The 'shaft is machined from steel bar, hardened and then ground to size and finish. For this highly stressed part, it is best to use an alloy steel with high toughness and core strength. These steels are more expensive, harder to obtain and far more difficult and slower to machine than common free-cutting machining steels. They are generally supplied with a black-scale finish that is unsuited to holding in the collets of production machines. I use EN36A, a chromium-nickel steel, the best I know of for model engine crankshafts. From within a large quantity made for a corporate user, I was able to buy enough 13 mm diameter bars with a bright, reeled finish to make all the engines that I shall.
Most of the 'shaft machining is done on my Feeler FHR-68 manually controlled turret lathe, a clone of the similar Hardinge lathe. This particular lathe was presented to Gordon Burford by Aling Lai, the Chairman of Thunder Tiger Corporation, in appreciation for assistance that Dad gave. A nice touch for me to continue to use this machine. I use also various attachments and fixtures with, my King Rich turret milling machine, Overbeck grinding machine and a Raytech vibratory deburring machine.
One machining operation on the Feeler can involve many tools, mounted on cross slides and on the eight sided turret that is indexed and presented to the work manually. An automatically controlled machine would do much the same without my participation. The modern automation of choice is Computer Numerical Control. It is just that though; a method to control the machine. Drills still must drill the holes. My lathe is cost-effective for my production. A manually controlled lathe could cost $25,000.00, a basic CNC version $50,000.00 and a sophisticated CNC, machine, $500,000.00. Even this complex machine would not be able to complete the crankshaft. There would still need to be subsequent machining, hardening, precision grinding and washing operations.
The first turret lathe machining operation from the bar involves,
Feeding the bar
out to a length stop
Drilling the centre
Drilling the # 39 hole and chamfering the bar end
Turning 5.20 diameter
Turning 3.98 diameter
Tapping 4:40 thread
Turning 4.80 diameter recess
Forming the undercut steps and lengths
Parting-off the machined blank
I then repeat this for the next crankshaft of the batch. Then, I work the machined blanks through each of the next 18 listed sequences, all of which can similarly involve several or more sub-operations.
Face the head
Polish the centre
Mill the web counterbalance
Mill the crankpin boss
Turn the crankpin diameter
Mill the crankpin chamfer
Drill the inlet passage
Deburr the counterbalance cutouts
Mill the thrust washer driving flats
Deburr the milled flats
Drill the inlet port pilot hole
Mill the inlet port
Vibratory deburr the 'shaft
Send away for carburising and case hardening
Polish the centres
Grind the journal diameter and thrust face to size
Grind the crankpin and web face to size
Ultrasonically clean and oil
The diminutive size of the part means that the processes are more delicate than those on a crankshaft for a larger engine. Some must be done under magnification, with devices so that it can be held firmly. Tolerances and finishes are generally more exacting than those of larger crankshafts.
Some crankshaft design features are: The thrustwasher is driven by flats, rather than pressed to a spline, so that it too can be made of hardened steel. A milled inlet port gives better timing opening and closing and the rounded ends ramp the rate of stress change. The angled crankpin relief permits con-rod assembly and allows lubrication, with minimum clearance. This relief is opposite the power-stroke loading. The 'shaft is robust for the engine size. Using a screw to retain the propeller minimises crash damage. The generous web thickness allows proper counterbalancing.
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