INVITATION TO ROLLING 
INVITATION TO ROLLING
© COPYRIGHT ESCOFIER TECHNOLOGIE
CONTENTS
A. Rolling
III.
ROLLING CYCLE
IV.
ESCOFIER
ROLLING PROCESSES
1. HZ series machines
2. Syncrollâ
3. ALS
4. RDB
1. HZ series machines
2. Incrementalâ
V.
FEASABILITY
1. HZ and Syncrollâ machines
2. ALS machines
3. RDB machines
4. Incrementalâ machines
VI.
PRODUCTIVITY
1. Thru-feed rolling
2. In-feed rolling
VII.
"TRADE" TERMINOLOGY AND SPECIFICITIES OF ROLLING
1. A rolled thread can be recognised
2. Rolled splines can be recognised
VIII.
TERMINOLOGY SPECIFIC TO ESCOFIER
IX.
BASIC NOTIONS ON PROFILES AND SYMBOLS USED
XI.
TOOL LIFE
1. Profile
2. Material
3. Condition of material
4. Cleanliness1. Material
2. Treatment1. Mechanical condition
2. Adjustments made
Rolling is the term used to designate forming (generally cold) of a round part (cylindrical or tapered) by tools, rotating or otherwise, in various numbers and arrangements around the work-piece. Forming can also be carried out at medium temperature on certain materials (in the aeronautical industry in particular).
There are generally two tools, but one, three or four tools are also regularly used (even more in certain cases). The reason for increasing the number of tools is to reduce the force applied locally to the workpiece in order to avoid deforming it too much (for instance, a hollow part).
The tools are generally arranged symmetrically around the workpiece (except in the case of a single tool). This:
There are three possible tool shapes:
The tool has one (or several) relief profiles and is driven in rotation. Contact between the tool and the workpiece drives the latter in rotation and a mirror profile of the tool is progressively impressed on the workpiece:
Between two tool profiles, the part material is displaced to form the outer diameter of the profile.
This displacement is parallel to the surfaces in contact between the tool and the workpiece. It flows all the less easily as the angle between the tool and workpiece contact surfaces and the direction of the force (or the variation of profile) approaches 90°.
If not used to develop a profile, displacement of the material can be used to tie two parts. This operation consists of crimping. The tie can be either in translation or in rotation with or without leaktightness between the two workpieces.
Certain rolling operations are designed only to create a surface finish on the workpiece or to calibrate a particular shape. In this case, there is no or very little displacement of the material. This is called burnishing or finish rolling (in particular used for the pinions of gear boxes to avoid costly operations such as shaving, grinding or honing).
Rolling is used increasingly because it brings many advantages compared with machining:
- Improved surface condition (which can be further improved by burnishing).
- Increased mechanical strength by work hardening (see below), by avoiding fracture starts generally due to tool marks or scratches and by fibring obtained on rolling.
- Economy of material since the blank diameter (the one whose volume corresponds to that of the finished part) is less than that of the finished part (outer diameter).
- Shorter cycle time on machining in many applications (above all on spline rolling).
- Better quality is achieved than with conventional machining: rolling is therefore occasionally used to obtain a better quality than machining and to avoid milling.
III.
Rolling cycle
The rolling cycle breaks down into three parts:
The tool is brought to the workpiece and presses against it either to "print" its profile on the previously smoothed blank, or for burnishing or calibrating the surface.
The workpiece is made round, which is not the case momentarily during penetration.
The tool and the workpiece move apart in order to free the workpiece and machine elasticities. This phase ultimately makes a workpiece circular or not. Failed decompression will induce a "signature" in the part, which is no longer round (visible on measuring its circularity).
IV.
ESCOFIER rolling processes
All the Escofier processes have circular tools. All the machines have a roll center distance which is adjustable in order to adapt to the size of the part. However, during rolling, some of them work with variable distance, others with fixed center distance.
1. HZ series machines
So-called "conventional" machines with two tools moving symmetrically one relative to the other (the workpiece is at the centre). The available forces are from 15 to 240 tons. Applications range from thread rolling to serration rolling (precision involute splines = Incrementalâ, see below). The rolling operation can be either in-feed (rolling with a variable roll center distance) or thru-feed (rolling with a fixed roll center distance).
In-feed: the width of the tool is slightly greater than the one of the workpiece. The tool axes and workpieces are parallel and in the same plane, and the tools, with constant section, are fed into the part to print their profile on them. The tool and workpiece helical values are identical.
2. Syncrollâ
This is a variant of the HZ machines with numerical control and which can be equipped with the synchronised drive system (with tools) of the rotating part. The principal advantage is to be able to roll splines accurately on a machine with a less dedicated function than an Incrementalâ machine, more flexible, more multipurpose (by retracting the Syncrollâ system, it can be used to roll in in-feed or thru-feed modes).
3. ALS
This machine is dedicated to rolling back-angled dog gears (for gear box sliding gears). A single tool with axis parallel to that of the part is used. The back angles can be on inner or outer teeth. The part with inner teeth, previously spindle-mounted, arranged in a die, is forced against the tool in order to "print" the tool shape on the part teeth. In the case of a part with outer teeth, the tool is forced against the outside of the part. There is also a numerical controlled version.
4. RDB
This machine was originally developed to roll bearing cages from tube ring blanks. There are two tools: an outer roll and an inner roll. The tool and workpiece axes are parallel and in the same plane. Rolling takes place at a point like on the ALS, but since the workpiece is not inside a roll, its outer diameter increases (20 to 100% or more expansion can be achieved). The forces used are from 25 to 50 tons.
1. HZ series machines: thru-feed rolling
The width of the roll is less than the one of the part. The roll and part axes are not in the same plane. The rolls are set at the final centre-to-centre position and the part (generally a bar) is inserted between the rolls. The roll profile is axially evolutive and presents three zones (penetration, calibration and decompression). The roll profile is impressed on the workpiece progressively as the part passes through the roll. The part is fed through the roll by the fact that the helixangle value of the roll and the one of the part are not identical (the difference is the compensation or thru-feed angle). The three main tools are:
- Circular grooves : the tool takes the form of circular grooves. Its axis sloping by the workpiece helix.
- Compensated helix for machines with inclinable shafts: the tool axis is inclined by the helix angle difference between the part and the tool (which is voluntarily modified on design and execution).
- Compensated helix for parallel axis machines: the tool axis remains parallel to the part axis, whereas its helix is modified as in the case above.
Comment: In certain cases the rolling cycle can begin by in-feed rolling on the part (for instance, near a shoulder, and then continued, once the tools are at the final center distance, by thru-feed. This is then called in-feed-thru-feed.
2. Incrementalâ
The two tools only execute one rotation in order to roll the part. Their centreline is parallel to the one of the tool and the operation lasts four seconds. The tool profile evolves radially in order to penetrate the part in its "penetration" zone. It is withdrawn, also radially, in its "decompression" zone. This process is widely used in the automobile industry for large production runs of splines, threads and lubrication grooves on transmission, gear box, torsion bar parts, for instance.
Note: There is a similar competing process which uses straight tools called "racks". Escofier also designs and manufactures these tools.
V.
FEASIBILITY
There are three levels of feasibility:
Generally, it is considered that a material with a yield strength (also called hardness) which does not exceed 1400 MPa (~ 140 kg/mm²) and whose elongation is not less than 10% (elongation calculated based on the conventional tensile strength test) can be rolled. However, the hardness limit for splines is 1000 MPa due to the precision to be obtained and the service life of the tools (hence the economic viability of the operation).
Almost all metal materials can be rolled, but with limits which are related to the profile and the accuracy to be achieved. Competing materials are steels of all kinds (carbon, case-hardened, nitrided, stainless steels) and certain bronzes (Cu Zn 34 Pb 2 or Cu Zn 36 Pb 3). Other more exotic materials can also be rolled but with varying degrees of success (titanium, inconel, aluminium alloys).
In addition to the theoretical characteristics of the material, care must be taken to ensure a good condition of the material before rolling. Operations such as drawing, straightening, burnishing can make a material unsuitable for rolling or reduce the use life of the tool by prior work hardening (see specificities).
It must be possible to roll the profile. Its shape, its length, its angles, its shoulder radius all contribute to part rollability. The profile combines with the material; a profile which is declared rollable on a low carbon steel may be rejected with a cold drawn, and therefore work-hardened, material.
Generally speaking, a spline can be rolled from a 30° pressure angle (the angle which characterises the tooth flank slope) and a thread from 5°. Nevertheless, for various reasons, a 10 to 12° pressure angle is required. One aspect is the difficulty in grinding very straight profiles, another is the fact that on rolling, a very vertical flank involves more machining (therefore shavings) than a rolling operation on the part.
Once the profile and the material have been accepted, an appropriate machine for executing the work has to be selected:
1. HZ and Syncrollâ machines
The force applied on the machine conditions rolling feasibility. Insufficient force will not produce a full profile on the part. A theoretical calculation (deduced from the "material coefficient") can be used to predict the force and therefore to choose the appropriate machine.
If the in-feed length to be rolled exceeds the limits of the machine (from a standpoint of the force applied), a thru-feed solution must be sought.
This is also true for the Syncrollâ machine. However, on the Syncrollâ machine, once feasibility is accepted, for example for the splines of a given module, there is almost no limit on the number of teeth that can be rolled (in the limit of the machine roll center distance). It is simply necessary to increase the rolling time in order to increase the number of teeth rolled with the same tool. This differentiates the Syncrollâ process from the Incrementalâ process.
2. ALS machines
The force available on the machine is always sufficient to roll the parts. Indeed, very little material is displaced on the tool flank. It simply consists of reworking the existing teeth.
3. RDB machines
The force available on the machine also conditions feasibility of rolling, but there is no formal calculation for predicting the force.
4. Incrementalâ machines
The tool only executes one rotation. Therefore it only has a certain developed length in which to roll the part. The reasoning is in terms of the tool action (penetration value per workpiece half-rotation) and the drive torque of the tools based on the profile and the length to be rolled, which determine the limits of workpiece acceptance.
The rolling force is indeed induced by the base stiffness on the Incrementalâ machine, and not by a hydraulic system as on most Escofier machines.
VI.
PRODUCTIVITY
Productivity depends on the type of machine and the type of operation:
1. Thru-feed rolling
Productivity is related to the helix angle and to the rotation speed of the tools (constant tool diameter): the helix angle also has limits relating to the profile, material, quality and the machine. The rotation speed is in theory limited only by the power available at the tools. The higher the power, the faster the tools can be rotated and therefore more parts produced.
In fact, the limit on the speed is decided by the part. This can reach fairly impressive levels. The problem is therefore in terms of the part holding system which has to be constructed to industrial production requirements (no early wear on the workpiece due to friction), to ensure that the part can be correctly guided, and its rolled profile is not deteriorated by friction. With a bar feed system arranged in front of the machine and a rolled bar reception zone behind the machine, the bar loading and unloading times do not greatly affect the cycle time.
2. In-feed rolling
The power at the tools and therefore their rotation speed have an impact on productivity. This is also affected by the rate of penetration and withdrawal of the tools in the workpiece. Last but not least, the part loading and unloading times have an enormous impact on the final production rate, all the more so now due to the legal obligation to place guards round the machines. This reduces the work rates obtained previously when access to the rolling station was totally unhindered, but with higher safety risks for the operator.
The time required to roll one part is around 4 seconds. Productivity is therefore dependent on this time (which cannot be reduced) and the associated time needed to load and unload the part. Rates of 600 parts per hour have been obtained (in very special cases, even 900 parts per hour).
The tool feed and rotation speeds, the no-load movement of the tools (mandrel advance in the blank on the RDB, indexing of the tool in the part on ALS), part loading and unloading times all condition the work rate on the other machines.
VII.
TRADE TERMINOLOGY AND SPECIFICITIES OF
ROLLING
This is the diameter at which the volume corresponds to the volume of the finished part on which an appropriate profile has been printed. A calculation taking into account the profile enables a theoretical value to be determined. Certain parts are elongated on rolling, notably when rolling an important profile in terms of the part diameter. It is not unusual to have to use a larger diameter in practice than that indicated in theory.
In order to measure the volume of a spline or a thread, one or several measuring devices are placed in the profile and the dimension is measured (in general at the diameter, therefore on two measuring devices for splines and three for threads). The measuring devices are in contact with the profile flanks.
Work-hardening occurs during rolling (it is very high at the surface and then gradually decreases towards the inside of the part). The material is deformed because energy is applied. For a certain period of time the material does not change, and then it hardens and gradually begins to refuse the deformation. This refusal ultimately translates into a poor appearance of the part (decohesion of the material, flecking which gives a poor surface finish and even splitting of the part from the centre outwards).
A coefficient "z" (used in the material coefficient calculation) characterises the ease of a material work-hardening, and therefore its risk of hardening on rolling until it becomes unsuitable for deformation.
The higher the z coefficient, the more the material can be work-hardened, and the more the force to be applied will need to be high and rolling will have to be carried out quickly to avoid reaching the work-hardened point.
Certain metal working operations (drawing, straightening, burnishing) induce prior work-hardening in the workpiece and can make it unsuitable for rolling, or at least make rolling difficult.
Saturation occurs when the workpiece material entirely fills the tool profile. This type of rolling gives a finished appearance to the part which many clients appreciate because the summit of the profile is entirely closed and does not present any risk on handling the piece. However, after saturation, three phenomena may occur:
The dimensions cannot be controlled: the difference between the measured dimensions and the outer diameter change, the division of splines becomes incorrect, the profile pressure angles are deformed.
As the material which resists the tool cannot move, a poor surface appearance is developed on the part (decohesion of the material following by flecking and even splitting of the part from the centre).
The tool service life may be shortened because the tool resists much more strongly due to the fact that the compressed material cannot escape.
Before reaching full saturation, the crest of the profile has a hollow which we call the "saturation indicator" (in fact indicator of non-saturation) or margin (in the case of threads with very sloping flanks and due to the direction of rotation when rolling, the margin tends to be on the side and not in the middle of the summit). Rolling with a "saturation indicator" is correct rolling which will not reduce the service life of the tool.
Rolling in a given direction of rotation directly influences the pattern of the profile flanks. It determines one flank which is so-called "pushed" by the roll (or penetration for the Incrementalâ machine) compressed and less high, and one flank so-called "drawn" by the tool (or calibration for the Incrementalâ machine) drawn and therefore higher than the other.
The drawn flank presents a so-called "cat's ear", i.e. point of material which is characteristic of a rolled spline. In thread rolling, this dissymmetry results in a position of the margin which is not centred on the crest of the thread but spread on one side. Before the crushes the summit of the part thread, the presence of two "cat's ears" can be observed.
The following reasoning is valid for all the machines apart from the ALS, and as long as saturation is not reached.
If the roll center distance decrases, the measuring device dimension and the minor diameter reduce and the major diameter increases.
If the roll center distance decrases, the measuring device dimension and the minor diameter increase, and the major diameter is reduced.
With a fixed roll center distance decrases, if the blank diameter increases, the major diameter will increase and the measuring device dimensions and minor diameter will not change (in theory; in practice, as the machine force increases, so does elasticity and these slightly increase but not in the same proportion as the major diameter). The relationship is 2 to 5 between the increase in the blank diameter and the major diameter (this depends on the profile and its angles of pressure).
With a fixed roll center distance decrases, if the diameter of the blank decreases, the major diameter decreases, the measuring device dimension and the minor diameter do not change (in fact they reduce very slightly). The relationship between the reduction in blank diameter and the outer diameter is the same as previously.
We noted previously that a change of blank diameter and its consequence on the outer diameter is amplified. Therefore the tolerance on the diameter will have to be above the one of the blank diameter (2 to 5 times depending on the profile). When this is not the case, the blank diameter tolerance can be copied on the major diameter by rolling while bearing on the material.
To do this, the machine has to be adjusted to apply only the necessary force for rolling and no more (using the hydraulic control system pressure to control the movement of the tool carriages). When the required force has been reached, the roll is bearing on the workpiece material (in equilibrium). It can no longer penetrate into the part and to do this, more force is required.
If the blank diameter of a part is greater than previously for instance, bearing on the material will take place sooner and the part will be larger overall (major diameters and minor diameters and on measuring devices). An increase of 0.05 in the blank diameter therefore in theory results in the same increase on the major, minor and measured diameters.
Bevels on the blank diameter are designed to protect the roll. Without the chamfers at the end of the parts, the last tool thread will be working in contact with the material on one side and not in contact on the other. Dissymmetric lateral thrust therefore occurs and causes the roll breakages. With a chamfers, the roll threads in contact with the material are submitted to more balanced lateral forces which protect it from breaking.
The harder the material to be rolled and the less its potential elongation, the less must be the chamfers angle measured relative to the part axis (10°). For less hard materials, 15 to 20° and 25° maximum may be selected for very soft materials (400 to 500 MPa steels).
As the tool applies pressure to the material, this is displaced upwards insofar as there is no other point of relief. At the end of the part, the matter is able to flow axially and therefore not to move upwards. Therefore, we observe that the major diameter is not constant over the entire length rolled. There is a zone at the end where the major diameter gradually reduces. This phenomenon is particularly visible on splines, and certain standards have taken it into account.
The division mainly concerns splines. In theory, the teeth should be regularly arranged around the part. In practice, this is not the case. Certain teeth or splines are not exactly in the correct angular position (due to the quality of the tool, machine adjustment, differences in material hardness).
This error of division is the maximum deviation of the angular position of one tooth. In practice, the dimension is measured using a micrometer on a given number of part teeth (measurement on K teeth), around the part. The division error is the difference between the maximum and the minimum dimension measured. A good division on testing at Escofier is 0.01 to 0.03 (at the customer's, in production this will be 0.02 to 0.06).
This is matching the machine so that the forms made by a roll on the workpiece are found opposite the forms made by the other tool after one half-turn of the part (when using two tools). Matching adjustment is partly responsible for the quality of division of the splines;
1. A rolled thread can be recognised by:
- its shiny surface,
- the presence of the saturation indicator,
- "cat's ears" if the outer diameter of the part is not flattened by the tool,
- at the bar start (thru-feed) if this is available.2. Rolled splines can be recognised by:
- the shiny surface (except with milled rolls and electroerosion cut rolls : see "roll execution"),
- the outside diameter reduction,
- a single "cat's ear",
- burrs at the crest of the teeth for the Incrementalâ and Syncrollâ processes, and the rack machine,
- occasional saturation indicators,
- an axial band at the end of the part below the inner diameter of each tooth,
- by deformation of machining striations at the end of the part close to the inside diameter.
VIII.
TERMINOLOGY SPECIFIC TO ESCOFIER
This is a coefficient which is used to calculate the minimum theoretical force needed to roll one part. This coefficient is based on the profile to be executed and the material used (characterised by its hardness, its elongation and its work-hardening coefficient). It also defines the ideal conditions for rolling in terms of the number of part rotations for the machining operation.
In practice, this coefficient, although it provides a basis, is not accurate enough. The actual force can be different from the theoretical force. This can be explained by the fact that the ideal rolling conditions do not in fact exist on rolling, that the material does not have the predicted characteristics, and by the inaccuracy of the mathematical model.
In fact this is the "hammer blow" value received at one point on the part on passage of a roll. It may be considered similar to the depth of cutting on a machining operation.
Let us consider one point on the part which has just been deformed by the tool. By rotation of the part, this point is found at the end of a half-rotation (case of two rolls) opposite the other tool which, due to its variable profile and machine center distance variation, will exert a counterforce of a certain value. This value (measured at the radius) is the tool action. It can be quantified in hundredths and even tenths of millimetres. If it is too large, the roll will be prevented from operating and the part will stop rotating (the part will have two flat surfaces).
This is the term used to designate the rolling tools insofar as they are circular.
This term designates the form of the key of the Incrementalâ dies used to take up play.
It also designates the shape of the material forced to the crest of a thread or a spline before the roll crushes it. If it is not crushed by action of the roll it is called "rabbit's ears" in the profession.
IX.
Basic notions on profiles and
symbols used
Normal profile: this is the profile which is observed in a perpendicular section cut at the helix angle. It is designated by the index "n".
Frontal (or apparent) profile: this is the profile observed in a section perpendicular to the part axis. It is designated by the index "t".
Axial apparent (or axial) profile: this is the profile observed in a cut through the part axis. It is designated by the "x" index.
dD : blank diameter. See definition in Section 7.
d: reference diameter. This is the reference diameter used to define the profile database (module, angle of pressure, helix, for instance).
Z1: number of teeth of the part threads.
Z2: number of teeth of the roll threads.
pitch: distance between two adjacent threads or teeth.
px: unit axial pitch of a part (measured parallel to the axis). px is always a linear pitch.
pz: lead (= Z1*px). pz is always a linear pitch.
pt: frontal transverse pitch. pt is always a circular pitch.
pn: normal pitch.
mn: normal module of a screw or a spline. This module characterises the size of the thread or the tooth: mn = pn / p.
involute : a particular tooth profile obtained from the base circle. Any rolled profile is an involute.
a : pressure angle (normal if accompanied by the letter "n"). This is generally indicated for the reference diameter. It is different for each diameter.
threading: angle between the tangent to the profile on the diameter and a line perpendicular to the axis of the part.
involute to a spline: angle between the tangent to the spline on the diameter and the straight line from the centre of the workpiece through the point of intersection of the profile and of the diameter considered.
b : helix angle (measured relative to the part axis). This is generally given for the reference diameter. b is different on each diameter.
g : helix angle (measured relative to the part axis perpendicular). It is generally given for the reference diameter. g is different for each diameter.
R: resistance to break of the material.
A or A%: elongation in percentage of the material used.
z: coefficient of work-hardening.
Escofier is able to design and execute all convex circular tools which correspond to the Escofier processes and the competition, with one exception to date: tools for the ALS machines.
Moreover, Escofier has the capacity to design and execute rack tools.
The tool profiles are obtained:
By cutting before heat treatment whenever the accuracy required on the part allows this.
By numerical control hard turning. This is the case for certain tools produced for the ring rolling (RDB machines) or finning discs, for instance.
By precision grinding for most circular or straight tools: thread rolls or racks.
By electroerosion on Incrementalâ rolls for splines.
XI.
TOOL LIFE
The tool life is related to three factors:
Mainly due to its profile, its material, the condition of the material and its cleanliness before rolling.
1. Profile
The profile has an obvious impact on tool behaviour. Certain profiles are much stronger than others. A ball screw thread profile will be less easily damaged than an ISO-type profile. Similarly, on splines, a 45° pressure angle on a tool will result in longer life than a 30° pressure angle.
The crest and root radii also have an impact. Small radii, involving high pressures, are more quickly chipped than larger radii. When they are chipped, they lead to other deteriorations of the tool and reduce its life (even more so when the tool leaves marks on the part, which may a cause for its rejection).
2. Material
The tool life is inversely proportional to the hardness and proportional to the elongation. For materials with more than 1000 MPa strength and less than 11% elongation, the service life will not be exceptional.
3. Condition of the material
The hardness and elongation capacity of the material are one thing, but the condition of the material before rolling may influence not only the rolling operation itself (make it impossible), but the life of the tool by increasing the forces on the tool due to prior work-hardening of the workpiece (e.g. drawing or straightening).
4. Cleanliness
Cleanliness has a direct influence on tool life. Two known examples will illustrate this: the case of unwashed and ground blanks (the wheel grains (abrasive) remain on the surface of the part and gradually destroy the tool), and the case of blanks with machining chips (or rings of non-detached ground material) which damage the surface of the tool and reduce its service life.
Mainly through its material and its treatment.
1. Material
Depending on the application, different tool materials can be used.
Conventional tooling steel with 12% chromium for applications such as thread rolling.
High-speed steel: conventional (grade Z 85 WDV 6 5 2): vacuum cast, sintered (obtained by powder metallurgy). High-speed steels can achieve hardnesses of 66 HRc. It is used on conventional machine tools when it is sought to increase the hardness of the tool, Incrementalâ, RDB, thread rolls on very hard workpieces.
2. Treatment
There are two separate notions under the term "treatment".
Heat treatment which increases the hardness and produces a suitable structure for tool steel. Two techniques are used to treat the steel in at core (identical hardness throughout the tool). Salt baths and vacuum. The fused salt technique has long had the edge on the vacuum technique due to the rate of cooling after austenisation of the steel (a phase which brings it to high temperature (~ 1000°C) in order to change its structure and hardness). This difference has now been overcome by high pressurised gas cooling which increases the rate of cooling.
Surface treatments and finishes: these are treatments which affect the surface of the material and a slight depth. They are designed to produce particular properties (notably friction resistance in rolling): polishing, chroming, carbide and nitride linings.
Mainly through its mechanical condition and the adjustments made.
1. Mechanical condition
In mechanical engineering, tolerances are required and even essential. However, machine tolerances which are too high become dangerous for the tool life.
Example of a tolerance which is too high in the Incrementalâ machine shaft line: apart from the poor quality of division, tools are able to move angularly and this causes additional vibrations which are detrimental to good tool working.
2. Adjustments
Incorrect adjustment of the machine shortens the tool life by causing it to work in poor conditions. Example: incorrect tool matching.
Instead of receiving balanced flexure forces on one side and the other (in fact incorrect since the fact that the tool pushes the part in a given direction induces larger stresses on one side), the teeth undergo additional flexure forces which lead to early teeth breakage. Another example is saturation rolling (studied previously).