Hi Fred!
It really all depends on the desired pattern. I think each maker has there "go to" arrangement in material thickness and how it's stacked/situated. Personally, my philosophy is that I want to get the maximum number of layers built up, with the least amount (times) exposing the billet(s) to the heat. I base the size (length and width) of my materials on my equipment. My press has flat dies that are 8" long..... so the pieces I stack up are 8" long. What I start out with when building a damascus billet is 1/8" thick 1080, 1 1/2" wide X 8" long. For the 15N20, it's .050-.070" thick X 1 1/2" wide X 8" long. The very basic recipe is to just stack one layer of 1080 or 1084 to one layer of 15N20, and build the billet in that manner.
Nickel alloy materials are compression resistant, meaning that they simply will not "squash" or "thin out" at the same rate that a non-nickel alloy will....that's why I use thicker 1080 or 1084 and thinner 15N20 ..... generally by the time I get to 300 layers of so, all of the layers are basically the same thickness. Commonly, I will start off with a stack of 40-50 layers..... but my equipment is large enough to handle that..... I think when it comes to the size billet a person can start with.... the capacity of your equipment is the limiting factor. If I had the muscle to handle it, a forge large enough to heat it, and a press or hammer that would handle it, I'd go to 100 layers in a starting billet.
Beyond the basic recipe of stacking "one to one", I always encourage folks to think outside the box when stacking up billets.... many times I will double or triple up on layers of 1080 or 15N20 when stacking up a billet....depending on how I want the light/dark contrast to appear in the finished piece. There are times when I will do one a full 1/3 to 1/2 of a billet in a single material, and stack the remainder in a 1 to 1, 2 to 1, or even a 3 to 1.... it just all depends on what I'm looking for in th end.
There are just so many possibilities.....just don't be afraid to think outside "the box". Sorry, I got off subject...... As long as the forging technique is good, and a person isn't wrecking steel, I think it's best to go with thicknesses that will allow you achieve the most layers, with the least amount of exposures to the heat of the forge as possible.
Since I've gone that far, I might as well also share what I consider some other important "procedures" when it comes to building billets. When it comes to building the initial billet, I use an old milling vise, and clamp the stack as tightly as possible. I then use a MIG welder and run at least three beads/welds across each "edge" of the stack.... one near the front, one near the middle, and one near the back (or handle end). DO NOT weld the ends of the stack....if you do, it increases the risk of trapping flux/slag inside the billet, and increases the chances of inclusions and/or "cold shuts".
I always forge the initial billet out as long as possible.... for a 40-50 layer stack, that often gives me a billet of 4-5 feet long. I allow that billet to cool a bit, and then using an angle grinder, clean off both sides of the billet. Here's the important part.... the scratch pattern that is produced by grinding with the angle grinder should go ACROSS the billet's face and be slightly "scarfed" (slightly higher in the middle/centerline of the billet) This lets any flux or slag escape out the edges when you restack and reweld. If you have grind lines running with the length of the billet, cold shuts and inclusions happen at a dramatically higher rate. After I grind both faces of the long billet/bar clean (the cleaner the better), I then measure, divide and mark the bar into equal lengths, and using a chop saw, cut, and then restack for the second (which is usually the final time for me) weld.
Sorry, I got long winded, but thought it might help someone.
