Technical Information

Higher Precision for Machining Centers in AutomatedLarge-Batch Production
Automobile customers demand a variety of vehicle types and engines in accordance with highly diverse framework conditions, asking
about differing vehicle sizes, motor types or engine power within the individual markets, depending on fuel costs, emission standards
and tax measures. For automobile manufacturers this hampers any estimation of quantity progressions or capacity planning and thedesign of production lines.
To meet customer needs, automobile manufacturers offer engines with the same number of cylinders but in differing performanceclasses, which also leads to an increase in variant diversity. Ever shorter vehicle product life cycles in combination with volatile markets
therefore lead to changed production challenges for automobile manufacturers [1]. To meet such challenges, the aspect of flexibility isadded to the classic production parameters such as quality, time and costs.
For production lines for the machining of vehicle powertrain components (engine, transmission and axle components), this means
departing from classic transfer lines with fixed timing and high productivity, and adopting lines with linked machining centers and higher
levels of flexibility. In addition to the challenge of responding to fluctuations in demand in automotive sales markets, the future will also
bring increasing legislative and end-user requirements regarding the emission of pollutants and fuel consumption of vehicles. One
approach to dealing with such a situation consists of reducing friction in the powertrain, which in most cases would lead to increased
quality requirements for the components to be produced.
The interlinked machining centers used in production lines also have the task of implementing the demands of manufacturers in processsafe ways with large vehicle quantities (see Figure 1). The use of linear encoders in Closed Loop control in the feed axes of the machining
centers contributes to meeting such diverse requirements.

November 2015

Figure 1: Cylinder head machining in large-batch production (photo: MAG IAS GmbH)From transfer lines to linked machining centers
Greater flexibility for variants, machines and capacity

Transfer lines lack adaptability
Because individual stations within atransfer line are designed for machining
specific workpieces, these fixed, chainedlines cannot be simply expanded by adding
additional stations. To make available themaximum planned capacity requirement, itis therefore necessary to implement themain part of production line investment at
the start of production (SOP). Because
quantities slowly increase (line ramp-up) at
the beginning of production or fall away
(line ramp-down) at the end of the productlifespan, excess production capacity is theresult (Figure 2), and as a result the
installed capacity is not fully exploited.

Quantity capacity [u./a] Additional challenges come about if capacity requirements during regular operation in
the transfer line are larger than originallyplanned, and workpieces required by themarket can no longer be produced with the
installed line capacity. Duplicating a transfer
line to absorb such quantity peaks, however,would lead to significant over-capacity.

Flexibility through linked machining
centers
Production lines with smaller capacity
ranges can be installed by using linked
machining centers, enabling a wider
spectrum of production tasks and therefore
higher production flexibility. Adaptation toactual quantity needs during line ramp-up
can then be achieved by integrating
additional machining centers into theexisting production line at free stations or
by duplicating the complete line.
This method of approximating available
capacity to capacity needs is aimed at
expanding the production system with asmuch cost-efficiency as possible [2]. As aconsequence, the large-batch productionof vehicle powertrain components is now
distancing itself from transfer lines and isincreasingly adopting machining centers ortransfer centers [3].
The increased flexibility of linked machining
centers is directly related to the requirement for highly rapid and economic responses to fluctuations in demand. As an
example, one demand is that differing variants of a workpiece should be machinedon the same production line without needing to adapt the setup (variant flexibility).

Actual quantity progression

SOP

Ramp-upTransfer line

Normal operation

Ramp-down

Machining centers

Figure 2: Quantity flexibility—capacity availability vs. capacity needs [4]2Time 

Another approach is using additional
machines to simply expand productionlines. This could be called reuse flexibility,
as the use of standard machines and theirstandardized linking enables simplified
integration and reuse of such machines inother production lines.
If line capacity is seen with respect tothe complete lifespan of a manufactured
workpiece, the fluctuations in demand
outlined above also lead to differences
between capacity needs and capacity
availability in the production line. While
with transfer lines only comparatively large
jumps in production capacity can beachieved, production lines with linked
machining centers enable capacity to be
increased in smaller steps at production
ramp-up. Such production lines can also be
decommissioned again thanks to their highflexibility at production ramp-down [4].
Figure 2 shows a step-by-step investment
during the ramp-up phase of a workpiece
to be produced, and the reduction in the
ramp-down phase (quantity flexibility).
Figure 3 shows an example of a production
line layout for the mechanical machining ofa powertrain component. The machining
centers are illustrated linked by gantry loaders and grouped into individual machining
operations. A machining operation therefore consists of several machining centers
operated in parallel with the same processing content.

Figure 3: Example of a production line for large-batch manufacturing (photo: MAG IAS GmbH)
Advantages of horizontal machining
centers
Horizontal machining centers are usuallyused for production purposes, and they
have an advantageous chip fall, whereby
less cooling lubricant or even minimum
quantities of lubricant (MQL) are used towash chips. Such machines can also bevariably automated via easy front or top
loading for workpiece supply, enabling useof the same machine model for various
production lines even with different loading
systems. Double-spindle machines, with
only slightly larger support base areas, aresometimes used due to their higher
productivity (see Figure 1 on page 1).

Features of large-batch automotive
production
In summary, the following characteristics
for machining cubic workpieces in largebatch automotive production can be
identified as follows:• Automated production with linked
horizontal machining centers• One machine operator supervises eight
to fifteen machines• A machining operation usually consistsof several identically built machining
centers operating in parallel withidentical processing content• A workpiece is machined sequentiallyacross several machining operations• A single workpiece is produced withseveral machining centers, meaning thatseveral centers are responsible fortolerance conformity of the workpiece

The last point in particular makes clear thatthe produced tolerances of a workpiece
depend on the individual machines or
sub-processes of the entire production system. In contrast to die and mold production
where workpieces are produced on one
machine and supervised by a single machine operator, with large-batch production
dependencies exist between the individualmachining steps in the process chain. This
fact must be taken into account when designing the process as well as the production line.

3

Cubic powertrain components
Machining process and tolerances

The powertrain of a vehicle can be sepa­
rated into engine, transmission and axle
modules [1], and these modules in turnconsist of further component groups suchas the valve train and its components. The
cylinder head, crankcase and transmission
housing are examples of classic cubic
components in powertrain production.
Because of high quantities and related
cost-efficiency, workpieces are normallymanufactured to near net shape before
mechanical machining. This means that the
primary geometry of workpieces is alreadypredetermined by the casting or forging
process, e.g. when forging connecting
rods or crankshafts, and this is reflectedin the machining operations for cubic
workpieces [5, 6].
Focus on drilling
Figure 4 shows the machining operations
for a cylinder head. Milling is relatively low
compared to drilling or similar machining,
and this is understandable because theprimary geometry of the workpiece has
already been cast, apart from a few areas.
Specified holes in castings, however, aremanufacturable only at high cost or are
not possible at all for complex geometries.
They have to be integrated into the mechanical produc­tion line using drilling operations, so that their share of the machining
process clearly predominates.

4

12.3 %

0.4 %Brushing

44.5 %

Turning*)
Reaming

8.7 %Thread-cutting
Countersinking
Milling
10.5 %

Drilling*) Finish-boring process

17.4 %
6.2 %Figure 4: Machining operations and their share of the manufacture of a cylinder head [5]The various machining operations alsoenable conclusions about the axis movements required for machining. Because of
the drilling operations, most of the movements consist of positioning the tool to theworkpiece and a single-axis feed and retraction motion. And because the workpieces usually have no free-formed sur­faces, during milling operations the motionin the working plane (via traverse in two
linear axes) is sufficient in addition to a
positioning motion.

The predominant number of geometricaltolerances on the workpiece to be pro­
duced is therefore determined by the
majority of drilling operations required.
In addition to diameter tolerances mainly
influenced by the dimensions of the tool,such as reamers, workpiece drawings
also contain tolerance specifications for
position, distance and hole depth.

Figure 5: H
 EIDENHAIN linear encoder in a horizontal machining center(photo: GROB-WERKE GmbH & Co. KG)

Linear axes determine dimensional
accuracy
Complying with specified tolerances isdetermined among other factors by thecapability of the linear axes of a machine
tool to achieve precise positioning (Figure5). Depending on the workpiece or set ofdrawings, differing tolerance values andtolerance divisions for production features
may exist.
Figure 6 shows the distribution of tolerance(position, distance and depth tolerances)
for a crankcase. The example shows thatsomewhat more than 30 % of the toler­
ances analyzed are ≤ ±0.1 mm, andproduction features also exist on cylinder
heads and transmission housings withtolerances of ≤ ±0.1 mm required by thedesigners. Examples are the position anddepth tolerances of drill holes in the area ofa cylinder head valve train as well as thebore axis distances of the main and neckbearing axes on a differential housing.

60 %
50 %
40 %
30 %
20 %
10 %
0%
T  0.1 mm

0.1 mm  T  0.2 mm

T  0.2 mm

Figure 6: Distribution of position, distance and depth tolerances for a crankcase

Reliably complying with tight tolerance
valuesTo achieve the above-specified need for
improved flexibility, producers make it theirgoal among other things to use standard
machine tools in manufacturing, making it
unnecessary to use additional specializedmachines for individual production dimen­
sions. This in turn means that, even if
tolerance values are ≤ ±0.1 mm, they muststill be complied with in a safe and reliable
way with the machining centers installed in
the production lines.

5Cubic powertrain components
Capability analyses supply specifications for machine capability andprocess capability
The tolerance values specified previously
seem large compared to requirements inthe die and mold production sector. It must
be considered, however, that values forautomotive workpieces initially correspondto the designer’s drawing specifications.
Automotive manufacturers carry out
statistical capability analyses on themachining centers to make sure that themachine tools and implemented production
processes are capable of meeting requisite
drawing tolerances over a long period witha defined safety margin to the tolerance
limits [7].

Capability tests for machine andprocess
For acceptance testing of the machine
tools, 50 workpieces, for example, are produced on the system or single machines
and the distribution of produced tolerancesis then evaluated using statistical methods.
Capability specifications or capability indices are defined for the capability tests.
To determine the machine capability theindices Cm or Cmk are defined, and forrecording process capability the indices
Cp or Cpk. The indices Cmk and Cpk take
into account a non-central distribution of
measured tolerance values captured on
several workpieces.

(±) [mm] 

Determining the machine capability provides knowledge about the temporary scattering behavior with tolerances produced
by a machine tool subjected to constraints
that are as constant as possible (≡ shortterm testing). The process capability aims
to determine the long-term scattering behavior of a machine under process constraints (≡ long-term testing). See [7] forequations for calculating the indices basedon the measured tolerance values of produced workpieces, as well as for further information.

Equation 1 specifies the variables Cm
and Cp:Cm,Cp = T / (6 ∙ s)		(equation 1)

T = 	tolerance range in accordance withdrawing dimensionss = standard deviation
Manufacturers specify capability indicesfor the quality-relevant tolerances, andproduction machines supplied by themachine tool builder must at least achievethese during acceptance testing. With
regard to the acceptance test, determining
the indices leads to a further limitation oftolerance values specified in the workpiecedrawing. Figure 7 demonstrates this withthe example of a tolerance of T = ±0.1 mm
under consideration of the machine
capability specification Cm. Automotive
manufacturers also specify feature classes
for tolerance that are critical (Cm = 2.00),
important (Cm = 1.67) or less important(Cm = 1.33) in relation to component
functionality.

0.12
0.1
-25%

-40%

-50%

0.08
0.06
0.04
6·s
0.02
s

0

Tolerance T

6

T_rest @ Cm=1.33

T_rest @ Cm=1.67

Figure 7: Correlation between the drawing tolerance specification and capability index Cm

T_rest @ Cm=2.00

Target: Minimum scattering
For uniformly high workpiece quality andminimum scrap rates, the target is toachieve lowest levels of scattering withtolerances produced, and therefore low
deviations from the standard. Equation 1
can therefore be construed as follows:
Cm,measured = T / (6 ∙ s) ≥ Cm (equation 2)or
6 ∙ s ≤ T / Cm 			(equation 3)

Cm,measured corresponds to the index
derived from measuring a tolerated
dimension on e.g. 50 workpieces produced
during machine acceptance and thestandard deviation calculated from them.

Capability tests check compliance withmaximum acceptable scattering
Equation 2 demonstrates that the capability index resulting from the measurements
becomes greater the lower the standard
deviation is. The value (Cm,measured) frommeasurements on the workpieces must be
greater or equal to the value of Cm specified by the manufacturer. If this is taken
into account, specifying a capability index
corresponds to specifying the maximum
acceptable standard deviation by the manufacturer for a tolerated dimension (equation 3 and Figure 7).
For the example in Figure 7, with Cm = 1.67
this means that for the original tolerance of±0.1 mm, 99.73 % of all tolerance dimension values measured on various workpieces must be in the range of ±0.06 mm. As aconsequence, the acceptable standard deviation must have a maximum value of only
±10 µm. Achieving such values is a significant challenge for the linked machining
centers in large-batch production.

7

Error limits for machining
Flaw sizes in the process and their effects

The previous pages outlined the order ofmagnitude of tolerances on workpieces forvehicle powertrains. The question arises as
to the percentage of available tolerance ofa production feature that flaw size arising in
the process depletes.
If the total flaws are greater than theavailable tolerance, the machine tool orprocess cannot fulfill the specified
machining task. In the following, the error
limits that can occur on a machine tool e.g.
due to workpiece clamping or thermal drift
behavior on axes are therefore estimated,
and the aim is to estimate the residual
tolerance reserve by comparing theavailable tolerance with the existing error
limits. This reserve enables the machine
tool or process to compensate forunpredictable or difficult-to-control
production influences.

¹Z (uncompensated) [µm] Index tolerance of workpiece clamping
The loading of machine tools is usuallyautomatic, using special loading modules
such as gantry loaders. Before a workpieceis machined, it needs to be positioned andfixed in the clamping fixture of the machine,and because this must be carried out
without manual intervention, positioning is
usually done by index pins in the clamping

fixture. This means that the mechanicalfitting tolerance between the index holesof a workpiece and the index pins of theclamping fixtures in the production line
must be considered.

To avoid this, a transition fit of Ø 12 H7/js6
is assumed in the following. These
specifications achieve the following
maximum error limit for workpieceindexing in the clamping fixtures:

According to the drawing specifications,
identical tolerance for the index holes isavailable for each workpiece produced on
the production line, but the actually
produced dimension varies from workpiece
to workpiece. This also applies to theindividual index pins of the clamping
fixtures. As a result, constantly identicalindexing deviation cannot be expected. The
specified maximum tolerance ranges forthe index holes and index pins should,
however, be complied with in each case as
rejects will otherwise be produced. This
makes it possible to estimate an error limit.

Index hole
+0.018¬ 12 H7 = ¬ 12 0mm

Index pin
+0.0055¬ 12 js6 = ¬ 12 -0.0055 mm (equation 5)¬ (12.018 – 11.9945) mm≡ ±11.75 µm (radial)		

X

-10

Y
Z

-20
-30
-40
-50
-60
-70
0

1

2

3

4

5

6
Time [h] 

10 000 rpm

16 000 rpm

Figure 8: Measured, uncompensated Z axis drift due to heating of the main spindle

8(equation 6)

To manufacture features with tighttolerances, the workpieces are sometimes
moved in a defined direction using thrust
cylinders in the clamping fixture prior toclamping. This means the workpieces areradially pressed onto the index pins, so thatfrom the central drilling axes only amaximum of half the error limit occurs(≡ ±5.88 µm). Because sliding units often
make clamping fixtures more complex andtherefore more expensive, they are notintegrated in all machining stations.

With the above-specified cubic automotive
powertrain components (cylinder head,
crankcase, transmission housing), index
holes with a diameter of 12 mm are often
drilled. Clamping of the workpiece within
the indexing pins is not desired because
this can lead to excessive loads on theloading systems or workpiece clamping
errors, and therefore to possible idle times
in the production line.

0(equation 4)

Tool change tolerances
On the tool side, inaccuracies also occur
during tool changes and depending on thetool interface used. In the machining
centers of the above-specified productionlines, main spindles and tools with hollow
taper shank interfaces are normally used.
In [8], the predominant repeatability of thehollow taper shank connection is specified
as < 1 µm (axial) and < 3 µm (radial)(≡ ±0.5 µm axial; ±1.5 µm radial).
Thermal drift of the spindle nose
Axial drift on the spindle nose during
operation has an effect for holes withdefined depth dimensions as well as onface milling if two plane-parallel surfaces
must be machined with a defined distance,
for example the combustion chamber andhood faces on cylinder heads. During
operation, heat development of the main
spindle strongly affects expansion of thespindle shaft and subsequently the Z axis
structure. Motor spindles are usually
implemented in the above-specified
production machine tools. The various
machining conditions lead to fluctuating
performance constancy and thereforechangeable thermal loads from theintegrated spindle drive and spindle
bearings. Because the motor spindle ishighly integrated into the axis structure of
the machine, heat is dissipated via themotor spindle housing and mounting
surfaces onto the machine axis structure.
The motor spindle is therefore usuallyconnected to a cooling unit.

Figure 8 displays drift in the Z axis in
the warm-up phase as a result of drift
measurements taken from a machining
center with motor spindle. The drift of thetool center point is shown relative to themachine tool table, at idle spindle speedsof 10,000 rpm or 16,000 rpm. The meas­
urements do not include supplementary
thermal loads and resultant drift due tospindle loads occurring during cutting.
Depending on the load and speed ofthe main spindle though, significantly
increased drift values compared to idle
speed should be expected.
Based on the measurement curve shownin Figure 8 for load with an idle speed of
16,000 rpm, a thermal drift at the spindle
nose of around 64 µm in the Z direction
occurs relative to the machine tool table.
Thermal drift is often reduced even further
by use of compensation models saved in
the control, achieving an expected reduction of the drift shown by around 80 %.
When applied to the measurement curve
shown this means a residual value of
12 µm (≡ ±6 µm).

9Error limits for machining
Flaw size due to the position detection principle

The further considerations require a differentiation regarding determination of the actual position value on a feed axis with useof a ball screw drive. The main difference inthis case occurs due to the mounting location of the axis encoder and its integration
into the control loop of the feed axis. Figure 9 shows a feed axis design with SemiClosed Loop control compared to an axis
with Closed Loop control.
Semi-Closed Loop and Closed Loop
Semi-Closed Loop means the position
control loop of the feed axis is closed
via the encoder of the feed motor. The
encoder measures the angular position of
the feed motor shaft here, and not theposition of the axis slide to be positioned.
The slide position is calculated by using themeasured angle and the pitch of the ballscrew, and it is assumed that the correlation of angle position, spindle pitch andslide position while operating the feed axis
or machine tool remains constant. If thiscorrelation changes, e.g. due to thermal expansion or wear in the ball screw, then thecalculated position value is erroneous. The
control and encoder are “blind” to changesof the feed axis powertrain.

With Closed Loop operation of the feed
axis, the slide position of the feed axis ismeasured with a linear encoder and returned to the position control loop of theaxis as an actual position value. Any deviations occurring during operation, such as
thermal drift of the ball screw drive, are detected on the slide and adjusted with axis
control.

Connecting a linear encoder to the axis
slide and feedback of the actual position
values into the position control loop reduces
the effects of friction in the axis and of linear guides on positioning. Measurements
show that the position variation range with
Closed Loop control can typically be reduced by 40 % to 50 % compared to
Semi-Closed Loop (≡ ±0.5 µm).

Systematic and random position
deviations
Positioning inaccuracies result during positioning of the feed axes ([9], [10]). Systematic deviations occur that can be reproduced with identical boundary conditions
and can therefore be compensated. Random deviations also occur that are not reproducible, but have more or less statisticalbehavior. The effect of random deviations
on the positioning of a machine axis isspecified as the position variation range.

Reduction of thermally induced drift
Measurements specified in the “Accuracyof Feed Axes” and “Machining Accuracy of
Machine Tools” Technical Information
documents show that a feed axis operated
in Semi-Closed Loop with a ball screw andtraverse range of 800 mm can produce
thermally induced drift of over 40 µm(≡ ± 20 µm) ([11], [12]). If the problem isapproached by cooling the ball screw, adrift reduction of 50 % can be achieved byusing a high quality cooling unit (with atemperature constancy ≤ 1 K).
As well as integrating the cooling unit,
additional pipes for coolant fluid for theindividual feed axes must be installed,
together with rotary unions for the trans­mission of coolant to the rotating ballscrew spindles.

If a machine is to drill a hole in a workpiecewith position tolerance, then the position
variation range as a non-reproducible factor
of error must be considered. Measurements carried out demonstrate that typicalvalues for position variation ranges withfeed axis lengths of 500 mm to 800 mm
are around 2 µm (≡ ±1 µm) if the axes areoperated in Semi-Closed Loop.

Semi-Closed Loop (SCL)
Xnom +

Positioncontroller+

Velocity controller–– Xactddt
φClosed Loop (CL)
Xnom +– XactPositioncontroller+

Velocity controller

–
d
dt
φ

Figure 9: The principles of Semi-Closed Loop control and Closed Loop control

10

Estimated error limits [µm]Semi-Closed Loop

Closed Loop

Workpiece clamping indexation±11.75±5.88 (with sliding unit)

Tool change inaccuracy(hollow taper shank)±0.5 (axial)±1.5 (radial)

Thermal main spindle axis drift(compensated)±6

Position variation range(500 mm < X  800 mm)

±1

±0.5

Thermal drift due to heatingof ball screw±20±10 (cooled ball screw spindle)±1

Linear measurement deviation of±1.5
coordinate measuring machine (CMM)
Table 1: Estimated error limits for Semi-Closed Loop and Closed Loop
A significantly higher reduction of thermallyinduced drift on a ball screw axis can,however, be achieved by using Closed
Loop control. The linear encoder detects
the drift of the ball screw spindle and thisis adjusted by the axis control—residual
thermal drift is then ≤ 2 µm (≡ ±1 µm) [11].
Table 1 lists the previously estimated error
limits, comparing these for a feed axis with
Semi-Closed Loop and Closed Loop. The
position of the workpiece and the tolerated
production features in the workspace must
be taken into account when estimating
which share of the tolerance to be produced
the specified error limits will deplete. Thus,
only those errors in the direction of theindicated tolerances require a share of theavailable tolerance budget.

Influences of workpiece measurement
Evaluation of the dimensional accuracy ofworkpieces is usually carried out in suitablemeasuring rooms with coordinate measuring machines. In its geometric construction
a coordinate measuring machine is similar
to a machine tool and is also subject to influences of error, with measurement deviations occurring when evaluating workpiece
dimensions. The extent of measuring deviations is influenced by:• Temperature fluctuations in themeasurement room
• Geometry errors of the coordinate
measuring machine• Probing errors• Measurement method selected• Errors from workpiece clamping andoperator errors• ...

Because the residual measurement devi­
ation must be significantly lower than thedimensional tolerance to be evaluated,
coordinate measuring machines are usuallyoperated in temperature-controlled meas­
uring rooms. High precision is aimed forwhen producing and assembling the coordinate measuring machines, and linear encoders are used for Closed Loop control.
An additional reduction in residual deviations (e.g. from bending of the probe
during probing) is achieved via computedcompensations. To ensure reliable meas­
urements with tolerated length dimensions, the linear measurement deviation ofa coordinate measuring machine should be
only approximately one tenth of the tolerance to be evaluated. So for a tolerance of±0.1 mm as specified in a workpiece drawing this therefore means ±10 µm or less.
However, to reliably measure smaller tolerances and to make available universally usable measuring machines, for example incentral measuring rooms, in practice coordinate measuring machines are used thathave linear measurement deviations in therange of ±1.5 µm.

11Tool tolerance vs. error limits
Closed Loop increases the tolerance reserve

The comparison of required workpiece tolerance and estimated error limits specified
on the previous pages is demonstrated in
the following using an example geometry.
Figure 10 illustrates the holes for the bearing assembly of a shaft in a transmission
housing.

Errors to be considered
The error that can occur due to indexing of
the workpiece in the clamping fixture must
therefore be taken into account when machining Z1 and Z2. Since other tools areused at other positions on the workpiece inthe machine during this machining operation, the tool change inaccuracies must be
integrated into the error budget. Furthermore, errors in the direction of the Z1 andZ2 dimensions, caused by thermal main
spindle axis drift, position variation range
during positioning of the Z axis in the machining direction, and thermal drift in the
Z axis direction resulting from heating of
the ball screw drive, should also be taken
into account. The linear measurement deviations of the coordinate measuring machine are calculated into the error budget
analysis independent of the clamping direction of the workpiece in the machine tool.

12  

Z2  

 

Z1  

Y  

To reduce ancillary times and therefore thecycle time itself, both hole diameters canbe produced using a combined tool. The
tool edges for machining the diameters
and Z dimensions are both located on one
tool for this purpose. One of the index
holes required for aligning the workpiece isalso shown. The tolerance direction of theindex hole is in the same direction as thetolerated production dimensions Z1 and Z2.
This does not apply to the Y dimension.

Machining direction  
Index hole

Figure 10: Bearing holes in a transmission housing (example geometry)
Under the assumption that tolerated
production dimensions have a mean
machine capability index of Cm = 1.67, it
can now be ascertained how large thedepletion of available tolerance is due toestimated errors in the worst case. Figure
11 demonstrates this for tolerances in theZ1 and Z2 dimensions with Semi-Closed
Loop control.

Fewer errors and more tolerance reserve
It becomes clear that a reduction of error
variables directly leads to an increase intolerance reserve. The use of linear encoders on the linear axes, meaning operation
in a Closed Loop, is able to specificallyreduce thermally induced ball screw drift(Figure 12), and the result is a significantly
increased tolerance reserve of ±39 µm.

It can be seen that the largest consumer ofavailable tolerance is uncontrolled thermal
drift of the ball screw spindle. The residual
quantity of available tolerance (tolerance
reserve) at the end of the analysis is only
±19 µm.

As outlined above, dependencies occur
among individual production steps in theprocess chain with large-batch manu­
facturing. The error limits listed in Table 1
only indicate some of the potential error
variables that can occur along the entire
component production chain. The aim istherefore to achieve minimum flaw sizein the individual machine tools, so as tomaximize the tolerance reserves for difficult-to-influence errors within the process.

(±) [mm] Figure 11:
Available tolerance(±0.1 mm) with
Cm = 1.67 vs. error
limits with SemiClosed Loop

0.07
0.06
~ ±19 µm

0.05
0.04
0.03
0.02
0.01
0
T=0.1 @ Cm=1.67

Structure deviations

Thermal drift of position

CMM & tolerance reserve

Workpiece clamping

Thermal drift of position

Tool clamping

CMM linear measurement deviation

Position variation range

Tolerance reserve(±) [mm] 

Thermal drift of work spindle

Figure 12:
Available tolerance(±0.1 mm) with
Cm = 1.67 vs. error
limits with Closed
Loop

0.07
0.06
0.05
~ ±39 µm

0.04
0.03
0.02
0.01
0
T=0.1 @ Cm=1.67

Structure deviations

Thermal drift of position

CMM & tolerance reserve

Workpiece clamping

Thermal drift of position

Tool clamping

CMM linear measurement deviation

Position variation range

Tolerance reserve

Thermal drift of work spindle

Factors of influence that are difficult to con­trol in the production chain may consist of:• Hall temperature fluctuations• Coolant temperature fluctuations• Workpiece temperature fluctuations andtherefore thermally induced workpiece
expansion during transport between
machines• Static distortions on workpieces due tovarying clamping forces and clamping
tolerances in individual clamping fixturesin the production line
• Thermal distortion of tolerance
dimensions due to differing workpiecetemperatures in the individual machines(production of tolerance dimensions) andin the measuring room (evaluation oftolerance dimensions)• Tool deflection due to inaccurate premachining (i.e. deflection of a reamer
due to tolerance deviations in pilot
drilling)• Chips (i.e. stuck chips, chips from pilot
drilling, etc.)• Tool wear (i.e. variable cutting forces, tool
deflection, etc.)• …

This list is not comprehensive but clearlyshows that designing a production process
is not in any way trivial for the automatedmechanical machining of vehicle powertrain
components. Many possible error sources
exist that are able to influence the quality,
duration and costs of production.

13

Tool tolerance versus error limits
Tolerance reserves increase productivity

To gain a feeling of what tolerance reservesof ±19 µm in a Semi-Closed Loop or ±39 µm
in a Closed Loop mean, the extent of tool
wear is looked at closer below. With alumi­
num workpieces, burr formation on a work­piece often leads to a tool change despite
the appearance of highly sharp cuttingedges, whereas the machining of work­pieces from cast iron (e.g. crank cases) or
high-temperature resistant cast steel (e.g.
exhaust gas turbochargers) causes higher
levels of tool wear. This wear is usuallyseen as the increasing flank wear (VB) overtime (Figure 13).
Tool wear must be permissible
Due to the tool edge geometry (clearance
angle Þ, rake angle γ), a corresponding
cutting edge offset (SV) occurs withincreasing flank wear that then directlyreduces the tolerance reserve. The cuttingedge offset can be calculated according to[13], depending on the flank wear VB as
well as Þ and γ as follows:sv =VB · tan Þ
1–tan Þ · tan γ(equation 7)

Presupposing the latest linear layout incorporating linked machining centers, tool
costs for the mechanical machining of cylinder heads, crank cases and transmission
housings consist of 8 % to 12 %, which isa significant portion of the total system investment. This includes both non-wearing

Rake facetool components (e.g. tool holder and boring bars) and the parts subjected to wear
(inserts, drills, etc.). The proportion depends among other factors on the numberof replacement tools used (sometimes up
to three sets) and the cutting materials(e.g. PCD blades for aluminum machining).
To save on running tool costs the tool life
should be as long as possible, and a certain
level of tool wear must therefore be acceptable before carrying out tool changes
or modifications. A flank wear VB of 200 µm
to 400 µm is specified in [14] as a benchmark for carbide tools and for the finishingcycle relevant for generating tolerance.
To gain a magnitude for the cutting edge
offset, the typical cutting edge angles ofÞ = 5° and γ = 4° are applied. Equation 7
then gives a cutting edge offset range of
18 µm to 35 µm.
Assuming a very sharp edge in the centerof the tolerance zone, in a Semi-Closed
Loop the targeted minimum tool wear values already require the largest part of theresidual tolerance reserve. Under consideration of the larger tolerance reserves withthe use of linear encoders, more workpieces
can be machined before tool wear needs
to be compensated, which in turn means
less testing effort on the workpieces when
detecting the right time to compensate orchange the tool.

Tool changes interrupt production
Changing a tool used to produce a criticaltolerance often requires a workpiece thatneeds to be directly tested and that was
found to be in order (OK part). To prevent
rejects, the machine sits idle while this first
workpiece produced with the new tool ismeasured, and the machine tool is only
approved if the produced and measured
workpiece corresponds to an OK part. The
waiting time until approval can be equatedto a loss of parts because production timeon the machine is lost.
Because the testing and compensation
complexity can be reduced and machine
run-times increased, a large tolerance
reserve in large-batch production
contributes to an increase in productivity
levels. In analogy to automotive driving,
this would correspond to driving in a wide
lane where little correction effort by thedriver is needed to stay within the lane—significantly more effort for correction isneeded in narrower lanes.

Cutting edge

Tool flank

Figure 13: Flank wear (VB) and cutting edge offset (SV) – Drawings on the basis of [13] and [14]14Summary

In addition to the challenge of responding
to fluctuations in demand in automotive
sales markets, the future will bring increasing legislative and end-customer requirements concerning the emission of pollu­
tants and the fuel consumption of vehicles.
One possibility of meeting such a situationconsists of reducing friction in the powertrain, which in turn requires narrow workpiece tolerances as well as higher quality
surface parameters.
Because of the sequential workpiece machining implemented in automated largebatch manufacturing, dependencies occur
between the individual production steps inthe process chain. For reliable adherence
to workpiece tolerances throughout theentire process, the aim is to expend a minimum of the available tolerances by the individual machine tools, with the remaining
tolerance reserve being available for difficult-to-control factors that influence theprocess, meaning this reserve should be
as large as possible.
Using linear encoders in the feed axes ofthe machining centers contributes to an increase in machine accuracy, and resulting
from this an increase of the tolerance reserve as well. Inspection complexity andloss of parts can be reduced, leading toincreases in productivity levels.

Literature[1] Kasper, G.: Challenges for FuturePowertrain Production – Insights to
Global Manufacturing. 12th Powertrain
Manufacturing Conference, Darmstadt,
September 26th – 27th, 2013.
[2] Abele, E.; Hueske, B. and Kuhn, S.:
Overall Equipment Flexibility –Entwicklung einer Methodik zur
Messung der Gesamtanlagenflexibilität
für die spanende Fertigung. ZWF 103
No. 5, 2008.
[3] Dörr, J.: Powertrain Machine Tool
Concepts: Comparison from an
Economic Point of View. 10th
Powertrain Machining Conference,
Fellbach, November 11th – 12th, 2009.
[4] Abele, E.; Wiegel, F.; Kuske, P. andHueske, B.: Kleine Schritte für große
Flexibilität – Eine Methodik zur
monetären Beurteilung derExpansionsflexibilität vonFertigungssystemen. wt
Werkstattstechnik online year 100,
2010.
[5] Stürenburg, B.: Optimierung derSpanbildung und Minimierung desSpäneeintrages in das Werkstück für
das Bohren von Al-Legierungen.
Production reports from the FBK,
Volume 02/2009.
[6] Abele, E.; Ellermeier, A.; Hohenstein, J.
and Tschannerl, M.: Untersuchungenan langen VHM-Spiralbohrern –Einfluss der Werkzeuglänge auf das
dynamische Verhalten vonVollhartmetallspiralbohrern. wt
Werkstattstechnik online year 97, 2007.
[7] VDMA 8669 : 1999-01:
Fähigkeitsuntersuchungen zur
Abnahme spanender
Werkzeugmaschinen. Verband
Deutscher Maschinen- und Anlagenbau
e. V. (VDMA), 1999.
[8] MAPAL: MAPAL Kompetenz –Spanntechnik. Corporate publication
MAPAL Präzisionswerkzeuge Dr. Kress
KG, Order No. 10138532 | V2.1.0, 2015.

[9] DIN ISO 230-2 : 2000-05: Testing Rules
for Machine Tools – Part 2:
Determination of Positioning
Uncertainty and Repeat Accuracy of
Positioning with Numerically Controlled
Axes. DIN Deutsches Institut für
Normung e.V., 2000.
[10] VDI/DGQ 3441: Statistische Prüfung
der Arbeits- und Positionsgenauigkeit
von Werkzeugmaschinen - Grundlagen.
Verein Deutscher Ingenieure /Deutsche Gesellschaft für Qualität,
March 1977.
[11] HEIDENHAIN: Technical Information –Accuracy of Feed Axes. March 2006.
[12] HEIDENHAIN: Technical Information –Machining Accuracy of Machine Tools.
September 2011.
[13] Paucksch, E.; Holsten, S.; Linß, M. andTikal, F.: Zerspantechnik – Prozesse,
Werkzeuge, Technologien. Vieweg +Teubner Verlag, 12th edition, 2008.
[14] GARANT: GARANT ToolScout –Zerspanungshandbuch. ISBN 3-00016882-6, modified ND 04/2013.

15

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 kinematic 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 number of 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 screw

Therefore, linear encoders are indispensable for machine tools on which high positioning accuracy and a high machining
rate are essential.
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 milling machines, machining centers, boringmachines, lathes and grinding machines.
The beneficial dynamic behavior of thelinear encoders, their high permissible traversing speed, and their acceleration in the
direction of measurement predestine themfor use on highly-dynamic conventionalaxes as well as on direct drives.

LC 485 up to 2040 mm

LC 185 up to 4240 mm

LB 201 up to 28 040 mm

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
1151304-20 ∙ 3 ∙ 11/2015 ∙ F&W ∙ Printed in Germany

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