CNC System Design of the Roller Machine

• 1
  What CNC System Design of the Roller Machine Actually Controls
• 2
  Motion Control and Interpolation of the Bottom Rolls
• 3
  Feedback Devices and Why They Decide the Real Tolerance
• 4
  Sequence Logic, Safety, and the Operator Interface
• 5
  When a Simple Control Design Is the Right Choice
• 6
  How the Mechanical Parts Behind the Control Are Made
• 7
  Control Architectures for a Roller Machine

A plate rolling machine is not a milling machine. There is no tool path in the usual sense. The axes move slowly, the loads are large, and the geometry is defined by three rolls squeezing a plate. A CNC system design of the roller machine has to manage that geometry, not a spindle.

The controller commands two bottom roll positions and one top roll position, plus rotation of the main drive. The goal is a target radius on the finished cylinder. The math is simple. Holding it under load is not.

Most designs split the work. A motion controller handles position and velocity loops. A PLC handles sequencing: pinch, clamp, tilt, eject. The human interface sits on top and shows the operator the current radius and the roll positions.

That split matters. If the PLC tries to close the position loop, cycle times stretch and the radius wanders. If the motion controller tries to run the sequence, the logic gets rigid and hard to change.

• 1
  Two layers, one machineMotion control for axes, PLC for sequence and safety.
• 2
  Radius is the outputEvery axis command exists to hit a target curvature.
• 3
  Load changes the answerSpringback and roll deflection shift the real radius.

The two bottom rolls must rise together. If one leads the other, the plate twists and the cylinder comes out conical. The motion controller interpolates both axes from a single command so they share a trajectory.

Typical designs use a position loop in the controller and a velocity loop in the drive. The controller updates the command at 1–4 kHz. The drive closes current at a much higher rate. That division keeps the controller free for geometry.

Parallelism is the hard part. A small offset between the two bottom rolls translates into a visible taper. Mechanical linkage helps, but it wears. Electronic synchronization has to be tuned so both axes reach the target within a tight window.

On a machine that rolls 4,000 mm plate, the two bottom rolls may travel over 200 mm. A 0.05 mm mismatch at the start becomes a measurable gap at the seam. The controller has to correct for that during the pass.

• 1
  Single command, two axesInterpolation keeps the bottom rolls level.
• 2
  Fast inner loopCurrent loop in the drive, position loop in the controller.
• 3
  Tune for the loadGains set on a free roll will overshoot under plate load.

The controller only knows what the feedback tells it. On most roller machines, each bottom roll carries a linear encoder or a glass scale. The top roll often uses an indirect measure, such as a motor encoder plus a lead screw pitch.

Indirect feedback is cheaper and easier to mount. It also hides backlash and screw wear. If the machine rolls thin plate, that error may be acceptable. If it rolls 20 mm plate, the same error shows up in the seam.

Direct measurement is better when the tolerance is tight. A linear scale on each bottom roll removes screw error from the loop. The trade-off is cost and protection. Rolling machines see scale, grit, and vibration. The scale has to be shielded.

A common mistake is to put a high-resolution encoder on a weak structure. Resolution is not accuracy. If the frame flexes 0.1 mm under load, a 1 μm encoder will report a position that the roll never reaches.

• 1
  Direct beats indirectLinear scales remove screw and backlash error.
• 2
  Resolution is not accuracyFrame stiffness sets the floor.
• 3
  Protect the scaleShielding matters more than resolution on a roller.

The PLC owns the sequence. It checks that the plate is clamped, that the top roll is down, and that the safety gate is closed before it enables the main drive. Those checks are interlocked, not advisory.

The operator interface shows the target radius, the current roll positions, and the pass count. On a good design, the operator can correct the radius after the first pass without editing the program. That correction is stored as an offset.

Data logging helps more than most builders expect. If a batch drifts, the log shows whether the drift came from roll position, load, or material. Without it, the operator blames the machine.

Do not put safety logic in the motion controller. Safety functions belong in a dedicated path that can remove power independently. Mixing them slows the motion loop and complicates certification.

• 1
  Interlocks are hardSequence checks must block motion, not warn.
• 2
  Offsets per passLet the operator correct radius without a new program.
• 3
  Keep safety separateA dedicated safety path is easier to certify.

Not every roller machine needs full interpolation. A small machine rolling thin sheet at low volume can run on a PLC with analog drives and limit switches. The radius is set by mechanical stops.

That design is cheap and easy to service. It also has limits. Changing the radius takes time. Repeatability depends on the stops, not the control. It works when the shop rolls a few radii and the plate is thin.

Full CNC design pays off when the machine rolls thick plate, when radii change often, or when the seam has to close without rework. In those cases, the cost of the controller is small next to the cost of a scrapped shell.

There is a middle path. A PLC with servo drives and a simple position loop covers many shops. It gives repeatable roll positions without the complexity of full interpolation and radius calculation.

• 1
  Stops and PLCBest for thin sheet and a fixed set of radii.
• 2
  Servo PLCA middle path for repeatable positions.
• 3
  Full CNCJustified by thick plate and frequent radius changes.

The control design sets the requirements for the mechanics. Roll journals, bearing housings, and the frame that carries the load all have to hold their geometry. If the housing bores are out of line, no amount of tuning will fix the roll parallelism.

Bearing housings for roller machines are usually machined from steel or cast iron. The bore tolerance and the perpendicularity of the mounting face matter. A housing that is 0.02 mm out of square tilts the roll and shows up as taper.

At GreatLight, we machine these parts on 3-axis, 4-axis, and 5-axis centers. We hold ±0.005 mm on critical features and inspect 100% before shipment. Reports are available on request.

We work from 6061 and 7075 aluminium, 304 and 17-4PH stainless, and 4140 and 4340 steel. For wear surfaces, hardcoat anodizing or electroless nickel is common. The finish depends on whether the part sees sliding contact or just locates.

• 1
  Housing bores set parallelismLine boring or single-setup machining keeps them true.
• 2
  Material follows the loadAluminium for light frames, 4140 for loaded journals.
• 3
  Inspection before assemblyCatch the geometry error before it becomes taper.