Most press brake purchases go wrong before the quotation stage — in the way the machine is specified. A workshop compares tonnage figures and prices, picks whatever looks like the best value per ton, and then spends the next five years working around the limits: a bed too short for the longest job, a backgauge that will not hold tolerance on thin gauge, a controller that nobody on the floor can program.

The sequence below is the one we use when specifying a press brake for a customer. It runs backwards from the part to the machine, not the other way around — because every specification that matters is derived from what you actually bend, not from what the catalogue offers.

1. Start with the part, not the machine

CNC press brake selection is a derivation, not a comparison. Before you look at a single machine, write down five numbers about your work. If a supplier quotes you a tonnage without asking for these, they are guessing.

Part audit worksheet — the five inputs that determine every downstream specification.

One more number worth recording, even though it is not a specification: the largest part you physically need to get onto the machine. A box-shaped part with a return flange needs clearance behind the tooling, and that sometimes rules out a machine that is otherwise perfectly sized on tonnage and bed length.

2. Press brake tonnage — how to calculate it properly

Tonnage is the specification buyers most often get wrong, and usually in the same direction: undersized, because the calculation was done on an average part rather than the heaviest one.

The working rule for air bending

For a 90° air bend in mild steel, with a V-die opening of roughly 8× the material thickness:

In other words: about 8 tons per metre of bend line, for every millimetre of thickness. The table below turns that into numbers you can read off directly.

Mild steel, 90° air bend. Tonnage rounds to the nearest whole ton. Figures are for sizing only — confirm against the tonnage chart supplied with your tooling.

Material factor: the step most buyers skip

Eight tons per metre per millimetre applies to mild steel only. Force scales with tensile strength, so the same bend in stainless or high-strength steel needs substantially more.

Approximate multipliers used by tooling manufacturers. For stainless, a wider V opening (10–12×t rather than 8×t) is also common, which reduces the force per metre — check both.

The trap: tonnage is rated for the whole bed, not per metre

This is where most undersized machines come from. A press brake's nominal tonnage is the total force available across the full bed. Divide it by bed length and you get the capacity per metre — the number that actually matters.

The reverse is also worth knowing: a rule of thumb you will see quoted — "one ton per millimetre of thickness per metre" — understates an air bend by roughly a factor of eight. Always check any rule against a V-die tonnage chart before you size a machine.

Two worked examples

Example A — 3 mm stainless steel, 2500 mm bend line

• Mild-steel base: 24 t/m × 2.5 m = 60 t
• Stainless factor 1.5: 60 × 1.5 = 90 t
• Headroom 25%: 90 × 1.25 = 113 t
• Specify: a 135-ton machine on a 3200 mm bed. The bed is longer than the part so tooling can be positioned with clearance at both ends.

Example B — 6 mm mild steel, 3000 mm bend line

• Base: 48 t/m × 3.0 m = 144 t
• Headroom 20%: 173 t
• Specify: 200 tons. On a 4000 mm bed that machine is rated 50 t/m — enough for 48 t/m, but only just. On a 3200 mm bed it is rated 62 t/m, which leaves real margin.

Tonnage is not the same as thickness capacity

A 200-ton machine does not "bend 20 mm steel". Maximum thickness is set by the V-die opening you can physically use, the punch radius, and the tooling's own load rating — often well before the machine reaches its tonnage limit. When a supplier quotes a maximum thickness, ask which V opening and which tooling that figure assumes.

3. Bed length and bend length

Bed length is usually chosen as "the longest part plus a bit". That is the right instinct but the wrong arithmetic, because three things have to fit: the bend line, the tooling, and the clearance for the part itself.

Two practical points follow. First, sectionalised tooling lets you run short parts on a long bed without losing accuracy — so a long bed is not wasted on small work, it is just diluted on tonnage. Second, if your long parts are thin and your short parts are thick, the economics may favour one long low-tonnage machine plus one short high-tonnage machine over a single compromise.

4. Drive type: torsion bar, electro-hydraulic or full electric

The drive and synchronisation system determines how consistently the ram stays parallel — which is what "bending accuracy" actually means in practice. There are three families, and the choice is driven by tolerance, batch profile and budget rather than by tonnage.

Model families shown are Botai's; the same three-way split applies across the industry.

Three questions that settle it

1. What tolerance do you have to hold? If ±1° on 1–2 mm sheet is acceptable, torsion bar is enough. If you are holding ±0.3° on thin gauge across a long bend, you need closed-loop servo with linear scales and crowning.
2. How many setups per day? Above roughly 10–15 setups per shift, programming speed and backgauge axes matter more than tonnage. Below that, a simpler machine with faster tooling change wins.
3. What is the cost of a bad part? Scrapped stainless or a rejected batch of painted panels changes the arithmetic fast — the accuracy premium is cheap against rework.

5. Controller and axes — what you actually need

Controller discussions tend to start with brand names. That is the wrong starting point. The brand matters mainly for operator familiarity and local service; the axes matter for what the machine can physically do. Get the axes right and the brand becomes a support question, not a capability question.

On thin gauge especially, the single most valuable option is usually motorised crowning, not extra backgauge axes. Without it, a long bend in 1.5 mm sheet will be over-bent in the middle and under-bent at the ends, and no amount of controller sophistication will fix that.

6. Tooling, crowning and the costs nobody quotes

Tooling is routinely left out of the comparison spreadsheet, and it is not a rounding error — a full set of segmented punches and dies, plus crowning, can add a meaningful percentage to the machine price. It also constrains what the machine can bend.

V-die opening and minimum flange

• V opening ≈ 8 × thickness for mild steel air bending; 10–12 × thickness for stainless.
• Inside radius ≈ V/6 in air bending — you cannot get a tighter radius than the die allows, whatever the punch shape.
• Minimum flange ≈ 0.6 × V opening. A 32 mm V die therefore needs roughly a 19 mm flange. Shorter flanges need a narrower die — which raises tonnage.

That last point catches people out: specifying a small flange can force a narrow V opening, which increases the tonnage required, which may push you into the next machine size. It is one reason the part audit in section 1 asks for the shortest flange, not just the thickest material.

Crowning: mechanical, hydraulic or motorised

Under load, the bed and ram deflect, so the middle of a long bend receives less force than the ends. Crowning compensates for this. Manual wedge crowning is cheapest and adequate for short bends in thicker material; hydraulic crowning is fast and repeatable; motorised (CNC) crowning is controlled from the program and is what you want if you change material and length often.

Items to put in the budget explicitly

• Segmented punch and die set — sized to your thickness range, not just one V opening
• Crowning system, if long or thin-gauge bends are in scope
• Quick-clamping / tooling changeover hardware — pays back in setup time
• Backgauge fingers and support arms for the part sizes you actually run
• Front support / sheet follower for large or heavy blanks
• Installation, foundation, lifting and commissioning
• Operator training and a spare-parts starter kit

7. The selection sequence, in order

Working through these seven steps in order avoids the circular arguments that make machine selection drag on for weeks.

1. Audit your parts. Five numbers: heaviest material, longest bend line, smallest radius, shortest flange, material mix.
2. Calculate tonnage from the worst case, apply the material factor, add 20–30%.
3. Fix bed length from the longest bend line plus tooling and part clearance.
4. Re-check tonnage per metre at that bed length. This is the step that changes the machine size most often.
5. Choose the drive type from tolerance and setups per day, not from price per ton.
6. Choose axes and crowning from the parts that are hardest to hold tolerance on.
7. Price the tooling and installation before comparing machine prices — otherwise you are comparing incomplete numbers.

8. Six specification mistakes that cost real money

1. Sizing on the average part

The machine has to handle the heaviest job in the order book, not the typical one. Sizing on the average guarantees you turn down work in year two.

2. Ignoring tonnage per metre on a long bed

The most common cause of an undersized machine. A long bed at the same nominal tonnage is a weaker machine per metre, not a bigger one.

3. Choosing bed length for the part but forgetting the part's shape

Box-shaped parts with return flanges need clearance behind the tooling, and sometimes a gooseneck punch. A machine that fits the blank can still be unable to form it.

4. Buying a controller brand instead of a support network

A well-known controller with no local service is worse than a less fashionable one with a responsive distributor and a stocked spare-parts channel.

5. Leaving tooling out of the comparison

Two quotations that look 10% apart can reverse once tooling, crowning and installation are included — or once you discover that one machine needs a die set you do not own.

6. Assuming tonnage equals thickness capability

Maximum thickness is limited by V opening, punch radius and tooling rating. A machine can be fully rated on tonnage and still unable to bend the thickness you had in mind.

Frequently asked questions

How do I calculate press brake tonnage?

For a 90° air bend in mild steel with a V-die opening of about 8× material thickness, allow roughly 8 tons per metre of bend length per millimetre of thickness. Multiply by a material factor (about 1.5 for stainless, 0.5–0.6 for aluminium), then add 20–30% headroom. A 3 mm mild steel bend over 2.5 m therefore needs about 60 tons, and roughly 75 tons with margin.

What tonnage do I need to bend 3 mm stainless steel over 2.5 metres?

Mild steel at 3 mm requires about 24 tons per metre, so 2.5 m needs 60 tons. Stainless needs roughly 1.5× that, giving 90 tons, and a 20–25% margin brings the requirement to about 110 tons. A 135-ton machine with a 3200 mm bed is the practical choice.

Is a 100 ton press brake enough for 4 mm sheet?

It depends on the bed length, not only the tonnage. Four millimetre mild steel needs about 32 tons per metre of bend line. A 100-ton machine with a 2500 mm bed is rated 40 tons per metre and can handle it; the same 100 tons on a 4000 mm bed is rated only 25 tons per metre, so it cannot bend 4 mm across the full bed.

Should I choose a 2500, 3200 or 4000 mm bed?

Match the bed to your longest bend line plus tooling and clearance, then check the resulting tonnage per metre. 2500 mm suits brackets and small cabinets, 3200 mm is the general-purpose default, and 4000 mm and above are for long panels and structural work — but a longer bed dilutes available tonnage, so higher tonnage is needed to keep the same capacity per metre.

Torsion bar or electro-hydraulic press brake — which should I buy?

Choose torsion bar when you bend general sheet metal of roughly 1–3 mm, work to normal commercial tolerances and need the lowest capital cost. Choose electro-hydraulic servo when you need consistent angle accuracy on thin gauge, frequent setup changes, or CNC crowning and multi-axis backgauge control. Full electric is worth the premium only for thin-gauge, high-volume or clean-room production.

How many axes do I need on a CNC press brake?

Y1/Y2 ram control and an X backgauge are the minimum. Add R (backgauge height) if you bend parts with different flange heights or box shapes, Z if you use two gauges or offset bends, and motorised crowning if you regularly bend thin gauge over long lengths. Extra axes only pay back if your operators will actually program them.