Those who have fitted digital read outs for 'X' and 'Y' movement on their milling table can locate the precise position of each bolt hole on the cylinder covers
Well I have to say that after boring something like 20 cylinders since I started in model engineering this latest attempt is by far the best in terms of machining accuracy and surface finish. I put this down to the use of my new boring bar purchased from RDG Tools and used with automatic feed. I finished up with a super smooth bore of 19.50mm – at both ends ! – now there’s a novelty. I believe reaming will be unnecessary and maybe just a little gentle lapping will give me a very serviceable finish.
The next task was to drill the inlet passageways which feed steam (or air) to either end of the bore. It’s no problem tilting the column of my mill to the specified angle of 16.5 degrees but it’s a major undertaking resetting it afterwards (although I have fitted a laser positioning device which alledgedly makes repositioning a doddle).
Instead, I opt for using a Clarke 2.5″ tilting vice purchased from Machine Mart for £15. Not the greatest precision instrument you will come across but it does the job. This combined with the clineometer app on my iPod and the required angle is soon sorted. Take great care at this critical stage as it is only too easy to get the airway out of position. When slot drilling the air inlets on the cylinder face I would suggest going a little deeper (from 4.75 to 6mm) which will increase the chance of your airway connecting up with the inlet.
For the cylinder cover bolt holes I followed the plan for the bottom cover with 6 bolt holes but for the top cover I increased the number to 8 which, in my view, looks more authentic in my larger sized interpretation of Elmer’s engine.
You may find this free bolt circle calculator of assistance during this next stage.
remember, you can click on each photograph for an enlarged image
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- The cylinder block is held in a tilting vice and is spot drilled for the start of the airway.
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- Aim for a spot drilled point like this to ease entry of the drill bit for the airway proper.
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- Set up the required angle for drilling the airways. Here I am using a handy app for my iPod
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- Once the air passages are drilled the surplus material is milled off the cylinder block. This area will eventually be covered by lagging.
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- The square outboard head was machined from aluminium round after turning the cylinder bore locating lip. Centre drill a location datum point before parting off.
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- The cylinder base plate or end cover seen prior to drilling for the six hold down bolts and machining into the required square format.
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- Using a piece of on-line software the co-ordinates for the six bolt holes were established working from the central datum point. A digital read out system ensures accuracy.
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- Offsetting the first bolt hole by 30 degrees will ensure that none of the bolt holes will interfere with the air inlet position.
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- The first straight edge is milled before rotating 180 degrees to square off the opposite edge. When the two sides are formed the piece is positioned flat to mill the remaining two sides.
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- Cylinder block with outboard head is trial fitted to the engine chassis and alignment carefully checked.
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- The cylinder top cover is drilled and tapped to take the gland nut. The blue tape is a marker to guide depth of thread.
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- The cylinder top cover, drilled and tapped, is turned down to size.
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- A close fit is essential between the top cover and the cylinder for correct alignment.
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- The top cover is temporarily superglued to the cylinder and the centre finder used to locate the central datum point. (Application of heat will free the superglue when required)
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- Following the bolt hole co-ordinates each of the eight holes are centre drilled and drilled to 2.25mm to a depth of 11mm. The holes in the cylinder are then tapped M3.
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- Each hole is now drilled to M3 clearance for the depth of cover. The number of bolt holes was increased from 6 to 8 for appearance sake.
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- M3 bolts with two washers and nut are ‘Loctited’ into position then the threads cut off flush with the nut using a diamond wheel on a Dremel type tool.
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- With one of the two washers removed and the nuts inverted it just remains to clean up the thread ends with a small needle file.
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- The piston blank is mounted onto the piston rod for final turning to size to ensure concentricity.
It occurs to me that my descriptions may not be too clear to some. If anyone would like me to expand on any points on this, or any other posting, just drop me an email at john(spam)@start-model-engineering.co.uk removing ‘you know which bit’ from my email addy. Please remember I am not an experienced model engineer and my methods may not be the correct way of doing things.