As can be seen a frame extension (orange) is not simply a flat piece of steel plate. It is profiled to fit beneath the cylinder block but in front of that it has to be cut away to clear the leading pony truck wheels before being shaped to fit the front buffer in due course. With the cylinders upside down on the floor the LH extension is seen in position having been aligned and fitted to the block ready for all bolt holes to be reamed ready for the securing bolts to be made.

This view of the front of the extension frame shows how it is not part of a flat plate but has also to  be tapered  in thickness from the front of the block and joggled inwards to increase clearance for the pony truck wheels. This part of the work was done by our supplier. Apart from not wasting steel the reduction in thickness helps to keep weight distribution under control.

At the back of the cylinder block the frame extension is deeper where it will be bolted to the main frame plate in due course. In the photo. this section is still full thickness but this has to be reduced in order to provide clearance for 4561’s leading sandbox which will be fitted between the frames. This machining has to be finished in such a way as to minimise the possibility of creating any stress raisers that could cause frame cracking under heavy shunting impacts. The old extension frames are shown in the next photo. where it can be seen how this reduction in thickness has to be  blended into the main part of the extension frame plate.

The old plates do show evidence of past cracking and bending under shunting impacts and the new plates are being thickened in this area to minimise this possibility in the future.

This photo shows the back of the other, R.H. frame extension set up on our large milling machine for the thickness to be gradually reduced in a series of steps using a slab mill. Once this has been done down to finished size the plate will be laid down flat on the machine table to be profiled to removed the cut edges and provide a smooth transition from the thinner section to full thickness.