Technical Information

Sustained Accuracy in Small andMedium Batch Production
The capability of a machine tool to cope with rapidly changing operating conditions is a decisive factor for its accuracy. A transition fromroughing to finishing completely changes the mechanical and thermal load on the machine, which can cause considerable changes inaccuracy. Similar load changes occur during machining of smaller production runs. Permanent changes between setup processes andorder-specific machining cause varying heat inputs with the corresponding effects on accuracy. Particularly in small production runs,however, the profitable manufacturing of orders with tight tolerances depends on accuracy from the first to the last part. In such anenvironment the thermal accuracy of machine tools plays a decisive role.
The feed drives are of particular importance in this context. High traversing speeds and accelerations put a heavy load on the feed drives,causing much heat to be generated. Without suitable position measuring technology, this rise in temperature can quickly lead to positioning
errors of unexpected magnitudes. The rise is temperature of a one-meter long steel recirculating ball screw from 20 °C to an entirely
typical 45 °C can result in an expansion of 250 µm (coefficient of thermal expansion for steel: 10 µm per meter length and degree oftemperature change).

Thermal stability of machine tools
Solutions for avoiding thermally induced
dimensional deviations of workpieces havebecome more crucial than ever for themachine tool building industry. Active
cooling, symmetrically designed machine
structures and temperature measurement
are common practice today.
Thermal drift is primarily caused by feed
axes on the basis of recirculating ballscrews. The temperature distribution alongball screws can be changed very rapidly by
feed rates and moving forces. On machine
tools without linear encoders, the resulting
changes in length can cause significant
flaws in the workpiece.

February 2017

Figure 1

Typical machining situation

Position measurement of feeddrives
The position of an NC feed axis can bemeasured through the ball screw in
combination with a rotary encoder, orthrough a linear encoder.
If the slide position is determined from thepitch of the ball screw and a rotary encoder(Fig. 2), then the ball screw must perform
dual tasks: As the drive system it must
transfer large forces, but as the measuring
device it is expected to provide highly
accurate values and to reproduce thescrew pitch. However, the position control
loop only includes the rotary encoder.
Because changes in the driving mechanics
due to wear or temperature cannot becompensated, this is called Semi-Closed
Loop operation. Positioning errors of thedrives become unavoidable and can have aconsiderable influence on the quality ofworkpieces.
If a linear encoder is used for measurementof the slide position (Fig. 3), the position
control loop includes the complete feedmechanics. This is referred to as Closed Loop
operation. Play and inaccuracies in thetransfer elements of the machine have noinfluence on position measurement. Thismeans that the accuracy of the measurement depends almost solely on theprecision and location of the linear encoder.

Proof of drive accuracy

Measuring theposition via ball-screw
angle and pitch

Figure 2

Position feedback control with a recirculating ball screw and a rotary
encoder in Semi-Closed Loop mode

Measuring the position directly atthe machine table

Figure 3

Example: contour milling and drilling
The differences in drive accuracy between
the Closed Loop and Semi-Closed Loopmodes are well illustrated in this exampleof a moderately difficult operation without
especially high traversing speeds for themachine table. The maximum feed rate forthis job is 3.5 m/min. Two holes 350 mm
apart are to be drilled in 40 parts and thecontour milled. The job takes five and a half
minutes; the tolerance for the distance
between the holes is ±0.02 mm.

Position feedback control with a linear encoder in a Closed Loop.02

350

mm

±0

After 40 operations, thefirst part is milled again
with 10 mm infeed.

The ball screw drive heats up continuously
during machining. After conventional
position measurement in a Semi-Closed
Loop, the subsequent quality control
shows that, of the 40 parts to be produced,
only the first 25 are within the specified
tolerance. After the 25th part, the heatincrease has reached the critical point
when the thermal expansion of the ballscrew drive causes the tolerance of±0.02 mm to be exceeded. The deviationon the last part was even 70 µm.

Constraints:
Machining time: 2 hours
Max. feed rate: 3.5 m/min.
Mean feed rate: 2.8 m/min.

Figure 4

Manufacturing the coupling lever= Fixed bearing of the recirculating ball screwAfter finishing the 40th part, a second run
on the first part during which the infeed in
Z direction is halved makes this error
clearly visible. The second holes made inthis way in the finished part leave a clearlyvisible edge in the existing holes, likewise
the second milling operation on the contour.
This is the result of the 70 µm deviation
due the thermal expansion of the ballscrew drive.
On the other hand, position measurementwith a linear encoder in the Closed Loop isindependent of the thermal expansion of
the ball screw drive—and any otherinfluences. The comparative machining run
demonstrates the example describedabove. There is no scrap from a machinewith closed loop control; all the parts arewithin the specified tolerance. Aftermachining the 40th part, a second machining
run on the first part at half the feed setting
in Z direction leaves no visible edge.

Semi-Closed Loop:
Thermal offsets(70 µm)Figure 5

Coupling lever, machined twice from one blank form

Summary
The successful fulfillment of manufacturing
orders requires machine tools with highthermal stability. Machine accuracy must
be maintained even under strongly varying
load conditions. Consequently, feed axes
must achieve the required accuracy overthe complete traverse range even withstrongly varying speeds and machining
forces. Thermal expansion in the recirculatingball screws of the linear feed axes adversely
affects accuracy and varies depending on thevelocity and load. Position errors of 200 µm
and more may result during a machining
operation if the slide position is determined
only from the spindle pitch and a rotary
encoder on the motor. Because essential
drive errors are not compensated in theClosed Loop:
No thermal offsets(3 µm)

control loop when this method is used, thisis referred to as Semi-Closed Loop operationof the feed drive. These errors can beeliminated by using linear encoders. Feeddrives with linear encoders are operated ina Closed Loop because errors in therecirculating ball screw are considered inposition measurement and compensatedin the position control loop. Angle encoders
used on rotary axes provide similar benefits,
since the mechanical drive components arealso subject to thermal expansion. Linearand angle encoders therefore ensure highprecision of the components to be manufactured even under strongly varying
operating conditions of the machine tools.

Linear encoders for machine tools

Linear encoders for position feedback areindispensable for high positioning accuracyof machine tools. They directly and immediately measure the actual position of thefeed axis. Mechanical transfer elements
therefore have no influence on position
measurement—both kinematics errors andthermal errors or influences of forces aremeasured by the linear encoder and taken
into account in the position control loop.
This makes it possible to eliminate a numberof potential error sources:• Positioning error due to thermal behaviorof the recirculating ball screw• Reversal error• Errors due to deformation of the drive
mechanics by machining forces• Kinematic errors through pitch error in
the recirculating ball screwLinear encoders are therefore indispensable
for machine tools on which high positioning
accuracy and a high machining rate areessential.
Linear encoders from HEIDENHAIN fornumerically controlled machine tools canbe used nearly everywhere. They are ideal
for machines and other equipment whose
feed axes are in a servo loop, such as millingmachines, machining centers, boringmachines, lathes and grinding machines.
The beneficial dynamic behavior of linear
encoders, their high permissible traversing
speed, and their acceleration in the directionof measurement predestine them for use
on highly-dynamic conventional axes aswell as on direct drives.

LC 485 for up to 2040 mm

LC 185 for up to 4240 mm

LC 201 for up to 28 040 mm

DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany{ +49 8669 31-0| +49 8669 32-5061
E-mail: info@heidenhain.dewww.heidenhain.de1151723-21 · 2 · 3/2017 · H · Printed in Germany

More information:• Brochure: Linear Encoders for
Numerically Controlled Machine Tools• Technical Information Accuracy ofFeed Axes
• Brochure: Measuring Systems
for Machine Tool Inspection andAcceptance Testing