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


Castings, Materials and Fastenings


Cylinder Block

Top Cylinder Covers

Lower Cylinder Covers

Steam Chests

Crosshead Guides and Bracket

                 Crankshaft (this page)



Connecting Rods and Crossheads

Main Bearings


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


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(c) John R. Bentley 2010.