Sizing a polishing line looks like a simple question — how many square metres per month do you need — and turns into a difficult one as soon as a supplier quotes a line speed in metres per minute. Those two numbers are not the same thing, and the gap between them is where most capacity surprises come from.
This is the calculation, in the order it should be done.
Step one: convert line speed into theoretical output
Theoretical output in square metres per hour is line speed in metres per minute, multiplied by sixty, multiplied by the working width in metres. A line running at 8 metres per minute on a 1250 mm width produces 8 times 60 times 1.25, which is 600 square metres per hour, per pass.
That figure assumes continuous running, full width utilisation and no stoppages. It is a starting point, not an answer.
Step two: understand what a pass is
On a multi-head line, the material passes every head in one run, so the finish is produced in a single pass through the machine. The number of heads determines what finish can be produced, and the line speed determines how fast it is produced. A four-head line does not run four times slower than a one-head line — it produces a different finish at whatever speed the whole sequence supports.
What does change with the finish is the achievable speed. A coarse grinding stage can run fast. A mirror sequence cannot, because each successive stage needs more contact time to remove the previous stage scratches without introducing new ones. Mirror lines are typically run substantially slower than No.4 lines of the same width, and that difference is the main reason mirror capacity is more expensive per square metre.
Step three: subtract the four losses
Theoretical output has to be reduced by four factors before it means anything.
- Availability — scheduled and unscheduled downtime. Belt changes, cleaning, maintenance and breakdowns. A realistic figure for a well-maintained line is high, but it is never one hundred percent.
- Width utilisation — you rarely run full-width material all the time. Narrow strip on a wide line wastes capacity, and it is common in job shops.
- Yield — material scrapped or reworked. The first months of a new line always run below mature yield.
- Changeover — coil changes, abrasive changes, and finish changes. A line that runs one product all week is a different asset from one that changes finish three times a day.
Applying those four factors to the 600 square metres per hour above, a realistic planning figure might land somewhere between 350 and 450 square metres per hour of acceptable output. The exact number depends on your product mix, and the point is that it must be calculated rather than assumed.
Step four: work backwards from the monthly requirement
Take the monthly requirement in square metres, divide by the realistic hourly output, and you have the hours the line must run. Then compare that with the hours available in a month at your shift pattern.
If the requirement needs more hours than you have, the answer is more capacity, more shifts, or a narrower product range. If it needs far fewer, you are buying capacity you will pay for and not use — which is exactly the situation that makes a payback calculation fail.
Step five: find the real bottleneck
The polishing line is rarely the only constraint. In a coil operation the bottleneck is often the decoiler, the cut-to-length shear, or the crane that moves coils. In a sheet operation it is usually handling and stacking, because finished sheet cannot be stacked until it has cooled and been filmed.
A line that polishes faster than you can feed it or stack it does not increase output. Before sizing the line, measure the actual rate of the steps before and after it. The slowest step sets the plant throughput, and it is usually not the one being quoted.
Step six: size for the finish mix, not the average
Most shops produce more than one finish. If 70 percent of volume is No.4 and 30 percent is mirror, the line has to be sized for the mirror speed during the mirror runs, and the No.4 runs will finish early. Sizing on a weighted average speed produces a line that is always either waiting or overloaded.
The practical answer is usually to size the line for the slowest finish you must produce in-house, then decide whether the faster finishes could be run on a second, simpler line. Two specialised lines sometimes cost less than one universal line of the same total capacity.
What to ask a supplier for
- Recommended line speed for each finish, on your material, not a generic figure
- Number of heads and the function of each
- Achievable output per hour at that speed, with the assumptions stated
- Changeover time between finishes and between coils
- Abrasive consumption per square metre
- Typical yield during the first month and after stabilisation
A supplier who can answer these in writing has sized lines before. A supplier who answers only in metres per minute has given you a specification, not a capacity.