One measurement, and the rest is arithmetic
The only input on this page that cannot be reasoned about is the gain per heat, and it is also the only one that matters much. Take a bar, mark it, put it in the fire, work it for one full heat until the colour has gone, and measure it again. That difference is your number, for that section, that hammer and that day. Everything else the page does is counting.
The number is not stable across a draw. Early on there is a lot of steel and a lot of heat and the bar moves fast. By the time the section is half what it started at there is much less of both, and the same heat gains less length. If the draw is a long one, measure once near the start and once near the finish, run the page twice, and treat the two answers as the range rather than believing either.
The rotation result, which is the useful one
Hammer time per piece is fixed. Heats times minutes at the anvil, and no arrangement of the work changes it. Fire time is not fixed at all — it is either dead time you stand through, or it is time somebody else is being hammered.
Work one piece and every heat costs you the full reheat. Work two and the second one absorbs one work slot of the wait. The wait disappears entirely once the other pieces in the rotation take at least as long to work as one piece takes to reheat, which is a condition you can write down: the reheat divided by the work time per heat, rounded up, plus one for the piece itself.
With a three minute reheat and ninety seconds at the anvil, that is three pieces. Two is not enough — one round works two pieces for three minutes and then leaves you standing for the last minute and a half. Three is enough, and four buys nothing at all, because past that point the hammer is the constraint and adding pieces just makes the batch longer.
Blows per inch, and what it is for
Total blows on its own is not very interesting. Blows divided by the length actually gained is, because it is the only figure on the page that lets you compare two ways of doing the same job. Move to a heavier hammer, run the fire hotter, use the far side of the anvil instead of the face, put the piece under a striker — each of those changes blows per inch, and the change is visible immediately even on one heat.
What it will not tell you is whether the change was a good idea. A heavier hammer that halves the blows and doubles the time you need to sit down afterwards has not helped, and no arithmetic can see that.
What the page deliberately leaves alone
Working temperature. Every steel has a range it should be forged in, and both ends of it matter — worked too cold it takes damage that does not appear until much later, worked too hot it takes a different kind. Those figures belong to the data sheet for the specific steel and this page does not carry any of them.
Fuel is the other omission. What a session burns depends on the fire and the fuel, and there are pages here that handle burn rate arithmetic properly for solid fuel and for gas; the useful version of that number is one you measure over a real session rather than one derived from a forging time estimate.
Questions people ask
How do I measure the gain per heat?
Mark the bar at the two ends of the section you are going to draw, measure between the marks cold, take one full heat and work it until the colour is gone, then let it cool enough to measure again. The difference is your gain for that heat. It is worth doing three times and taking the middle one, because the first heat off a cold fire and the first heat of the day are both unrepresentative.
Why does the batch time barely change when I add more pieces to the rotation?
Because once the rotation is big enough that the reheat is fully hidden, the hammer is the constraint and the hammer time per piece never changed. The page prints the point where that happens. Below it, each extra piece removes real waiting time from every round; at or above it, the extra pieces only make the group bigger and the batch takes proportionally longer to finish.
The last heat only needs a fraction of a gain. Does the page round it up?
Yes, and it says so — the heat count is rounded up and the output prints how much length the final heat actually has left to gain. A heat is not divisible: if there is a tenth of an inch left to move, that is still a heat, still a trip to the fire and still the reheat that goes with it. Rounding down would understate every job by up to one full heat.
Does the number of blows per heat affect the timing?
Not in this arithmetic. The minutes at the anvil are entered separately, because how long a heat lasts is set by how fast the piece loses colour rather than by how many times you hit it. Blows are tracked as a load figure — blows per inch of length gained is the number that changes when you change hammer or fire, and it is the one to watch if you are trying to work out whether a change helped.
Can I use this for work under a power hammer?
The structure holds — heats, gain per heat, work time, reheat, rotation — but the numbers you feed it will be completely different, and the rotation result usually inverts. A power hammer makes the work slot so short relative to the reheat that you need a lot of pieces in the fire to keep it busy, and at that point the fire rather than the hammer is what limits the day.