by Bradypus Nick
General view setup for punching the top row of rivets on the tank. The short silver bar lying on the tank is the punch.
The fundamental requirement for a punch and die holder is to locate them concentrically in the x and y planes. Using hinges means that the holder can be raised for changing the die and used as a clamp to hold the work.
Looking at a pair of cheapo 6in. gate hinges, there was plenty of slop in the vertical direction but very little fore and aft movement. A light tap with a hammer closed up the clearance around the pin a bit more.
The base is an offcut of 5mm thick steel. Attach the hinges as shown (in my case with a couple of plug welds) so that they overlap just behind the end hole. Insert a packer about 3mm thick under the overlap, big enough to project beyond the ends of the hinges. Kink the hinges at the pin end so they overlap flat and true with the packer in place. Mark out through the end holes, drill through the packer and base and use suitable setscrews to hold the hinges firmly.
Now mark out the centre where the hinges intersect and drill through everything about 6mm dia. Open out either with a reamer or the next +0.5mm size drill (the poor man's way of getting reasonably accurate size holes) and remove the screws and packer.
The above is a long winded way of describing how I got the holes in the hinges and base to line up adequately!
Cross section through.
The punch has a steel tophat guide, the OD of which is a tightish fit in the holes in the hinges. My punches are 5mm dia silver steel, with a turned pip on the end to suit the rivet size. The length of the pip defines the projection of the rivet. The diameter is about 0.5mm less than the OD. I haven't hardened the punches but haven't had any trouble yet after punching tanks and cabs for 2 locos. Most of my body plates are 0.9mm brass (20SWG ) but some bits have been punched in 0.5mm tinplate, 1mm styrene and 0.5mm cardboard.
The die is another steel tophat which fits snugly in the baseplate. The diameter of the hole defines the diameter of the rivet but the depth is made with a generous clearance. The reduced upper diameter of the die is equal to the minimum distance between rivets (A) and the height is such that the finished rivet heads don't touch the baseplate. For quick (freelance?) jobs there is no need to mark out, simply bump the edge of the previous rivet against the shoulder and bonk away!
An important feature of the punch profile is the flat face 'B' which closes the plate against the die after the rivet is formed. This forms a crisp semicircle and prevents 'molehilling' and keeps the plate flat.
The final holes in the base are the ones at the corners to hold one end of the steel straightedge which aligns the row of rivets with the edge of the part. Fore and aft errors aren't too noticeable but one side to side misalignment in a row sticks out like a sore thumb.
Punching the vertical row of water baffle rivets
Usually I print out paper templates for parts with the rivets marked with bulleyses, as shown in the photos. The target circle is just smaller than the hole in the puch guide and is lined up by Mk1 eyeball. The template is stuck to the brass with clear varnish so it can easily be stripped off afterwards. An alternative would be to scribe the positions and have a separare punch with a pointed end to feel the centre, but that would be proper engineering, and I try to avoid that!
Having got a setup which accepts different tooling makes it easy to change the rivet size. Most of the rivet heads on the Isle of Man locos are 1 1/4in OD , with a few 1 1/2in OD in odd areas (not only do I count, I measure too!). For the real purist, imperial rivet heads have an almost hemispherical head. Metric rivets heads are truncated some way short of the equator.
Another possible use is punching bolt heads. Drill and tap a die to accept a socket head grubscrew, clean the end off flat and use a punch with a cylindrical pip. I got a few passable ones to use on a mud hole cover, but the diameter of the punch is a bit critical to achieve a clean hexagon. I need to experiment further because not all the lumps on loco bodies are rivets. Cabs are bolted to tanks and smokeboxes are bolted to frames.