Measurement of Forging Plate Movement

Something like a pair of internal clipers with a stiff pivot point so it will stay at the mininimum. Some sort of holding jig tacked onto one leg would be a good adition.
But I quite like the ball of foil, simple and effective.

Del

I'm with Del on this. Your ball of foil is simple and repeatable.

Milo

We used to use lead shot to check the parallelism of our laminating presses. One in the middle and one at each corner.

Sounds like aluminum foil is as good.

The combination of Del's calipers and your crumpled aluminum might work. Sandwich the aluminum foil between the calipers, such that when the press goes down it crumples the foil. Because this is a permanent deformation, there will be minimal bounce back (coefficent of restitution) so the measurement can be accurate. Instead of relying on a tight pivot, the calipers are held apart to preserve the measurement by the crumpled aluminum foil.

How about making your own "plasti-gage" like is used for sizing bearings when you rebuild an engine? Take Bruce's playdough and roll it out into sheets of different colors and known thickness, roll them one on top of the other, and then place a slice of the roll on the die before you press. After you press, the play-dough will be the same thickness you want to measure and the colors will separate into bands that can give you a quick read on the measurement. Do a few times at known thickness/squish and you can make a chart.

You can use a pasta roller machine to make your slabs of playdough and if you use artists modeling clay it will keep for a long time once you make a log or two of "plasti-gage".

If you eventually wish to have live measurement of the platen position on your press, you might want to consider a magnetostrictive sensor such as those manufactured by MTS:

http://www.mtssensors.com/products/linear-position-sensors/index.html

At my former job we used a similar technology to measure the platen position on a press for compacting carbon coke into anode blocks to be used in electrolytic aluminum smelting. It was dead-reliable and quite accurate.