Opus Proximum – a vertical engine by Stan Bray published in a special issue ‘The Best of Model Engineer’.
Fellow model engineer and regular visitor to my site Malcolm Tompkins (now sadly deceased) was attracted to this vertical engine as his next project. The plans and build notes are published in ‘The Best of Model Engineer’ an Autumn 2009 special.
The plans show construction in both metric and imperial measurements but don’t try to mix them – they won’t work !
Having decided that a vertical engine would also be a welcome addition to my line up of model engines I felt that upping the size by 50% would produce a more impressive result. At the same time both Malcolm and I felt that metric was the way to go with easier calculations and a better choice of fasteners.
The plans were enlarged to fit on A3 sheets. All the original dimensions were tipexed out and the new metric dimensions, upped by 50%, were inserted and final A3 copies run off. I would hasten to point out that both Malcolm and I bought a copy of the magazine so there was no distribution of freebies !
A study of the article and plans did throw up one or two teasers. The most obvious being the photograph on page 29 which showed a completely different arrangement of the main bearings, crankshaft and connecting rod. I came to the conclusion that this was an error and will therefore be ignored in our build. I also came across one or two other anomalies. For example the instructions on positioning holes for the cylinder (2) PCD – should have read (12). Similarly the valve chest was given as 16mm wide yet the cover was given as 15mm. Care is therefore required as we machine each piece to ensure that it corresponds with related components.
remember, you can click on each photograph for an enlarged image
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- The Best of Model Engineer, volume one, available from W H Smith contains several interesting plans including Stan Bray’s Opus Proximum
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- Plans and build notes for Opus Proximum an attractive vertical engine. These engines were typically used to drive factory line shafts.
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- This close up from the magazine article shows the finished model and is a useful general shot for reference during the build procedure.
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- The plans were copied, enlarged to A3 and all metric dimensions increased by 50 per cent which will produce a model aproximately 200mm in height.
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- Where did this come from ? Ignore this photograph which was included in the article as it does not match up with the plans. (It does look rather good though !).
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- As Stan Bray’s notes suggested a start was made by machining the three frame plates. The base plate being increased in size and fluting added around the edge for extra interest.
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- Machining the valve rod slot in the top plate. Note the use of the vice ‘back stop’ which will allow the next plate to be accurately inserted into the correct position.
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- The fly cutter was used to bring the cylinder block down to size. The flycutter does produce a super fine finish although covering your workshop in fine brass chips !
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- The cylinder block was transferred to the self centering 4 jaw chuck, faced off and brought to the required length. Note protection of workpiece in jaws.
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- Before transferring back to the mill for boring the centre of the bore was located and marked a little deeper than normal with the centre drill.
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- A centre finder was used to accurately position the cylinder block below the drill chuck. Adjust until there is no ‘ridge’ on the centre finder.
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- Before machining the cylinder bore the position of each cover screw hole was centre drilled using a bolt circle piece of software.
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- The cylinder bore was drilled using a succession of drill bits then changing over to the boring bar to take the bore up to 15mm diameter.
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- On the final setting the boring bar cutter was run up and down several times to produce a fine finish before lapping with fine emery.
Machine cylinder and top cover ensuring alignment of bolt holes without the need for digital readout facility on rotary table. Note dimensions are for engine increased by 50% from plan.
The bolt circle programme referred to in the captioned photographs does require digital read out capability on ‘X’ and ‘Y’ axis. There is an alternative method of tackling the cylinder and cover best illustrated with this sketch and notes
1. Square up a 50mm length of brass to 24mm square.
2. Bob it in your lathe’s self centering 4 jaw chuck, face both ends then turn the first 5mm down to 24mm dia.
3. Machine locating lip to 1.5mm deep.
4. Transfer to R.T. spot the position of each of the eight bolt holes and drill 2mm to depth of 8mm.
5. Transfer back to lathe, centre drill the 3mm piston rod hole and part off the cylinder head cover. This will now be aprox 3mm deep including lip. (f your parting cutter is wider than 2mm then increase the 5mm turning in point 2 as necessary.
6. You may need to skim the top of your cylinder cover later on your mill to clean up and bring to required overall depth.
7. Similarly I should take a fine facing cut off the top of the cylinder so everything is nice and square.
8. You can now drill and bore your cylinder to 15mm (or a whisper under the size of your most appropriate reamer).
9. Return back to R.T. and machine away two corners until the desired shape has been achieved.
Email me if you require further information or assistance with this procedure john@start-model-engineering.co.uk
Moving on to the shaping of the cylinder. This was done utilising the rotary table on my milling machine. It would be possible to form the shape by careful hand filing or, alternatively, leaving the cylinder in its square shape.
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- A jig to hold the cylinder on the rotary table was turned on the lathe then threaded M6 to take a securing bolt.
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- To ensure machining accuracy the rotary table was carefully squared up on the mill prior to machining the cylinder block.
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- Aligning the rotary table chuck with the mill chuck with the aid of a simple bar which is a sliding fit in the brass cylinder.
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- Ensure that you select an end mill cutter at least equal to the depth of the cylinder and take very light conventional cuts. Take your time !
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- Be careful to take only conventional milling cuts – avoid climb milling which could rip your cylinder from the chuck and hurl it across your workshop.
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- The Opus Proximum cylinder now completed. The next stage will be the cylinder top cover, base plate and silver soldering the side lugs.
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- The cylinder top cover was turned down to 24mm diameter from a length of brass bar.
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- After turning the locating register the cover was centre drilled as a reference point for drilling the cover bolt holes.
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- Using the same co-ordinates employed for the cylinder studs each bolt hole was centre drilled and drilled 2mm.
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- An alternative method to achieve symmetrical shaping of the glands and gland covers is with the use of of drill bits for positioning.
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- Drill and tap piston and add oil retaining grooves before parting off. Turn to final size with piston mounted on piston rod.
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- Aim for a close sliding fit between piston and cylinder that produces a satisfying ‘pop’ when the piston is quickly pulled free.
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- Machining the square base from the round on the milling machine after turning the retaining lip in the lathe.
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- Take extra care in correctly positioning and sizing of ports and steam passages on cylinder.
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- The steam chest blank was machined to dimensions given on the plan and was clearly oversize.
The project was brought to a halt at this stage for adjustment of the dimensions on the plan.
