Lesson 4.4DeformChapter 4 · Sheet and tube forming

Flow forming

Rollers stretch a spinning tube thinner and longer over a mandrel

Flow forming squeezes the wall of a spinning tube or cup against a mandrel with rollers, making it thinner and longer. Many lightweight car wheels are made this way, along with gas cylinders and rocket motor cases. It gives precise, seamless tubes whose metal gets stronger as it is squeezed.

forward flow forming · side viewThe figure cuts the part and mandrel in half along the axis; circled numbers match the steps below.

How it works

Load and clamp. A short, thick preform (a starting part, usually a machined, forged or cast cup or tube) slides onto a hardened steel mandrel. The tailstock clamps it in place.
Spin. The headstock spins the mandrel and the preform together.
Squeeze. Usually three rollers, spaced evenly around the part, press the wall onto the mandrel. The wall gets thinner, often by more than half in total.
Travel and flow. The rollers move along the axis, and the squeezed metal can only go lengthwise, so the tube grows longer. Finally the tube is pulled off the mandrel and trimmed to length.

In this forward version, the metal flows the same way the rollers travel. In backward flow forming, the tube is pushed against a stop and the metal flows the opposite way, which suits long open tubes.

Key facts

Materials: Steel and stainless steel, aluminum, titanium and nickel alloys; ductile grades work best
Typical parts: Wheel rims, gas cylinders, rocket motor cases, drive-train drums, precision tubes
Part size: Diameters from a few centimeters to about a meter or more
Precision: Wall thickness and diameter to about ±0.05 mm in precision work
Surface finish: Very smooth bore that copies the polished mandrel; fine roller marks outside
Batch size it suits: Medium to high volume (wheels, cylinders); also small runs of high-value parts
Tooling and machine cost: Each inner diameter needs its own mandrel; machines typically cost hundreds of thousands of dollars or more
Speed: Typically one to a few minutes per part

Strengths and limits

Strengths

Seamless tubes with very even, precise walls
Metal gets stronger as it is squeezed
Little waste: metal is moved, not cut away
Flow-formed wheels are lighter than plain cast ones

Limits

Only round parts
Costly machine, plus a mandrel for each inner diameter
Needs a preform made by another process first
Hard alloys may need heating or annealing between passes

Where it's used

AutomotiveWheel rims, clutch and transmission drums, drive-train parts
Energy and gasGas cylinders and pressure vessels
Aerospace and defenseRocket motor cases, missile bodies, shafts
IndustryPrecision hydraulic cylinder tubes

How to recognize it

Long, seamless tubes or wheel barrels with thin, very even walls.
A mirror-smooth bore and fine spiral roller marks on the outside.
Aftermarket wheels sold as flow formed or rotary forged: the cast center stays thick while the barrel is thin.

Compare with

Metal spinning4.3the same lathe idea, but it reshapes a disc and keeps the thickness roughly the same; flow forming deliberately thins the wall.Extrusion3.3also makes tubes, by pushing metal through a die; flow forming gives thinner, more precise walls from a short preform.

Watch it

BBS of America, Inc. · How a wheel maker flow forms a rim, explained and shown

Leifeld Metal Spinning · A tube stretched on a mandrel as the rollers travel

WF Maschinenbau · Real footage of a wheel rim machine at work

Leifeld Metal Spinning · Flow forming automotive drive parts in production

See real photos

Wikipedia: Shear forming — covers flow forming; mostly text, so use Commons for photos
Wikipedia: Metal spinning — the parent family, with photos of spinning lathes and parts
Wikimedia Commons photo search

Remember

Rollers squeeze a spinning tube onto a mandrel: the wall thins and the tube grows longer.
The result is a seamless, precise tube whose metal is stronger than before.
It needs a thick preform first and makes round parts only.