Marine Compound Engine

March 2010.


John R. Bentley 2010.

Machining the
Crankshaft, Eccentrics and Flywheel
-  for the Stuart Compound Launch Engine  -




This is a two-throw crankshaft with 90-degree separation and double eccentrics at each end



Here is the basic forging as supplied with the castings set



I machined an identical crank for a Stuart Twin Launch 20 years ago.
It had a built-up crankshaft instead of a forged one but the final result looks identical to me.



This one consists of two forgings fused together at 90 degrees at the centre bearing journal







The entire assembly (including the fettled areas) is a uniform colour
I assume that indicates post heat treatment



That should relieve any stresses produced during the fusion of the two parts



My first step was to place one end in a 3-jaw chuck







Here I am trying to locate an approximate center on the other end by rotating slowly







Once the centre dimple was formed I punched it to mark it better







Centre drilling







Back in the lathe to rough-turn one of the main shaft sections







Flipped end-for-end and supported in the steady rest







Facing the end and turning the last bit of shaft (originally in the chuck jaws)







Moved back - "close up" in the steady rest arms







Drilling a centre hole in this end







After rough-turning the other main shaft section







Now I'm ready to go to work...







Turning one set of web ends







Face turning the outside of the nearest web







Starting on the centre journal







A view part way through the job



(it's an easy matter to take the work out of the lathe
for close inspection or photos when turning between two centres)



It is necessary to make turning fixtures in order to machine the crankpins
(and the other ends of the webs)



The two small holes take the lathe centres - each located for a separate crankpin
The large hole will clamp the end of the main shaft of the crank



After milling a large notch in the periphery, I cut a long slit from the outside to the shaft hole







Splitting the unit and making two



This is Naval Brass from the discarded propeller shaft of an old lobster fishing boat



Now it is just a matter of adding clamping screws at the top







Threading for the clamping screw







The completed fixtures







Although this shot was taken later, you get the idea!







I put a rubber band to protect the fixture from the lathe carrier (dog)







Setup so the crankpin (to the right) is along the lathe axis







Here's the proof of that last statement







Ready to face the inner sides of the webs







One crankpin finished







Milling the edges of the web in the Taig mill







The Lomo stereo microscope assisting in finishing the journals







A closer view







Working on the filleted area with a round nose tool similar to a parting tool.







The completed shaft







All this stuff must go together somehow...






Compound Launch main page

or

Castings, Materials and Fastenings

Soleplate

Cylinder Block

Top Cylinder Covers

Lower Cylinder Covers

Steam Chests

Crosshead Guides and Bracket

                 Crankshaft (this page)

Eccentrics

Flywheel

Connecting Rods and Crossheads

Main Bearings

Pistons

Fittings: Oil Cups

Fittings: Drain Cocks

Fittings: Exchange Pipe, Flanges and Glands

Stephenson Link Reversing Gear (5 pages)

Completing and Erecting the Compound Launch Engine

or

Return to main website home page

ModelEngines.info




(c) John R. Bentley 2010.