Machining

Question 1

In which sectors can milling be implemented?

Answer 1

-Aerospace (bladed disks-inconel)
-Automotive (engine block)
-Oil and gas (valves-SS, inconel)
-Medical (implants, Ti-alloys)
-Die and mould (competitor with plunge EDM)
-Power generation
-Wind power

Question 2

Which are the milling parameters?

Answer 2

-Dc : diameter of the part which is in contact with the workpiece.
- Dcap : equivalent diameter of a part which is not cylindrical
-n : rotational speed of the spindle
-Vc : cutting speed (its the most important patameter and as it comes from the combination of the target material with the tool material)
Vc=πDcn/1000 –> n=1000Vc/πDc

-fn: feed per revolution (length accomplished by the tool along the feed direction)
Z: number of teeth (for micromills usually Z=2)

fz : feed per tooth ( fz=fn/Z)
-Vf: feed rate
-hd :uncut chip thickness (radial distance between the trajectories of two consecutive teeth)
-Ad : nominal cross section area of the chip.

-ap : axial depth of cut
-ae : radial depth of cut
both of them are defined by the tool and not the workpiece

Question 3

What are the angles in milling?

Answer 3

• Rake angle (Kre): angle between the cutting target workpiece surface and the surface (rake surface) on which cutting chips flow away when the cutting tool abrades the target workpiece.

Kre=90 deg in flat-end mills

Question 4

Differences between up milling and down milling

Answer 4

In down milling (climb), the mill pushes down the workpiece against the fixture.
In up-milling (conventional), the tooth applies a force that tends to detach the part from the fixture.

The revolution of the spindle is typically clockwise. However, in modern machines, anti-clockwise rotation can be used but specific machine configuration must be done.

Question 5

Shape of the tool

Answer 5

The geometry of the tool plays an important role to avoid defects. It should be found, considering the angle of the thrust force.

-The mill must be relatively short, otherwise it would bend under the feed force.
-Also, the mill can’t be too thick, because it would deform greatly.

Question 6

Differences between micro milling and macro milling when it comes to size effect

Answer 6

The size effect phenomena only become relevant when a certain dimensions threshold if overcome.
-In micromilling, the tool is sometimes of the dimensions of the target material’s grains( while in macrocutting the tool is by far larger than thte target material’s grains). This makes the force fluctuate when passing thourhg grains with different orientation or composition .

The tager material can’t be considered homogenous. That’s why quality materials should be used when high accuracy is needed. Also, thermal treatements can be used to homogenize the material.

Another type of size effect, comes from the fact that in a small portion of material, it it less probable to find defects as dislocations and inclusions. This makes the target materials more resistant in the macroscale. In other words, it’s expected that the cutting pressure kc is higher in microcutting.

Question 7

Minimum chip thickness tc for orthogonal cutting

Answer 7

The chip thickness is only related to the feed per tooth and not to the depth of cut. (tc=fz in orthogonal cutting)
tc,min=30-40% re , re cutting edge radius

We can disrtinguish 3 cases:
1)Plastic shearing tc>tc,min
2)Mixed elastic-plastic deformation tc,e<tc<tc,min
no material removal, the surface of the material is compressed leaving residual stresses and spingback occurs after the tool passage, producing flank wear at the tool.

3)Elastic deformation tc<tc,e
no permanent effects occur

tc,e is the elastic chip thickness

Question 8

MInimum chip thickness (milling)

Answer 8

We divide the uncut chip thickness in slices. Every slice has its own local γ,eff which can be obtained by considering the tangent to the cutting edge profile.

Another phenomenon is that when the side mill is engaged, the chip is not formed in the first part of the engaged arc as the chip thickness cannot overcome tc,min. At a certain point, after some revolutions, tc>tc,min and then the chip is formed.

Question 9

What is the Quick Stop Device ?

Answer 9

The QSD can be used to freeze the chip formation and take a micrography of the chip during its formation in quite regime conditions.

Question 10

Stable built-up edge (BUE)

Answer 10

BUE refers to the term of the cut material being bonded/welded to the tool surface and to the machined surface and can lead to change of the tool geomertry and the mechanics of the process.

Increasing Vc in macrocutting, increases the working temperature and the strain hardening effect, which is the base of the BUE formation.

In microcutting, BUE tends to be stable which makes it not detrimental for the cutting quality.

The stable BUE is claimed by some researchers to be always present when machining with an uncut cheap thickness lower than the tool cutting edge radius.

Stable BUE generates an effective rake angle.

Question 11

Forces in orthogonal cutting

Answer 11

In microcutting, Fc is not the main component anymore, because when the chip thickness is low, the feed force (thrust force) Ff dominates the chip formation.
Fc (cutting force) regularly increases with the chip thickness, while Ff fills the passage through the minimum chip thickness value.

When the chip thickness is low, its increase produces a sharp increase of the feed force, which makes it even higher than the cutting force but, when the minimum chip thickness is overcome, Ff comes back to a lower value and the chip formation returns to be regular as in macrocutting,

Question 12

Interminent chip formation in micro milling

Answer 12

This means that at the first passes, ploughing takes place, meaning that no chip is formed. After a number of passses, the chip that is formed has increased volume.
This phenomenon happens for low feed per tooth velocity fz.

Question 13

Common defects in micromilling

Answer 13

1)Mechanical deformation: It’s suggested to use the buggest possible tool to minimize the deformation. However this implies, that a sharp cutting edge (small re) should be used.

2)Thermal deformation: Big parts tend to deform more because of the thermal elongation of the materials. The temperature of the procedure should be controlled because otherwise some parts go out of tolerance.

3)Surface integrity: Surface quality does not proportionally decreases with the workpiece dimensions.

4) Workpiece reference: When the parts are too small, mechanical contact is not effective. The best strategy is limiting the part placements during the machining cycle.

Question 14

Burrs

Answer 14

Burrs can be detrimental for the parts, as sometimes their magnitude is the same as the machined features.

Cryo lubricants can be used to make them more fragile and easy to brake and remove.

Question 15

Causes of tool breakage

Answer 15

1)Chip clogging: When there is not enough room to remove the chips from the area close to the tool.

2)Fatigue: Gets worse due to wear

3)Buckling: Wear reduces MRR, but feed remains the same causing tool buckling and breakage

4)Run-out: Spindle, tool and tool holder run-out is one of the main causes of tool breakage. The action of the spindle is different and some teeth can be more exposed than others during cutting.
The run-out can be observed using visual tools. They acquire a picture of the tool at each revolution and then it calculates the tool diameter and runout. When the runout is too big, one should remount the spindle.

Question 16

Hardness of the spindle

Answer 16

Microcutting is suitable to machine hard and brittle materials due to the state of compression it generates in the machining area. This allows machining in ductile mode even on these materials, which means creating chips without crack formation.

Question 17

Tool holders

Answer 17

Tool holders play an important role in the milling process and they accoung for the possibility of run-out. The tool holder aims to enhance the accuracy thanks to the torsion reduction obtained by placing a ball bearing at the nut.

Shrink fit tool holders can be used too.

Question 18

Fixtures on micro machining

Answer 18

Fixtures are used to place small parts that need to be machined. Their advantage is the ease of replacement if needed to be placed in different machines.
However, we should be careful about the residual stresses once the part is removed from the pallet, otherwise it will deform. To do so, the parts should be thermally treated prior to the machining and the process parameters should be kept realatively low in order not to induce residual stresses.

Question 19

Peripheral feed rate

Answer 19

As the tool has to work on a corner or a curve, the peripheral feed rate has to be higher than the feed rate at the tool center because it has to travel a bigger distance. However, this imposes a risk of overcoming the tool’s static performance under the cutting force. Thus, we should reduce the feed rate at the tool center and thus keep the peripheral feed rate under the limit proposed by the manufacturer.

Question 20

Radial engagement

Answer 20

In order to make milling as smooth as possible, we should keep the engagement angle as constant as possible along the tool path. This can be done by varying ae (radial) along the tool path.
When the engagement angle increases, the cutting force, torque and power increase too, which could lead to the tool breakage.

The radial engagement together with the consideration about the peripheral feed rate should be taken into account together for the milling processes.

Question 21

Which technique should be followed for sharp edges?

Answer 21

Instead of turning around the edge, its better to machine one plane and then the other.

Question 22

Choice of feed rate, rotational speed and Dc

Answer 22

Firstly, we have to choose Vc, which is suggested by the manufacturer and its a function of the workpiece material and the tool material.
Then the tool’s geometry (Dc) is chosen based on the geometry of the target piece.

Micromachining has generally better perfomance for high rpm of the speed, which implies using small tools.

fz cannot be too small because of the minimum chip thickness effect. This means that Vf tends to be high in micromilling.

To maximize the MRR, we should have high Vf, but ap and ae must be considered too. ae (radial) should be limited but ap can be increased according to the tool’s manufacturer.

Question 23

Interpolation error

Answer 23

The machine has to draw the trajectory to be followed for completing the machining process. It carries out an interpolation of the points that come from the various constraints. The feed rate has to be kept as high as possible for high productivity. The interpolator uses all the allowed tolerance around the nominal tool path and used the so called look-ahead funciton to better calculate the interpolated trajectory to respect all the constraints.
This means that the interpolated trajectory not always passes from the desired points, thus creating the so called interpolation error.

The machined tries to reduce the acceleration and the derivative of the acceleration (jerk) which are however need for high feed rate.

Question 24

Chord error limit

Answer 24

The feed rate optimization is achieved by varying the feed rate along a curve to maintain precision defined by a chord error limit.

In other words, the chord error limits the error generated by the interpolated path to limit the offset of the desired product.

Question 25

Size of tool

Answer 25

In macromilling, the size of the tool can be decreased in order to have the required productivity.

However, in micromilling the size of the tool can be decreased up to a certain point because of the technical difficulties to produce tools smaller than some μm.

Question 26

Computational time needed to calculate the trajectory

Answer 26

In micromilling, the machines use circular and linear interpolation, which compared to macromilling, are more efficient because they require less computational time and have smaller error.

If a machine has low sampling time Ts, the maximum feed rate can be increased. This choice must be made when buying a new machine.
Ts cannot be lower than the time needed for the machine to calculate the trajectory.

Question 27

Accurate model of chip thickness and milling force

Answer 27

A more accurate model can be obtained by dividing the mill into horizontal slices in order to consider the action of each single portion of the cutting edge, which has a delay respect to the tool tip.

Also, the chip thickness must be divided in slices to consider the local effective rake angle.

The contribution of each slice along both of hte directions has to be summed up to get the resultant force in the three coordinate directions.

Question 28

Which are the two milling strategies for thin walls?

Answer 28

1)When the wall is too slender, the “step-support” strategy is followed. A small step from each side is removed until we reach the bottom.

2)On the other hand, if the wall is stiff, the classical strategy can be used, where the one part of the wall is removed first and then the other,

3) For intermediate structures, the “waterline” strategy is followed, where 2 steps are removed from one side and 1 from the other and then the opposite.

Question 29

Inconel

Answer 29

Inconel 718 and 625 are mostly used because of their high mechanical properties.

If they are aged, their hardness is increased a lot. Thus, when machining they tend to plastically deform the tool material. Notch wear is the main cause of tool wear. It’s a kind of crack generated at the tool flank especially when the same ap is applied during the tool life.

Question 30

Which kind of tools are used to cut Inconel ?

Answer 30

1)Solid carbide tools
2)Solid ceramic tools -> They can withstand the temperature of 1000 deg C which is needed to achive the softening of Inconel. To increase the temperature at such values high feed rate is used (Vc=1000m/min) and thus also high n. However, this can damage the machine because it has to provide high torque and power in order to achieve high n and Vc. Subsequently, this leads to rapid wear of the tool. The machining cycle has to be properly designed, usually the roughing is done with ceramic tools and then the finishing with carbide tools.

When using ceramic tools, up-milling should be used. Coolant must not be used with ceramic tools as it would probably create cracks, detrimental for the tool because of the thermal shock.

For carbide tools, Vc is smaller but fz higher.

Ceramic tools have Z>2 which increases the MRR significantly but decreases the tool’s life.

Depths of cut tend to be smaller for ceramics as they involve higher forces.

Question 31

Ceramics vs Carbides productivity

Answer 31

Overall, the productivity of ceramics is higher. The MRR is inceased 115%.

Be careful that even for ceramics, a finishing operation is needed.

The most suitable situation for ceramics is when big quantities of material should be removed in roughing in a relatively short time.

Question 32

Ceramic vs Carbide working conditions

Answer 32

Ceramics: low ap, high ae, high fz

Carbides: high ap, low ae, low fz

Question 33

Solid ceramic end milling tools

Answer 33

The new comers in this topic, are special silicon nitride ceramic tools (Z=4-6) with excellent heat resistance that can withstand flute tip temperatures of more than 1000deg C.

Cutting speeds can be a lot higher than the normal ones.

By achieving cutting in the softening temperature region of the work material, tool life is greatily improved.

Strategies at the entrace of the tool to the workpiece can save the mill from breakage.

Question 34

Tool breakage and re-sharpening

Answer 34

Tooth breakage can occur giving a temporary decrease of cutting pressure which then increases because the material under the broken tooth is also sharp and acts as a new tooth (re-sharpening). However, this changes the tools dimensions and geometry.

This cycle can occur a couple of times before the final breakage of the tool.

Question 35

Ceramic tools for soft materials

Answer 35

Ceramic tools can also be used for soft materials if the cutting forces are kept under control.

For soft materials, the tools can maintain their soft edges which means higher tool’s life and less downtime for the machine.

This also provides good performance in terms of burr formation.

This technique is used in the watch industry.

Question 36

Burr formation in milling

Answer 36

Higher feed per tooth (fz) produce less burr in slot milling.

Low fz also produces ploughing and smearing so it’s also the cause of burr formation.

This is also a reason why reducing fz is not a concervative action.

Question 37

Diamond tools

Answer 37

Diamond can be used in various abrasives and non-ferrous materials. Thus we can;t machine steels with diamond because of the bonding of carbon and diamond.

Because diamond is very expensive, the diamond coating of other tool materials is used.

Question 38

Peck drilling techniques

Answer 38

While drilling holes, we enter with a step of 0.2mm and then we use steps of 0.1 mm each. We stop the feed rate without stopping the rotation to reduce the thermal load and evacuate the chip.

fz should be small and lower than the minimum chip thickness.

No cooling is used because of the high fragility and sensibility to thermal shocks of the involved materials. Just fresh air is used to remove the chips and cool down the tool and the workpiece.

Question 39

Heat resistant alloys

Answer 39

When machined, the usage of coolant (oil+water) is mandatory (high pressure and high flow rates). They are machined with low productivity rates.

Question 40

Cryogenic machining

Answer 40

Cryogenic machining is an alternative of the classic cooling methods. It uses liquid N2 or liquid CO2, with nitrogen being more effective due to its lower boiling point of -196 degC (vs -78 of CO2).

The cooling happens because of the fluid’s phase change from liquid to gaseous which subtracts heat from the environment. If the tool has been deposited onto the tool surface, the tool will be cooled very effectively.

Another advantage of N2 is that its expansion is safely distant from the solid phase ( which is not the case for CO2 which when expanded, it gets close to the triple point, and as a result it sometimes cloggs the nozzles)

Question 41

Advantages of cryogenic machining

Answer 41

1)Increased tool life
2)Increased productivity
3)Increased surface and subsurface hardness of the machined piece
4)Increased surface finish
5) No need for pumps and filitration
6)Clean chips and workpiece
7)Shorter and more controlled chip
8)Negligible environmental impact and reduced health issues.

Question 42

Cryo machining vs conventional machining

Answer 42

In ocnventional mschining, a compromise is found that minimizes either the machining cost or the machining time.

Cryo machining aims to allow the increase of the cutting speed by prolonging the tool’s life, thus increasing the productivity.

Question 43

Number of nozzles for the liquid N2 in cryo machining

Answer 43

There are 3 nozzles in total in cryo machining. One for the rake face, one for the flank face and one for the secondary cuttting edge.

Nozzles are located close to the machining zone to ensure a consistent cooling action.

Question 44

Which law holds for calculating the tool’s life?

Answer 44

Taylor’s loaw parameters identification allows to calculate the tool’s life (T), taking into account the cutting speed, the feed rate and some empirical exponents.