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1. ABSTRACT
Rapid Manufacturing (RM) processes have evolved from the Rapid Prototyping (RP) paradigm
and are increasingly being used to manufacture parts, tools and dies in addition to prototypes.
The advantages of RP methods to produce complex shapes without the use of specialized
tooling can naturally be extended to RM processes. For RM to be accepted as a mainstream
manufacturing process, parts created by RM have to consistently satisfy critical geometric
tolerances specifications for various features of the part. The process parameters, namely slice
thickness, orientation, support structures and 2D tool path planning, directly impact the final
part accuracy both individually and in combination with each other.
The generation of form errors in parts manufactured by RM depends upon the process
parameters chosen by the designer during the part build.
Form errors in layered manufacturing are:
1. Flatness & straightness errors
2. Cylindricity errors
3. Staircase errors
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2. INTRODUCTION:
Rapid Prototyping (RP) is the process of building prototypes in slices using a layered approach.
Several popular RP processes such as Selective Laser Sintering (SLS), Stereolithography (SLA)
and 3D Printing have been developed during the last couple of decades and are extensively
used in industries. Although these processes differ in their method of building the slices and the
materials used, all of them follow the same basic steps. The main process parameters in RP are
orientation, slicing, supports and tool path planning.
Orientation refers to the direction with respect to the part in which the slices are built in the
machine. Slicing refers to the segmentation of the part into layers. Slicing can be performed
uniformly by using slices of equal height or by adaptive slicing using slices of unequal heights.
Supports are additional materials which are added during manufacturing to support holes,
overhangs and unsupported features. Tool path planning refers to the determination of the
geo-metric path for building each individual layer. RP allows prototypes with complex and
intricate shapes to be built very easily. The time associated with building prototypes in RP is
significantly less compared to the traditional methods of clay making or building a one of
prototype by machining or casting. RP also eliminates the need for specialized tooling or dies to
manufacture prototype parts, thereby reducing the cost considerably. The cost and time
advantages associated has made RP extremely popular with product designers during the early
design stages. The popularity of RP has naturally led to the extension of using additive layering
manufacturing techniques to manufacture tools, dies and industrial parts in a rapid manner.
This family of RP-based manufacturing processes is called Rapid Manufacturing (RM), Layered
Manufacturing (LM) or Solid Freeform Fabrication (SFF). The successful and widespread use of
RM processes to fabricate industrial parts depends upon the quality of parts being
manufactured. The parts should conform to the required geometric tolerancesspecified in the
design of the part. Therefore, there is a need to study RP parameters that influence tolerances
on critical features of the manufactured part. Due to the nascent stage of RM technology, the
effect of the RM process parameters on form errors has not been researched in detail.
For Additive Manufacturing to be accepted as a mainstream manufacturing process, parts
created by AM have to consistently satisfy critical geometric tolerances or the parts should be
accurate. Accuracy in additive manufacturing (AM) is evaluated by dimensional errors, form
errors and surface roughness of manufactured parts. Types of form errors are Flatness errors,
cylindricity errors, straightness errors, staircase errors etc.
LAYERED manufacturing (LM) has been quickly adoptedby industry in the past decade. From the
initial applicationfor concept modelling, it has been extended to low-volume
prototypeproduction. However, LM processes have their inherentdrawbacks that limit their
applicability. The staircase effect isone of the most serious problems that affect the surface
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3. qualityof the LM-built parts. As the LM process approximates theobject by layers with vertical
edges, the stacking of the layersdoes not match the original CAD model very well, thus
causingthe formation of staircase-like surfaces. The development ofa different slicing strategy is
a possible solution to solve thisproblem, and much research has been carried out for
thispurpose.
In rapid prototyping (RP) processes, the tessellated standard triangular language (STL) model is
“sliced” into layers, each of which is reproduced physically by the RP machine in building the
prototype. This is an additive process where each slice is placed on top of the preceding one,
resulting in the creation of the prototype. The thickness of the slices used to manufacture the
prototype would bring about an effect called layering error, stair stepping error, or staircase
effect. This study demonstrates the effect of slice thickness on the surface finish, layering error,
and build time of a prototype object.
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4. FORM ERRORS:
HOW FORM ERRORS ARE PRODUCED???
The generation of form errors in parts manufactured by AM depends upon the process
parameters chosen by the designer during the part build. The process parameters, namely slice
thickness, orientation, scan speedetc.
LITERATURE REVIEW:
 Hanumaiah and Ravi studied form errors in rapid tooling in two processes: (a) Direct
Metal Laser Sintering (DMLS) and (b) SLA. They concentrated on straightness, flatness
and circularity errors. They manufactured eight sample parts with the six most
commonly used features: plate, hole, boss, boss with hole, and ribs. They sampled
points on each feature and calculated the relevant errors by the Least Square Method
(LSM). They developed a regression model of the form error at a certain point on the
surface based on the errors calculated from the sampled points.
 Ollison and Berisso studied cylindricity errors and the effect of build direction, printhead
life and size of feature on cylindricity in 3D printing. They created two parts with
diameters 0.75 and 1 inches at three build orientations of 0, 45 and 90 degrees. They
conducted an ANOVA study on the parts and found that at a build angle of 0 degrees,
the cylindricity error was minimum while it was maximum at an angle of 90 degrees.
 Yang et al. minimized the staircase error in the SLA process using cusp height as the
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error criteria.
 Lynn-Charney and Rosen used Response Surface Methodology (RSM) to understand the
effect of process parameters on six tolerances: positional, flatness, parallelism,
perpendicularity, concentricity and circularity on a variety of geometries. Once the
relationship between process parameters and the tolerances were calculated, they used
a Compromise Decision Support Problem (CDSP) to find the optimal process parameters
to achieve the best tolerances.

5. STAIR CASE ERROR:
The staircase effect has been the major concern for industry to widely adopt rapid prototyping
technologies. It will not only worsen the surface quality but also create errors on the parts
built.AM process approximates the object by layers with vertical edges, the stacking of the
layers does not match the original CAD model very well, thus causing the formation of
staircase-like surfaces. The reduction of staircase effects can be categorized into different
classes contour shaping, and optimization of the build orientation.Processes approximate
objects using layers, therefore the part being produced may exhibit staircase effect. The extent
of this staircase effect depends on the layer thickness and the relative orientation of the build
direction and the face normal. The minimum layer thickness for a given process is known.
Therefore for a given process, the primary factor that determines the extent of staircase effect
is the angle between the build orientation and theface normal.
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METHODS TO REDUCE STAIR CASE ERROR
 Adaptive Slicing:
The staircase effect is closely related to the layer thickness. The staircase error increases with
the layer thickness and can therefore be reduced by using thinner layers.

6. However, simply using thin layers will increase the number of layers and thus the build time. A
number of adaptive slicing methods have been developed by considering the geometric
features of the model, so that the error can be reduced without a corresponding increase in the
build time. An adaptive slicing algorithm that can vary the layer thickness In relation to local
geometry was proposed. Four criteria (cusp height, maximum deviation, chord length, and
volumetric error per unit length) are identified, and the layer thickness is adjusted such that
one of the four is met. Some approaches adaptively slice the STL file of a part model to deposit
a variable- layer thickness according to the geometric information extracted from the STL file.
Another class of adaptive slicing methods use the original CAD model for the adaptive slicing
and then employ piecewise-linear approximation for the slices in that case, the surface
curvature contained in the CAD model is used to determine the slicing error .
OBJECTS SELECTED FOR INVESTIGATION
Three objects were modeled using the Solid works package fordemonstration of the research
issues. Three STL files of different tolerances were generated for each object. One of these STLs
for each object was used to show the effect of slice thickness on the layering error and surface
finish. All of the STLs were used to investigate the tessellation-slicing relationship.
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7. Results
The sliced images show the effect of slice thickness on the layering error and, hence, the
surface finish of the object
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PROBLEMS WITH ADAPTIVE SLICING:
Thin layers increases the build time.

9. The potential savings of adaptive slicing is lost by conventional adaptive slicing methods which
slice all parts of given build with same resolution regardless of their dissimilar surface
characteristics.
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 OPTIMIZATION OF BUILD ORIENTATION

10. As the staircase effect is affected by the build direction, orientation optimization approaches
have been proposed and are aimed at minimizing the staircase error. Optimization of build
orientation has been a popular research topic in LM process planning area. Usually, build
orientation is optimized based on the following objectives: support structure, trapped volume,
building time, and surface finish. Another approach is based on the empirical model derived
from data collected experimentally.
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ORIENTATION BASED ON PROCESS PLANNING ERROR
ORIENTATION BASED ON NUMBER OF LAYERS

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 Contour Shaping
The undesirable feature from the staircase effect is primarilydue to the layer edge being vertical
in most LMprocesses. Thus, it can be reduced by contouring the actual edge of each layer to be
close to that of the desired part geometry, or the sloped layers, through a combination of
additive and subtractive manufacturing processes.
The “stair-stepped” effect can only be combated by contouring the actual edge of each layer to
match that of the actual desired part geometry.
The approach taken with the CAM-LEM project [Cawley et. al, 1996],[Zheng et al., 1996] is to
use sloped rather than vertical edges. The CAM-LEM process uses a high-powered laser with a
5-axis positioning system to machine sloped surfaces on the edges of each layer.

12. CYLINDRICITY ERRORS:
According to ANSI standardsCylindricity Cylindricity is a conditionof a surface of revolution in whic
points of the surface are equidistant
from a common axis.
which all
The analysis of cylindricity error on RM parts is performed by the following three methods:
(a) Simple geometric model
(b) Simulating the cylinder er surface from a CAD model
(c) Simulating the cylinder surface from an STL file.
 In the geometric model, a mathematical analysis of the relation between
cylindricityerror and part orientation isdeveloped
simulated mulated both from the CAD model
and the STL file.
 SIMPLE GEOMETRIC MODEL:
isdeveloped. The manufactured cylinder is
In an AM process, when a cylinder is built in slices in a
particular orientation, a staircase effect is introduced on the cylinder surfacedue to the
thickness of the slices. The build orientation in the case of a cylinder is defined as the
angle between the cylinder axis
(−→ca) and the build direction ( v).
If the slice thickness is infinitesimally small, then the amount of material added to each slice
would not have any significant contribution to the cylindricity error. However, since this is not
the case, for practical purposes, some additional material is
deposited on each slice.This added
material in effect leads to additional point on the
coaxial cylinders on the surface.
tcyl =
Where tcyl is the cylindricity error, AB =
surface. This phenomenon generates two
= slice thickness, = build orientation.
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nder

13. Thus, there is a sinusoidal relation between the build orientation and cylindricity error,
provided the slice thickness is kept constant.
FIG
FIG-Cylindricity error in RM.
FIG- METHODOLOGY TO MINIMIZ
MINIMIZE CYLINDRICITY ERRORS
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 Simulation of manufactured surface using CAD model
In this approach the CAD model of the cylinder is tilted about the x-axis by a small arbitrary
angle at the start of the process. The circular edges of the top and bottom faces of the cylinder
are discretized into numerous points,. The points at the top surface are joined to the
corresponding points at the bottom face to form a set of lines
Simulation of manufactured surface using STL model
In this section, the CAD model of the cylinder is converted toan STL file for building the cylinder.
In STL file, the cylindrical surface is tessellated into triangular facets, as shown in Fig. 6b. The
STL file is less accurate than a CAD file but all RP/RM machines in industry typically use STL files
as input. The cylindricity error is obtained by slicing the STL file and simulating the
manufactured part. The simulated surface for an STL file is generated in a process similar to
that for the CAD file. The STL cylinder surface consists of a set of triangles Tcyl. The minimum
and maximum points, Pmin and Pmax respectively are calculated from the set of vertices V of
the triangles. These points provide the height h by which the part platform will move during the
manufacturing of the part. The number of slicing planes, ns, is calculated and the set of slicing
planes S are generated. The lines of intersection of Si and the triangle set T, (Lint)i are
calculated which form the contour.
EXPERIMENTAL RESULT:
Test part:
Optimal part orientation from experiments

15. FLATNESS ERRORS:
The errors in flatness was introduced due to the staircase effect which in turn depended upon
the slice thickness and the face inclination. The relation between flatness error slice thickness
and build orientation is given by.
tfla = ΔZ*cos θ.
The flatness error of a nominal flat feature is defined by ASME as the minimum tolerance zone
between two offset planes which completely enclose the points sampled from the
manufactured feature.
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16. Warpage has an adverse effect on the geometric accuracy of the fabrica
a form due to upwards curling of the part at either edges or only one edge.
fabricated parts. It could be in
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ted

17. Using proper temperature setting for particular materials is important to fabricate a good part.
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MINIMIZING FLATNESS ERROR
 Using proper temperature setting for particular materials is important to fabricate a
good part.
 Optimizing orientation and slice thickness.

18. CONCLUSION:
In this report three types of form errors have been quantified:
(i) Stair stepping error due to slicing of components, (ii) cylindricity error, and (iii) flatness or
straightness error
It has been shown that the layering error increases, and the surface finish quality and build time
decreases, with increasing slice thickness. The form error is shown to decrease for certain
orientations of the part, for which the number of layers is also reduced. The applicationof the
algorithm objects shows that it is possible todetermine preferred orientation of objects using
these error criteria.
Various methods of minimizing different forms of form error are discussed as: adaptive slicing,
preferred orientation, contour shaping etc.
These are essential for the growth of application of rapid prototyping in manufacturing sector.
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REFRENCES:
 Paul R, Modelling and optimization of powder based manufacturing processes, 2013,
M.S.M.E University of Cincinnati, B.Tech National Institute of Technology, Warangal
 P. Ratnadeep , Anand S. Optimal part orientation in Rapid Manufacturing process for
achieving geometric tolerances , Journal of Manufacturing Systems 30 (2011) 214– 222
 Sahatoo DR, Chowdary BV, Fahraz F. Ali , Bhatti R. Slicing Issues in CAD Translation to
STL in Rapid Prototyping, Proceedings of The 2008 IAJC-IJME International Conference,
ISBN
 Taylor JB, Cormier DR , Joshi S, Venkataraman V, Contoured Edge Slice Generation In
Rapid Prototyping Via 5-Axis Machining, Department of Industrial Engineering, North
Carolina State University
 Karunakaran KP,Shanmuganathan PV, Jadhav SJ, Bhadauria P,Pandey A, Rapid
prototyping of metallic parts and moulds, Journal of Materials Processing Technology
105 (2000) 371-381
 Bablani M, Bagchi A ,Quantification Of Errors In Rapid Prototyping Processes,And
Determination Of Preferred Orientation Of Parts, Product Realization Laboratory, Centre
for Advanced Manufacturing, Clemson University, Clemson, South Carolina.
 Mahesh M, Rapid Prototyping And Manufacturing Benchmarking,National University Of
Singapore 2004