Machining and Grinding Flashcards
What is machining?
It is a subtractive process, that involves cutting a desired shape out of a piece of material(called a billet or a workpiece)
What is Grinding?
A cutting process that uses abrasive grits to profuce smooth surface finishes and close tolerance dimensions
Note about machining and grinding being used together
They are commonly used to prepare castings and forgings, and to finish additive metal parts to their final shapes
What are the three main machining types?
Single point cutting
- Shaping
- turning
- planing
Multiple cutting
- drilling
- milling
- tapping
- reaming
Abrasive cutting
- Grinding
- Honing
- Lapping
What are the design implications of turning? ie things to be considered
- Internal corners will need to have a minimum radius of 0.4mm corresponding to the size of the end mill
- External burrs/edges will be quite sharp- We usually deburr these, and optimally chamfer them
What is grinding?
In essence, a big turning wheel has an abrasive substance in its matrix.This spins fast and on the surface to give a good finish
When do we use grinding/abrasive machining?
When parts:
- Require a very good surface finish
- Require really close tolerances
- Made of hard, difficult to machine materials
How to handle grinding wheels?
Why do we need to handle carefully?
Grinding wheels are very fragile
- Before use, visually inspect the wheel for cracks
- Ring test it using non-metal object: literally tap it and listen for a clear ringing sound. A good wheel will have a clear ring. If cracked, it won’t
- Keep it at or below the maximum RPM
- Always place wheel guards to avoid injury
- Eye protection
- Balance the wheel well. If unbalanced, can create shit surfaces, excess vibration, and early wheel failure
What do you call the waste material in machining?
What size of this do we aim for?
How do we try to ensure a good finish?
- swarf
- We aim for small pieces so we don’t tear or rag edges by accident
- We decrease the rake angle as the shear strength of the material increases. If the material is brittle, we use a tiny rake angle(aluminium rake angle is about 30 degrees, while brass is around 1)
- We can also add certain substances to the material, e.g. culphur to steel, to decrease the shear strength and even possibly acting as a lubricant, prolonging the life of the tool
What factors may affect the quality of surface finish?
- relative speed of tool/workpiece
- Cutting depth
- Materials and starting temperature
- Lubricants/coolants used
- dynamics of tool and workpiece holder, e.g. dampening/stiffness
- rake angle, clearance angle
- sharpness of tool
Formula to calculate the power needed to machine something
(Specific Energy x Material Removal Rate)/Drive Efficiency
Relationship between mean temperature, cutting speed and feed rate on a lathe
k(v^a)(f^b)
v=cutting speed in mm/s
f=feed rate in mm/s
k=constant
the value of k is derived empirically
What are the types of tool failure? What does each occur from?
- Fracturing/chipping: Excessive stresses caused by high cutting forces generated by using an excessive feed rate and/or an excessive depth of cut
- Premature blunting or deformation: The workpiece material is too hard, or there is softening due to lack of coolant
- Gradual wear/erosion: Friction causes gradual, abrasive wear- most wear at the highest temperature.
What is the Taylor tool life equation?
V.T^n=C
n and C are constants depending on the specific cutting conditions of the operation. These are found empirically and we usually use a log-log plot
What characteristics do we look for in the material used to make the cutting tool?
Some commonly used materials?
- Harder than the workpiece
- Good high temperature strength
- Good heat conductor to remove heat from the work area
- High speed steel
- Sintered carbides
- Ceramics: little resistance to shock, but can operate at high speeds. Used for long continuous cuts
