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As per this reddit thread https://www.reddit.com/r/factorio/comme ... eel_steel/. I tried to create a blueprint for Prod3 furnaces to smelt iron ore into steel without losing any productivity.
The math is pretty easy with 20% bonus, 5 crafts makes 6 products.
So 5 charges of iron is required to make 6 steel, aka the first extra steel from the prod bonus.
5 charges of iron plate is 25 iron plate.
25 iron is not a multiple of 6 so it would waste some prod making the iron plate. The lowest common multiple of 25 and 6 is 150.
150 iron plate needs 125 iron ore.
So the numbers are:

125 iron ore makes 150 iron plates makes 36 steel plates.
25 prod cycles of iron ore to plate and 30 prod cycles of iron plate to steel.

All things considered these are relatively low numbers.

The bug in Question

When it comes to execution I run into an issue. By the last cycle of ore to plate the furnace switches off and stalls the prod cycle before it can finish, resulting in only 149 plates instead of 150. (Tested on normal game speed without beacons)


Copy blueprint Blueprint attached for testing

The 30 steel cycles dont have this issue.
The earliest I've been able to recreate this is 11th prod cycle.


Ive also tried increasing the ratio to
250:300:72 (Steel also fails)
375:450:108 (Steel passes if primed with 1 iron plate)
500:600:144 (Steel passes if primed with 1 iron plate)
All fall 1 iron plate short by the end of the iron ore phase.


Ive considered priming the system with 1 iron plate but the whole cycle resets when the furnace changes recipe and the steel phase sufferes the same issue.

Thanks for the report. This is a known issue due to floating point not being able to represent fractions perfectly. The logic will slightly over or under produce when a series of floating point values are added together.

There is no perfect answer to this and so this is how it has been for the last 5+ years.