Gauge 1 Steam Locomotive - Cylinders.

The cylinder assemblies were machined from a piece of cast iron bar. The port face drillings were simplified by making them hole rather than slots.

The pistons were machined from brass.










To minimise waste material a CAD sketch was drawn up to calculate what diameter bar would be needed. This showed that a piece of 32.15mm bar was needed as a minimum so a piece of 35mm bar was used.

Click to enlarge









This photo sequence shows the process for cutting the cylinder blank from the piece of cast iron bar.

The three outside straight cuts were made using the milling machine and a fly cutter. The step was then machined with a 10mm slot drill. The bar was then cut with a large slotting disc to create two slightly over long cylinder blanks.









Each cylinder blank was placed in the 4 jaw chuck and bored to a final size of 12.5mm diameter.

Also in this setting the front faced was machined to ensure it was square to bore.

Then the cylinder was reversed in the chuck and tuned to final length.










The port drillings were marked out and drilled on the drilling machine using the depth stop to ensure that the holes didn't break through to the cylinder bore.

The drawing called for slots to be machined for the steam ports in the valve face. To simplify this, holes were drilled instead. The diameter of the holes was equal to the widths of the slot indicated in the plans ie: 1/8" for the exhaust hole and 1/16" for the inlet ports.









Milled cylinder port

The angled port drillings were made by just drawing out the required angle on the side of the cylinder blank and lining it up by eye on the drill press.

However before this, a small nic was made as shown, on each end of the cylinder. This feature was cut with a 2mm milling cutter and then centre punched to locate the 1/16" port drill









The mounting holes for the cylinder were transfer drilled from the locomotive frames as shown. The holes were drilled and tapped 8BA.









To machine the curve under the cylinder a rotary table was used on the milling machine. The tool had to be used fully extended in the collet to reach the full length of the cylinder so careful cuts had to be taken.

The radius of the curve was reduced until it met smoothly with the outside of the cylinder.









The front cylinder cover was a simple turning job. First a piece of cast iron bar was reduced in diameter to suit the outside radius of the cylinder. Then a sharp tool was used to make a shallow feature that would locate the cover in the cylinder bore. Before parting off the disc, a small spirit level was used on the chuck jaws, to mark 3 evenly spaced holes around the egde.

Three holes were chosen instead of the 4 mentioned in the plans, partly because it was easy to divide in the lathe and partly so that the front end cap would match the rear one which had been re-designed to make it easier to machine (see below).

Process for marking out holes









The three hole markings were centre punched and then drilled on the drill press.

Then the cover was used to transfer drill the holes to the cylinder. The first hole to be drilled was the one at the bottom of the cylinder because it was easy to locate centrally.

Once this hole had been drilled and tapped, the cover was held in place with one bolt while the other holes were machined.

The front end covers were located on the non-square face of the cylinder, leaving the square face for the rear cover which would also need to locate the piston rod.

Drilling the cylinder to take the end cap.










The rear cylinder over was redesigned to make it easier to machine. By reducing the number of mounting holes to 3, it was possible to machine a square block on the cover face to mount the piston guide rod. This removed any need to machine a complicated profile using the rotary table, which would be necessary to make room for 4 bolt holes.









Tapping the rear cylinder cover.

First, the bar was reduced in diameter to match the outside cylinder radius.

Then a hole was centre drilled, drilled and reamed to take the piston rod (3mm).

Next, part of the hole was opened out to take the 1/4" tap, which would be used to locate the gland packing nut.

This was tapped 1/4" x 32 tpi.









Milling the rear cover profile

The piece of bar was removed from the lathe chuck and placed vertically in the milling vice. 5.7mm side and lower edge cuts were made to a depth of 4mm to create the final cover profile. The remaining thicker section would be used to hold the piston guide rod.

Cutting this profile left room for 3 mounting screws to be used to hold the cover on to the cylinder as with the front covers.









Next the bar was returned to the lathe to mark out the 3 mounting holes. The orientation of the bar in relation to the chuck jaws was important if the spirit level was to be used to mark the holes. So a line was scribed down the middle of the raised feature on the cover and this was lined up at lathe centre height (ie level with the tool tip) at the rear of the lathe.

The spirit level was then used on the front jaw as before to mark the 3 holes.

The holes were centre punched and drilled on the drill press.









To finish the rear cylinder cover, it was parted off from the bar and a 3mm arbor was machined and threaded to take a nut.

The cover was mounted on the arbor as shown and light cuts were taken to form a locating shoulder on the inside of the cover. The arbor ensured that the piston rod hole and cover shoulder were concentric.

The holes in the cover were then transfer drilled on to the cylinder.









The mounting holes were transfer drilled from the locomotive frames and tapped to take 8BA bolts.

To make assembly in the final situation easier the exhaust was to be connected using o-rings. The cylinder was drilled to 6mm and a piece of brass bar was turned to length and shouldered to hold the o-rings.

Cylinder mounting holes and exhaust hole.

Exhaust pipe.


After the 6mm exhaust hole was drilled a connecting holed could be drilled at an angle to the exhaust hole in the port face.