TOC in Operations – Advanced · Lesson 4 of 12
When touch time is not small
Forging, heat treatment and long processes break the rule that touch time is under 10% of lead time. Halve the lead time there and you remove the buffer, not the waiting. Cut it to touch time plus half the buffer, keep the buffer at least a third, and compute buffer status from the buffer, not from the elapsed time.
The rule that quietly fails
The standard solution cuts the production lead time by half, and it is safe because touch time is under 10%. Most of an order’s lead time is waiting, so half the lead time is still almost all protection.
Some plants are different. In forging, in products that sit in heat treatment for many hours, in long chemical or curing processes, the touch time is far more than 10%. What happens there if you apply the standard cut?
Buffer and lead time are not the same thing
The U-curve shows why the standard cut works. On the right of the curve, too much work in process causes jams and missed priorities. Cutting the lead time by half moves the plant onto the flat bottom, the plateau, while leaving plenty of buffer.
But that picture silently treats buffer and lead time as the same. They are not:
Production lead time = touch time + buffer
When touch time is negligible, buffer and lead time are practically equal. When touch time is large, halving the lead time can take away most of the buffer and push the plant over the left side of the U-curve: too little time to react, and orders late again.
Cutting lead time when touch time is large
The rule for a plant on the right side of the U-curve:
New lead time = touch time + half of today’s buffer.
If the new buffer comes out at less than a third of the new lead time, raise it to a third.
Three products, each with a 60-day lead time today:
| Touch time | Buffer today | New lead time | New buffer | Lead time cut |
|---|---|---|---|---|
| 12 d (20%) | 48 d | 12 + 24 = 36 d | 24 d (67%) | 40% |
| 24 d (40%) | 36 d | 24 + 18 = 42 d | 18 d (43%) | 30% |
| 36 d (60%) | 24 d | 36 + 12 = 48 d, buffer only 25%, so raised: 36 + 18 = 54 d | 18 d (33%) | 10% |
The lead-time cut gets smaller as touch time grows, which is honest: there is less waiting to remove. And all of the cut comes from the waiting, none from the protection.
Pause and think: why keep the buffer at least a third of the lead time?
- A third is the length of the green zone, so a shorter buffer would have no green
- Customers expect a third of the lead time to be safety
- Below that, normal disruptions can’t be absorbed and orders go late again
The buffer exists to absorb variability. When touch time is long, its own variation is large too, so the protection can’t be squeezed below a minimum.
Buffer status when touch time is large
In a normal make-to-order plant, the lead time consumed is practically the buffer consumed, so buffer status is simply the share of the lead time gone. With large touch time, that misleads. An order on day 10 of 13 that has only one day of work left is in better shape than one with three days of work left, though both have used the same share of their lead time.
Compute the buffer directly:
- Buffer remaining = lead time remaining − touch time remaining
- Buffer consumed = buffer − buffer remaining
- Buffer status = buffer consumed ÷ buffer
Colours as usual: up to 33% green, 34–66% yellow, 67–99% red, 100% and more black.
An order with a 13-day lead time, 4 days of touch time (four steps of one day) and a 9-day buffer:
| Day 2 | Day 6 | Day 10 | |
|---|---|---|---|
| Steps completed | 0 | 2 | 3 |
| Lead time remaining | 11 | 7 | 3 |
| Touch time remaining | 4 | 2 | 1 |
| Buffer remaining | 7 | 5 | 2 |
| Buffer consumed | 2 | 4 | 7 |
| Buffer status | 22% green | 44% yellow | 78% red |
The status now responds to progress, not just to the calendar. An order that is behind turns red early enough to save it; one that is ahead stops calling for attention.
Check your understanding
Answer all five. Get four right to complete the lesson.
1. Why is halving the lead time risky in a forge?
- Forges can’t run with less work in process
- Touch time is a large share of lead time, so halving it removes most of the buffer
- Forging lead times are fixed by the customer
- Because heat treatment needs large batches to be economical, so lead time can’t fall
Lead time = touch time + buffer; when touch time is large, the cut falls on the buffer.
2. Lead time 60 days, touch time 24 days. The new lead time by the rule is…
- 30 days
- 48 days
- 36 days
- 42 days
24 + half of 36 = 42 days, with an 18-day buffer (43%).
3. Lead time 60 days, touch time 36 days. Why is the result 54 days, not 48?
- At 48 the buffer would be only 25%, under the one-third minimum
- Because the customer accepts 54
- Because 36 + half of 36 is 54
- Because heat treatment always adds six days to the touch time of a forging
Raise the buffer to a third of the new lead time: 18 days on top of 36.
4. Buffer remaining is computed as…
- Buffer minus days since release
- Lead time minus the days already gone since the release
- Lead time remaining minus touch time remaining
- Touch time minus the steps done
Only time not needed for work is protection.
5. On day 10 of 13, with 3 of 4 steps done, the order is…
- Green, since most of its work is done
- Red: 7 of its 9 buffer days are used
- Black
- Yellow: 10 of 13 days gone
Buffer remaining = 3 − 1 = 2; consumed 7 of 9 = 78%.