Lathe – concentricity

One of the wonderful features of the lathe is that any number of cuts of any diameter, all done without moving the workpiece in the chuck, will all be concentric. Furthermore other operations such as drilling a hole will also be concentric.

There are three examples of this that are very common and are totally dependent upon this feature. The first is turning a wheel and drilling a hole in its center so the outside of the wheel will be concentric with the axle. The second is fitting a piston rod to a piston. The third is drilling a hole in a cylinder cover so it is concentric with the cylinder.


Concentricity – wheel and axle

A wheel is often made from a cast iron casting. In any case the wheel usually has either a flat back or a back with a recess in it. Either way this back does not have to match the edge or axle perfectly. The casting is held using any convenient method – probably a three jaw chuck.

The wheel is now turned round. If the back is flat it can be held on a faceplate. It is usually necessary to shape the edge of the wheel right round to the back so a space is needed between the wheel and the faceplate. This can be done by using double sided tape to hold pieces of metal acting as spacers to the faceplate. The wheel is then clamped onto the face plate taking advantage of the holes between the spokes to place bolts to clamp it.

If the wheel is recessed at the back this can be used to hold the wheel in a three jaw chuck.

The profile need on the edge of the wheel can now be turned. The hole for the axle can be center drilled, drilled and reamed. Doing this without moving the wheel will mean that the circumference of the rim of the wheel and the bore will be concentric.


Concentricity – piston and piston rod

Making a piston the right size to fit a cylinder is covered elsewhere. Here we are looking at the problem of getting the piston rod concentric with the outside of the piston.

Firstly as soon as the outside of the piston has been turned to just fit the cylinder it must be left in the chuck for the next part.

One way of doing this is by making the piston rod so it has a shoulder on it to fit on the surface of the piston. The end of tthis narrow part is fitted with a thread. The piston is center drilled, drill for tapping the thread all the way through, drilled for reaming the guiding bit, reamed for the guiding bit and tapped.

The piston rod which has already been machined as required, is held in a drill chuck in the tailstock and is screwed into the piston but turning the chuck holding the piston whilst feeding the piston rod in.


Alternatively the piston is machined oversize. It is faced on one surface. The hole is centered, drilled reamed etc as above. The piston rod is fitted as above. The assembly is removed. It is turned round and held by the piston rod. This can be done using either a collet in a collet chuck, a four-jaw chuck with the rod precisely centered or a split bush. The piston is now machined to the diameter to fit the cylinder. (See Model Locomotive Building LBSC)


Concentricity – cylinder cover with hole for piston rod

Cylinder covers are often made from bronze castings. These usually have a spigot for holding the cover during machining. The cover not only holds the steam in the cylinder but also guides the piston rod holding it straight in the cylinder. This means the cover must be aligned properly in the cylinder. This is done by machine a projection on it that is a perfect fit in the cylinder. At the same time as this is machined it is necessary to machine the hole for the piston rod. This is done by center drilling the cover, drilling a hole suitable for reaming to the necessary size.

When this has been done the cover is turned round and the other side of the hole is drilled and tapped to take the gland which hold the seal in. Notice that this cannot be expected to be perfectly aligned simply by means of a screw thread so it might have to be slightly bigger than the diameter of the piston rod.



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