Estimating Shop Workload When Product Mix Changes Weekly

Why calculations break when the product mix changes frequently

A monthly average almost always smooths over what disrupts daily work. On paper the shop looks evenly loaded: there are 176 hours per machine, there’s a parts plan, so everything adds up. On a short week that falls apart quickly. One holiday, one urgent insertion and two changeovers are enough for the time buffer to disappear.

The problem is the scale of the calculation. A month shows an average picture, while production runs by weeks and sometimes by shifts. If long runs come in the first week and small batches with many setups in the second, the monthly plan hides that. It doesn’t show where the shop will hit a ceiling by midweek.

In a turning shop this becomes obvious fast. When the product mix changes almost every week, time goes not only to cutting. You need to remove fixtures, install others, set up tooling, check the first part, tweak the program, wait for blanks or inspection. For small batches those losses eat a noticeable part of the shift, even though they look minor in a monthly plan.

Most often the calculation omits hours for changeovers between batches, the first part and post-start inspection, short stops for tooling and fixtures, urgent orders injected into an existing plan, and idle time between short batches.

Small batches also change the logic of loading. With a long series most time is spent cutting. When positions switch frequently, the share of preparatory work grows. So the same 40 hours in a plan can mean very different real loads. In one case the machine cuts nearly the whole shift; in another, half the time is taken by transitions.

That’s why a weekly load estimate is usually more honest than a monthly one. A week better shows the real picture: how many shifts are actually available, how much time changeovers will take and how many hours to leave for urgent orders. This calculation isn’t perfect, but it’s closer to how the shop actually works. That makes it easier to promise deadlines without constant postponements.

What to collect before calculating

Calculations fail not because of a formula but because of gaps in source data. If the product mix changes every week, first gather a short but accurate set of numbers. Then you won’t have to redo the plan every day.

For a turning shop five data blocks are usually enough:

• the list of machines that will actually run that week;
• hours per shift for each machine separately;
• the week’s orders with quantity and due date;
• average changeover time by groups of parts;
• known losses in advance: service, first-part inspection, warming after downtime, setup after maintenance.

It’s better not to collapse everything into one line. One machine may run 10 hours on Monday and 16 on Tuesday. If you immediately set “week = 80 hours,” the calculation will almost certainly lie.

The same applies to orders. One urgent order for 40 parts and three small orders of 5–10 pieces load the shop differently. The second situation is often harder because time is lost on transitions. That’s why it’s useful to note the part group next to the order quantity. You can see what can be combined into one run and what must be launched separately.

You don’t need perfect timing for changeovers. An honest average time for 3–5 typical groups is enough. A simple tool change takes one amount of time; switching fixtures and a new program takes another.

Also mark separately all work that doesn’t produce output but consumes a shift. These are most often forgotten. First-part inspection can take 20 minutes or stretch to an hour if QA is involved or dimensions need tweaking.

If this data sits in one weekly table, the calculation takes little time. Errors start earlier, when available hours are counted by calendar rather than by real shop operation.

How to calculate available hours

Available hours are not all scheduled hours. It’s the time when a specific machine can actually work according to plan. If this number is wrong, everything else will be off: output, reserve and promised deadlines.

It’s better to count per machine. One may be down for scheduled repair, another will run the whole week, a third may lose hours to operator training. If you sum everything into one shop-wide number, the picture becomes too even and therefore useless.

The formula is simple:

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Regular pauses usually include lunch, shift handover, daily workstation cleaning and other fixed breaks. Planned stops are better counted separately. These include scheduled maintenance, machine checks, operator training, setup after service and internal acceptance after installation. For the shop these are part of the week, not a randomness you remember on Friday.

Example for one machine. There are 2 shifts of 8 hours and 5 working days. The calendar fund for the week is 80 hours. Subtract 30 minutes of lunch per shift, 10 minutes for handover and 10 minutes for cleaning. That’s another 8.3 hours per week. Then subtract 4 hours of scheduled repair and 2 hours of training. About 65.7 hours of available time remain.

Do not mix available hours with losses inside production. If the machine waited for a blank, the operator searched for a tool, or a batch went to scrap, that is already a loss inside available time. Changeovers are also better kept as a separate line. Then you can see two things at once: how much time you had on the schedule and how much was eaten by frequent product changes.

In practice a simple table is handy: one row per machine, one week per block. This accounting gives an honest base for the plan. After that you can comfortably calculate changeovers and reserve for urgent orders.

How to account for changeovers without a complex table

If you hide changeover time inside the per-part norm, the plan almost always lies. On paper the shop looks evenly loaded, while in reality people spend half a shift changing chucks, tooling and programs. So count changeovers separately as a standalone task with their own hours.

The simplest approach: each order has not only processing time but also an entry time. That line usually includes removing the previous fixture, installing the new one, setting tooling, a trial run and first-part check if it takes noticeable time. Then you immediately see where hours go to preparation rather than cutting.

Rely less on old standards and more on the latest similar batch. Check how long a real changeover took, how long the first-part check lasted, and whether programs needed fixes. Facts almost always beat an averaged old estimate, especially when the product mix changes weekly.

Grouping orders works well too. If two batches use the same material and similar fixtures, run them back-to-back. For a turning shop this often saves 20–40 minutes per change. Over a week the difference becomes several hours previously lost unnoticed.

For each order it’s usually enough to track four fields: batch processing time, changeover time, time for the first part and its check, and a note which order it can be combined with.

There’s a quick test. Look at a shift by the number of transitions, not by part count. If in 8 hours you have 5 changeovers and only 3–4 short processing runs, the shop lives in constant switching mode. In that case the problem isn’t machine speed but launch sequencing.

A simple example: a bushing series runs 1 hour 40 minutes, the changeover to the next part takes 35 minutes, and 15 minutes go to the first part and measurement. If in one shift you have three such product changes, you lose 2 hours 30 minutes not to processing but to entering orders. Without a separate line this time dissolves inside the plan.

When accounting for changeovers becomes habit, shop planning immediately sobers up. You see which orders to combine, which batches to enlarge, and where an extra product change eats the whole point of an urgent run.

How to leave a reserve for urgent orders

Urgent orders break the plan not because they’re urgent but because the shop is often loaded to the limit and people hope new work will “fit somehow.” With that approach any unplanned part shifts deadlines for regular orders.

Reserve should be counted in advance as a regular part of weekly capacity. Two common simple options are used:

• fixed hours, e.g., 6–10 hours per week;
• a percentage of the week, e.g., 10–15% of available time.

Fixed hours are convenient when urgent requests come nearly every week and their volume is roughly the same. Percentage works better where weekly load varies a lot.

Don’t pick the number from thin air. Look at the last 6–8 weeks: how many urgent orders actually arrived, how many hours they took and on which days. Very often shops think urgent work takes 5% of time, while in reality it’s 12%.

Keep the reserve on the most flexible equipment. Usually that’s the machine that accepts more different parts and needs less time for changeovers. In a turning shop it’s smarter to keep the buffer where the operator changes tooling and fixtures faster, not at the most loaded bottleneck.

A typical mistake: free hours are promised to “if we have time” tasks or small internal jobs. Then an urgent order arrives and the reserve is already spent. If a task doesn’t affect shipments and can wait a couple of days, don’t occupy the reserve with it.

Review the reserve. If urgent orders come in waves, keep one norm for calm weeks and another for peaks. For example, keep 8 hours in normal periods and increase to 12–14 hours before month-end or major shipments. That’s easier than rebuilding the whole plan every time.

A simple step-by-step calculation

You don’t need a huge table of hundreds of rows. It’s enough to count time in the same order each week. Then you quickly see where the plan runs smoothly and where the shop will hit hours by Wednesday.

Count per machine and then roll up to the shop. For a turning group this is especially important: one machine can be free while another is overloaded due to frequent changeovers.

1. Record available hours per machine for the week. Take not the calendar time but the time that can actually be worked: shifts, breaks, scheduled repair, operator absence.
2. Sum the machine time for all orders that week. If a part needs 12 minutes and the batch is 40 pieces, that’s 8 hours of pure machine time.
3. Add the changeover time and first-part checks for each order. This point most often breaks the plan when the mix changes daily.
4. Separately subtract the reserve for urgent orders from available time. If urgent work comes regularly, don’t count on squeezing it in without consequences.
5. Compare results not only by week but also by day. The week total can look fine while overload appears on Tuesday and Thursday, when the longest changeovers occur.

Two simple formulas are handy.

Planned weekly load = machine processing time for orders + changeovers + first-part checks.

Available time for the plan = available hours - reserve for urgent orders.

Suppose you have two turning machines, each providing 80 available hours per week. Total 160 hours. Orders require 118 hours of machine time; changeovers and first-part checks add 24 hours. If you keep 16 hours reserve for urgent orders, 144 hours remain for the plan. That leaves only 2 hours buffer. That’s not a comfortable schedule but work with almost no room for error.

If you see overload, don’t hide it at the week’s end. It’s easier to move part of a batch, combine similar parts into one setup, or shift a job to another machine if the route allows it.

Example for a turning shop

Take a normal week: 4 turning machines, 2 shifts, 5 days. With 8-hour shifts the total machine-hour fund is 320. On paper that often seems more than enough, but the true picture changes after the calculation.

There are 9 orders, 3 of them small batches that consume time most of all—not by cutting but by frequent changeovers.

A simple framework:

• gross shop fund: 4 x 2 x 5 x 8 = 320 hours;
• urgent reserve: 6 hours;
• changeovers: 11 events of 35–50 minutes, average roughly 42 minutes, about 7.7 hours total;
• net time left for processing: about 306 hours.

Now break down the load by machine. Suppose the process engineer estimated processing times like: 75 hours on machine 1, 74 on machine 2, 72 on machine 3 and 71 on machine 4. Add changeovers and the even picture disappears.

• Machine 1: 75 hours processing + 2 changeovers at 40 minutes = 76.3 hours
• Machine 2: 74 hours + 3 changeovers at 45 minutes = 76.25 hours
• Machine 3: 72 hours + 2 changeovers at 35 minutes = 73.2 hours
• Machine 4: 71 hours + 4 changeovers at 50 minutes + 6 hours reserve = 80.3 hours

Here the bottleneck appears. Overall the shop still has a buffer, but machine 4 is already overloaded. Looking only at the shop total, that overload is easy to miss.

The reason is simple: small batches, more changeovers and the reserve landed on one machine. As a result it slows the whole weekly plan even if three other machines haven’t hit their limits.

Usually one of two solutions helps: move one small batch to machine 3 or split the reserve across two universal machines by 2–3 hours each. Even moving work for 3–4 hours often relieves the overload and makes the plan achievable without crises.

Where mistakes happen most often

Even a careful plan collapses quickly if you count only pure machine time. A part may need 12 minutes of processing, but the batch often adds 30–40 minutes for tooling change, chucking, a trial part and first inspection. If that isn’t in the calculation, the shop looks free on paper while in reality it’s already overloaded.

Another common error is one “average” changeover for all parts. That’s convenient in a table but not true in practice. A simple bushing and a housing requiring different fixtures take different times. It’s better to split the product mix into at least three groups: simple repeats, parts needing partial changeovers and new or rare positions. Even such a coarse split usually gives a more accurate plan than one averaged number.

Urgent orders are often placed on top of an already full schedule. That doesn’t make the plan flexible; it makes it unrealistic. If the product mix changes weekly, reserve is needed in advance. For many shops it’s reasonable to keep at least 10–15% of time free. Otherwise one urgent order shifts everything and people save the situation with overtime.

Another mistake is not recalculating available hours after downtime. A machine stood for half a shift, an operator is on leave, a tool change stretched out, but the plan still shows the old 80 hours for the week. That immediately skews the shop load assessment.

The last trap is looking only at the weekly total. The sum may show the shop fits, but a collapse is already inside the schedule. For example, Monday and Tuesday may be booked to the limit while a free window appears only on Friday. For an urgent order such a plan is useless.

If you want to quickly catch weak spots, check five lines: machine processing time by batch, changeovers by part type, reserve for urgent orders, hours lost after downtimes and load not only by week but by day. Errors usually lie not in formulas but in too-even assumptions.

A 10-minute plan check

A quick check isn’t for reporting. It’s to spot weak points before the shift starts. When the product mix changes each week, the plan usually breaks not because of volume but because of small things: a forgotten changeover, an urgent order placed tight, or checking load only by week and missing an overload on Tuesday.

For this check one sheet or a simple table is enough. Each machine should have its own available hours for the week and by day. Not a shop-wide number but individual ones. One machine runs 40 hours, another 32, a third loses time to maintenance or a short shift.

Then compare available hours with the plan. Include not only pure processing but all changeovers as a separate line. If changeovers are hidden inside part norms, errors accumulate quickly. In a turning shop this is especially visible when the week has many small batches.

In 10 minutes check five things:

• each machine has available hours by day and by week;
• changeovers are recorded separately with clear time for each batch switch;
• reserve for urgent orders is non-zero even if the week looks calm;
• overload is visible by day, not only by the total week sum;
• each conflict already has a solution: move a batch, enlarge a batch or change sequence.

The daily split often shows the truth faster than any overall number. By the week a machine may look 92% loaded and that seems normal. But if on Monday and Tuesday it already runs 11 hours per shift, the plan won’t hold without missed deadlines.

A simple example: one machine has 40 hours per week. The plan shows 34 hours processing, 4 hours changeovers and 4 hours reserve. On paper that’s 42 hours. Overload is obvious. Decide before launch: move a batch to another machine, combine two small batches into one setup or shift a less urgent order.

Such an assessment takes little time if the numbers are in one place. If you have to collect them from memory, the plan is already unreliable.

What to do after the calculation

Numbers alone change little. Value appears when the same imbalance holds for 3–4 weeks in a row. Then you see where the plan breaks not by chance but for the same reason.

If overload repeats, don’t rush to add a shift or buy a new machine. First look at how many hours extra changeovers consume. Often the issue isn’t the volume of parts but too-frequent switching between small batches. Combining similar orders and cutting switches can return several hours per week without cost.

When the urgent reserve is used almost every week, it’s no longer a reserve but a permanent part of the plan. In that case allocate a fixed window: e.g., keep 4–6 hours mid-week on one turning machine for urgent parts rather than trying to squeeze them into any free gap. That makes dispatching easier and helps the foreman keep a rhythm.

If one machine is consistently overloaded, check the calculation against reality. Sometimes a part’s standard says 18 minutes but the operator spends 24. Or the machine works faster than the table records. Without this check the shop load estimate stays pretty only on paper.

Usually after such analysis one of four actions is taken: enlarge batches of similar parts, move some work to a less loaded machine, fix separate hours for urgent orders or update time norms based on actual output.

A good sign is when planned and actual available hours start to match within a couple of weeks. If the gap remains large, the cause is usually routing, downtime, tool waits or overly optimistic changeover accounting.

If overload persists, sometimes the problem is not calculation but equipment capability for your part mix and shift pattern. At that point it’s worth discussing selection of turning machines, commissioning and service with EAST CNC to match the real shop schedule rather than an averaged plan.