CNC Tool Magazine Capacity: How Many Pockets Does Your Work Really Need?

A machinist with a 21-tool umbrella changer on a Fadal asked others which tools they leave in the magazine for good. The answers split.

One keeps a face mill, a spot drill and a chamfer tool in the last three pockets and loads the rest for each job.

Another empties the changer every time and loads tools in program order, because he finds waiting for the umbrella to index costs more than reloading it. A third, running dual-arm changers, said the next tool is always waiting, and admitted that on his 48-pocket machine he seldom knows what’s in there.

Each answer was a way of working around the magazine in front of them. When you’re the one buying, size the magazine for the stretch of production you want to run without reloading, and check how it readies the next tool.

Count pockets for that set of parts, add sister tools and tools outside the programs, then check how many tools are too big to sit next to a neighbor. That count, not your longest program, tells you whether 24 pockets will do.

1. Count by part family, not by program

For example, your longest program might use 18 tools. That number only matters if you’ll run that part alone. If you want to switch between three related parts without touching the magazine, you need every tool any of them uses, with the shared ones counted once.

Then add any resident tools that aren’t already on that list, and decide how many pockets you’ll keep free for the next job’s specials, so you’re not unloading resident tools to fit them.

2. Add sister tools and tools outside the programs

If the machine runs overnight, a drill that wears out at 2 a.m. stops the job unless there’s a second one in the magazine for the control to switch to when the first reaches its life limit. Each sister tool takes a full pocket. Our article on broken-tool detection for unattended running covers how the machine knows when to switch.

Then add the spindle probe, which doesn’t cut but lives in a pocket, and any deburring or rework tools you keep loaded that aren’t in this batch’s programs. They’re easy to forget when you add up a program.

3. Check the big tools

Pocket count assumes every tool fits its own pocket. Large face mills and boring heads often don’t. Take a typical BT40 arm-type disc magazine from OKADA, the maker that builds ours.

• Every pocket full: tools up to about Ø70–80 mm
• Both neighbors left empty: one tool up to Ø140–150 mm
• Length up to 300 mm, weight up to 8 kg

So on that magazine, one 125 mm face mill doesn’t cost you one pocket. It costs three. Two of them can cost six, unless your magazine and control let you place them to share an empty pocket. A tool over the empty-neighbor limit doesn’t fit at all. List your large tools before you count, and check them against your magazine’s own figures, not just the spindle’s.

4. Check how the next tool gets ready

On that Fadal, as on most umbrella magazines, each tool goes back to its own pocket before the magazine turns to find the next one. An arm-type magazine swaps the tools in one move, and while the current tool is cutting, the magazine can turn the next one into the change position.

That’s also why you should read tool change times carefully. Tool-to-tool time covers the change itself. Chip-to-chip time runs from the last cut of one tool to the first cut of the next, so it adds the spindle stopping, the moves to and from the change position, and the spindle getting back up to speed. When you compare quotes, make sure the numbers are the same kind.

Before you order

• List every tool for the parts you’ll run without reloading, shared tools once, plus resident tools not already on the list.
• Add sister tools for unattended hours, the probe, and loaded tools outside the programs.
• Check every large tool against the magazine’s full-pocket and empty-neighbor limits, and count the pockets it really takes.
• Leave a few pockets free for the next job’s specials.
• Ask for tool-to-tool time, and whether the next tool is staged while the current one cuts.

Then compare the total with what you’re being offered.

If you reload for each job and don’t run unattended, a bigger magazine mostly holds tools you’d have swapped anyway.

How we size magazines on our machines

Our V- and L-series vertical machining centers and LW-series horizontals all run 24-pocket magazines, and each of them can take a larger magazine when your count comes out higher. On our BT40 machines, tool-to-tool time is as low as 1.8 seconds, with the next tool staged while the current one cuts.

For small parts with lots of holes, our T-series drill-tap centers change BT30 tools in as little as 0.8 seconds; our comparison of drill-tap centers and VMCs covers when that’s the better fit.

Send us your tool list for the parts you plan to run, including the large tools and how long you want the machine to run unattended, through our inquiry form. We’ll count the pockets with you, recommend the magazine and machine, and quote it. See the full range on our machining center page.

FAQ

With a random-pocket changer, does T1 stay in pocket 1? No. A dual-arm changer swaps the tool in the spindle with the next one, so tools end up in different pockets, and the control keeps track of where each one is. T1 is still T1; it just may not live in pocket 1.

Can my program use tool numbers higher than the number of pockets? It depends on how the machine builder set up the control. Some machines, often those with random-pocket magazines, keep an editable table that assigns tool numbers to pockets; others fix tool numbers to pockets or block numbers outside a set range. If you want permanent tool numbers across a large tool library, ask about this before you order.

Sources

• Practical Machinist forum, VMC tool changer mandatory tools, January 2024
• Practical Machinist forum, Program tool numbers greater than machine’s number of pockets, January 2019
• OKADA, Servo vertical disc tool magazine (BT40 capacity, tool diameter, length and weight limits)
• CHANSIN CNC V-, L-, LW- and T-series specifications