CNC wire forming vs four-slide: volume, complexity and cost

Two ways to move a tool into the wire

Both machines start with wire from a coil, straighten it, feed it and cut it. The difference is how the tooling moves. A CNC wire former uses servo axes to position the wire and the tools. The wire can be fed, rotated and bent in sequence, and the next bend can be in a different plane without a new cam.

A four-slide machine uses four slides that move toward a central point, driven by cams on a main shaft. The tools are shaped to the part, and the sequence is built into the cam and the slide setup. The machine runs the same motion thousands of times an hour, which is why it is fast and why the tool is specific to one part.

The distinction matters commercially. A CNC former carries its knowledge in a program. A four-slide carries it in a set of hardened tools. One is cheap to change and slow per part. The other is expensive to change and fast per part.

How each process makes a bend

On a CNC former, a bend pin or roller is driven into the wire while the wire is held and rotated. The controller applies an overbend to compensate for springback, and the value is tuned at the trial. Because the axes are independent, the machine can form a loop, reverse the bend direction and change plane in one cycle.

On a four-slide, each tool comes in from one of four directions and forms the wire against a centre tool. The bend is created by the shape of the tool and the stroke of the slide. Springback is corrected by tool geometry rather than by a program value, so a material change usually means a tool change or a tool adjustment.

This is the reason a four-slide is happiest with parts that can be formed in a plane, or in two planes with simple tooling. A part that needs a bend in a third direction, a long leg that must clear the tooling, or an internal loop may not fit the slide envelope at all.

Cycle time and the volume crossover

Cycle time is where the four-slide earns its tooling. A simple part can run in a second or two on a four-slide, while a CNC former may take several seconds per part because it forms one feature at a time. On a ten bend clip the difference can be three or four to one.

The crossover is the volume where the lower cycle time has paid back the higher tooling charge. Take an illustrative comparison: a CNC route with £900 of tooling and a nine second cycle at £54 per hour, against a four-slide with £6,500 of tooling and a 2.5 second cycle at £48 per hour. The CNC piece price starts near £0.14 in conversion and the four-slide near £0.03. The tooling difference is £5,600, which takes about 55,000 pieces to recover.

Below that volume the CNC route is cheaper in total. Above it, the four-slide pulls ahead and the gap widens. At 200,000 pieces the CNC conversion cost is still £0.135 per part while the four-slide is nearer £0.03, so the annual saving is real money.

Those numbers are illustrative and will move with the part, the machine and the local rate. The method is what matters: build both routes with their own tooling and cycle time, then compare at your volume.

Geometry: where each process stops

The CNC former wins on complexity. Bends in three planes, loops, reverse bends, long legs and hooks are all within reach, subject to tool clearance. A part with fourteen bends and two changes of plane is a normal CNC job and a difficult four-slide job.

The four-slide wins on simple, high volume parts: clips, terminals, small brackets and retainers with a sequence of bends in one or two planes. It is also strong where the part needs a cut and form in the same cycle, because the tool can shear the wire and form the end without a second operation.

There are limits on both sides. A CNC former cannot make a part smaller than the tooling can reach, and very small parts can be hard to grip and eject. A four-slide cannot easily produce a bend that needs the wire to rotate about its own axis, and long parts can exceed the slide stroke. Ask for a tooling review on the actual drawing rather than judging from a family photograph.

Tooling cost and lead time

CNC tooling is mostly engineering. A program, a set of standard pins and a trial, with a small number of part specific items. The charge is usually measured in hundreds rather than thousands, and the lead time is short because most of the work is on a computer.

Four-slide tooling is a machined set: slides, cams, form tools and a cut off. The charge is closer to a stamping tool and the lead time runs to weeks, because the tools are hardened and tried. A revision after the trial can mean reworking a hardened tool, which is slower and more expensive than editing a program.

The tooling comparison should include the change rule. Ask what a bend revision costs on each route and how long it takes. A part that is still moving in design is a poor candidate for a four-slide, even if the volume looks right, because every change carries a tool cost.

Changeover, revisions and programme risk

A CNC former changes over by loading a different program and swapping a few tools. That makes it suitable for a family of similar parts, for kanban supply and for a programme with several variants. The machine can also make a small replacement batch without a long setup.

A four-slide changeover is a tool change. It takes longer, and the machine is only efficient when the run is long enough to justify it. That is not a fault; it is the economics of the process. A supplier who puts a 2,000 piece order on a four-slide will either charge a setup that reflects the changeover or lose money on the job.

Programme risk follows the same pattern. A CNC part can move to another supplier with a program and a drawing. A four-slide part needs the physical tool, and the tool may not travel well or may not fit another machine. If continuity of supply matters, the tool ownership and transfer terms belong in the contract.

The comparison in one place

The table compares the two routes on the questions that decide a quote. The figures are illustrative and assume the same ten bend clip in 2.0 mm music wire.

How to choose

Start with geometry, then check volume. If the part needs bends in three planes, a loop or a long unsupported leg, the CNC route is the natural first quote. If the part is a planar clip with a simple bend sequence and the volume is above the crossover, ask for a four-slide quote as well.

Ask both suppliers to quote the same volume and the same release pattern, and to separate tooling from the piece price. A four-slide quote that looks cheap at 200,000 pieces can be twice the CNC price at 20,000. The comparison only means something when the volume is fixed.

For a programme with uncertain volume, the CNC route carries less risk. For a stable, high volume part, the four-slide pays for its tool several times over. A few suppliers run both machines and will recommend the route that fits, which is usually a better answer than a buyer choosing from a brochure.

• Count the bends and the number of planes before comparing prices
• Ask for tooling as a separate line on both routes
• Compare total cost at your annual volume and release size
• Check the cost of a revision if the design is still moving
• Confirm tool ownership and transfer if continuity matters