How I tackled the connecting rod from scratch...

Hello! If you're still following this build blog, you've made it to the explanation of the connecting rod. I had been working on several other parts, still with the mindset of fitting each component to the next through a process of trial and adjustment. In this natural progression, the next part to tackle was the connecting rod.

At first, I thought it would be simple—a rectangular piece with a turned section (actually a taper), so just some straightforward lathe work followed by a bit of drilling. But here’s the thing: the piece of material I was planning to use turned out to be slightly too big.

Also, it wouldn't fit in the chuck, the internal bore is just under 1" on both of my lathes then after looking at some pictures from the http://eastgrinsteadmodelengineers.co.uk/projects-gallery/3-scale-burrell-agricultural-engine   I saw a connecting rod being produced on a CNC milling machine. 

From a round bar !!

This got me thinking what if I used my milling machine and dividing head to produce the square ends and start with a round. What size bar would I need to produce the 22.22 x 12.7mm rectangle. So there is a formula to calculate this :

Use the Pythagorean theorem:

Round Bar Minimum Diameter = Square Root(Width² + Height²)

To determine the length of the diagonal and thus calculate the diameter of a rectangle.

Alternatively go to this site pop your numbers in and it does it for you.

https://corrosionmaterials.com/technical/round-to-flat-hex-or-square-bar/

 

The answer was the moment of genius.   25.5933mm

So! here we have the possibility to start with a 1" bar give or take a gnats pecker and produce the rod. 

The order of machining was :-

• Decide on the length, need approx. 30mm to hold each end as scrap.
• Face a centre both ends
• Mark the positions of the squares (roughly with a lathe tool scratch).
• Mill the 22.22 x 12.7mm end to check the concept.
• Use a round tool to remove some material from the side of the rectangle.
• Repeat on the other end ensuring the two squares are aligned with each other. (Note : that is the reason for the right angle block in the 2nd photo, to check when re-fitted because I removed it.
• Drill the cross hole at the same setting to ensure its at 90deg to the boss.
• Replace in the Lathe and remove all the metal in the middle, and finally Taper turn the finished shape.

Above some milling was done on one end to prove the maths, this is the larger of the 2 rectangle sizes. So to check, that after milling the flats I end up with are the correct size, and that the corners were not clipped off.  Next step drill the hole and then carefully machine the middle down to the larger taper size. 

In the picture above (on the left hand side) is the big end bearing. I will leave it over length for now, as the exact length is not known yet and the position of the other component's on the boiler may mean a mm or 2 longer or shorter many be necessary.  This part is still ongoing I may at some stage in the future come back and add to this post, so even if you have seen it as I write now, please come back later and check the final completed rod.

Partially completed as of 3rd June but still on going. 

Until next time, stay safe in the workshop, make plenty of swarf, and keep building!
Stephen.