PB Engine Australia

A quality micro diesel model aircraft engine

Model Aeroplane Engine

 

Construction

My air-conditioned workshop is situated in an idyllic valley of our sub-tropical mountain hinterland. The machine tools are all of good quality and have been chosen specifically for the task. I produce almost every stage of each component. This brief description of the components and manufacturing processes will demonstrate the extraordinary care that is taken.

There are more extensive descriptions of the Crankcase, Crankshaft and the critical Finishing and Fitting of the Cylinder/Piston assembly. 

 

Propellers

There are two sizes, 6 x 3 and 6 x 4, designed specifically for the pb 0.33 engine and moulded in fibreglass reinforced polyhexamethyleneodipamide resin. I made the models and patterns by hand and pantographed the moulds in the traditional manner. The plastics injection gates are central to align the glass fibre reinforcement and to avoid the weakness of weld lines. The moulds were precisely polished to give balance to the propellers. Immediately after moulding on my NESTAL machines, each propeller was carefully packed to protect the fine edges.

Engine Crankcase

I mould the aluminium crankcase using investment casting.  A set of injected wax mouldings is encapsulated within a ceramic investment.  Aluminium,  A 356  alloy is vacuum cast into the void remaining when the wax is subsequently burnt out.  The process is a craft and each crankcase bears a surface that is unique.  I heat treat the castings before commencing the multitude of machining operations needed to complete the crankcase.  During machining, the casting is mounted and carried upon a precise hardened steel fixture to maintain the squareness and position of each feature.

The following is a more detailed description of the process.

The crankcase is machined from an aluminium casting made by the investment process.  While this requires several steps, using equipment made primarily for the jewellery trade by Kerr Corporation, it enables the forming of the internal transfer ports and the fine webbing detail,

Liquid wax, injected into a machined aluminium mould, sets quickly into a firm crankcase form.  The mould cores forming bores and ports are removed and the mould halves split to access the wax moulding.  The transfer ports are formed by a tiny, five piece core that allows the three ports forms to collapse inward, once the centre is withdrawn.  This could not readily be achieved by other casting methods.  Five wax crankcase mouldings are attached to a separately formed branch moulding to make a tree that is next mounted inside a cylindrical stainless steel flask.  The tree is encapsulated within a plaster-like investment, poured into this flask and consolidated under vacuum.  When solidified, twelve of these flasks are stacked in an electric burnout oven, which ramps up to heat overnight to vaporise the wax and harden the investment.  Next day, I melt aluminium in a gas-fired furnace and ladle it into each flask in turn.  The molten metal is pulled, under vacuum, into every crevice left in the investment by the burnt out wax.  When cool, I wash the water soluble investment from the aluminium to expose sixty crankcase castings.  These are reheated in the oven, held for a period at temperature and then quenched in water to give added strength and a fine grain structure to the metal.  There are now thirty more sequences to finish the crankcase ready for assembly.  

As explained under the crankshaft heading, a 'serious' and very expensive CNC controlled machining centre could carry out many of the needed thirty sequences without intervention, but each would be done in much the same manner as with my manually controlled machines.

I cut the journal diameter to length with a rotary slitting saw and mount the casting within a bespoke spindle fixture on my Feeler Turret Lathe in order to machine the crank and back plate bores.  Tooling mounted on the eight sided turret is accurately indexed into place and used in this following sequence of operations, each with it's specific selection of spindle speed and slides positioning.

Face across the engine rear mounting face.  Centre the bores.  Bore 13.70 mm diameter.  Bore 14.70 mm diameter.  Bore 16.75 mm diameter.  Bore 6.75 mm diameter.  Ream 7.00 mm diameter.  Thread 1/2" x 40 tpi.

I remove the casting, next drill the engine mounting holes, spot face the journal front to length and press in the bronze journal bushing.

I use the drilled engine mounting holes to bolt each 'case on to an accurate, hardened and ground tooling block.  By using the faces and edges of this block as locations though subsequent operations, I keep the crankcase faces and bores precisely aligned.  

Many of the following operations involve a series of sub-operations.

Machine the cylinder diameters and faces on the Feeler.

Machine the tank diameters and faces on the Feeler.

Mill the exhaust face.

Mill the carburettor mounting face.

Drill the inlet hole.

Ream the inlet hole.

Drill the exhaust flap screw hole.

Thread the exhaust flap screw hole.

Drill the cylinder location screw hole.

Counter-bore the cylinder location screw hole.

Thread the cylinder location screw hole.

Drill the carburettor mounting screw holes.

Thread the carburettor mounting screw holes.

Drill the fuel filler tube hole.

Drill the fuel tubing hole

Drill the tank air vent hole.

Remove from the tooling block.

Wash, de-burr and check.

Stamp the serial numbers.

Spot-face the mounting holes.

Vibratory de-burr.

Hone the bronze journal to size.

Hone the cylinder diameter to size.

Press in the fuel filler tube.

Ultrasonically wash.

I drill many of the small holes using my Steinel, high speed sensitive drilling machine with air-mist cutting fluid.  I use thread-forming rather than thread-cutting taps for the #1-72 threads so as to tap closely to the hole bottom without generating swarf. 

 

 

Crankcase Bushing

This is machined from bronze alloy bar and pressed into the crankcase. It is micro-honed to a fine, accurate size and finish.

Crankshaft

I machine the shaft from one piece of EN 36 A alloy steel bar. It is case hardened before I precision grind the journal and crankpin diameters and faces. The milled, rotary inlet port is integral with the crankshaft.

The following is a more detailed description of the process.

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 especially 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 will.

Most of the 'shaft machining is done on my Feeler FHR-68 manually controlled turret lathe, a clone of the similar Hardinge machine.  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.  For the crankshafts, I use also various attachments and fixtures with my King Rich turret milling machine, Overbeck cylindrical grinding machine and a Raytech vibratory deburring machine.

One machining sequence 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, albeit a very fine and accurate method.  Drills must still drill the holes and turning tools form the diameters.  My lathe is cost-effective for my production.  A manually controlled lathe with tooling could cost $25,000.00, a basic CNC version $50,000.00 and a sophisticated CNC mill-turn 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 to a length stop.  Drilling the centre hole.  Drilling the #39 hole and chamfering the bar end.  Turning 5.20 mm diameter.  Turning 3.98 mm diameter.  Tapping 4:40 thread.  Turning 4.80 mm diameter recess.  Forming the undercut steps and lengths.  Parting off the machined blank.

I 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 involve several or more sub-operations.

Face the crank head to length.

Polish the centre, so as to give smooth support during the final grinding operation.

Mill the web counterbalance.

Mill the crank-pin boss.

Turn the crank-pin diameter.

Mill the crank-pin chamfer.

Drill the inlet passage.

De-bur the counterbalance cut-outs.

Mill the thrust-washer driving flats.

De-burr the milled flats.

Drill the inlet port pilot hole.

Mill the inlet port.

Vibratory de-burr the 'shaft.

Send away for case-hardening.

Polish the centres.

Grind the journal and thrust-face to size.

Grind the crank-pin 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 design features are; -  The thrust-washer is driven by flats, rather than being pressed to a spline, so that it too can be made of hardened steel to run against the bronze bushing.  The milled inlet port gives better inlet timing opening and closing and the rounded ends ramp the rate of stress change to prevent breakage.  The angled crank-pin relief permits con-rod assembly and allows lubrication with minimum clearance.  The 'shaft is robust for the engine size.  Using a replaceable screw to retain the propeller minimises crash damage.  The generous crank-web thickness allows proper counter-balancing to reduce vibration.

 

Thrust Washer

Machined from steel bar, this is cased hardened to run against the bushing if pusher propellers are used. It is driven by two flats on the crankshaft and has a spigot to centralise the propeller.

Propeller Washer

Made from polished aluminium, it is recessed to enclose the socket head propeller fastening bolt.

Extension Spool

This accessory is machined from aluminium bar and can be used to extend the propeller mounting further forward. It is supplied, with a longer propeller fastening bolt.

Cylinder

I machine this from steel bar. The exhaust port and three fuel transfer ports are accurately milled before the hardening process. The cylinder is then precision ground on the location flange and on the outer surface that locates within the micro-honed diameter of the crankcase.  I precision grind the bore at 65,000 RPM before diamond honing to match with a specific piston.

Finishing and Fitting the Cylinder/Piston assembly 

This is the most critical sequence of the engine construction.

The stage begins with a fully machined and case-hardened steel cylinder, a machined and case-hardened steel piston, a machined and case-hardened contra-piston, a case-hardened and ground alloy steel gudgeon pin and a fully machined aluminium connecting rod.

The chosen materials combination is certainly not the easiest to machine to this stage, nor to finish and fit, but the final properties compensate for this difficulty.  A hardened steel cylinder has an extremely long operating life.  A steel piston can be thinner and have less mass than an equivalent one made from cast iron.  A hardened steel piston will expand in operation at much the same rate as a hardened steel cylinder.  It can therefore be fitted without interference, or 'pinch' at top-dead-centre of the engine stroke.  Then there is always a lubrication film between cylinder and piston, there is less tension on the connecting rod when the engine is new, there is little break-in of the engine needed and there is minimal wear of the piston and cylinder bore.  My prototype engines show almost no wear after countless hours of operation.  Of course, once hardened, these two components must be finish machined by more costly processes and their accuracy must be of a far higher standard to seal against compression.

The S12L14 steel cylinder is carbonitrided and hardened to 62 RC.  This process causes the dimensions to 'grow' and distort slightly.  It must be resized by an abrasive grinding process.  I firstly cylindrically grind the outside diameter to fit the crankcase bore and at the same operation, grind the locating flange face square to this diameter.  I use an Overbeck Zetto 20 Precision Grinding Machine that I bought used and then reconditioned.  I was familiar with this highly respected production machine, as we used a slightly different model at the Taipan factory  The cylinder is held while grinding, by expanding an O ring inside the cylinder to hold it on a mandrel, firmly against the locating flange.   This clamps without distortion and dampens any possibility of flutter while grinding it to size.  Next the bore is internally ground to size in another operation on the same machine.  The grinding machine is now set for semi-automatic operation and reciprocates and infeeds mechanically while the 6 mm diameter abrasive wheel, mounted in a precision air-spindle, sizes the bore at 65,000 RPM.  The machine traverse accelerates just before reversing instantly at each stroke end, to minimise overcut; clever!  The cylinder is nested on the previously ground outer diameter and clamped on only the flange so as to avoid distortion.  The flange is at the top-dead-centre height of the piston stroke to give stability at this point of maximum compression.  The cylinder will later be retained in the engine by clamping on this flange only.  This prevents any distortion that would occur if it was threaded or clamped under compression.  I grind the bore to a slight taper and measure the size using a hard-chromed plug gauge inserted from the lower end.  I next diamond-hone the bore using a Sunnen Honing Machine.  I bought this machine well-used, reconditioned it and then bought all new honing mandrels.  It appeals to me that it bears a plaque stating that it once was Machine 71621, Property of North American Aviation, Los Angeles, California.  Maybe it was used to make large aeroplane engines?  I hold the cylinder freely in a lightweight plastics fixture that I reciprocate manually.  I hone the bore, above the ports, to almost parallel, checking the size with a plug gauge.  At the Taipan factory we finished many tens of thousands of cylinders of different materials, tough, hard and hard-chromed; ground and honed by automatic, mechanical and manual processes.  The best were of hardened steel, internally ground and then finish-honed manually by a highly skilled operator, -- my Father.

The S12L14 steel piston has been carbonitrided and hardened, then tempered to 50 RC.  Two sliding surfaces will not gall if one differs by more than 7 points of Rockwell hardness from the other.  I firstly diamond hone the 1.6 mm diameter gudgeon pin bore.  This bore was honed before heat treatment, so now needs a fine resizing only for clearance on the gudgeon pin.  I then cylindrically grind the outer piston diameter parallel and to size on the Overbeck, clamping the piston on a mandrel by pulling back from the gudgeon hole and expanding an O ring internally at the skirt, to prevent any flutter.

The hardened and tempered S12 L14 steel contra-piston is ground to size on the Overbeck, using a similar set-up to the piston.  The ground diameter has a slight taper, larger at the top.  When coupled with a high length/diameter and a thin wall, this gives a slight 'spring' interference to the fit.  Being tempered also to 50RC it will adjust smoothly without sticking or backing-off.

The gudgeon pin is a standard, hardened alloy steel dowel pin.

The 2 mm diameter connecting rod big-end is honed to a closely measured size to run on the crankpin.  A delicate operation on a part this small!  The honed cross-hatched finish allows a good oil film between the 'rod and crankpin.

I join the piston, gudgeon pin and connecting rod.  The pin is a tight fit in the small end of the 'rod and it's ends oscillate within the piston.  I assemble the three so as to eliminate any possibility of distortion, by using a press fixture that applies force only at the end of the pin and at the position where the 'rod contacts the lower edge of the piston gudgeon hole.  The precise hand press I use was once used in the Taipan factory to insert ball races in crankcases.  Once assembled, there is little sideways float of the 'rod within the piston.  The gudgeon pin ends cannot contact the cylinder wall.  This restriction also prevents the connecting rod big-end from floating off the crankpin and contacting the backplate face while the engine is running.

All parts are now ultra-sonically cleaned in a solvent bath.  The cylinders, pistons and contra-pistons are remeasured and graded, lubricated and assembled in matched units.  The piston operates within a trumpet-shaped bore, very slightly tapered below the ports and almost parallel at top-dead-centre.  It has little clearance below the ports and an infinitesimal amount above.  This gives good crankcase and combustion seal, but requires that all components be accurately aligned.  The contra-piston is within a parallel section of the bore.

All of this takes much longer than it may appear, when written down in this abbreviated description.

 

Contra Piston

This too, is from hardened and tempered steel, precision ground to an accurate fit within the cylinder.

Connecting Rod

I machine the rod from 7075 T6 high strength aluminium alloy. The big-end is micro-honed to an accurate size and finish.  The hardened and ground gudgeon pin is pressed into the rod to locate it within the piston.

Head

The head is machined from aluminium bar.  The cooling fins are radiused at the root to promote heat transfer.  A nylon lock screw is incorporated to give added retention to the compression screw.

Compression Adjustment Screw

This is machined from brass to screw freely within the very fine thread of the aluminium head.  The hard alloy steel tommy bar has domed ends.

Back Plate

Injection moulded from high temperature glass reinforced poly-phenylene sulphide plastics, this incorporates an 0 ring seal.

Single Speed Carburettor

Again, this is moulded from glass reinforced PPS plastics, with the inlet and mounting holes drilled after moulding to avoid weakening weld lines.  It incorporates a surface jet spray bar and smooth venturi for good fuel atomisation.  I bolt it against a machined crankcase surface with socket head cap screws.

Variable Speed Carburettor

This alternative is mounted in place of the single speed carburettor using brass studs and nuts. The body and lever are moulded from ‘glass reinforced PPS plastics. The engine speed is varied by restricting the air intake with a hardened and ground drum that can be rotated by the attached lever until stopped by the adjustment screw. Fuel mixture is firstly adjusted using the needle of the stationary spray bar and then with the air bleed screw within the body.

Needle Valve Assembly

The outer components are finely machined from brass bar while the metering needle is machined and taper ground from hardened alloy steel.  This needle seals against an internal seat and can be opened to adjust the fuel flow through a cross drilled surface jet. The fuel tubing can be led either to the integral plastics tank, or to a tank within your aircraft.

Fuel Tank

I mould this from transparent, high-impact polyamide plastics.  It is retained and sealed by an O ring.  The capacity can be reduced by cutting the tank to the lower retention ring.

Exhaust Flap

This engine speed control is supplied with the single speed carburettor option.  It is not supplied if you choose the more effective, variable speed carburettor.  The stainless steel flap is rotated on the bronze bushing to restrict the exhaust.  It will then decrease the engine power and speed sufficiently for many applications.  There is a small outlet to allow continuous, low speed operation with the flap closed.

Exhaust Manifold

The optional, machined aluminium manifold can attach a shorter or longer length of flexible tubing to lead the exhaust from the ‘plane.

Assembly

The parts are ultrasonically cleaned before assembly.  Each engine is hand started and test run just prior to dispatch.

Peter Burford, PB Engine Australia