HEIDENHAIN ROQ rotary encoders in INDEX lathes

TECHNOLOGY REPORT

Error-Free Workpiece Transfer

ROQ rotary encoders in INDEX lathes
Why does INDEX install a second rotary
encoder on the Z axis in its lathes for fullsurface machining? The answer is safety andaccuracy—two unbeatable arguments thatcustomers treasure.
Multifunctional production centers for full-surface machining are the hallmarks of the INDEX Group from Esslingen
am Neckar in Germany. Their state-of-the-art CNC lathes
offer the user quality and accuracy on the one hand, andeconomical production costs on the other hand throughshort door-to-door times. One of the reasons for this is
the precise transfer of workpieces from the main spindle
to the counter spindle during six-sided full-surface machining.
ROQ model HEIDENHAIN rotary encoders with DRIVE-CLiQ interface
play a decisive role here.

Transfer manager: the ROQ model HEIDENHAIN rotary encoder sits directly on the Z axis, which is driven by a motor
through a toothed belt. (Photo and Photo cover: INDEX-Werke)

2Like a relay race
“The transfer of a workpiece in the lathe from the main spindle to the counter spindle is like handing over a baton in a relay in athletics: a tricky thing at
which even professionals at world championships or in the Olympics repeatedly make mistakes. Highly complex processes are at play that we wantto master on our machines,“ Ulrich Baumann explains, Manager of INDEX‘ssingle-spindle design department. Now, no lathe operator has to worry abouta workpiece falling out of the spindle like in a botched baton pass. Reliablytransferring a workpiece from the main spindle to the counter spindle haslong ago been solved. Here the devil is in the detail, or more precisely, in theaccuracy of the transfer.
When the clamping elements are loosened and clamped on the mainspindle and the counter spindle, it exerts forces on the workpieceand thus on the entire slide unit. This results in changes from theoriginal workpiece position on the main spindle to the workpiece
position on the counter spindle after transfer. “We have to measurethese position changes so that we can compensate them again,”continues Ulrich Baumann in his statement. “Otherwise, at the end ofa six-sided full-surface machining operation, the overall accuracy of theworkpiece simply cannot be correct if dimensions have to be compliedwith beyond the first and second clamping.”

Live ascertainment of all factors
Rules of thumb, experience, standard compensation values or a specified allowance cannot
help here. Too many factors influence the displacement: Which clamping devices are used? Whatis the workpiece material? How are the surfaces of the chuck and workpiece? Does the workpiece havethick, stable walls or thin, sensitive walls? With what forces is the workpiece clamped in the main spindle onthe unmachined side, and what forces can be exerted during the transfer from the counter spindle to the side that has
already been machined? Even the cooling lubricant used has an influence on the position shift. “All these factors are machining-specific and highly individual. Friction, too, is always a factor—and friction is never constant. So we have to measure
the influences in real time if we want to compensate them,” concludes Ulrich Baumann.

TECHNOLOGY REPORT
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That’s where the additional ROQ rotary encoder from HEIDENHAIN on the Z axis helps. In order to transfer a workpiecefrom the main spindle to the counter spindle, the machine initially moves the counter spindle over the workpiece and thecounter spindle then clamps it. During this clamping process, the axis moves due to the forces that are ascertained by therotary encoder. The encoder then transmits the measured values to the control via its DRIVE-CLiQ interface, which offsets
them during further machining. “With the second rotary encoder on the Z axis, we are closer to the cutting edge and cantherefore measure more accurately than with the motor encoder. Thanks to this arrangement, we are able to reliably realizea length deviation of less than 0.02 mm during the workpiece transfer process,“ adds Eberhard Beck, Head of Control Technology at INDEX.

There’s a lot happening in an INDEX G200: if you see the full working space, one can well imagine the consequencesof a crash of the tool carriers with the opposing spindle. (Photo: INDEX-Werke)

4HEIDENHAIN rotary encoders offer safety as added value
When searching for a suitable encoder for this application, the INDEX developers quickly came upon the ROQ encoderfrom HEIDENHAIN. “It is small, can be built in everywhere and is completely insensitive to contamination,” as Eberhard
Beck explains the choice. Also, thanks to the DRIVE-CLiQ interface, it can be easily connected to the machine control andhas sufficient reserves with a singleturn resolution of 24 bits to ensure the required accuracy.
Eberhard Beck also brings up another argument for the additional encoder: the gain in safety. In order to save space, INDEX
integrates the motor for the Z axis backwards, so to speak, under the axis and drives it with a toothed belt. Depending on
the amount of chips and the machine load, this toothed belt can tear and stop the spindle. The rotary encoder already on
the motor would not detect such toothed belt failure. The machine would continue machining. “In the worst case, the tool
carriers could crash into the stationary counter spindle,” Eberhard Beck pointed out: “Then up to one week of machine
downtime and damage of several thousand euros are possible—a catastrophe for any company.” Preventing such disasters
is one of the HEIDENHAIN rotary encoder’s tasks.

Freedom of movement: for six-sided full-surface machining to become possible, a modern lathe such as the INDEX
G200 must coordinate highly complex motion sequences in all axes. (Photo: INDEX-Werke)

TECHNOLOGY REPORT
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ROQ 436 S¢ Absolute rotary encoder①¢ 
Clamping flange with additional slot for fastening with fixing clamps¢ Solid shaft for separate shaft coupling②¢ 
Siemens DRIVE-CLiQ interface

①

①

②

②

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