Wheel treads are turned down when wear for service causes the profile of the tread to vary from the shape of a new tread in excess of certain established standards. Most often, this is due to the tread wearing hollow where it bears against the rail, or the flange wearing thin where it bears against the rail. Gages are used to determine how much the diameter of the wheels need to be reduced to restore a proper profile. All of the wheels in a given set need to be the same size when finished, so one badly worn flange in a set of six driving wheels condemns the other five to be turned down the same amount.
Of the several methods used to restore the profile of wheel treads, we use our wheel lathe with form tools. The Strasburg Rail Road’s wheel lathe was built in 1913 by Niles. It will swing 90”, has a 25 hp motor, and when in gear for tread turning has spindle speeds ranging from 0.45 to 1.89 RPM. That’s right, just under ½ a revolution per minute. Both the headstock and tailstock have driven faceplates, and there are two tool posts, so both treads can be cut simultaneously.
Driving wheel tires are made of extremely tough high carbon steel, which is essentially spring steel. Using high speed cobalt steel cutters, the standard cutting speed is 10 to 12 surface feet per minute. Using a single point roughing tool to bring the treads down to near their finished size, we usually feed the tool at 1/8” per revolution feed, and up to 3/8” depth of cut. The chips come off the tires a royal blue from the frictional heat. I have read some railroad’s standards that called for roughing cuts taken at ½” feed, and ½” depth of cut.
Once the treads are roughed to size, three form tools are used in sequence to finish them. The first cuts the finished shape of the flange, the second cuts the radius at the outside edge of the tread, and the third cuts the tread surface in between. The treads are measured for size, and if accurate, the wheel set is finished. The AAR standard tolerance for variation in diameter of all the wheels in a given set, fresh from the lathe is .039”. If when turning six driving wheels, the machinist turns the last one too small, the others will need to be put back in the lathe and additional material removed to bring them within tolerance.
In this photo, Brad is using roughing tools to reduce the diameter of this wheel set to match one already done, prior to finishing it with form tools. On the outer edge of the left wheel, you can see where the tool is just starting a new cut. The chalk marks show the point where he was measuring the diameter from the last cut. The grey castings in between the face plates and the tires hold the dogs that transmit power from the faceplates to the wheel set.
Strasburg Rail Road
