Marine Compound Engine

January 2010.


John R. Bentley 2010.

Machining the
Soleplate Casting
-  for the Stuart Compound Launch Engine  -






The bed is cast from Gunmetal



An alloy of copper and tin, it would be considered to be a bronze in North America.




Here is the accompanying view of the top of the base before machining




As shown elsewhere on these pages, this was my first step in preparing the castings




Next I roughed out the holes with the side of an endmill to remove flash




Checking the edges to see that they are parallel with the X-axis of the mill prior to scoring the centre line




Scoring the line using a small centre drill in the mill




Double-checking that the scored line went in the correct place



The position of the crankshaft and the entire upper part of the engine depend on this line for their final location.



A view of the finished crank recesses





A good sturdy casting, offering generous support for the crankshaft main bearings





A view for size reference





I accurately located the hole for the middle column at the back
(The other four will be referenced to this one)





At first, drilling these holes was a breeze...





But then I realized where the properly-located corner holes are required to be placed!





I started out by drilling undersize holes, in case any small corrections were needed. One hole was slightly off, so I used a small endmill to change the location of the entry - then drilled it out to size, leaving it correctly positioned and showing no evidence of my error.







The final part of this job was taping the holes 5/32" x 40 t.p.i. to match the columns



I used the drill press as a tapping station



After the tap has been threaded in a few turns, it is time to lock the upper nut on the drill press's stop bar to prevent the spring in the quill from trying to rip the tap out of the hole if the quill handle is released.





Then it is a simple matter to use the key in the chuck to release the tap in order that a small tap wrench can be used to continue tapping by hand with greater sensitivity.







If you are used to looking at North American Unified threads (UNC, UNF etc), the British BA threads will come as a bit of a surprise at first glance.



The Unified threads are quite like Metric threads  -  a 60° V-thread form. However their BA counterparts are at 47.5° and are quite rounded at the crests and roots.

At a casual glance they will look "worn" to someone not familiar with this type. Don't be fooled - they get a surprising grip for the depth of the threads due to the steeper angle.





Now just a quick pass with a 3/8" endmill over the top surface to prepare for marking and drilling the bearing cap stud holes





Using the Taig mill as a hand tapping station in much the same manner as with the drill press





I took a flat file and removed some of the high spots before going to the next step







I wanted to try a sanding drum on a high speed flex shaft (Dremel)







It worked very fast, but I needed a small rotary stone to get into the corners
(not done yet in this photo)







I needed to get the sloping end of the casting parallel to the mill table so I braced it out at the bottom with a brass rod







This view explains the need when milling the ends of the soleplate







Gunmetal is hard to come by in this country - I should have cut this off and saved it!







Getting closer to the final shape







Flattening the bottom prior to bringing the top down to size



..an odd method of clamping but it worked







Now - here's where it starts to get funny...


Gunmetal is an excellent bearing metal and very slippery
(that doesn't make it easy to machine - even for razor sharp endmills)







It produces a shiny surreal surface that is surprisingly "unflat"







Just look at that!  - the weirdest thing I have ever seen!



OK, I machined an identical soleplate years before - but that was by hand with a large file



- and that is exactly how this one will be completed



Note the dull flat patches where the file has already hit the high spots



At this point it is ready for polishing







Trimming the perpendicular parts with the side of the endmill







The drawings depict a more rounded curve than this at the top edges







So I have complied with the drawings







Now a quick hand rub with some rouge on a flannel cloth











The soleplate now awaits the carving of the three main bearings


BEARINGS:

Much more work was done on the engine in 2010, but the bearing job was left for another year - until Jan 2011.
If you wish to jump ahead for a look Click Here.




Making the cylinder block supporting columns


I started by turning one end of the 1/4" steel rods to accept a 2BA thread
- then making a centre hole for later finishing the column with tailstock support







Cutting the 2BA thread with a die in the Taig lathe







Preparing the other ends for 4BA threads - not according to the drawings for the Compound







Applying the No.4 BA thread







I decided to make new long 4BA nuts - 1/4" across the flats
(which I felt were more appropriate in size than the large flat 2BA nuts provided)









Tapping the hex for the 4BA column ends







The completed set of long pattern nuts







Tightening down on thin stainless steel internal shakeproof washers
(not provided or needed, but I happened to have some)







I really can't see the need of using larger nuts and I think these look more sensible



(it is interesting to note that the company suggests this smaller size in their almost identical Twin Launch engine)



After the exact position of the (crosshead slider/reversing shaft) bracket is ascertained
I may turn the columns down in a "waisted" style




Back to

Compound Launch main page

or

Castings, Materials and Fastenings

                 Soleplate (this page)

Cylinder Block

Top Covers

Bottom Cylinder Covers

Steam Chests

Crosshead Guides and Bracket

Crankshaft

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.