A Flexible Line Or A Dedicated Machine

Two quotes, same part list, four hundred thousand apart. One machine is a dedicated line that runs the part in 45 seconds and cannot do anything else. The other runs 20 seconds to 20 minutes depending on what is loaded, and costs forty percent more per part at full volume.

Both suppliers are answering the question they were asked. The problem is that the question was written before anyone knew which of these two machines the plant actually needs, and the answer depends on three numbers that are usually not in the enquiry.

The Problem: Volume Is Not A Number, It Is A Shape

A dedicated machine is designed around one part at one cycle. Every second of its cycle is allocated, every nozzle is aimed, every fixture is fixed. That is why it is fast and why it is cheap per part at volume.

Its weakness is not capacity but shape. It exists for the part it was designed for, and when that part is revised, replaced or joined by a variant, the machine either adapts at the cost of cycle time or does not adapt at all.

A flexible line trades cycle time for range. Running a part at 20 minutes because it needs a long plug-sealing hold and a vacuum dry is not a defect; it is the machine doing two different jobs with one set of hardware. The cost appears as investment, floor area and a longer cycle on the parts that did not need the extra.

The Diagnosis: Three Numbers Decide It

Before comparing quotations, three numbers have to be known, and all three are usually available inside the plant rather than from the supplier.

The first is the ratio between peak and average volume. A plant that runs one shift at steady output has a flat profile and a dedicated machine fits. A plant that runs two shifts in season and one out of season has a profile that a dedicated machine handles by standing idle half the year.

The second is the number of variants expected within three years, not today. A part list that is stable for five years justifies a dedicated machine. A part list that has changed twice in the last two years will change twice more.

The third is the cost of a changeover, expressed in minutes and multiplied by how often it happens. A flexible line that changes over in thirty minutes is not flexible if it changes over twice a shift; the same line is excellent if it changes weekly.

What Each One Is Actually Good At

What The Numbers Look Like In Practice

Two plants, both running the same family of aluminium housings, both asking for one machine.

The first runs two variants, has run them for four years, and expects to run them for four more. Its output is flat at around 300 parts per shift. A dedicated machine at 45 seconds delivers that with margin, and the flexibility premium would buy range that will never be used. This plant should buy the dedicated machine and should not be talked out of it.

The second runs the same two variants but is launching a third and a fourth within eighteen months, and one of those needs an oil circuit cleaned to a limit the current parts do not have. A dedicated machine would need a rebuild or a second machine. A flexible line runs the existing parts slower than necessary and handles the new ones without new hardware, which is the correct trade even though the cost per part on today's volume looks worse.

The difference between these two plants is not the part. It is what they know about the next three years, and that knowledge is worth more to the decision than any quotation.

There is a third path worth naming: a dedicated machine for the high-volume part, plus a small flexible cell for everything else. It costs more than either single option and it is the right answer more often than the industry admits, because it puts each machine where its economics actually work.

The Decision

Take the dedicated machine when the part is stable, the volume is high and flat, and the cleanliness requirement is consistent across the family. In that case every hour of flexibility you buy is an hour nobody uses.

Take the flexible line when any one of the three numbers is unknown or moving — a new programme, a platform that is still being rolled out, a customer that has not confirmed volumes. The premium is an insurance premium, and it is cheaper than a machine that becomes scrap.

The mixed case is more common than either: a high-volume part that pays for a dedicated machine plus a low-volume family that does not. That is usually two machines, and the mistake is trying to make one machine serve both by slowing down the fast part.

Where The Comparison Goes Wrong

The most common error is comparing cost per part at design volume. A dedicated machine wins that comparison by construction, and it wins it whether or not the plant ever reaches design volume.

The second is treating changeover time as a fixed property. It is a property of the fixture design and the programming, and a flexible line with a poor fixture can change over more slowly than a dedicated machine being rebuilt.

The third is assuming the flexible line is slower on every part. It is slower on the parts that need slow processing, and on a simple part it can be faster than a dedicated machine sized for the worst case in its family.

If You Only Remember Three Things

Compare at the volume you will actually run, not the volume in the business case. Count the variants you will have in three years, not the ones on today's list. And measure the changeover in minutes per shift, because a flexible line used like a dedicated one is the most expensive of both.

A machine is not flexible because it can do many things. It is flexible if the plant actually asks it to.