Tool magazine: 24 or 40 positions — how to assess the tooling reserve

Why 24 slots run out quickly

On paper 24 slots often look sufficient. In the shop they disappear much faster, especially if the machine doesn't run the same part for weeks but works with a mixed part mix.

Magazines are often calculated per part or per operation. In real shifts they follow a different rule: today a shaft, tomorrow a flange, then a small batch of housings with a different set of cutters, drills and boring tools. Each such change takes free slots, even if the parts individually don't seem complex.

Some slots are permanently occupied by tools that are rarely removed: turning, grooving and parting tools, common drill diameters, holders and probes for typical operations. Formally these aren't a kit for one part, but in practice they form a constant minimum without which the shift quickly slows down.

Another hidden drain is duplicates. If a tool wears quickly or there’s a risk of chipping, an operator prefers to keep a ready replacement in the magazine rather than stop the machine at an inconvenient moment to change tooling. For a CNC lathe this is common practice when consistent output is important.

Slots usually run out for four reasons: different parts require different tool sets, some slots are taken by the constant kit, duplicates occupy specific positions, and you need to leave a small reserve for changeovers.

So you can't judge a magazine with 24 or even 40 slots solely by the number of operations in a process card. If the card lists 14 tools, that doesn't mean 24 slots will be comfortably enough. In real work you quickly add duplicates, holders, measuring tools and items held for neighboring orders.

A miscalculation hurts both ways. Too few slots increase manual changes and lengthen setup. Too large a magazine without real need means you pay for a configuration you don't use. Calculate not by "per part," but by order flow and your shop's changeover habits.

What to gather before calculating

To decide whether you need a 24- or 40-slot magazine, first collect a picture of normal work — not the best or worst day. The most common mistake is counting for a single part and then wondering why the magazine fills by mid-shift.

Base the analysis on orders for a typical 2–4 week period. That’s enough to reveal the everyday mix of parts, short runs and recurring operations. Looking at just one week often gives an overly optimistic result.

Look not only at part names but at the tooling sets used for them. Which cutters, drills, taps, parting and boring tools do you place in almost every order? That is the constant minimum you shouldn't remove without reason.

Identify rare tooling separately. These are fixtures for short runs, nonstandard diameters, special chamfers or threads needed every few days. Mixing them with the base set hides the true tool reserve.

A simple table with four columns is helpful:

• part or part group
• operations for it
• constant tooling
• rare tooling for short runs

Then add duplicates. If a position wears fast or often breaks during a shift, one slot is not enough. Duplicates are most often needed for common drills, parting tools and tools that take a long time to reset.

Another necessary metric is how many changeovers you do per shift. If there is only one or two changeovers a day the magazine can be filled more tightly. If the operator switches tasks several times per shift, extra free slots clearly save time.

A short example quickly checks the logic. Suppose over three weeks you ran 18 parts. For 13 of them the same 11 positions repeated, 6 were needed only for short runs, and two common tools required duplicates. That already makes 19 slots, and this is without reserve for measuring and unexpected replacements.

With honest data the calculation becomes easier. You'll see your real working set, not an abstract magazine capacity.

How to calculate tool reserve

Do the estimate based on your part mix for a typical week, not the machine spec or a guess. On paper the magazine may seem large, but in practice slots are eaten up by mixes of parts, duplicates and urgent insertions into the plan.

Simple calculation scheme

First identify the basic set needed for most parts. These are the most frequent drills, threading tools, cutters for typical grooves, roughing and finishing tools, and measuring probes if used in the cycle. Count actual positions that should stay in the magazine without removal.

Next, take the most complex family of parts in your current portfolio and list what it needs beyond the base set. Avoid double counting: don't add tools already included in the base.

Then add duplicates for operations where tools wear fastest. Usually this is roughing, deep drilling, threading in difficult materials and anything you don't want to change mid-shift.

The final layer is free slots for urgent jobs, trial runs or replacing a tool without a full rearrangement. For a live production leave at least 2–4 empty slots. A fully packed magazine is convenient only on a spreadsheet.

The working scheme looks like this:

1. Base set for most parts.
2. Plus unique positions for the most complex part family.
3. Plus duplicates for fast-wearing tools.
4. Plus 2–4 free slots for urgent tasks.
5. Compare with the actual weekly plan.

Short example: the base requires 16 positions. The complex family needs 7 more, but 3 of those are already in the base, so add 4. Duplicates take 3 more slots. Reserve for urgent work is 3. Total — 26. In that case the question "24 or 40 slots" is decided: 24 is tight, 40 gives comfortable reserve.

Then test the calculation on a real weekly plan. If the operator had to remove active tooling at least twice for a one-off job, your estimate was too optimistic. For mixed part mixes this is common.

When 24 positions are enough

24 positions usually suffice where the job flow isn’t too varied. If the shop makes 1–2 similar part groups, the operation set stabilizes and the magazine doesn’t get filled with rare tools "just in case."

This format works well on a CNC lathe when batches are similar by material, diameters and typical transitions. The same set of cutters, drills and parting tools covers most of the shift.

What matters is how much tooling you need in the machine at the same time. If today and tomorrow are shafts, bushings and simple flanges of similar sizes, 24 slots are often enough even with regular changeovers.

Typical picture: parts repeat rough and finish turning, groove, threading, drilling, boring and parting. One part may use 8–12 positions, but many tools overlap between batches. The magazine then retains a small reserve for 2–4 changeover positions without constant fighting for free slots.

Another good sign: the operator doesn't change the set every few hours. He runs a batch, does a planned changeover and swaps only a few positions instead of half the magazine. This reduces capacity requirements significantly.

24 slots also work when the shop doesn't keep many duplicates in the magazine. For example, worn tools are kept nearby in a prepared kit, and only the tools needed for the current batch are placed in the machine. It's a stricter approach but workable with a stable part mix.

A good example is a cell producing similar bushings and short shafts for repair orders. Parts vary by length and a few sizes, but the route is almost identical. In that case extra slots often remain idle and 24 positions are a reasonable choice.

But if you routinely keep duplicates, special tools for many small variations and separate slots for rare operations, 24 becomes insufficient. With repeatable work and disciplined changeovers that capacity usually holds; with a jagged job flow it does not.

When to choose 40 positions

40 positions are chosen not for a vague "just in case" but so the machine won't stop at every new batch. This magazine is needed where the shop runs several part families and switches between them quickly. If shafts, then flanges, then small shafts again in one day, free slots run out faster than expected.

With a mixed part mix tooling rarely stays compact. One part needs drills and threading, another needs boring and grooves, a third needs parting and finishing. When a cycle includes many different operations, a 24-slot magazine often hits its limit right after the base set is loaded.

Another reason for 40 slots is frequent changeovers during the shift. If the setter constantly removes part of the tooling and then returns it, the machine loses time and error risk rises. It's much calmer to keep ready sets for neighboring orders and only change what's truly necessary.

Wear is a practical factor: keep duplicates for tools that wear quickly. For drilling, threading and parting, a dull or chipped edge immediately affects size and finish. If a duplicate sits in the magazine, replacement takes minutes instead of a long pause.

40 slots usually make sense when several conditions align:

• you regularly run 3–5 different part groups
• there are multiple short batches per shift
• a single setup includes many different operations
• machine downtime costs more than the magazine upgrade

A simple guideline: if your working set plus duplicates and a couple of reserve slots totals 26–30 occupied positions, choose 40. The extra slots won't stay empty long; they'll be used for changeable tooling, trials and urgent orders. For shops with frequent changeovers this capacity is the usual working reserve.

Example for a mixed part mix

Imagine a shop running small batches of shafts, bushings and housings. Today 20 shafts, after lunch 15 bushings, and in the evening 10 housings. At this pace the magazine must not operate "tight," or each task change becomes a rebuild.

The process engineer has a common set that is rarely removed: 12 positions (turning, parting, drilling, boring, threading and a few usual tools). This set covers most shaft work and parts of the bushing operations.

The number of positions grows quickly. Bushings need 6 more slots: thinner boring holders, tools for internal grooves, sometimes a separate threading tool for small diameters. Housings add 8 positions for other diameters, face operations and extra steps.

Counting simply: 12 common + 6 for bushings + 8 for housings + 4 duplicates for roughing and parting = 30 positions. Duplicates matter most here: they save you when an edge fails mid-batch and you can't stop the machine for an urgent replacement.

Trying to fit this into 24 slots forces one of three compromises:

• remove a rare but necessary tool and rush to put it back
• skip duplicates and suffer downtime on replacements
• move operations to another machine even though it's more efficient to do them in one cycle

Formally 24 can be "stretched." In practice the reserve vanishes as soon as two different part groups come in the same shift. With frequent changeovers and mixed parts the choice usually leans to 40. A larger magazine doesn't speed the machine on its own but reduces swaps and provides a real buffer for production.

Common mistakes

Many people count only turning tools and the picture becomes too optimistic. Once drilling, boring, parting and threading enter the route, tool reserve melts much faster.

A typical simple example: one part needs 7–8 positions (roughing, grooving, parting, a couple of boring tools and a threading tool). The next part requires a drill, a tap or a different threading insert and sometimes a duplicate for the most loaded tool. The magazine that "should have been enough" fills almost immediately.

Another frequent error is calculating for one part instead of at least a week's plan. With a mixed mix this solves most problems. Today shafts, tomorrow bushings, then a small repair batch — by part the magazine may seem sufficient, but on the real schedule changes happen back to back.

Completely filling the magazine is also a bad approach. With no free slots any urgent job breaks the rhythm: the operator removes set tools, puts them back and rechecks sizes. Practically it's better to keep 2–4 slots free.

Duplicates are often forgotten. If one tool wears faster, it's simpler to have a ready replacement than to stop the machine for an urgent swap. This is especially noticeable in series work where drills, parting tools or roughing inserts wear quickly.

A quick self-check uses four questions:

• how many tools are needed not for one part, but for all batches in the week?
• which positions are required for drilling, boring and threading besides the usual turning tools?
• how many slots do you leave for an urgent job?
• are duplicates needed for fast-wearing tools?

The opposite mistake exists too: sometimes 40 slots are chosen "just in case" while parts are very similar. If you have one part family, a stable route and rare changeovers, the extra magazine won't pay back. It's better to honestly calculate the working set than pay for empty slots.

Quick check before ordering

Average loading is misleading. To choose, look not at an average week but at the busiest day when different parts follow each other and the setter changes tooling several times. That day shows whether you need 24 or 40 positions.

A good check takes under an hour if you use data from one or two real shifts. Don't calculate by memory. Pull routing cards, the changeover log or at least the operator's notes from a day when the machine was near capacity.

Check five numbers:

• how many different tools were in use on the busiest day, including wear spares and repeated holders
• how many positions you remove and reinstall with each batch change
• how many minutes a changeover takes from stop to first good part
• how often an urgent order disrupts the normal set and forces you to hold extra slots
• whether you plan to expand the part mix in the next year by a few new parts

If you usually change 8–10 tools per batch and a changeover takes 20–25 minutes, the benefit of a larger magazine becomes clear. On a mixed mix losing half an hour two or three times per shift often costs more than the price difference between the two machine options.

Urgent jobs are easy to underestimate. If one off-plan job per week needs 4–6 extra positions, the average hides it but reality shows the problem. The operator either removes tools needed for the next batch or keeps part of the set outside the magazine and loses time restoring it.

Reserve for growth should be calculated without optimism. If today you occupy 22–24 positions on a peak day and add two or three more parts in six months, free space disappears. For a cell with frequent changeovers this is not trivial.

A practical rule: if the peak day fits into 18–20 positions and batch changes are rare, 24 is usually enough. If you regularly approach the limit, keep a reserve and constantly face urgent orders, choose 40. Most mistakes come not from arithmetic but from underestimating a real production day.

What to do next

Start with facts from your cell, not the machine catalog. Take the last month of work and record for each part three things: which operations were done, what tooling occupied the magazine and where duplicates were kept because of wear or stop risk.

Then consolidate this into a simple calculation:

• list all parts that actually ran during the month, not only the most frequent
• note the tooling set and any duplicates kept for each part during the shift
• find the maximum occupied slots on the busiest days, not the average
• add a small reserve, usually 2–4 slots or roughly 10–15% if the mix changes often

You will see your working peak. If on a peak day you occupy 22 slots, 24 is still possible but the margin is almost gone. If the peak is 26–30, the difference between 24 and 40 directly affects shift rhythm.

Compare not only machine price but also the cost of changeovers. If the operator spends 15–20 minutes twice per shift on tool changes and rechecks, monthly downtime becomes hours, often costing more than a larger magazine.

For a mixed part mix it helps to review the week with the most frequent part transitions. That week shows whether you need extra tooling space or a compact configuration will do. The average load often lies.

If you’re selecting a CNC lathe and deciding between 24 and 40 positions, it's useful to run the calculation with the supplier using your drawings and real operations. The same machine model can suit different shops differently depending on their part mix and changeover discipline.

At EAST CNC this calculation is a normal part of selecting a machine and magazine configuration. You present parts, operations and changeover losses, and it becomes clear where 24 positions suffice and where 40 prevents unnecessary stops.