Expensive constraints and batching machines

The expensive constraint

A worsted textile mill makes suiting fabric. Its constraint is spinning, on ring frames: 55 of them, with between 96 and 504 spindles each, about 25,500 in all. A ring frame’s daily output depends on the yarn’s count (thickness) and twist: the finer the count and the more twists per inch, the lower the output.

Different shades with the same count and twist can be clubbed on one frame, so the planners always try to fill every spindle. But some spindles are often forced to run idle, for two reasons booked as losses every day: no material available with the same count and same twist, and a shortage from the back process (dyeing, recombing, roving).

After TOC, the mill already did better: spinning efficiency rose from about 71% to 77%, and in recent months to about 82–83%, close to its best ever, mainly by using spare roving machines for fine counts. Losses fell from about 10% to 5%. Five points were still on the table.

How releases work, and where they slip

The mill accepted orders on a standard lead time of 70 days. Inside it, the production buffer was 55 days, with a constant load of about 20 days in front of the constraint; the rest was protective time after it. Releases were driven by the load in front of the ring frames: as it fell each day, new plans entered the floor.

So whenever the constraint lost output, central planning had to delay releases, even though the plans existed in the system. In months with big losses, the release date drifted past the system’s due date. Every lost spindle-hour was a lost day for some customer.

The conflict

Blue orders

A blue order is a colour below green.

• It may be produced only when there is no other order of any colour at that work centre.
• Every work centre treats it as the lowest priority, and its movement is controlled at each one so capacity is never diverted to it at the cost of regular orders.
• It is made only of runner yarns needed for regular make-to-availability products, controlled by a norm that it may not exceed.
• The yarn is stored after spinning with no further processing, and netted against the next plans that need it. Because the mill now holds those yarns, the products made from them have a shorter lead time, so their finished stock norms at the warehouse can come down.

The choking mechanism is defined department by department, always using idle capacity, which these departments have more of than the constraint:

Department	How the blue flow is choked
Dyeing	The blue replenishment plan is released at once, but dyeing starts it only when a vessel has no load. Idle vessels are reported in the daily flow meeting; the blue material is tagged with a blue sticker.
Recombing	Idle lines are reported in the daily flow meeting; only then is a blue shade card released.
Ring frames	When spindles would run empty, the frame team loads available blue stock and reports the consumption, which triggers replenishment (norm − stock − pending).
Post-spinning	Pulls blue yarn only when it has idle capacity, and moves it to the yarn store.

The negative branches to guard against: capacity stealing (making blue while a green order waits), raw-material stealing by blue orders, enough bobbins and trolleys for the blue stock, a missing choke at some work centre, and yarn-store stock growing if the wrong yarns are chosen for blue buffers.

The foundry: batching machines that aren’t the constraint

A foundry supplying engineering customers runs core making, pouring, fettling and finishing in its own plant, then machining at five vendors. Its shape is mixed: an A-shaped front, then a T/V-shaped back. Core making and pouring run large batches; finishing runs small ones. Space and trolleys are short; the warehouse is small.

The undesirable effects: poor capacity use after the A-shaped part, choking after the main batching machine, periods of uneven loading at the front, and long lead times. The batching machines are not the constraints, yet they batch as if they were.

The current reality behind the conflict is a chain any planner will recognise. With the full monthly forecast visible, and a belief that capacity is short, planning cherry-picks items to build big batches, by postponing current requirements or pulling future ones ahead. Castings are made beyond the customers’ immediate needs, and some customers take in excess to build stocks. Other customers’ orders are delayed and miss their assembly dates; they expedite, which means urgent set-ups, more unplanned set-ups and more lost capacity. Repeated delays make the foundry look unreliable; customers source from the other supplier, and new development goes to more reliable vendors. Sales is booked ex-factory while payment comes after the customer’s receipt, so receivables rise. And the potential business lost delays the filling of the new capacity the company is installing.

Check your understanding

Answer all five. Get four right to complete the lesson.

1. Ring-frame spindles ran idle mainly because…

• Operators were absent on some shifts, so frames could not be fully manned
• No matching count and twist was ready, or the back process fell short
• Customer orders were too small
• The frames were too old

Both losses were booked daily under those two heads.

2. A blue order may be produced at a work centre when…

• It is green, since green orders have time to spare
• The month’s target is short
• The plant manager approves it
• No order of any colour is waiting there

It is the lowest priority, below green.

3. Blue orders are limited to…

• Runner yarns for regular stocked products, capped by a norm
• Any yarn whose count and twist fits the idle spindles
• Export orders
• Samples for new products

So whatever is made will be consumed soon, and netted against new plans.

4. Losses at the constraint made the mill’s release date drift past the system’s due date because…

• Planners kept the old due dates in the system and ignored the losses
• Sales accepted too many orders in the months when losses were high
• Releases followed the load in front of the constraint, which fell more slowly
• The system had no buffer

Lost constraint output delayed every release behind it.

5. In the foundry, saving set-ups at the batching machines…

• Raises throughput, because every saved set-up adds output
• Saves nothing, since they aren’t the constraint, and creates stock
• Is the main improvement opportunity, since set-ups there are long
• Reduces receivables

Batch to the real requirement window where there is spare capacity.