On to the last lap with this Elmer variation. The cranks were machined out of stainless using plenty of cutting oil. I picked up a litre bottle of cutting oil from Machine Mart and as I work mainly in brass and aluminium the oil is lasting me a long long time. Turned the stainless down to size on the lathe then transferred to the mill for cleaning up. I always seemed to find that when parting off two things happen. The cut veers away from the face of the machined part and there is often a protrusion remaining as the parting tool breaks through. Both these unwanted formations can be cleaned up easily in the mill and then on to forming the web. An M3 screw with a shoulder to act as the bearing surface for the ‘big end’ of the con rod was machined up.
Next on the ‘to do’ list was drilling and tapping the back of the cylinder for attachment to the engine frame taking care not to drill through into the cylinder bore – 4mm seemed about right. Where possible I use a guide block to ensure my tap runs square to the hole. You can also replace your drill bit after preparing the hole to be tapped with the appropriate tap and turn the chuck a few turns by hand to get the thread started and running square. This can then be transferred to the vice for finishing off.
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- A length of brass bar was centred up in the 4 jaw independant chuck using a DTI (dial test indicator).
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- The offset hole is started for the cylinder bore with a centre drill then opened up with a succession of drill bits.
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- The critical depth of drilling the ‘blind’ bore is checked against the vernier scale.
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- A boring tool makes the final cuts to size and imparts a fine finish to the bore.
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- A profiling tool provides an attractive curved finish to the cylinder of Elmer’s Standby.
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- The completed engine mounted on a wooden base runs well on just 5 psi of air.
The next stage I am afraid didn’t get covered photographically. However, the piston was turned from aluminium, cross drilled for the gudgeon pin (or wrist pin as it is called across the pond). The con rod was turned up on the lathe and again transferred to the mill for putting the flats on both small and big ends. The con rod needs to be turned as narrow as reasonable to clear the base of the cylinder.
The crank shaft was machined from stainless rod to produce the flats which cleverly act as inlet and outlet ports. Clearly to be effective your crankshaft needs to be a very snug fit in the bearing to avoid air loss. I am sure it is not aproved engineering practice but as you will see from my picture the crankshaft was overhung from the vice for milling the flats. Very light cuts were taken and a satisfactory result achieved.
Care needs to be taken when drilling the exhaust outlet along the length of the crankshaft – slowly does it with plenty of backing off and cleaning and lubing of the fine drill bit. The crank was ‘loctited’ into position making sure it was set square to the inlet and outlet flats on the crankshaft – refer to your plan.
Now I would like to tell you that I assembled all the components, connected up the air and off she went. Well I found that I couldn’t quite get a full rotation due to the top of the piston making contact when approaching TDC. I used a milling cutter by hand to clean up and ‘bottom out’ the head of the cylinder. The engine now rotated by hand – a spot of oil added to cylinder bore and bearing surfaces, air connected and off she went and after just two or three minutes she was running a treat on around 5 p.s.i.