Closed loop control boosts workpiece accuracy

Thermal error is the main cause of position error on modern machine tools, mainly arising from the machining process itself. Significant error may result if the thermal expansion and contraction of machine components is not compensated for. Conventional control loops, however, are unable to detect thermal changes in the feed mechanism. Closed loop control is therefore recommended. It enables dimensional and contour accuracy in everything from one-off parts to large-batch production.

One-off parts

Complex, one-off parts require accuracy starting with the very first part. Highly accurate linear and rotary machine axes are the key ingredient, particularly in five-axis machining.

Small- and medium-batch production

In small- and medium-batch production, the temperature of the ball screw changes frequently along with the machining program. Linear encoders in a closed loop system enable consistent manufacturing accuracy.

Large-batch production

Linear encoders and angle encoders increase the working accuracy of interlinked machining centers and ensure consistently high quality in large-batch production.

Compensate for thermal error with closed loop linear encoders

Closed loop control in one-off part production

How position measurement affects 5-axis machining accuracy

The benefits of position measurement with HEIDENHAIN linear and angle encoders are evident on a Telstar workpiece. For flawless appearence of the Telstar ball, its grooves, pentagons, and hexagons must be milled to high precision during over two hours of machining. Due to the programmed angle of the cutter relative to the workpiece, the rotary and linear axes make expansive movements. This necessitates a high volumentric accuracy.

Semi-closed loop control

Mechanical factors from the drive system impair machining accuracy. The seam width varies, and seam intersections are visibly inaccurate.

Closed loop control

Precise linear and angle encoders from HEIDENHAIN eliminate the effects of mechanical error. The seam width is correct, and the intersections are accurate.

Closed loop in small- and medium-batch production

Sustained accuracy in small- and medium-batch production

The capability of a machine tool to cope with rapidly changing operating conditions is a decisive factor for its accuracy. A transition from roughing to finishing completely changes the mechanical and thermal load on the machine, which can cause considerable changes in accuracy. Similar load changes occur during machining of smaller production runs. Permanent changes between setup processes and order-specific machining cause varying heat inputs with the corresponding effects on accuracy

Semi-closed loop control

Thermal drift of feed axes

Closed loop control

No thermal drift of feed axes

Closed loop control in large-batch production

Greater accuracy for machining centers in automated large-batch production

Serial production in the automotive industry is currently changing. Rigid transfer lines for drivetrain components, for example, are being replaced by lines of linked machining centers for greater flexibility. 

The dimensional accuracy of finished parts depends on the individual machines and subprocesses of the entire production system. Linear encoders in closed loop configurations help increase the machining accuracy. Greater tolerance reserves, for example, can then be used to deploy tools for longer periods.