Buyer guide

Sheet Metal Bending Services: How to Pick a CNC Bending Supplier

This guide is for engineers and buyers sourcing sheet metal bending services on CNC press brakes. It covers the checks that decide whether a part bends clean the first time: bend radius limits, tooling, tolerance, finish, inspection and lead time. By the end you should know what to ask for and what to walk away from.

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±0.005 mm machining toleranceBend lengths to 4,000 mmNo minimum order quantityISO 9001 / IATF 16949

sheet metal bending services on CNC bending machines

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What decides a good bend

Geometry beats machine sizeA small brake can be wrong for a thin bracket and right for a 6 mm plate. Match tonnage to thickness.

Tooling sets the radiusAir bending gives roughly 0.16 × die opening as inside radius. Tighter radii need a different die.

Flat pattern is a shared jobSend the 3D model. The shop should return a corrected flat pattern with its K-factor.

Tolerance is per featureA bend angle and a hole position do not carry the same tolerance. Say which one matters.

Ask for the inspection recordAngle, flange length and hole position should be on paper before the parts ship.

Judging the job

Which bending route fits your part

Pick the row that matches your drawing, not the row that matches your budget.

Part condition	Route that works	What to watch
Thin sheet under 1.5 mm, long flange	Air bending on a CNC press brake	Springback; compensate with over-bend angle
Inside radius tighter than 1 × thickness	Bottoming or coining	Higher tonnage; cracking risk on 7075 and 6061-T6
Short run, one to fifty pieces	Press brake with standard punch and die	Setup time dominates; check the hourly rate
Panel with many holes and cutouts	Laser or punch first, then bend	Bend lines must clear every cutout edge
Box or enclosure with four bends	Press brake with gooseneck punch	Punch clearance against the previous flange
Thick plate over 6 mm	Heavy press brake, large die opening	Tonnage per meter; radius grows with die width
Tight angle tolerance ±0.5° or better	CNC brake with angle measurement	Material batch variation still moves the angle

Bend geometry

Bend radius, K-factor and the flat pattern

Most quoting arguments start with the flat pattern. A designer draws a 90° corner in 3D, exports a flat DXF, and assumes the shop will fold it back into shape. It will not. When steel bends, the neutral axis shifts inward and the outer surface stretches. The flat length has to be shorter than the sum of the two legs.

The K-factor is the ratio that describes where that neutral axis sits. For air bending in mild steel it commonly lands between 0.33 and 0.45 of the thickness, depending on die width and material. Aluminum runs lower, around 0.30 to 0.40. If the shop folds your flat DXF without recalculating, every flange comes out long or short by the same amount.

Inside radius is the other half of the problem. Air bending produces an inside radius of roughly 0.16 × die opening. A 1.5 mm sheet in a 12 mm die lands near R1.9. If the drawing says R0.8, the shop either switches to a narrower die, which raises tonnage, or coins the bend. Coining forces the punch into the material and can crack high-strength grades.

Rule of thumb: keep the inside radius at or above the material thickness for aluminum 6061-T6 and 7075, and at or above 0.8 × thickness for mild steel and 304 stainless. Below that, expect orange-peel surface on the outside of the bend or a full fracture on the first article.

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Send the 3D model, not only the DXFA STEP file lets the shop rebuild the flat pattern with its own K-factor and die set.
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State the bend radius you actually needIf R1.5 is fine, say so. A tighter call adds tooling cost for no function.
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Mark the critical bendOne or two bends usually set the assembly fit. Flag them so they get checked first.

Tolerance and fit

What tolerance can sheet metal bending services hold

Bend angle and bend position are different animals. A CNC press brake with angle measurement holds about ±0.5° to ±1° on a 90° bend in 1.5 mm mild steel. Position of the bend line along the flange is tighter, usually ±0.1 mm to ±0.2 mm, because the backgauge sets it directly.

Cross-section dimensions stack up fast. Four bends at ±0.2 mm each can put a box corner out by nearly a millimeter if every bend runs the same direction. That is why the critical dimension should be measured from a datum, not from the previous bend. A shop that measures from the datum will hold the assembly fit.

Holes and slots need clearance from the bend line. Keep hole edges at least 2.5 × thickness away from the start of the bend radius, or 3 × thickness for holes that carry a fastener. Closer than that and the hole distorts into an oval, and a bolt will not seat.

Material batch matters more than most buyers expect. Yield strength varies between heats, and springback varies with it. A shop that keeps its die sets and its press brake program unchanged between batches will see the angle drift. Good practice is a first-article check on every new heat, not only on the first order.

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Give the datumDimension from a machined face or a tooling hole, not from a bend.
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Keep holes off the bend2.5 × thickness minimum from the bend radius start.
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Expect angle drift±1° is normal. If you need ±0.3°, plan for a post-bend check.

Supplier checks

How to screen a bending supplier before you send the PO

Ask what press brakes they run before you ask the price. Tonnage and bed length tell you the real size limit. A shop with a 4,000 mm bed can bend a full-length panel in one hit; a shop with a 2,000 mm bed will either weld two pieces or turn the job down. Bed length is the first thing to confirm, because it cannot be worked around.

Tooling inventory is the second. Standard punch and die sets cover most work. Gooseneck punches and sectional dies are what make a box or a return flange possible. If a shop has to buy a special punch for your part, that cost lands on your quote and adds a week.

Ask how they check the first article. Angle gauge, calipers, or a CMM? For an enclosure that has to fit a PCB, the hole pattern matters more than the angle. A shop with in-house measurement can report hole position to ±0.05 mm; a shop without it will send the part and hope.

Finally, ask about the material certificate and the finish. Bending alone rarely ships. Most parts get anodizing, powder coating, zinc plating or laser marking afterward. One vendor handling both bending and finishing removes the shipping step where parts get scratched, and it removes the finger-pointing when a coated part no longer fits.

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Confirm bed length and tonnageThese set the maximum part size. Everything else is secondary.
2
Ask for the tooling listGooseneck and sectional dies decide whether your box can be formed.
3
Check the finishing chainIn-house anodizing or powder coating shortens the route and the risk.

Step by step

From drawing to first article

Seven steps that keep a bending job on schedule.

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1. Send the 3D model and the drawingSTEP or IGES plus a PDF with the critical dimensions marked. Note the material grade and temper, for example 5052-H32 or 304 stainless.
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2. Ask for DFM feedback before quotingA useful reply points out bends tighter than 1 × thickness, holes closer than 2.5 × thickness to a bend, and any feature the die cannot reach.
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3. Agree the flat pattern and K-factorLet the shop rebuild the flat from the 3D model. Confirm the bend allowance on one corner before the whole nest is cut.
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4. Confirm tooling and tonnageAsk which punch and die will run the job, and the tonnage per meter. This is where a tight radius gets flagged before the steel is cut.
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5. Run the first articleBend one part and measure angle, flange length and hole position. Compare against the drawing, not against the flat pattern.
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6. Approve finish and markingDecide the finish before bending if the part will be anodized, since masking and rack marks change the layout. Laser marking needs a character height of at least 1.5 mm.
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7. Release the run and hold the recordKeep the first-article report with the batch. If a later lot drifts, the report tells you whether the die or the material moved.

FAQs

Questions buyers ask before ordering

Can you bend a part from my flat DXF only?

We can, but we will rebuild the flat pattern first. A DXF exported from 3D CAD usually carries the designer's K-factor, which may not match our die set. Rebuilding takes a few minutes and avoids a first article that is long or short on every flange.

If the part is a simple L-bracket with one bend and a loose tolerance, the risk is small. For a box with four bends and a mating cover, the flat pattern has to be recalculated.

What is the tightest inside radius you can hold?

Air bending gives an inside radius of roughly 0.16 × die opening. With a narrow die we can go close to 0.8 × material thickness in mild steel and 304 stainless, and about 1 × thickness in 6061-T6 and 7075.

Below that we would coin the bend, which needs more tonnage and raises the risk of cracking on high-strength aluminum. If your drawing calls for a hairpin radius, a machined or cast part is often the better route.

How do you hold the bend angle on a long panel?

CNC press brakes with angle measurement compensate in real time, and a long panel is usually bent in several hits with the ram crowned. We hold about ±0.5° to ±1° on a 90° bend in 1.5 mm mild steel.

Springback is the variable that moves the angle between material batches. We check the first article on each new heat of material rather than trusting the program from last month.

Can bending and machining be combined on one part?

Yes, and it is common. A bracket often gets its holes milled after bending, or a bent panel gets a machined boss welded on. Running both operations under one roof means the datum carries from the press brake to the mill.

We run 127 CNC machines alongside the sheet metal work, so a bent part can go straight to 5-axis machining without a second shipping step.

What lead time should I plan for?

Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours of the order, and parts ship in 3–5 days for typical runs.

Long panels and special tooling add time. If your part needs a custom punch or die, add a week for tooling before the first article.

Do you sign an NDA for new parts?

Yes. Uploads are secure and confidential, and we sign an NDA on request before drawings are shared. That covers automotive, medical and defense-adjacent work where the drawing itself is sensitive.

We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022, so the documentation trail matches what these industries expect.

Send the drawing, get a bending quote and DFM notes

Upload your 3D model and drawing. You get a quotation and a free DFM analysis within 12 hours, with the bend radii and hole clearances flagged before anything is cut.

12-hour quote100% inspection before shipmentNo minimum order quantity

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