ACCUR ACY

Manufacturing accuracy
regardless of circumstances
Closed loop position measurement provides
high accuracy in any situation with small series

Accuracy often suffers in small series production with constantly changing tasks
and processing procedures. In many cases the reason for this are the permanently fluctuating and unforeseeable temperature changes in the machines anddrives, which lead to the thermal expansion of the ball screw drives. Closed
loop position measurement with HEIDENHAIN linear encoders
eliminates these fluctuations in the drive train. It
determines the exact position of the machine table at all times. This resultsin consistently accurate workpieces
and strict adherence to tolerances.

Unaffected by the temperature development on the ball screw drive:
Closed loop position measurementwith a linear encoder

Position measurement

Flexible small series continue to demand a lot of even the most up-to-dateenterprises if they are to run economically and accurately. The organizationand logistics, in particular, devour timeand personnel. After all, the preparations as well as the production and further processing steps all have to be synchronized with the greatest of accuracy.
If the actual machining is much fasterthan the setting up of the machines andplant, then delays have serious consequences. The calculation is torpedoedjust as much as the time-consuming
and intricate planning of subsequentmachine occupation.
No wonder that with all this organizationand fine-toothed planning, the topic ofaccuracy is prominent in flexible small
series production. In fact, modern machines usually do have an acceptable
8

intrinsic accuracy. However, the devil isoften in the details and in this case he isthermal expansion due to internal heatsources in the machine and therefore
from the machining itself.

Thermal expansion has
surprising effects
We all know that materials expand
when heated. In the case of the linear axes it is mainly the ball screw
drive which is affected. Due to theinitial stress and associated friction
between the ball screw and the nut, itheats up each time the machine table
traverses during machining. The so called fixed /floating bearings of theball screw drive allow for the associated expansion so as to prevent damage to the bearing.

The expansion for a steel ball screw
drive can be easily calculated taking thethermal expansion coefficient of steelof 10 µm per meter length and degreeof temperature difference. In the caseof a ball screw drive 1 m long, a rise intemperature of 1 °C causes a deviationof 10 µm. Since temperatures of 45 °C
are quite common on the ball screw
drive—an increase in temperature of25 °C above the ideal temperature of20 °C—it is easy to imagine the actual
dimensions of the deviation.

Scrap due to overheating
On Monday morning the machine is at
its ideal temperature of 20 °C after aweekend of standing idle. Now setup
and preparation begin for a small series production run of 40 parts. It in-Clearly visible after the second machining:
The deviation of 70 µm due to the thermal
expansion of the ball screw drive

volves machining of average difficulty
without any particularly high traversing speeds for the machine table.
The maximum feed is 3.5 m /min. Twoholes 350 mm apart are to be drilled in
the parts and the contour milled. The
machining takes five and a half minutes, the tolerance for the distancebetween the holes is ± 0.02 mm.
The subsequent quality control showsthat of the 40 parts to be produced only
the first 25 are within the specified tolerance. About 40 percent of the production is scrap—what a catastrophic
result! What happened?
The ball screw drive heated up continuously during machining. After the 25thpart the heat increase had reached thecritical point when the thermal expansion of the ball screw drive causes thetolerance of ±0.02 mm to be exceeded. The deviation on the last part waseven 70 µm.
You can clearly demonstrate this deviation with a simple trick: After machining of the 40th part, the first part is put
back on the machine and the feed set-

Invisible second drilling with closed loop control:
A twice machined part with no corners and edges

ting in Z direction is halved. The second
holes made in this way in the finishedpart leave a clearly visible edge in theexisting holes, likewise the second
milling operation on the contour. Thisis the result of the 70 µm deviation due
the thermal expansion of the ball screw
drive.

Constant switching makes
expansion unpredictable
However, the problem in small series
production is not the linear expansion,
which is easy enough to calculate. The
problem is caused by the permanently
changing requirements and conditions,which makes the temperature development unpredictable. For the first small
series production on Monday morning
is followed in the afternoon by a quickchange of settings for the next machining run. But what are the temperature
conditions in the machine now? Has
everything cooled down again to 20 °C
during the switchover or is there still residual heat in the ball screw drive?
Nobody knows, and with every subse25 °C45 °C

Ball screw drive in action:
The ball screw reaches temperatures of up to 45 °C.

quent small series production the conditions will become increasingly inestimable. There is no way that you can derivevalues for future machining runs. The
same machining run could produce moreor less rejects the next time round depending on the initial temperature of theball screw drive at the start of machining
and how the temperature develops.

Everything under control
with precise position
measurement
On the other hand, position measurement with a linear encoder is independent of the thermal expansion of the ballscrew drive—and any other influences.
This so-called closed loop control always determines the precise position of
the machine table. The result is stableproduction with a constantly high levelof workpiece quality.
A comparative machining run demonstrates the example described above.
There is no scrap from a machine withclosed loop control; all the parts arewithin the specified tolerance. A second
machining run with the first part after
machining of the 40th part with half thefeed setting in Z direction leaves no visible edge.
In particular for companies who specialize in small series productions, the implementation of a machine with closed
loop position measurement with a linear
encoder is a profitable investment.

Klartext 62 + 10/20159