

First of all, chuck a piece of bar in 4-jaw chuck and turn to a good running fit in eccentric strap. Leave enough stock projecting from chuck to provide for facing a boss (if required) eccentric, and cut off.

Now mount a test indicator in the tool post, actually I used a small C-clamp and attach it right to the tool holder, and by slackening one jaw of the 4-jaw chuck and tightening the opposite jaw shift the work approximately one half the total throw required. To check for proper throw, run indicator up to work until it zeros at maximum run out of workpiece. Release the belt tension on the lathe and rock the chuck back and forth to pick up this point. With indicator zeroed at this point, set cross slide dial at zero. Now swing the chuck 180 degrees and, using cross slide dial, run cross slide forward an amount equal to total required throw of eccentric. Again rocking chuck back and forth, see if indicator touches zero at extreme point of swing. If so, you are ready to bore shaft hole and turn boss, if required, knowing your eccentric throw is correct to the nearest thousandth. If indicator does not zero out the second time, adjust chuck jaws and try again, resetting cross slide dial to zero each time.

After boring shaft hole, facing, etc. either part off with parting tool or cut off with hacksaw and reverse in chuck--probably 3-jaw this time--and face off to correct length.

If a dial indicator with sufficient throw is available, the operation is simplified since the actual throw can be read directly from the indicator without the necessity of advancing the cross slide. Those of you who have long admired the small dial test indicators available, but hesitate at the prices asked should look into the Starrett "Universal Junior" indicator. While not a dial type indicator, it is just as versatile and, in fact, more flexible. And the price is most reasonable.

See also Turning Eccentrics in Three-Jaw Chuck.