Connecting Rod

The connecting rod has one critical parameter. The 2 holes (one for the crank pin, the other for the cross head) need to be parallel in both axis. The distance between the holes is also important but not critical if you are building the components in the same order as described here. I planned to machine both the critical holes using a boring tool on the face plate, to keep things as parallel as possible.

First the con-rod was mounted in the 4-jaw chuck as shown below and set to run true. Then the locating spigot at the far end was faced off and centre drilled so the tail stock could be added for support. Then the main column was turned, along with the top and bottom faces of the foot. The outer diameter of the foot was tidied but left oversize. I used the top slide set over by 1 degree to get a tapered shaft but you can set the tail stock over if you prefer.

Turning the foot of the con-rod

Turning the column of the con-rod

Before the con-rod was removed I marked the base of the foot for the 2 bolt holes.

Next the piece of bronze to make the big-end bearing was cut in half and the 2 halves mounted together in the 4-jaw chuck. An indent to match the spigot on the base of the con-rod was drilled in the middle of the exposed face and then the front surface was machined to take one of the bronze parts to the final thickness. A 2.5mm hole was then drilled right through the 2 parts. Then the 2 parts were reversed in the chuck and the other half was machined to its final thickness.

Facing and drilling the indent in the big-end bearing

Facing the second half to thickness

The 3 parts were now assembled together. I used the shank of a 2.5mm drill to hold the 2 halves of the big-end together and the 2 bolt holes were drilled right through the set of parts so that they could be bolted like so.

The assembled con-rod and big-end

I planned to machine the 2 critical holes on the face plate but the top of the can-rod wouldn't fit through the gap in the bed. It was also about 6mm oversize so I placed it in the vertical slide and machined away about 5mm of this excess material.

Thinning down the top of the con-rod.

To bore 2 parallel holes in the con-rod I mounted the assembly on a piece of gauge plate which was then bolted to the face plate. The gauge plate could be moved around on the face plate whilst still maintaining a common reference, so that both holes could be bored parallel despite re-centring the workpiece. I supported one end of the con-rod in a small block of aluminium using the 2BA bearing bolts. In the other end I drilled a single hole in the top of the con-rod and supported this with another aluminium block. This hole would later be machined out when the top of the con-rod had the slot cut into it, to take the cross-head.

The images below show how the holes were drilled and bored. I drilled the cross-head bolt hole first and faced the side of the top of the con-rod to thickness. I had to use a reamer for the cross-head bolt hole as this was too small for the boring tool. The big end bearing was made using the boring tool and again the bearing was faced to thickness and also had the shoulder machined. In addition to the drawing I machined a slight recess on the inside of the big-end bearing. This was to allow for the small fillet of silver-solder that was at both ends of the crank-pin.

Facing and reaming the cross-head end of the con-rod

Boring, facing and machining the big-end bearing.

 

To complete the con-rod the part was reversed on the gauge plate and supported on suitable packaging. Matching profiles were then cut on this second side.

Next the con-rod was returned to the 4-jaw chuck and the bevel on the side faces was turned. The through hole drilled in the big-end bearing meant that it ran quite true from the outset and require no other set-up.

Shaping the side faces

When the con-rod was clamped on the crank-pin for the first time it was a little too tight although not very far out. I had to rub the side shoulder on light emery paper to make it fit between the webs. Then I found that progressive tightening of the big-end bolts whilst working the 2 assemblies together with Brasso in the joint helped it bed-in in no time. Once a smooth turning fit was achieved the Brasso was cleaned out and replaced with oil.

Bedding in the bearing using Brasso (and elbow grease)

The final operations on the con-rod were to shape the cross-head end to its final size. First some filing buttons of the correct diameter were made and put in place. Then in the vertical slide, the 2 sides of the cross-head end were milled right up to the filling buttons. This ensured that the 2 sides were equidistant from the centre hole.

Milling the thickness of the cross-head end of the con-rod

Then in the vice the end was filed to shape again using the filing buttons as a guide.

Final end profile

The assembly was placed back on the vertical slide to machine out the forked end profile. This was done by first drilling a hole of the correct diameter and then opening out the hole using a milling cutter. In practice I actually found that a milling cutter was better for drilling the hole because a normal drill tended to wander off line. This problem was created because this hole intersected with the cross-head bolt hole drilled earlier.

Drilling/milling the inside profile of the forked end.

The finished Con-Rod and Big-End bearing.

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