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					                                 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!

				
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