A Boring Story--The Cylinder Kind

By Jim Eng, ©2002 MCRHS

Over the years, we’ve opened the Underwood Cylinder Boring machine storage coffin several times to apply oil and marvel at all the parts stored in the box. You may question the use of the term “coffin” to describe the wheeled box the machine is stored in. In paper and other large mills, the term coffin was used to describe large coffin-shaped toolboxes and crates where you stored specialized equipment.

We were not able to ascertain if the Underwood boring mill was ever used at Mid-Continent. We believe it was but no one can confirm that. We didn’t find an instruction book or such in our searches through the old documents at the museum. We did find photos of Underwood Boring Mills in operation in several books. The one that comes to mind first is the book on the Allegany, C&O’s 2-6-6-6, and a Super Power book about Lima locomotives. These photos certainly helped us with the industrial archeology we had to do to set the unit up.

We found that all the parts where in the coffin except for one of the feed gears. Everything was in decent shape, some rust and such but nothing so bad that it couldn’t be taken care of by oiling and cleaning. We purchased a feed gear; it should have been larger for a finer feed, more on this later. We bored, keyed, and fit the new gear to the lead screw.

Our particular Underwood Boring machine is capable of boring cylinders for a locomotive where the back cylinder head is removable; this is the case with the Saginaw Timber #2. If the back cylinder heads are not removable, as on most newer locomotives, there is an adapter bearing assembly that pilots in the piston rod packing bore. We also have the proper tool holders to bore piston valve bushings in place, such as those on C&NW #1385.

The cylinders on the #2 were tapered and out of round by about 0.090", which has to be corrected. We decided to begin with the engineer’s cylinder as it is the worst one, and then move over to the fireman’s cylinder. The wear in both cylinders is not so severe that the counterbores are missing, so we were able to use these to line the boring bar to the cylinder. The counterbores are usually about 0.125" larger in radius, 0.250" in diameter. We will recut these counterbores after we finish boring the cylinder.

On the #2, it is interesting to note that there are no cylinder bushings. Apparently the prospective locomotive purchaser could specify if they wanted to have bushings installed in the cylinders. That would only be if the cylinders were of cast iron, steel cylinder castings would always have bushings installed as steel would not last very long with a heavy piston rubbing on it. Cast iron works well in these cases as it holds lubrication in its somewhat porous surface.

During disassembly we found that both of the back cylinder heads had a steam leak. The engineer’s leak was much larger than the fireman’s. We will have to take steps to eliminate these leaks during reassembly. We found one additional interesting thing. It was previously thought that the guide bars bolted directly to the back cylinder head with a single bolt. We found that there is a stepped adapter block that bolts to the cylinder, with two fitted bolts, and the guide bars bolt to these. All four of these bolts are worn and working. They will require replacement during reassembly.

nce we had the support brackets bolted into place, we were able to slide the 4-inch diameter boring bar in. We then used indicators on both ends to align the centerline of the boring bar with the centerline of the cylinder. The indicators were positioned on the cylinder counter bores. Interestingly, these turned out to be round within several thousands of an inch. As the boring bar was inserted through the supports and bearings, we threaded the proper tool holder onto the bar. The machine has an assortment of six triangular tool holding castings.

The boring bar has an acme lead screw set in its side with its outside diameter at the outside diameter of the boring bar. One end of the lead screw has a thrust collar arrangement and feed unit. Once things are set up, a special square halfnut is placed around the screw and under a setscrew in the tool holding casting. Turning the lead screw feeds the tools through the cylinder. We adjusted the three cutting tools to take a 0.060" cut with two tools cutting approximately 80% of the material with the third tool making a lighter finishing cut.

A compound open gear drive unit hangs on the exposed end of the boring bar. In order to get it on this particular locomotive we had to derail the pilot truck off the other side if the rail. The drive is held from rotation by bars hooked to the engine pilot platform. We used our best air motor, which gives a boring bar RPM (revolutions per minute) of 2—not very fast. But with an 18-inch diameter cylinder, the maximum boring speed would only be in the range of 18 to 20 RPM.

It took us three four-hour visits to bore once through the cylinder. The final finish we attained was not what we wanted so we had decided to bore through again with an eye toward attaining a better finish. However as we approached the end of the first cut, we found that an area some ten inches wide at the cylinder head, to one inch wide about six inches back into the cylinder was heavily worn. This is the point where the full weight of the piston is bearing on the cylinder. We will take one more cut through the cylinder to remove this uncut area and attain a more acceptable surface finish.

We have considered going from the slow air motor drive to an electric drive, which would give us the 18 to 20 RPM. However in these cases speed is not of the essence as it will be some time before the locomotive runs again. If we are pushed we can quickly speed up the process. Clearly 75 or 80 years ago it would have been done as we are doing it now: slow.

I previously mentioned that there was one missing component: one of the feed gears. We made a new 32-tooth gear and after using the boring bar, we realized that this should have been much larger, say in the 46-tooth range to further slow the feed. However we only feed a small amount at each revolution of the boring bar. It takes some concentration, but it works. We have ground up new cutting tools to improve the finish.


Views of cylinder honing (click for larger view). Clockwise from upper left: cutter tool hones cylinder wall; foreman Jim Eng operates the Underwood Cylinder Boring Machine; view of Machine from front of cylinder; view of Machine from back of cylinder.