John R. Bentley 2010.

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

This is a two-throw crankshaft having a 90-degree separation with double eccentrics attached 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.
An unmachined built-up crankshaft was supplied instead of a forged one but the end 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







Locating an approximate center on the other end by rotating slowly







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







Centre drilling







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







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







Facing the end and turning the last bit of shaft (previously held 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



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



I needed 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 turning a separate crankpin
The large hole will clamp on to the end of the straight shaft section



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







Splitting the workpiece to make 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 below the slit for the clamping screw







The completed fixtures







Although this picture was taken after completion, you get the idea!







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







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







Here's the proof of that last statement



(everything is flying around eccentrically but the crankpin is rotating on the lathe axis)



Ready to face the inner sides of the webs







One crankpin finished







Milling the edges of a 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.