Making The Connecting Rod for the Bassett Lowke 3/4″ Scale  Model Traction Engine

The Finished Connecting Rod And Comparisons

This connecting rod by virtue of it’s small (3/4″: foot) scale is very small, about 2 1/2″ (65mm) long, yet has most if not all of the details and components of the full-sized prototype engine.

First a few photo’s of the finished connecting rod:-

The first thing to mention is that the connecting rod itself is machined from solid stainless steel bar.

The first photo, below, shows the finished connecting rod – the tiny size and fine detail are obvious:-

The next photo shows the same item but with the big-end oiler cap removed:-

Did you notice the 10BA fully shaped heads to the strap bolts with lock-nuts on top?

The next photo shows the connecting rod from my half-sized Little Samson traction engine. It’s about 3ft (1m) long and the BLTE connecting rod is about as long as the big-end bearing brasses in the photo below.

And here is the same Little Samson Connecting Rod with the Bassett Lowke Connecting Rod sitting on the Big-end Bearing:-

Finished Component Parts

Here is a photo of all the finished component parts of the Conn Rod:-

Yes, I know that the 10BA screws (yes, they should be bolts) are a bit rusty and I might make new ones. Problem is sourcing the hexagon stock to make the heads for the screws and the nuts. As far as I know this is unavailable. If I make my own from scratch, and I have a method planned to enable that to be done, then I’ll make them from stainless steel.

Drawings

The Connecting Rod drawing by Bassett Lowke:-

You can clearly see what a very simple slab-sided design this is. Here is a photo of an actual Conn Rod made to the Bassett Lowke drawings:-

The full-sized Allchin Connecting Rod looks nothing like that, of course, and here is an extract from the Allchin Book by W J Hughes:-

Also, I can show a 3″ Scale Allchin Conn Rod, at least showing the Big End:-

Compare it to one of the many, many, CAD drawings that I made to make my Conn Rod look more like the prototype:-

I tried to get as much detail and possible from the WJ Hughes drawing into my own drawing but bear in mind that my Conn Rod is half the size of the WJ Hughes drawing.

Machining The Components

The next section shows how the various components of the Connecting Rod were machined. note that not all operations are covered. There are over 75 photos taken during the machining process, far to many to show here. However, if  I have missed a point and you have a query, please do not hesitate to send me a quey vi the Contact Form, or leave a comment below.

The Machine Tools Used

Apart from the Oiler and lid, the main parts for the BLTE Conn Rod were machined on my two big machine tools, namely the Argo mill (a Bridgeport clone):-

and the big KINWA CH-450 X 1100 (11″ X 28″) lathe:-

Machining such tiny parts on big machine tools does have it’s drawbacks but once the principles are understood there are advantages. The main thing to bear in mind is to keep plenty of bulk in the stock from which the part is being machined and to avoid placing cutting-tool pressure on small machined areas.

I learned a trick or two about machining small parts from Joe Pieczynski’s YouTube Channel and also there is a whole article is a whole article dedicated to making very small parts on big machine tools on the ModelEngineeringInThailand.com website under the Gauge 1 LMS 2F Tank Locomotive section.

Machining Photos 1 – Turning the Conn Rod

In the above photo gallery you’ll see at least one benefit of having large lathe – a long-travel topslide to turn the taper on the Conn Rod.

Machining Photos 2 – Machining the Big-End Bearing

As in the full-sized prototype, the Big End Bearing is split vertically to allow for adjustment as it wears. I did this by splitting the stock in half and soldering the two halves together before machining.

In order to get the sides of the bearing at 90 deg to the split line I had to line up the split line with the milling machine bed. This wasn’t too difficult but would have been much easier if I had left the loose part of the two bearing pieces short before soldering together.

Another point to note is the use of a silver-steel ‘D’ bit to ream the big end bearing. I didn’t have the requisite 7/32″ dimeter regular reamer and found them hard to source, but for some unknown reason I did have the ‘D’ bit of the right size. This is the first time I have successfully used such a tool.

Machining Photos 3 – Milling the Strap

In the following photo gallery note how the centre of the strap is machined to size before touching the surrounding stock. The surrounding stock gives plenty of support for the drill and milling cutters and allows the central part of the strap to be machined to size.

Only then is the outer stock cut away and this is done by a slitting saw thus no machining forces are applied to the central part of the strap and distortion is prevented.

Machining Photos 4 – Drilling and Tapping

Apart from making the oiler, drilling and tapping various holes in the Conn Rod are the last machining operations.

Conclusion

That concludes the photographic description of the Bassett Lowke 3/4″ traction engine model Connecting Rod, including comparisons with the Bassett Lowke design and the Connecting Rod on the Allchin prototype, the design and manufacture.

Instead of simply copying the simplistic design portrayed on the Bassett Lowke drawing the following tasks were included in the exercise:-

• Prepare designs using CAD to incorporate where feasible as many of the details of the Allchin prototype as possible.
• Use stainless steel for the Connecting Rod to prevent corrosion
• Incorporate proper lubrication facilities to bearings

Countless CAD drawings were produced. These were updated as the manufacturing progressed to incorporate dimensional  changes as they occurred due to machining allowances thus ensuring that subsequent dimensions were valid.

Including CAD drawings, making the Conn Rod took about 4 weeks, some time of which was spent by illness and working on the crank shaft. So although, at the time, it seemed to be a very long job, at (say) 2 weeks working seven days a week, half a day at a time.