[TheForge] slitting chisel / drift

[Andrew Vida]

Dave Mudge wrote:

> 2. This is a railing. Imagine the afore mentioned 3/4" x 1-1/2" flat
> bar as a wave running the length of a section of railing.
> The 1/2" square pickets will pierce the wave on 6" centers.
> The holes to be slit & drifted to size will have to be on an angle
> that would be on tangent with the straight pickets and the
> curves of the wave. Piercing the bar on the flat would be one thing,
> but piercing it on an angle is quite another.
> Any advice on how to do this would be appreciated.

	Well, I cannot say how the old timey smiths did it, but it must have 
been pretty virtuous stuff.  In modern terms I would say a fixture would 
be in order to hold the rail section fast and provide stern guidance for 
the punch.    The movement of the rail  during punching has to be 
seriously constrained.  You could build a jig where the die swings into 
place only at the time of punching operations.  This way you can use a 
torch for localized heating.  The punch has to have a tooth on the high 
side such as what follows.  This way it grabs into the metal and shears 
it rather than wedging it away, which will probably result is flow along 
the top surface.  The tooth at the top and the edge at the bottom should 
make contact the former just prior to the latter.  In fact, concave 
cutting edges all around would be a good idea, but perhaps not necessary 
and extra work.

Lessee...



             |    |
     \   \   |    | <--- punch
      \   \  |    |
       \   \ |/\  |
        \   \   \ |
         \   \   \|
          \   \   |
           \   \  |
            \   \ *
   rail----> \   \

	

Such a fixture will be a lot of work, so unless you are making a 
shitload of this railing or it is herniatingly expensive, it is probably 
not worth the trouble.  It will, however, do the job perfectly if you 
design everything properly.

The only other thing I can think of is to punch the holes normal to the 
surface and then rotate the punch to the desired angle and trim things 
up with tooling.  You could make a monkey tool to get the metal that has 
flowed out of place in the bending to go back into its proper location.

Imagine you punch or drift normal to the surface and then rotate the 
tool to the desired angle.  Metal will flow such that you will get gaps 
between the pickets and the rails in places you may not want them.  But 
if you make the monkey tool with the profile of the rail, you can place 
it on the rail and strike from the other side with hammer and punch to 
drive the metal back into place.




  \   \ Top of rail
   \   \
    \   \
     \   \
      \   \
       \   \
        \   \
         \   |
          \  |
           \ | <--hole
            \| __<---gap from bending
             | | \
               |  \
               |   \
               |_<----iron displaced from bending
                 \   \
                  \   \
                   \   \
                    \   \



  \   \ Top of rail
   \   \__________.
    \   \         |
     \   \        |
      \   \       |
       \   \      |
        \   \     |
         \   | |  |
          \  | |  |
           \ | |  |
            \| |X |<-- monkey tool
             | | \|
             | |  \   'X' = gap
             | |   \
   Pilot---> | |_   \
             | |^\   \
             | || \   \
                |  \   \
                |   \   \
                |_____________Drive metal up with punch
                               to fill gap

No idea if any of this fits your bill.  FWIW.

	-Andy