I could never do that Part 5: Slip eccentric valve gear By Markus Neeser As announced in the last Newsletter, “I could never do that” continues with the production of the slip eccentric valve gear. In case of the Midland single wheeler I have followed the drawings and went for the reliable and robust slip eccentric valve gear as suggested by Raymond Lewis but I could not resist and made some minor alterations compared with the original design. Photo 1: Reaming an eccentric blank in a normal self-centring 3-jaw chuck First I shall try to explain my method to produce eccentrics using the ordinary self-centring 3-jaw chuck that I have at hand. Paying tribute to the title of this series which should also be read as “Blimey, I can do that too” I shall try to describe a quite simple approach which is far away from rocket science. The whole “trick” of making accurate eccentrics is to do as many machining steps as possible without rechucking the work piece in between. Actually this goes for all lathe jobs: Always try to figure out how you can machine the important dimen- sions in one go without rechucking. This is the key to true running components. As you can see from the first photo there is a spacer that allows chucking the bronze rod eccentrically. In case of my 3-jaw chuck the spacer for an eccentricity of 1/8” (giving a valve stroke of ¼ inch) needs to be about 4.7 mm. This dimension depends a little bit on your chuck size and has to be found by testing. Drills are normally at hand in many different di- mensions and are therefore ideal to find the correct spacer width, but do under no circum- stance switch on the lathe, as this might turn the drill into a dangerous projectile! The eccentricity can be accurately measured using a dial gauge with a magnetic holder. Such instruments can be bought for a reasonable sum from the usual suppliers if you don’t have any at hand yet. A vernier caliper can be used too as long as you have a suitable sup- port to measure against. Once you have found the correct spacer width, you should prepare a suitable size piece made from brass or steel. Photo 2: Parting off after turning the grove The external diameters of the eccentric as well as of the grove for the strap are both turned in one go to ensure everything is perfectly concentric. When turning the grove make sure to use a sharp tool and you will get a very smooth finish that does not require any fur- ther finishing. Don’t forget to make the grove some 0.2mm wider than the strap to prevent from binding. Regarding the materials to be used for eccentrics and straps I have seen both variants: Steel eccentrics and bronze straps and vice versa. Further down in this article you will find the reason why in this case the choice of materials does not really matter. Brass can be used too but bronze is definitely the better choice and will last longer. Because I planned to make the straps from steel, I machined the eccentrics from a piece of bronze rod that I had at hand. The next components to make are the eccentric straps, which are made up from off cuts of steel bar. The individual parts are silver soldered together using a suitable jig (see photo 3). Photo 3: Silver-soldering the components of a strap using a jig The jig itself should fix the components to prevent them from moving around when the solder is applied. A better method is to apply plenty of flux and bed in a piece of silver solder followed by heating the components to red heat until the solder starts to flow and all will be nicely soldered together without misaligning. If you make your own flux paste from powder and water you will know the problem that the flux will not stay where applied to because the water starts to evaporate, spreading the flux all over. If you use some methylated spirit to make the paste instead of water, the meths will burn away and the flux will stay in place while heating. I shall not be tempted to loose too many words about how to produce decent eccentric straps as I have already described my favourite method in detail in newsletter 202 on pages 36-39. A few reminders follow here: After soldering, all remaining flux and oxides caused by the soldering should be removed before continuing with machining. The prefabricated strap is afterwards cut through the mid- dle and bolted together after cutting the threads. Make sure to cut the threads into the correct half of the straps: You need to have proper access to the screw heads after installation when fitted. Photo 4: Strap glued to the jig, ready for turning To turn out the strap it is simply glued on a bell-shaped gauge. For that purpose I use or- dinary cyano instant glue, preferably a type with a low viscosity (often sold as “thin” quality) as this type sets very quickly. The pin running through the hole in the eccentric strap also runs through the gauge and does centre align both parts while the glue sets. Of course it is removed before turning. If you do not trust cyano glue to stick the straps to the gauge, you can use soft solder instead (I have made many eccentrics that way and none came ever off the gauge). Photo 5: Turning out the strap The cyano glue dries very quickly. After a few minutes the eccentric can be turned out. Again use a sharp tool for this purpose. If you are using carbide tips it is good advice to go for a tip made for machining aluminium alloys as this type gives the best surface quality. Don’t forget that an eccentric strap also needs some play to prevent from binding. Some 0.05 mm in diameter should be sufficient. And now for the announced explanation why the choice of materials for the eccentric and the eccentric straps does not really matter: On the recent Spam Cans, Aster decided to put a strip of Teflon between eccentric and strap to prevent both from wear. This method works very reliable I decided to use this method for the Midland Spinner eccentrics too. To allow space for the Teflon strip, the internal diameter of the straps has to be made twice the thick- ness of the Teflon strap. Add some 0.05 – 0.1 mm play. Photo 6: Finished pair of eccentric straps Photo 6 shows how the finished straps will look like. Note the marking of the strap halves near the screw: These are made to ensure the straps can be taken apart and refitted as matching pairs. Photo 7: Marking the eccentric stops Now to the eccentric stops: The cutout section of the eccentric stops defines the valve tim- ing of the slip eccentric gear and should therefore be made accurately. Photo 7 shows a simple arrangement I use to mark the cutout. The vertical line is the position for the securing screw thread and the two upper lines mark the cutout angle. With some care, tolerances smaller than 1° can be achieved using this simple method. After marking the area to be cut away with a scriber, the excess material can be removed on a mill with a turntable or simply by using a hacksaw and file. Make sure that the cutout always clears the pin on the eccentric. As always, the proof of the pudding is the eating. On one engine I made I faced the prob- lem that the engine ran well in forward gear, but very sluggish in reverse gear. So I moved the eccentric stop backwards to get proper valve timing in reverse gear and the engine ran very well backwards. You guess it: This time I didn’t run well in forward gear, which was a clear indication for that the cutout angle of the eccentric stop was too small. After comparing the drawings against others with the same valve and port dimensions, I found that the angle . of the cutout was too small by about 20° So I made new stoppers and the engine ran both forward and backwards like a clockwork. Photo 8: Finished pair of eccentric stops Finally the eccentric stoppers do require a thread to fix them on the axle. This is drilled into the eccentric stop, preferably at the vertical scribed line to be seen on photo 8. Chose the set screws big enough so they can be well tightened (the set screws can be seen on photo 9). Photo 9: Valve gear fitted to the crankshaft Now it’s time to test fit of the eccentric stoppers on the crankshaft. To ensure that the valve gear will work properly afterwards, you have to make sure that the eccentrics move freely and do not bind. When I had a look at the crankshaft assembly at this stage, the idea came up to screw the driving wheels directly to the stoppers instead of gluing or pressing the drivers on the axle. I did this using two M1.6 (10BA) sink head screws and found the method to be completely satisfying. A big advantage of this method is that drivers can be very easily removed for painting. Photo 10 will certainly answer all questions. Photo 10: A close-up of driver and eccentric stop Provided you have accurately reamed the hole through the wheel and the eccentric stop, the wheel will nicely slip over the axle and will run as true as if pressed on. For perfect align- ment it’s good advice to spot the holes in the eccentric stops through the ones drilled into the wheel. Photo 11: Driver with screwed on eccentric stop Photo 11 shows the assembly. I particularly like this photo because big driving wheels are the most graceful Gauge 1 components to me and make Victorian and Edwardian engines look so attractive and charming. We are very lucky that Mark Wood is supplying us with such perfect lost wax wheel castings. Scratch building would only be half as interesting otherwise. Photo 12: Finished driving axle, ready for installation The last photo shows the complete driving axle with all but the connecting rods fitted. It is clearly visible that the outside bearings do allow making use of eccentrics and crank webs of reasonable width. The space between the two cranks is reserved for the pump eccentric. This one will have to wait until the engine is completed: By then it will be better visible where the pump can be fitted and where to route the pipe work. As you have certainly noted, the description of the valve gear took more space and words than expected before and so the description for the connecting rods will appear in the next issue. I hope you don’t mind too much. Good luck and enjoy your workshop time!