30cc Inline Twin 4-stroke Engine based on Westbury's Wallaby

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Eccentric

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This will be a quick update on the status of the crankshaft -- The only operations I have left are threading each end. I have not cut threads on my new lathe and don't want to learn on my crank with its many hours invested. I will practice on scrap and then finish off the crank later.



We left off on the design and placement of the counterweights

1626566309906.png


Since the pistons do move in unison the engine can be modeled as a single cylinder engine with twice the mass of one of the con rods, pistons, rings and wrist pins. This the two yellow rings in the assembly above. The formula I found is as follows: Weigh the top half of the connecting rod and add it to the weight of the piston, wrist pin and rings. Then take a percentage, say 55%, and add it to the weight of the bottom half of the rod. Place this weight in the CAD model at the center of both connecting rod journals on the crankshaft. Then adjust the weights of the counter weights to balance the entire rotating assembly.



The counterweights needed to be slightly wider than the crank webs, they could not extend toward the ends of the crank because they would interfere with the sides of the crankcase, but there is room inside the crank webs toward the conrod. I use two 4-40 cap head socket screws to secure each counter weight.

Before I set to work on the counterweights I did some cleanup and material removal on the crankshaft. I fabricated two plates that I clamped to the sides of the crankshaft to insure I did not knock the crank journals off center with further machining. I reduced the radius of the crank webs and trimmed the sides of the crank webs, all to reduce the mass at the conrod end of the crank. In hindsight I should have done this machining before I finished the main and center journals, but I was impatient and wanted to see how well the crank ran installed in the crankcase.


1626566327909.png



Then on to the counterweights.

First I rough cut some 1/4" mild steel plate.



1626566347112.png




I used the mill to drill three holes to align with three threaded holes in an arbor mounted in the lathe. I then turned the outside radius of the counter weights.




1626566364956.png



I used a cutoff wheel to slice the round disks in half to give me two counter weight blanks. I then mounted them in the mill, zeroed the Z-axis against the parallel under the blank and machined the top side as shown below.




1626566379737.png





Below is the first counterweight test fit on the crank.

1626566394724.png




I then used the following process to accurately locate the mounting screws for the counter weight where I wanted.

  • I marked each counter weight and its associated crank web so they would not get mixed up later
  • All of the following work was performed on the mill, I drilled two holes in each counterweight so they would be centered on the crank web, so the holes ended up slightly off center on the counter weight. The holes I drilled were the diameter of the drill bit used for the 4-40 tap.
  • Then I mounted the crank in the mill vise and match drilled one of the holes. through the counterweigh, into the crank web.
  • I then tapped this hole in the crank web.
  • I drilled out the one matching hole in the counterweight to a tight clearance fit and mounted the counter weight to the crankshaft with a 4-40 screw.
  • I then mounted the assembly in the mill vise again and match drilled the second hole in the crank web.
  • I tapped the second hole in the crank web
  • I mounted the counterweight in the mill vise and drilled out the second hole to 4-40 clearance size.
  • I used a 3/16" end mill to create my counter sinks for the heads of the socket head cap screws.
  • I then repeated this process for the remaining three counter weights.


Below is a close up of the mounted counterweights


1626566411501.png



The final operation I performed was drilling the two oil holes that will deliver lubrication to the large conrod end bearings from the crank center bearing.
1626566428291.png





This is the current state of the crankshaft. So far I have not screwed it up and it still turns true in the crankcase. :)

1626566445890.png
 

Eccentric

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I am going to present my design and fabrication process for the Wallaby conrod in three posts: one focused on the Computer Aided Design (CAD), one focused on the Computer Aided Machining (CAM), and one on the actual machining.

1626901338747.png


Above is the drawing for the Conrod with most of the critical dimensions, there are some minor dimensions such as fillets and radii that are not called out. These could be shown on another sheet and are defined by the choice of the end mill used.

1626901357902.png




Here are a couple views of the 3D model for the connecting rod.


1626901388715.png


The next series of pictures detail the steps I went through to create the 3D model. The dimensions for each were pulled from an initial sketch. The final drawing is a result of the final CAD model.
1626901405372.png

I first used a "revolve" feature to create the two barrels for the piston pin and the crankshaft main journal.

1626901415170.png


I added the web between the barrel sections, this will eventually be shaped as an "I" beam for max strength and min weight. The lightning cutout will be added later. This is a simple "extrusion" feature.

1626901432456.png

The "extrusion" feature on the end cap for the end cap screws to bear down on.

1626901443166.png


Above is another "extrusion" feature for the threaded end cap screws. Fillets have been added around the big end barrel. I used a "fillet" feature, but due to the complexity of the multiple surfaces that intersect, a blended surface would look better. I may go back and improve this area.

Below a "cut" cut feature gives us the side profile we need.

1626901454441.png




Below are three "cut" features: the counter sink for the head of the cap screw, the clearance hole for the cap screw and the threaded portion in the conrod.

1626901494050.png




Then finally the lighting cutout "cut" feature in the web.
1626901509816.png

Next I am thinking that I will show the steps to machine and insert the associated CAM process where it is needed.

Let me know if this is interesting, too much detail or not enough. thanks.
 

gadabout

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I am going to present my design and fabrication process for the Wallaby conrod in three posts: one focused on the Computer Aided Design (CAD), one focused on the Computer Aided Machining (CAM), and one on the actual machining.

View attachment 127757

Above is the drawing for the Conrod with most of the critical dimensions, there are some minor dimensions such as fillets and radii that are not called out. These could be shown on another sheet and are defined by the choice of the end mill used.

View attachment 127758



Here are a couple views of the 3D model for the connecting rod.


View attachment 127759

The next series of pictures detail the steps I went through to create the 3D model. The dimensions for each were pulled from an initial sketch. The final drawing is a result of the final CAD model.
View attachment 127760
I first used a "revolve" feature to create the two barrels for the piston pin and the crankshaft main journal.

View attachment 127761

I added the web between the barrel sections, this will eventually be shaped as an "I" beam for max strength and min weight. The lightning cutout will be added later. This is a simple "extrusion" feature.

View attachment 127762
The "extrusion" feature on the end cap for the end cap screws to bear down on.

View attachment 127763

Above is another "extrusion" feature for the threaded end cap screws. Fillets have been added around the big end barrel. I used a "fillet" feature, but due to the complexity of the multiple surfaces that intersect, a blended surface would look better. I may go back and improve this area.

Below a "cut" cut feature gives us the side profile we need.

View attachment 127764



Below are three "cut" features: the counter sink for the head of the cap screw, the clearance hole for the cap screw and the threaded portion in the conrod.

View attachment 127765



Then finally the lighting cutout "cut" feature in the web.
View attachment 127766
Next I am thinking that I will show the steps to machine and insert the associated CAM process where it is needed.

Let me know if this is interesting, too much detail or not enough. thanks.
Hi,
am liking the detail very much , please keep it up!!
regards
Mark
 

CFLBob

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Let me know if this is interesting, too much detail or not enough. thanks.
You know we could keep you busy writing and not building the Wallaby, right?

I have one question: what CAD program is this? Solidworks (based on the file extension)?
 

Eccentric

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Bob,

I use SolidWorks for CAD and Fusion360 for CAM.

For Gadabout it,

That is funny, I probably never would have noticed that "Westbury" is misspelled in the title of this thread. I will look into how to edit it, now it is bugging me.

Greg
 

Jasonb

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Yes keep it coming. Having done a few similar conrods it's interesting to see how others go about it. Judging by the colour of the 3D image you are going to be using bronze which can make a difference to the machining methods over an aluminium one which is mostly what I have made mine from.
 

GreenTwin

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I use SW 2011.
It is quite nice when it works correctly, which is most of the time.
I use it for work and modeling, and so I can justify the expense.
.
 

Weldsol

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Hi Eccentric not being a CAM or CNC user I'm interested how you will machine the conrod as I can't see how you will be able to do the big end bore as this will have to be done after splitting the cap otherwise you will end up with an oval bore.
I know when I did my conrod set I had start with split blocks drilled counter bored and tapped do both big & small end bores then make a jig so that I could do the profiling (material Dural)

Paul
 

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Jasonb

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Jason

6061T6 is stated on the drawing.
I missed that and was going by the colour of the part as the original used Gunmetal (bronze) castings. I thought the same was going to be used which makes it easy to soft solder the bearing cap on and machine as one.

If it is going to be aluminium then I tend to screw the two parts together, bore and then hold by the two holes to do the CNC external shaping, sacrificial brass screws can help as it does not matter if they are partly machined away. The alternative is to split the rod in two in the CAD package and machine each part separately then screw together and finally bore, I've done it that way once where shape of rod lent itself to that approach.

Also raises the question that 6061 is not a good bearing material so will really need bearing shells but drawing shows 1/2" big end hole and crankshaft is 1/2" too. If not using bearing shells I tend to use 2014 (2011 in US) as that can run straight on the shaft and is also less likely to stretch.
 

Weldsol

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Just been reading ETW's notes from ME 1962 on the Wallaby and he specified con rods in Bronze or Dural
Paul
 

Jasonb

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2014 which is the old HE15 spec is about as close as you can get to Dural. It's heavier than 6061 so could muck up the counter balance weight calcs but not as heavy as GM
 

Eccentric

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You all are correct, the conrods were intended to be made from cast bronze. I am building this engine without castings and I can't afford a chunk that big. Using aluminum with leaded bronze bushes would be ideal, but I am not ready for that level of complexity.

Solid bronze is heavy and not ideal for conrods. I have wondered about the suitability of aluminum without bronze bushes for the conrods, but Westbury seems OK with it. He states"..lighter rods machined from Duralumin (an obsolete term, originally a trade name for one of the earliest types of age hardened aluminum) may be preferable to some. In the Sealion build article he states that "duralumin or similar light alloy is a more suitable material for the job". Of course, he is a 50+ year old source.

It will be interestig to dissasemble the engine after a couple of hours running to see how the aluminum holds up. That of course depends upon: one that the engine actually runs and two that it ever makes it to two hours :)
 

Jasonb

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The 6061 won't hold up too well, there is a risk of it stretching around the holes and as I said earlier it is not that good a bearing material.

Although Dural is no longer made 2024 or 2014 both in T6 temper are a much better option and often specified on model engine drawings.They won't stretch as their about 50% stronger and as they have about 5%copper in will be OK as a bearing surface. Really any of the 2000 series alloys will be OK as they are similar to Dural, the 6000 series are not really similar. I've used 2014 as that's readily available in the UK for Glow, Diesel, 2-stroke petrol and 4-stroke petrol model engines all without issue
 

Eccentric

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I can readily get 2024 T3/T4 (not T6) at my local metal distributor, but I thought 2024 was really soft and malleable. I use some thin sheets of it on the lathe to protect a work piece from the chuck jaws, but I don't know what temper it is. I would have to order T6 online--totally doable.
 

Eccentric

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Conrod order of operations

  • square up the blank 1" X 3.5" X .438"
  • machine cap bottom shape on CNC
  • machine 3/16" counter bore on bottom of cap
  • Center drill two holes
  • drill 2 each .089 holes on bottom .5" deep
  • follow up with and drill both .113", but only .125" deep
  • tap the 4-40 holes using a spring tap guide in the mill
  • machine off the end cap
  • drill out holes in cap to .113 and install end cap
  • drill and ream the .501" main bearing hole
  • drill and ream the .251" piston pin hole
  • Mount to CNC machining fixture
  • Go to the CNC for contour machining both sides

1626983959024.png

Squared up blanks. The thickness is the only critical dimension.



Now I need to machine the cap bottom. Solidworks allows for "alternate configurations" which is a variant of a part with differences. I use it to crate simplified models to ease the generation of the CNC tool paths. Below I create a shadow solid that isolates the bottom of the rod so I can machine the curved bottom feature. I overlay the desired shape over the part and create an alternative configuration for the rod.
1626983975772.png

Sketch creating model for machining the conrod end

Below is the finished model, notice the small cut between the main body of the conrod and the end section. this allows me to isolate just this end part to target the machining and stay clear of the vise.

The CAD tool tells me the smallest radius I need to machine is .133", so I can use a 1/4" end mill to create this feature

1626983994732.png




Time to export the model for the CAM work. I use fusion360 and highly recommend it. It is free for us home hobbyists and does everything I need it to. It has a nice user interface, but getting all of the settings needed to create a tool path can be frustrating. It has taken me some trial and error to get the CAM to do what I want it to. Fortunately it has a "template" feature that allows you to save a setup for use in later parts. So once you get it dialed in, you are good to go.

I export an IGES file and import it into Fusion 360. I select "Manufacture".

1626984013427.png


This green highlighted geometry selection tells the cutter to only cut the area of interest and not interfere with the clamping on the rear portion of the work piece.

1626984026469.png




Contour Parameters

  • 1/4" flat end mill
  • 8000 RPM at 13 IPM
  • Step Down of .1"
  • Climb milling for smooth finished surface
  • 2 Passes, the first will leave .020" of radial material, the second will be a finishing pass.
The tool path is simulated to verify it is doing what we want, then exported to a Gcode file. This I put on a USB drive and carry out to the workshop and load onto my CNC router. I use LinuxCNC and am quite happy with it. So much of this software stuff it is finding something that works, learn its idiosyncrasies and making it work for you.

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Machining


1626984050331.png

Finished end cap feature. One down, One to go.


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Over to the mill, establish the center of the holes with an edge finder.




1626984074439.png

Counter bore 3/16" and center drill


1626984093289.png

Drill the two hole sizes. I like to drill the clearance hole in the same setup as the tapped hole to insure they are truly concentric.



Next I will mount the conrod in the mill horizontally and mill off the end cap.
 
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Weldsol

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I like to tap the bar prior to slitting as it is easier to hold plus doing the tapping at the same setting as the drilling (using a centre in the drill chuck to support the tap holder ) ensures the threads are true to the cap

Paul
 

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