Collet Marks on Thin-Wall Tube: Root Cause and Check

Surface indentations, radial steps, or galling marks left by the carriage collet on thin-wall tubing during feeding and spatial rotation indicate localized overload or micro-slippage between the gripping jaws and the tube surface. In rotary draw bending on machines like the CNC Boru Bükme Makinesi (Griffin), the carriage collet must transmit both axial thrust and rotational torque without exceeding the elastic limit of the tube wall. When clamping force is excessive relative to wall thickness, or when feed resistance forces the collet to slide across the tube skin, the outer surface deforms into jaw gaps or serration grooves.

Identifying radial impression and slippage marks on thin-wall tubing

Distinguishing between static clamping deformation and dynamic sliding scratches is necessary before adjusting operating parameters. Radial deformation appears as raised ridges or polygonal steps that correspond directly to collet segment gaps and jaw land geometry. This shape change occurs during the closing stroke when hydraulic or pneumatic clamping pressure forces thin metal into open gaps between jaw segments.

Sliding marks form longitudinal score lines or burnished friction tracks along the length of the collet contact zone. Micro-slippage happens when the carriage advances the tube into the bend head while carriage push force or rotational indexing encounters mechanical resistance. During torque transmission for spatial bends between consecutive curves, insufficient friction across the contact pad forces the collet to rotate around the tube circumference, creating circular scuff marks. During a routine field service check for collet marking on stainless steel exhaust tubing, measuring the depth of the jaw step across four quadrature points often reveals non-uniform jaw closure rather than excessive overall line pressure.

Evaluating collet clamping pressure and jaw contact profile

Carriage collets rely on hydraulic actuators or pneumatic cylinders operating through taper wedges or mechanical toggles. On thin-wall applications where the wall thickness to outer diameter ratio drops below 0.03, standard serrated steel collets exert narrow line loads that readily penetrate the material surface.

To maintain mechanical grip without marring the work piece, clamping pressure must be adjusted relative to wall thickness and material yield strength. Lowering proportional hydraulic valve settings reduces compressive stress, though setting pressure too low increases the risk of slipping during carriage feed movements. Machining smooth collet pads with a matching bore diameter distributes radial clamping loads uniformly over the contact area. Non-marking bronze or aluminum collet inserts are frequently selected for thin-wall stainless steel, copper, or aluminum tubing to prevent galling. Inspecting collet chuck housings for chip accumulation, dried coolant residue, or broken return springs ensures that all jaw segments collapse symmetrically against the tube wall.

Checking boost axis feed force and rotational drag

The carriage collet does not act in isolation during the automated bending sequence. During feeding or assist-driven movement, axial driving force is transferred through the collet mechanism. If carriage push force is set higher than necessary relative to pressure die assist settings, rearward thrust reaction forces on the collet increase, raising the likelihood of surface scoring.

Rotational torque required to index spatial planes between bend steps increases if the internal mandrel rod, wiper die, or rear tube support creates mechanical binding. On multi-axis machines operating under the ETU control system, axis synchronization regulates carriage advance and rotation. If rotational indexing begins before the clamp die or pressure die fully clears the tube, circumferential grooving occurs. Verifying that die stroke limit sensors confirm full retraction before carriage movement prevents mechanical interference between bend stages.

Adjusting collet serrations and internal mandrel support

When thin-wall tube cannot withstand the radial forces required to prevent slippage during acceleration, modifying jaw surface texture or adding internal support resolves surface marring. Smooth collets rely entirely on friction and require higher radial force, whereas fine bead-blasted or micro-grooved collet surfaces increase the friction coefficient without penetrating the tube wall deep enough to create functional defects.

In thin-wall applications where lower clamping force permits tube movement but higher pressure deforms the cross section, an internal support plug or extended mandrel shank can be placed inside the gripping zone. Positioned directly beneath the collet jaw landing area, a hardened internal sleeve provides resistance against radial compression. This internal support permits sufficient radial gripping force on automated systems like the Tam Elektrikli Boru Bükme Makinesi (EOS), ensuring accurate rotational indexing without crushing delicate walls. For tooling evaluation or process verification, provide a 3D STEP file, material specification, outer diameter, wall thickness, center line radius, and required annual production volume.