I was wondering if I use a hydraulic press for plate armor will it give the same effect as a hammer and stake or will it stretch and thin the metal?
I have only worked a little with a hydraulic press but will pass on what I know.
It should work fairly well for simple dished forms. So this is changing flat sheet into bowl shapes. You will have to be careful of wrinkles developing in the steel sheet. If a wrinkle starts immediately press it out or it will get out of control. Hydraulic press have the advantage of being very slow so you can see things develop. The extra pressure means that once an unwanted shape starts to develop it needs to be corrected right away.
The hydraulic press wont work well when using a raising technique on the sheet steel. You may have to do this by hand on a stake as usual. The raising techniques involve closing a piece of sheet over a form or stake with very light repetitive hammer blows. This is actually thickening the steel as the pipe or cone is created. The press tends to deform the steel either by thinning if it is trapped between two dies or by depressing it into open space between two dies. A lot will depend on the types of dies that you make for the press.
For simple armor forming I would think about a mushroom shaped top die and a matching bottom die that could be made from heavy pipe with 1/2 inch round bar welded around the top edge to provide a round surface of contact. The diameter of pipe I would try would be about 3 inch but this would vary depending on the work you were doing.
You will have to experiment here.
Like power hammers any dies that you make should have the edges well radiused so the edges of the dies do not cut into the steel. This will give a smooth transition to overlapping pressings.
Hydraulic Press as Power Hammers.
A hydraulic press is very different from a hammer and does have a different action. They will do some of the same things. Other things not as well. Advantages of the press is the tremendous force that is developed and there is no pounding so they are relatively quite. This can be important in a residential neighborhood. The main disadvantage is that the presses are slow.
You can work hot metal under them and many people use them to develop mosaic damascus with the controlled pressure. The press will work for drawing out but not as fast as an air or mechanical hammer. The press will work very well for punching holes or shearing (splitting) or for decorative veining or punching. All can be done hot provided your dies are set up for it.
Of course a press will work very well for flattening or controlled repetitive shaping if the the dies are exactly made for a specific purpose. Hydraulic presses also have a relatively small footprint in the shop.
In general I would say a hydraulic press is a useful addition to a blacksmith shop but it will mostly depend on the type of work that you are doing. For the armor it may work very well but you will have to experiment with die shape and size to make it work effectively for you.
Sample Hydraulic Presses
Hope this helps.