My next engine build will be a derivative of Edgar Westbury’s Wallaby, first designed for a model train, then updated for use in a model hydroplane. The engine is an overhead valve, water cooled, 30cc, inline twin cylinder 4-stroke. My version will use no castings.
I am a huge fan of Edgar Westbury’s work as a model engine designer. He was very prolific producing designs optimized for construction by the home machinist with minimal tools, typically a small lathe and a drilling machine. His first engine design was published in the 1920’s, a single cylinder 2-stroke for a 13 foot wing span model airplane. He joined Model Engineer magazine in the early 1930’s and went on to publish many wonderful engine construction articles as well as several books on machining.
This is a video of a classic Wallaby:
https://www.youtube.com/embed/BpmBdPD8euY?wmode=opaque&start=0
Construction articles for the Wallaby are available from the “Model Engineer” magazine and have been in the public domain for decades. This can be found in the download section
The engine has a bore of 1 inch and a stroke of 1-1/8 inches. It has a built in oil pump to provide pressure fed lubrication to the tappets, crank shaft center bush and the connecting rod big ends. I have redesigned the Wallaby to be machined from raw stock, using no castings, and I will be using ball bearings on the crankshaft, camshaft and timing gears.
I use SolidWorks for my computer aided design work and Fusion360 for tool path generation. I first fabricated 3D printed models before committing myself to cutting metal.
The original Wallaby had 5 main castings: the Body Casting, Sump, Cylinder Head, Cylinder Head Plate and Timing Cover.
I have split the Body Casting into three machined parts, the crankcase, rear timing plate and the block. I learned the technique of separating the crankcase and the block from Terry Mayhugh’s build of Ron Colona’s Offy. When designing a casting, a good engineer will strive to incorporate as many features as possible. This will reduce total part count and reduce cost of a mass production, but results in a very complex part, making it difficult to machine a one off. The Cylinder head has many internal passages that make it a good candidate for a casting, but difficult to machine, so it will likewise be made as two parts to be bonded later with high temperature structural adhesive.
Other design changes include the use of bearing housings for the crankshaft end bearings and the camshaft bearings. This will allow me to fabricate these precision components on the lathe independently from the machining of the crankcase.
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My first Goal – Machining the Large components:
- Sump
- Crankcase
- Block
- Timing Back Plate
- Crankshaft Main Bearing Housings
- Dummy Crankshaft
- Camshaft Bearing Housings
- Dummy Camshaft
Assembly model of only the large components
Another View
3D Printed Mockup of the engine
Sump
I have decided to machine the sump first. It is the second largest part and has many of the same shapes that will need to be machined into the crankcase, so will provide good practice developing the tool paths, tool selection as well as the speeds and feeds. The sump is less critical dimensionally than the crankcase. The crankshaft center bearing is held exclusively in the crankcase and the sump only holds the two bearing housings for the crank case, but will rely on the crankcase for alignment. There will be two locating pins where the sump and the crankcase mate to provide positive, repeatable alignment between the two and the crankshaft bearing housing mating surfaces will be machined from the same setup to provide the best alignment.
Datums for the sump, Top, Right side and Front
Machining Steps
- Flatten top and Right side of the work piece and insure they are perpendicular
- Mount the work piece in the vise with Top against the vise face and the Right side down against parallels – machine left side at least .05″ over dimension.
- Mount work piece in vise with the Top against the vise face and the Front down and machine the back side at least .05″ over dimension.
- Mount in the vise with the right side against the vise face and the top down. Machine the bottom square at least .05″ over dimension.
- Take to drill press and drill a 1/2″ hole from top, center of sump measured from Right side +.02″, from Front +.02″, and 1.5″ deep (which is more than 1/8″ from inside bottom of sump). this hole is the starting point for the end mill to machine the inside.
- Mount in the vise with the Right side against the vise face and the top up.
- Machine the inside of the sump. See detail below.
- Center drill all holes
- Drill all holes to depth including the two alignment holes. The alignment holes reamed to 1/8″ interference fit.
- Flip part over with Top against parallels, and the Right side against the vise face.
- Machine minimum clearance for crankcase bolts, leaving the majority of the material to be removed later.
- Lightly sand the top surface of the sump on a flat surface with 180 grit, then 320 grit and finally 600 grit to create a flat clean surface-don’t get carried away, enough to just remove tooling marks.
After these steps, the sump is compete for now and is ready to be attached to the crankcase for the machining of the front and back. 3D modeling programs allow “configurations” of models to be created that are derivative or different than the base part. I use this feature to create modified versions of the model to optimize the machining, creating both models specifically modeled for a specific machining operations as well as special configurations of the stock material. This is nice because as design changes are made to the base part, they are carried forward into the derived parts.
Detail of sump machining operations – A special 3D model is created for the machining of the top of the sump, it has the following modifications:
- All features are removed from the Front and Back.
- .020″ of material has been added to Front and Right side.
- Corner fillets removed.
- Oil drain hole in bottom removed.
A special model is created for the raw stock, it has the following characteristics:
- square block, .020″ over sized on the Front, Back, Left and Right sides.
- Has a 1/2″ hole 1.5″ deep in the middle of the sump for the 1/4″ end mill that will be used to remove the majority of the material to start in. End mills are great at cutting on their side, but not so good at machining down. By machining from the side of the part, or from a predrilled hole on an interior feature, the pocket machining operation can remove more material, quicker, with less stress on the cutter.
Work piece locked in the vise ready to machine the inside of the sump. The 1/2″ hole is used for an entry point for the end mill.
Modified model for machining the inside of the Sump. Compare to sump model above.
Finished inside machining of the sump
Finished bottom machining of the Sump. The sump can now be bolted to the crankcase for further machining.
Lessons learned:
- I added .020″ of stock to the top of the model that was machined off during the first horizontal milling operation. I then switched tools and touched off on the top of the model and my tool path was then .020″ too deep. I noticed quickly, but there is an area on the back where the main bearing mount is .020″ too deep.
- When machining the bottom I had problems with the work piece remaining securely clamped in the vise. It was moving during machining. I need to experiment, it may of been due to the fact that the work piece was hollow between the vise jaws, it might have been I just didn’t have the vise tight enough, it might have been I did not have a three point clamping points, or I did not have enough of the work piece in the vise.
- Should have used a smaller step over when using the 1/4″ ball end mill on the main bearing housing surfaces.
Things that worked well:
- I used a two flute 1/4″ flat end mill at 8,000 RPM spindle speed, coolant, 15 ipm and .050″ depth of cut. I was happy with the rate of material removal and the finish.
- I did not use a ball end mill on the inside of the sump. My thinking was no one would see the inside, so smooth surfaces were not worth the machining time. However, the surface turned out acceptably well with just with the 1/4″ flat end mill.
- When cutting deeper than the cutter flute length (.75″ in my case), I step the material out .010″ in the model to prevent the sides of the tool rubbing. This effect can be seen in the deep channels cut in the sump bottom.