Unconventional machining processes Flashcards
Which two types of water jets are there? How high is the pressure for them?
- pure waterjet - abrasive waterjet Both utilise a high pressure water 3000-4000bar
How does the pure water jet work?
Typical mortice diameter 0,1-0,3mm Important to keep a small stand off distance Depends on the material: shear stress goes in the material material is sheared away which can be smaller or larger then the jet.
How does the pump work for a waterjet
How does the abrasive waterjet work? How does the cutting head look?
The head is that the water orifice comes from the top and then its mixed with the abresive in the mixing chambier and then goes oout in the nozzle that has a diameter of 0,8-1,2mm.
The jet is about 95% air, 4%water and 1% abrasive
The speed is depending on the thickness of the material
What is the applications of a abrasive waterjet?
Plate material (2D)
not final machining
Plate thickness 10-50mm
What is the advantages and disadvantages of abrasive waterjet?
Advantages:
- Small diameter flexibility in geomerty
- Can cut thick diameter as well as thin
- Not sensitive to classically difficult to machine alloys
- No heat affected zone
- Relatively easy to learn
- Under water cutting of composites to minimize dust
Disadvantages:
- Relativielt low cutting speed
- Realtiviely expensive
- The jet sontinues after the cut
- The water oxidation a risk
- Can delaminate composites during piercing
What is important to consider if you want to have the water jet process as sheep as possible?
- The speed ned to be reduced in sharp corners so try to have as few as possible
- Also on surfaces where the surface rougness is not important try to have an lower quality for increased speed
- use the function nesting in CAM to fit as many parts as possible on one plate
- If not cutting time is important reduce the abrasive
How does the steps look for formation of the cut in waterjet?
The upper part:
- Grains hot at a shallow angle
- Chip (small!) formation
- So called cutting wer
The lower part of the cut:
- Grain hit at a not so shallow angle
- Erosion due to cyclic load of multiple grain
- Deformation wear
- Larger peces if material are removed
The real process is usally not as perfect as in theory. Describe three things that you need to concider about water jets?
- Lag of the jet: When running the pressure of the jet is not as high when it has goes through some material. which creates some lag of the jet. it only goes through the material beacuse of deformation. The surface in the lower part will look a little strange. this can be reduced with lower speed så the speed is related to the quality classes that goes from 1-5.
- Gemetrical defects: Due to the lag of the jet. at the end of a material can the jet flip over the edge which will result in that it dosent goes through the material. this is especially a problem in cornerns.
- Taper: when cutting in a hole the jet will have som lag which does that the hole will have not stright walls
What is the formula for the cutting speed?
Pw = water pressure [Mpa]
mw_prick =water mass flow [l/min]
ma_prick = abrasive mass flow rate [g/s]
h = plate thickness [mm]
D = diameter abrasive tube [mm]
q = quality
What are the design issues for the waterjet?
- Avoid small inner and outer radii, these requires low speed
- Unnecessary thick material will require low cutting speed
- Define which surfaces should be of good quality and it only “seperation” cuts OK on other surfaces
- Possible tolerance: Depends on (machine, part sie, quality class, thickness and taper, typically tolerance are +- 0,1 to 0,5mm)
- Choice of material, dont be afraid to choose difuícult material
What is the working principle of the laser?
Energy is conveyed ti the laser maedia
Excitation of the atom: Electon absotbs energy and jumps to next level
De excitation of atom: electrons releases energy and comes back tot its energy level
- Fotons are emitted
- These photons leads to excitation in other atoms
- By enclosing the madia an amplification is acoopmplished
What is the basic components of the laser?
- Fully reflection mirror
- Loss energy (heat)
- Active medium
- Semi- reflection morror
- Excitation energy or “pumping”
What is the important properties of laser light?
- Monochromatic (same wave-length)
- Coherent (in phase)
- Directed (close to parallel)
What is the important thing about CO2 laser?
- Wave length: landa 10,6 um
- Long length but still compact design
- Legth -> power
- Can be in multiple levels
The laser cutting head: How does is look?
What is the gas assistance?
What is the cooling?
Gas assistance:
- Blows away molten material
- Shields the cutting head
- Air is one alternative
- Can be inert (prevent oxidisation)
- Oxygen provides additional energy in Fe- cutting
Cooling:
- To avoid so called “thermal leansing”, diminish the atering of ahape and material properties of the lens.
Why is it good to use pulsed power?
if not used it will be excessive heating in corners which created bad surface
What is the basics of electrical discharge machining EDM? and what are the applications?
Applications:
- plastic molds: die for stamping (3D geom)
- small holes
- Complex geomertries.
Which parameters controls the surface finish ?
- Frequency (se picture)
- Grain size of grapfite electrode
- Improved wth “Diffused discharge machining”
- Power additive in diel fluid
What is wire EDM ? and what is the applications?
- Wire in Cu or Brass (0,04 to 0,4 mm)
- Wire spools:
- Continously fresh electrode
- Machine can be mun 500h +
Applications:
Thick dimensions, small innerradii, high tolerance demands
Dies for different applications including extrusion balnking (shearing)
