Machining the Connecting Rods

Two Connecting Rod forgings

Connecting Rods

The connecting rods are supplied as two brass forgings, and require considerable machining before completion.

I started off,by filing off any high points, then marking centre lines along the sides and the ends of the rods.

Carefully centre punching the ends, ready to centre drill for mounting between centres.

Centre Drilling the Con Rod

Centre Drilling the Connecting Rods

The photo shows a centre drill being used to, centre the rods.

The centre drill shown here is a Slocombe or Dormer drill and provides a much truer centre than just drilling with a standard drill, also easier to locate in the pre punched marking spot.

Make the hole deep enough to take the Lathe centre points, they don't need to be too deep, but there is plenty of leeway, so don't worry too much about depth.

Con Rod on the Lathe, Between Centres

Mounting the Con Rod Between Centres

If you have never turned an item between centres before, don't be too put off, in fact it is easier than it looks.

It also has the advantage of allowing you to remove and replace the work piece, with the work piece location, remaining accurate.

This image shows the work piece without the drive dog fitted, the work piece has just been mounted to check that everything is OK.

Also checking to see the chosen lathe tool will fit into the space with out snagging on anything.

Better to check at this stage, rather than when everything is whizzing around.

Faceplate Drive Dog and Drive Pin

Con Rod Between Centres showing the Drive Dog

The Drive Dog, (in black) clamps onto the work piece , then rests on the drive pin (in this case a Machine screw), allowing the lathe to rotate the job.

I sometimes place a small elastic band over the Drive Dog and the drive pin, just to hold the two together, saves a clonk, if the pin and the dog are out of contact, when the lathe is started.

Again turn the job on the lathe plate to check that everything can spin freely, without catching on tool, toolpost or carriage.

Machining the Con Rod

Here the machining process has begun, the spigot and the outside of the big end have been turned.

Facing the inside of the Con Rod

Here I've turned the con rod around, and gripped it in the chuck, I only used the chuck as I had put it back on for another job, you could still turn the work piece between centres to machine this part.

Turning the Stem of the Connecting Rod

You can clean up the connecting rod stem with a hand file, or even leave it as is, if you wish.

I wanted it to look a bit better so I have chosen to turn the stem to shape, so that I can polish it later.

So I'm using the top slide of the lathe, set to the angle of the stem, just to remove enough metal to clean it up, and remove any pits or blemishes.

Turning the Connecting Rod Stem

I'm using a parting tool to machine the work piece, note the rounded tip.

The rounded tip gives nice stress free rounded corners, which I like for several reasons.

It ensures stress free corners.
It is easier to polish.
It looks good.

If you take gentle cuts, there is no need to use the rotating steady, to support the small end, but use it if you think it is more secure.

The Con Rod Turned and the Faces Flycut

The Con Rod well underway now.

The faces have been cut with a Flycutter, just skimmed to get a flat surface, to ease marking out.

Marking Out, for the Big End Bolt Holes

Using a Digital Height Gauge on a Marble surface table, with the work piece supported on a V Block, laying on its side.

This can be done with a standard Scribing Block, but that would take a bit longer.

A thick piece of float glass makes a remarkably accurate surface table. This could also be done using a flat piece of wood.

Try to get the holes as central as possible, check by turning the work piece over and seeing if the scribe lines match up.

Big End Bolt Holes Marked and ready to Drill

Finishing the Marking Out

Measuring from the edge of the con rod mark out the centre lines, gently pop with a Centre Punch, and their ready for drilling.