[TheForge] switchplate

[Andy Vida]

Jerry Frost wrote:
> 
> Really nice George, not that I'm surprised.

> This is exactly the kind of project where doing it all by hand can put you
> behind the curve. You already use all kinds of dies to mass produce
> elements, textures and such, why not use dies to do the tedious precision
> work as well? It'll require a higher degree of precision than spring dies
> can but it's well within the range of alignment pins.

	Pick up an old die shoe that still serviceable and build dies
	for it.
> 
> I'm not suggesting you try stamping the entire part
> of course, just the screw and switch holes in the blank.

	That would certainly make sense.


> To prevent distortion of the blank part in the die we had a third (I'll call
> it the "sandwiched" plate as I don't recall the names for most of these
> things anymore <sigh>) matching steel plate and a piece of hard rubber sheet
> attached to the top plate. As the die closes, the first thing to make
> contact is the sandwiched plate, the press (or treadle hammer, etc.) drives
> the top plate down, compressing the rubber, extending the punches into and
> through the blank part.When the press, hammer, etc. retracts heavy springs
> raise the top plate, the rubber rebounds and pulls the punches back through
> the sandwiched plate and the part drops free without jamming.

	That's called a "stripper".

> The punches didn't make contact at the same time, they were all slightly
> different lengths so the press was actually only punching one or two holes
> at a time.

	This is standard practice for multiple cavity dies.  Machine
	cycle is going to be what it's going to be, so you may as well
	make life easier on the dies.

> The two screw holes can go at once but the large rectangular
> switch hole would be a bear all by itself, possibly impossible in a single
> shot under a treadle hammer.
> 
> If however, the rectangular switch punch is ground at an angle so it first
> makes contact at one corner then progresses through the blank like a shear
> rather than a punch, it'll go fine. It's not a sharp angle though or it'll
> distort the blank, the difference between first contact and final punch
> through is only 2-3 x the material thickness.

	Also, you may want to have the punching begin at the two ends of
	the slot, working toward the middle.

	Punching like this will require a fairly high tolerance fit between
	the die and the punch.  There is a formula for determining this
	clearance based upon material thickness, but I don't have it handy.
	For something like the main hole in a switchplate in, say, 16 ga
	sheet, I would feel comfortable with 0.005" clearance.  This is an
	easy tolerance to hold by hand and eye and should produce a good
	result.  It will also be a little more forgiving in setup, though you
	do not want to misalign the punch with the die in any event.  I did
	it just once on the 75T hydraulic and didn't catch it.  Square punch,
	two pieces of which departed the scene, one into my upper arm and
	the other we both heard richochet off the wall at the far end of the
	shop.  The noise shrapnel makes when it flies within a few inches of
	your ear is one you will not soon forget.  It's just eerie.