Pressure turning the smokebox tube plate to final dimensions ready for marking out and drilling.
Each hole is centre punch to locate the sheet metal drill centre and marked out to aid drilling.
The holes are drilled from the centre outwards to ensure maxiumum material condition around each hole;
Sheet metal drill profile can be seen in this photo, The centre of the drill locates in the centre pop mark and the drill cuts on the outside like a trepanning tool which cuts perfect round holes in thin materials without leaving large burrs.
