homework 1 procesos de manufactura

Universidad de Chile - FCFM

1. Home work on Materials technology for manufac-turing

1.1. Question 1

The graph shows the stress - strain behavior of two materials.

Figure 1: Stress-strain graph of two unknown materials.

They have similar densities and costs and they are equally easy to work with. For eachmaterial determine the:

Ultimate tensile stress: The ultimate tensile stress (or UTS) is the maximum stressthat the probe could withstand before failing (breaking) under tension. Thus, thehighest point in the graphs represents the UTS. Assuming that the y-label has[MPa] units and the graphs shows the apparent stress:

Material A: The highest point in the curve is located near 280 [MPa].

Material B: In this case, the highest point in the curve is somewhere around 205 - 210[MPa].

Ductility: The ductility is the mechanical property that indicates the amount ofplastic deformation that a material can withstand before failing (fracture). A ma-terial that does not exhibit plastic deformation (non ductile) is called brittle.

Material A: We can see that that the probe was made of a brittle material, because thefailure occurs suddenly during the elastic part, there is no necking or plastic deformation.

Joakin Ugalde Castro - ME5700 Procesos de Manufactura - Homework 1

1.2 Question 2 Universidad de Chile - FCFM

Material B: The curve shows a noticeable transition between the elastic and plas-tic behaviour at the yield stress point. Then, the plastic deformation that the materialwithstand is near 0.7 - 0.8 % of strain before failure. Although the probe exhibits certainductility, it is low.

Toughness: Is the resistance of a material to fracture propagation. It can be mea-sured by the amount of energy per unit of volume that the material can withstandbefore the failure. More accurate ways to characterize this property are the Charpyand Izod impact test. With the strain-stress curve it is possible to measure thearea under it to achieve an approximation for the toughness. With this, a toughmaterial must be both strong and ductile enough. We will use the area of a trianglefor the linear elastic behaviour and a square area fora the plastic one, as a firstapproximation:

Material A: Using the method described above, the material can withstand 75 [KJ/m3].It is not strong enough and too brittle to be called a tough material.

Material B: In comparison with the material A, the extended plastic behaviour andthe comparable UTS of material B makes him more tough than the A, 1.3 [MJ/m3],which can be noticed easily comparing the areas under the curves.

All other things being equal, which would you choose to construct:

A bridge: We do not want the bridge to fail in a catastrophic way, because it wouldnot be safe for the people using it in the moment of the collapse. A tough material wouldexpose any cracks long enough before failing to be noticed and acting accordingly to themaintenance procedure. A bridge also sustain a lot of cyclical stress and vibrations, sois desired that the cracks propagate and grow slowly. It also has to be strong enough toendure the weights that pass trough him and the environment temperature variation thatwill cause volume variations and possibly cracks.. By this, the ideal material would bethe B instead of the A, which is strong and tough enough.

A strong but breakable cover for an emergency release: An example of this need couldbe the safety pin that attaches a turbine to the wing of an aircraft, as we saw in assignment1. The component must be made of a strong material to withstand the stresses producedduring the work of the system, but not strong enough to resist the controlled force thatactivates the emergency release. In an emergency the process has to be swift, thus thefracture must propagate quickly in the piece instead of exhibit plastic deformation, thestrain could also affect the precise way the piece is meant to fail. By the above, the idealmaterial would be the A instead of the B, which is stronger and brittle.

1.2. Question 2

List 3 different Hardness tests and their respective formula for test.



Hardness: Is the measure of resistance that a solid material expose in his surfaceagainst a local stress agent like indentation, scratch or rebound trying to causepermanent shape change.

Scratch hardness: Measures the resistance of a material to the plastic deformationor abrasion caused by a sharp object inducing shear stress (mainly) in the surface byfriction. The Mohs scale is the most popular regards to this phenomena, but is relativelyto the hardest material known, diamond, being 10 in the scale, and the softest one, talc,being 1 in the scale. Basically, a material is scratched against another and the one whoexhibits a scratch is the softest one, thus being located under the harder one in the table.A sclerometer is the instrument used to observe and measure the scratch of a diamondagainst a surface under fixed load in the Turner-sclerometer scratch test.

Indentation hardness: It is a measure of the resistance of a material against beingpierced or deformed against a certain load inflicted by certain object, depending on thetest. The important parameters are the fixed load, the geometry and material of theindentation tool and the mark produced by the compression. Measuring hardness thisway it is possible to establish a linear relation between this and the strength of thematerial, and the test can be performed in a non destructive way, resulting in a practicalway of characterizing a material.

Vickers test: A pyramidal tool tip made of diamond is pushed against the sampleunder current load.

HV =0,01819F

d2(1.1)

Wheres F is the load and d the size of the diagonal mark in the indented surface. Thehardness is reported by the amount of time that the load is applied, the load, the scaleand the result.

Rockwell test: The tool tip can be a diamond cone or steel spheres with differentdiameters. A minor load is applied to the sample, and then a mayor one. The depth ofthe mark since the zero established by the minor load indicates the hardness.

Brinell test: The indenter is a steel ball or carbide ball for harder materials. To cal-culate the hardness, the diameter of the ball (D), of the mark (d) and the load (P ) arerequired:

BHN =2P

πD(D −√D2 − d2)

(1.2)

Rebound hardness: It measures the elasticity of a material measuring the rebound ofa diamond hammer falling freely on it. The scales used are the Leeb and Benett hardnessscales. A sclerometer is used in the tests.



1.3. Question 3

With the charts provided, what are the suggested spindle and feed speeds for drillingwith an 8mm diameter drill: (see Table 1 and 2 at the end)

In inches: 8[mm] = 0.314 [in], which is between 1/4” and 1/2”, so the feed would be0.004 to 0.008 [rev/in], following Table 2. The speeds are observed in Table 1, which areused to give the spindle:

RPM =12V

πD(1.3)

Where V is the cutting speed in [feet/min] and D is the diameter of the tool in inches.

1. (a) High Nickel steel: 50 [ft/min] ⇒ 606 [rev/min] ≈ 600 [rev/min]

2. (b) Heat Treated Steel (35/40 Rockwell C): 20 [ft/min] ⇒ 242 [rev/min] ≈ 250[rev/min]

3. (c) Titanium 64: 20 [ft/min] ⇒ 242 [rev/min] ≈ 250 [rev/min]

1.4. Question 4

What effects do imperfections have on the strength of materials?

There are several kinds of imperfections. We have point defects like vacancies andintroduced atoms in the structure, this will affect the passing of the dislocations whichare agents of plastic deformation. The motion of the dislocation requires less energy thanmoving the structure trough a slip plane, thus, the material is weaker in the presence ofthis phenomena, but not always.

Dislocations generate stress fields around the lattice, this stress fields affect even otherdislocations, so a certain amount of them is useful to prevent the strain of the material,making him stronger.

As we need that this dislocations remains in their places it is desired to introduceelements to retain them, called pinning points. Foreign atoms in the interstice will inducestress fields that will block the dislocation movement, such as the atoms introduced inan alloy. Substitution atoms will not induce stress fields but will put a different energybarrier to the movement of the dislocation trough the lattice depending on his elasticmodulus. A small amount of dislocations with no pinning point will make the materialweaker, because the dislocations will be free to move.



Work hardening will move this dislocations until they reach a pinning point, introdu-cing smalls amount of energy that are not capable of breaking bonds, making strongermaterials.

The grain boundaries also stops the dislocation movements, cause the planes are mis-matched and the usual movement can not be done and a small grain size will be desired.Other phases (solid precipitation) presents also would stop the dislocation movement,because it will have to bend around to pass trough, changing the dislocation form. Theprecipitations usually locates in the grain boundaries, so when the stresses are concen-trated there an intergranular fracture could occur.

An exceed of imperfections will decrease the ductility of a material, because the dis-locations will not flow easily, that is why pure materials have higher ductility.

1.5. Question 5

Explain grain boundaries in metals and how they form? If a pure single crystal metalif formed what does this give in terms of a less pure formed metal in terms of strength?

Grain boundaries are formed when there in the molten metal or alloy are severalnucleation sites that come in contact as they grow, having each crystal a different andrandom orientation. A pure single crystal of metal does not count with the grain boundaryimperfection that will stop the motion of dislocations (so is weaker), but it will reducethe corrosion and creep.

The single crystal may be anisotropic because his lattice will only have a fixed orienta-tion (depending on his microstructure) instead of the multiple that a polycristal presents.

A material with less grains is more ductile, cause the deformation (dislocations) andthe slips planes will move easily without the presence of obstacles.

1.6. Question 6

What tests can be done to measure the surface quality of test piece (what measuresshould be taken to ensure the results can be compared against other measurement takenin the past or possibly in the future)? What instrumentation can be used for such tests?

There are two kind of surface anomalies, geometrical and non-geometrical. Geometricalanomalies are differences that the machined piece will have with the ideal design; this canbe a less defined contour, material lost in certain parts, surface roughness and manyothers, like plucks, scoring, scratches, laps and cracks.

This defects can be easily seen in an optical way (microscopy) or with a profilometer,



an instrument that moves over a surface with a stylus and follows the path of the roughnessand the waviness and gets the deviations usually produced by the vibrations in the process.The surface can also be scanned by shock and vibration testing if more precision isrequired, which will depend on the quality desired by the manufacturer.

Any defects will lead to poor part life, or early/sudden failure, which can not beadmitted in safety critical parts.

Non-geometrical defects refers to the non intentional changes in the mechanical proper-ties in locations of the part, like recast layer, oxidation, contamination, residual stresses,material drag and others. This kind of defects can be detected with X-Ray diffraction andelectronic microscopy.

1.7. Question 7

If burn is detected within a ground (grinded) machined part due to grinding whatmeasures can be taken to ensure that machined part does not get this problem again andeliminate the burn areas?

Using cutting fluids reduce the friction and helps to propagate the heat out of thepiece, In this way high temperatures could be avoided and so the possibly burn marks.Eliminating burn areas can be done grinding again the surface with the cutting fluidsand/or less velocity. Grinding can eliminate thin capes of the surfaces so the material lossand precision of the geometry could be minimal.

1.8. Question 8

If chatter marks are detected on a ground (grinded) work piece what measures canbe taken to eliminate such phenomena? What is meant by process damping and tooldamping when milling?

Chatter marks are defects caused by the vibration of the tool in the machine and/orthe material being worked, causing a wavy surface by the relative movement. This kindof defect can be avoided by the using an effective fixture around the part and checking,as a way to achieve rigidity in the system (stabilizing), which is desired. Working atfrequencies that cause the less vibration in every part in the system is useful too.

Damping is to reduce the oscillation in the system, in machining case, the vibrations ofthe system machine and piece, trying to improve the quality of the work as the vibrationsare mesmerized.



1.9. Question 9

Give the machinability in terms of percentage for the following different materials:Inconnel 718, 347 Stainless Steel.

Machinability is defined as the easiness of working a material and the quality obtainedbased on parameters like the tool wear and the power consumption. For the named mate-rials, 12 % and 80 % of machinability respectively, a number obtained by the comparisonwith a fixed steel alloy under arbitrary conditions.

What is the cycle of events from design to the manufacturing stage when using amachine centre?

A machine centre is a device that can realize multiple machining operations and countswith a digital control system to automatize the process.

The part that is meant to be produced is first designed with a CAD (computer assisteddesign) having the limitations of the machine in mind, then the file is processed in thesoftware of the machine to produce the toolpath (CAM), which is the route that willfollow the tool over the material to produce the piece. After the post processing of this,the machine will receive the program of the component and will use computer numericalcontrol over the motors in his tools to manufacture the piece.

1.10. Question 10

1. How does one decrease grinding anomalies?

2. What causes high surface temperatures in grinding?

The non existence or low flow of coolant in the process, a high speed in the spindle.blabla

1.11. Question 11

What is the grinding wear process?

Grinding is a cutting processes in essence, in which materials are under abrasion bya rotary wheel or surface composed by small grains of hard material () moving at highspeeds. The grains in the cutting surface are so small that the cutting depth can beminimal in comparison to the rest of cutting processes, achieving more accuracy and lessmaterial loss.



1.12. Question 12

What are the mechanics (SG) in single grit cutting (unit grit analysis)? Discuss thematerial removal rates in terms of SG mechanics (show diagrams). What does the materialremoval mechanisms look like in terms of diagrams and equations.

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1.13. Question 13

Explain why the cutting force, increases with increasing depth of cut and decreasingrake angle.

Decreasing the rake angle reduce the shear angle and causes that the chip will sustainmore deformation since it will bend more before the fracture, requiring then greater forcesto surpass this energy barrier and produce the cut.

Increasing the depth of the cut implies a bigger chip. Removing a bigger volume ofmatter at a fixed speed will require more power since the amount of energy to producedeformation depends on this parameter (or mass), and thus, an increase in the force isneeded (P = F · V ) because more energy will be dissipated.

1.14. Question 14

What are the effects of performing a cutting operation with a dull tool tip?

A dull tool tip can not indent correctly in the material and will affect the shear forceas the rake angle decreases by the wear in the nose, increasing the energy necessary to thechip removal and the friction against the piece, the tool can even just ’rub’ over the piecewithout cutting and then generating a large amount of heat mesmerizing the performanceof the machine.

1.15. Question 15

Why does temperature have such an importance on the effect on cutting-tool perfor-mance?

The temperature affects in different ways the mechanical properties of materials.Usually a heated material tend to deform easily under stress (creep). If the tool pre-sent plastic deformation it will be easier to loose the definition of the cutting edge (bluntool), affecting the machining process. The temperature also affects the heat treatment



given to the steels (for example), loosing the mechanical properties given previously bythe grow of the grains and softening the material.

The localized volume changes also implies a minor control of the process and createsstresses and micro cracks.

1.16. Question 16

For machining safety critical devices if the measured residual stresses are tensile innature is this a problem for the part when in service?

It is, because tensile stress allows cracks to grow inducing an early failure in thecomponent, especially in cyclic stress work.

1.17. Question 17

An aerospace part requires 10 1mm DOCs (depth of cuts), the part size has a widthof 30mm and a length of 150mm which requires three passes per DOC as the wheel widthis only 30mm in width. If the feedrate is 4000 mm/min and wheel speed 4000RPM howlong will the job take? If the grinding wheel diameter is 150mm, what is the velocity?

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2. Tables


Universidad de Chile - FCFM

Material Speed [ft/min]Aluminum and its alloys 250

Bronze (high tensile) 100Cast Iron ( soft) 100

Cast Iron (medium hard) 80Cast Iron (hard chilled) 20

Hastelloy 20Inconel 25

Magnesium and its alloys 300Monel 25

High nickel steel 50Mild steel (.2-.3 C) 100

Steel (.4-.5 C) 60Tool steel 40Forgings 40

Steel alloys (300-400 Brinell) 30Heat Treated Steels35-40 Rockwell C 2040-45 Rockwell C 2045-50 Rockwell C 1550-55 Rockwell C 15

Stainless steel free machining 40Stainless work hardened 20

Titanium alloys 20

Cuadro 1: Recommended HSS Speeds for Common Materials

Drill Diameter [in] Recommended Feed [in/rev]under 1/8” up to 0.0021/8”to 1/4” 0.002 to 0.0041/4”to 1/2” 0.004 to 0.0081/2”to 1” 0.008 to 0.012

1”(and over) 0.012 to 0.020

Cuadro 2: Recommended Average Feedrates for 2 Flute HSS Drill


homework 1 procesos de manufactura

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