Stretching & Stretch-Forming Machine — Stretch-Straightening of Sections and Plate, Stretch Forming of Skins

Name: Stretching and Stretch-Forming Machine
Brand: Neometrix
SKU: NMX-STR-25

01

Overview

Two jobs, one piece of physics.

Straightening what is bent and forming what is flat are the same operation, pointed at different ends.

Fig · 02 — The same machine principle, twice. Straighten what is bent, or form what is flat over a block — both by pulling the metal past yield.

Straightening by pressing pushes the error somewhere else. Straightening by stretching takes the whole length past yield at once, so there is nowhere for it to hide.

Why a straight bar is not straight

Metal keeps a record of how it was made. It flows unevenly through a die, cools unevenly on a run-out, and pulls itself out of shape as it goes.

The result is a section that looks acceptable and is not. It has bow along its length, camber across it, and twist about it. Worse, it carries residual stress — a balance of internal tension and compression that is holding the shape together.

That stress is the real problem. A bowed bar can be seen. Locked-in stress cannot, and it waits. Cut into the part on a machining centre and the balance breaks. The part moves after it has been machined, which is the most expensive moment for it to move.

What stretching actually does

Pulling the whole length past its yield point makes every fibre give way at once. The metal stops remembering its old shape because it has been given a new one.

This is why the aluminium industry writes it into the temper itself. A plate marked T651 is not just heat treated and aged — the middle digits record that it was stress relieved by stretching. The same is true of T351. The stretch is part of the material specification, not a finishing touch.

And why the same machine forms

Bend a sheet and it springs back, because only part of it went past yield. Stretch the whole sheet past yield first, then wrap it over a form block, and there is almost nothing left to spring back.

That is stretch forming, and it is how large contoured skins are made accurately and repeatably. The contour belongs to the die. The sheet simply stops arguing with it.

02

The cycle

Five steps, and a window two percent wide.

The machine's whole difficulty is knowing exactly when to stop pulling.

FIG · 02THE STRETCH CYCLE · GRIP / TAKE UP SLACK / PULL PAST YIELD / HOLD / RELEASE — AND THE CURVE THAT DECIDES IT

The step people underrate is the second one. Until the slack is out and load has just begun, the head is moving but the metal is not stretching — so that instant, not the start of the stroke, is where elongation has to be counted from.

1 · Grip

Wedge jaws close on both ends. They are self-energising: the harder the machine pulls, the tighter they hold, which is what stops a slip at full load.

2 · Take up the slack

The moving head creeps out until load just begins to register. That instant is the zero point, and everything measured afterwards is measured from it.

3 · Pull past yield

The head draws out until the set permanent elongation has been put into the whole gauge length, typically a fraction of one percent to about three.

4 · Hold

The strain is held briefly while the section settles at its new length and the load across it evens out end to end.

5 · Release

The jaws open. The section is straight, its residual stress is evened out, and the gripped ends are cropped off as scrap.

Where it goes wrong

Under-stretch and the shape comes back within hours. Over-stretch and the metal necks, thins locally and loses ductility. Neither is recoverable.

03

Work content

What the machine contains, element by element.

Read it as a checklist: a machine missing a row will buy that row later, at integration prices.

Fig · 03 — The same machine as a drawing, with the parts named. The bed and its foundation carry the whole pull as a tension tie between the two heads.

Element	What it does	What matters
Fixed head	anchors one end of the pull	stiffness; it must not become part of the measurement
Moving head	applies the stretch	guided so the pull stays axial, not skewed
Bed & foundation	carries the full pull as a tension tie	engineered with the machine, never left to civil works
Gripping jaws	hold the ends without slipping or crushing	self-energising wedges; jaw sets per section family
Hydraulic pull cylinders	generate and hold the tension	synchronised, so the two sides pull together
Hydraulic power pack	feeds the cylinders	sized for a long slow pull, not a fast cycle
Elongation measurement	counts the stretch from the zero point	the machine's real instrument
Load measurement	records force through the pull	evidence and a safety limit, not the target
De-twisting head	rotates one jaw to take out twist	where bow is not the only defect
Controller & recipes	runs the same stretch every time	a recipe per alloy, section and temper
Form block mounting	carries the die for stretch forming	the contour is only as good as its support
Handling & feed	gets long sections in and out	where the working rate is actually won
Guarding & interlocks	keeps people clear of a loaded machine	stored energy is high right up to release
Records	ties each stretch to the piece	the temper claim has to be defensible
Training & documentation	operators run it; records survive audits	handed over with the machine

Fig · 04 — The hardest part of the machine. The jaw has to hold against the full pull without slipping, and without marking the metal — illustrative render.

The row that decides the machine is not the cylinder. It is the pair of jaws and the foundation: gripping a section hard enough without marking it, and reacting the pull without the bed breathing, are the two problems that make a stretcher difficult to build well.

Full specification — expand

Machine	Hydraulic stretching machine: fixed head and moving head on a tension-tied bed, self-energising wedge gripping jaws, synchronised pull cylinders and power pack, closed-loop elongation control with load recording, optional de-twisting head, optional form-block mounting for stretch forming, guarding, handling and records
The One Idea	A press controls force. A stretcher must control strain. The same pull yields one alloy and barely moves another, so what is set and held is permanent elongation, not tonnes
Job One — Straightening	Extruded sections, bar and rolled plate pulled past yield to remove bow, camber and twist, and to even out the residual stress that would otherwise appear later as distortion on the machining bed
Job Two — Stretch Forming	Sheet gripped at both ends, stretched past yield and wrapped over a form block, so the contour taken is the die's rather than the material's and springback is nearly eliminated — the route to large, accurate, repeatable contoured skins
The Window	Useful permanent set runs from about half a percent to three percent. Below it the shape returns; above it the metal necks, thins and loses ductility. The usable window is roughly two percent wide, which is what makes closed-loop control necessary rather than nice
The Zero Point	Elongation is counted from the instant load first registers, not from the start of the stroke. Until the slack is out the head is moving and the metal is not, so finding zero correctly is half the accuracy of the machine
Capacity Class	250 to 2,500 tonnes of pull. Capacity follows from the largest cross-section multiplied by the alloy's yield strength, plus margin — it is calculated from your product, never chosen from a catalogue
The Foundation	The whole pull is reacted between the two heads, so the bed and its foundation are a tension tie and part of the machine. They are designed with it, and the civil work is scoped at design review rather than discovered on site
Gripping	Self-energising wedge jaws that tighten as the pull rises, with jaw sets matched to the section family. Gripping hard enough not to slip, without marking or crushing the ends, is the hardest sub-problem in the machine
Tooling	Jaw sets per section family; form blocks per contour; stored recipes per alloy, section and temper, so a change of product is a change of parts and a selection rather than a rebuild
Records	Each stretch written against the piece and the recipe used — because a temper designation that records stretching, such as T651, is a claim that has to be defensible long afterwards
Scope Boundary	This page is the stretching machine as a machine in its own right. The 500 tonne stretcher supplied inside an aluminium extrusion line — in-line, hot, downstream of the press — belongs to the 10,000 tonne extrusion press page, including as a retrofit to an existing press
Status	Neometrix has quoted against successive stretching machine requirements across the 250 to 2,500 tonne class, for stretch-straightening of extruded sections and plate and for stretch forming of contoured skins, and engineers the class to order; no delivered machine of this class is claimed.

04

Configurations

One principle, three machines.

The physics is fixed. What changes is the length, the tonnage and what the machine is chasing.

Straightener

Section & bar stretcher

A long bed and a modest pull. It takes bow, camber and twist out of extruded profiles and bar, with a de-twisting head where twist is the harder defect.

Plate stretcher

Heavy stress relief

Wide jaws and a large pull, sized to the full cross-section of rolled plate. This is the machine behind a stress-relieved temper, and the tonnage climbs quickly with width.

Stretch former

Contoured skins

The same pull, aimed at a form block instead of a straight line. Sheet is stretched past yield and wrapped to the die, which is how a large skin holds its contour.

A large stretch-forming machine drawing a flat aluminium skin panel around a smooth convex form block, gripped at both ends by hydraulic jaw carriages that pull the sheet taut as it wraps the die

Fig · 05 — The second job. The sheet is stretched past yield first and wrapped after, so the contour it keeps is the block's — illustrative render.

And the thing that is not a machine at all but decides all three: the jaw sets and the recipes — grips matched to the section family you actually run, and a stored stretch for every combination of alloy, section and temper.

05

Where it is used

Wherever metal has to stop moving.

The common thread is a part that will be machined, assembled or flown — and must not change shape afterwards.

Extrusion and rolling plants

Straightening and stress relief on the way out of the plant, so a section can be sold against a temper that records the stretch.

Aerospace structures

Stretch forming of skins, leading edges and large contoured panels, where springback and repeatability decide whether parts fit at assembly.

Machining shops

Stock that has been stretched does not spring out of tolerance once it is opened up on the bed. The stretch is bought to protect the machining hours.

Defence and heavy fabrication

Long members and plate that must stay straight and dimensionally stable through welding, assembly and service.

06

FAQ

Common questions.

Longer answers, for readers who want the reasoning.

Q · 01 Why stretch at all — why not just press it straight?

Because pressing moves the error rather than removing it. A roller or gag press straightens by bending the section back the other way at the point where it is bent, which yields the metal locally. The visible bow goes, and the eye is satisfied, but the section is now carrying a new pattern of locked-in stress placed exactly where the press worked on it. Stretching does something different in kind. Gripping both ends and pulling takes the entire length past its yield point at the same time, so every fibre gives way together and the internal stresses settle into a far more even balance. That is why the result survives the next operation. Take a pressed bar to a machining centre, cut into one side of it, and the balance it was holding breaks — the part bows on the bed after it has been machined, which is the most expensive possible moment to move. A stretched bar has much less left to give up, so it stays where it was put. The distinction is worth stating plainly: pressing is a shape correction, stretching is a stress treatment that happens to also straighten.

Q · 02 How much stretch is right, and what happens either side of it?

Useful permanent set is small — typically from about half a percent to around three percent of the gauge length, depending on the alloy, the temper and how badly the section started out. Both edges of that range are unforgiving. Fall short and you have not really passed the yield point along the whole length: the section springs back, sometimes not immediately but over the following hours as it relaxes, and it arrives at the customer bent. Go too far and the metal begins to neck — deformation stops being spread evenly along the length and concentrates in one place, which thins the section there, reduces its ductility and can put the mechanical properties outside what the material specification allows. Neither error can be corrected afterwards; an over-stretched section is scrap. So the usable window is roughly two percent wide, and it is not in the same place for every job. That single fact is what dictates the machine's whole control philosophy: you cannot get there by setting a pressure and watching a gauge, because the same pull yields one alloy and barely moves another. The machine has to measure the stretch itself and stop at a number.

Q · 03 Why is controlling force not good enough?

Because force is an input and the thing you actually care about is an output. The load needed to take a section past yield depends on its cross-sectional area and on the yield strength of that particular alloy in that particular temper — and yield strength varies between alloys by a factor of several, varies with temper, and varies from batch to batch within a tolerance band. A pull that puts one percent into a soft alloy may not reach yield at all in a hard one. There is a second problem, which is subtler and catches people out: the section is not starting from a clean sheet. It already carries residual stress from extrusion or rolling, so it begins somewhere on the stress-strain curve that you did not choose. Setting a force therefore lands you at an unknown strain. The machine consequently works the other way round. It finds the zero point — the instant load first registers, once the slack is out — then measures how far the moving head travels from there, expresses it as a percentage of the gauge length, and stops at the set value. Force is still measured, continuously, but it is used as evidence and as a safety limit, not as the target.

Q · 04 What decides the tonnage of the machine?

Your product, not a catalogue. The pull required is essentially the largest cross-sectional area you intend to stretch multiplied by the yield strength of the strongest alloy you intend to stretch it in, plus a working margin for the fact that the metal strain-hardens as it goes past yield and for the friction and inefficiency of the grips. That arithmetic is why capacity climbs so steeply with plate width: doubling the width of a plate doubles the area and therefore doubles the pull, which is how machines that sound enormous become ordinary once the product is written down. It also explains the wide capacity spread in this class — a stretcher for slim extruded profiles and one for heavy rolled plate are the same machine in principle and very different machines in steel. The right way to arrive at a number is to start from the sections and plates you actually make, in the alloys and tempers you actually supply, take the worst case, and size from there. Choosing a tonnage first and hoping the product fits inside it is how a machine ends up either unable to do the job or paid for twice over.

Q · 05 Why is the foundation treated as part of the machine?

Because it carries the load. In most machine tools the frame reacts the working force and the floor merely holds the machine up. A stretcher is different: the pull is developed between two heads that are a long way apart, and the whole of it — hundreds or thousands of tonnes — has to be carried back between them through the bed and the foundation, which together act as a tension tie. Two consequences follow. The first is structural: the foundation is a designed, reinforced element sized for a specific tensile load and for the fatigue of applying it thousands of times, not a slab poured to a standard thickness. The second is metrological, and it is the one that surprises people. If the bed stretches under load, that movement is added to the movement of the head — and since the machine measures elongation by watching the head, a breathing foundation is silently measured as stretch that the workpiece never received. The section is under-stretched and nothing in the data says so. That is why the bed, the foundation and the measurement have to be engineered together, and why the civil scope is settled at design review rather than discovered on site.

Q · 06 Has Neometrix built one of these?

We would rather answer this plainly than let a page imply otherwise. Neometrix has quoted against successive stretching machine requirements across the 250 to 2,500 tonne class, for stretch-straightening of extruded sections and plate and for stretch forming of contoured skins, and engineers the class to order; no delivered machine of this class is claimed. What stands behind the offer is adjacent and real. We engineer and deliver heavy hydraulic machines and the closed-loop control that goes with them, and the 10,000 tonne extrusion plant we scope as a turnkey project includes a 500 tonne stretcher with a de-twisting arrangement and its own independent hydraulic system — the same heads, jaws, cylinders, control and foundation problem, sitting inside a line rather than sold as a machine. So the honest position is this: the class is engineered to order, the engineering is in the building, and the first machine of this exact type will be built to a customer's product rather than lifted off a shelf. If that matters to how you buy, say so early and we will scope it that way, with the reference work open to inspection.

Q · 07 What is the hardest part to get right?

The jaws, by a distance. Everything else in a stretcher is heavy engineering of a familiar kind — cylinders, a bed, a power pack, a controller. Gripping is where the machine is won or lost, because the requirement contains a contradiction. The grip has to hold the end of the section against the full pull of the machine without the slightest slip, since a slip at load is both a safety event and an instant loss of the measurement. At the same time it must not crush, mark or notch the material, because a notch at the jaw is a stress raiser and the section will simply break there instead of stretching. The answer is a self-energising wedge design: the geometry converts the pull itself into clamping force, so the harder the machine pulls the tighter the jaw holds, and the grip cannot be under-tightened by an operator in a hurry. Jaw faces are then matched to the section family — a round bar, a wide flat and an asymmetric extruded profile each need their own bearing surface to spread the load. This is why a stretcher is quoted with jaw sets rather than one pair, and why the list of sections you intend to run is one of the first questions we ask.

Q · 08 What do you need from us to quote?

Five things, and they are all about your product rather than about the machine. First, the sections: the largest cross-section you intend to stretch, with drawings or a profile family, since area sets the tonnage. Second, the alloys and tempers, because yield strength is the other half of that calculation and it is what stops the machine being guessed at. Third, the lengths you handle, which set the bed, the building and the handling — long sections often decide the layout before the tonnage does. Fourth, the job: straightening, stress relief to a specified temper, stretch forming to a contour, or more than one of those, since a de-twisting head or a form-block mounting is scoped in or out at this point. Fifth, the rate and the building: how many pieces a shift, and what floor, headroom and foundation depth you have to work inside. From that we come back with a capacity calculation you can check, a layout, a jaw-set proposal, the foundation loads, and a budgetary price. If you would rather start with a conversation than a specification, that works too — most of these projects begin with somebody describing a part that will not stay straight.

07

Three neighbours in the same heavy-hydraulics family.

↗ The line

10,000 Tonne Extrusion Press

Where a stretcher sits inside the plant — a 500 tonne in-line unit, hot, straight off the run-out.

↗ The press

Blanking & Cupping Press

The other way to move metal — force through a die, where a stretcher works by tension.

↗ The measurement

Servo-Hydraulic Fatigue Machine

The same closed-loop discipline, used to find the yield point rather than to cross it.

Browse all Neometrix product lines.

Get a quotation

Tell us the sections, the alloys,
and what will not stay straight.

The projects desk replies within two working days with a capacity calculation you can check, a layout, a jaw-set proposal, foundation loads and a budgetary quotation. Write to [email protected] or use the form.

Enquire — stretching machine Capability sheet (PDF) +91 7777 876 876

ISO 9001 / 14001 QUOTED CLASS — STRETCHING & STRETCH-FORMING MACHINE GRIP · TAKE UP SLACK · PULL PAST YIELD · HOLD · RELEASE — A PRESS CONTROLS FORCE, A STRETCHER CONTROLS STRAIN ENGINEERED IN NOIDA · INDIA

Metal remembers. Stretching makes it forget.

Two jobs, one piece of physics.

Why a straight bar is not straight

What stretching actually does

And why the same machine forms

Five steps, and a window two percent wide.

1 · Grip

2 · Take up the slack

3 · Pull past yield

4 · Hold

5 · Release

Where it goes wrong

What the machine contains, element by element.

One principle, three machines.

Section & bar stretcher

Heavy stress relief

Contoured skins

Wherever metal has to stop moving.

Extrusion and rolling plants

Aerospace structures

Machining shops

Defence and heavy fabrication

Common questions.

The line it came from, and the presses beside it.

10,000 Tonne Extrusion Press

Blanking & Cupping Press

Servo-Hydraulic Fatigue Machine

Tell us the sections, the alloys,
and what will not stay straight.

Similar Products

Share This Page

Metal remembers. Stretching makes it forget.

Two jobs, one piece of physics.

Why a straight bar is not straight

What stretching actually does

And why the same machine forms

Five steps, and a window two percent wide.

1 · Grip

2 · Take up the slack

3 · Pull past yield

4 · Hold

5 · Release

Where it goes wrong

What the machine contains, element by element.

One principle, three machines.

Section & bar stretcher

Heavy stress relief

Contoured skins

Wherever metal has to stop moving.

Extrusion and rolling plants

Aerospace structures

Machining shops

Defence and heavy fabrication

Common questions.

The line it came from, and the presses beside it.

10,000 Tonne Extrusion Press

Blanking & Cupping Press

Servo-Hydraulic Fatigue Machine

Tell us the sections, the alloys,and what will not stay straight.

Similar Products

Share This Page

Tell us the sections, the alloys,
and what will not stay straight.