MetalicaRap:Tool head processes discussion

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Discussion of Advantages and Disadvantages of different Tool head processes.

List of future tool head concepts FutureToolIdeas

External links for Processes comparison

Introduction to non conventional machining processes: J.P. Kaushish. "Manufacturing Processes". 2010. [1]

Introduction to machining and EBM 19 page: J. A. McGeough. "Advanced methods of machining". 1988. [2]

Quote " Their are many ways to belief "

Many routes need to be followed and researched, they all have their own value. Lasers are clearly more sexy than old TV tube technology though early comparison may not turn out to be the best guide . Never the less here we go!

N:B: "sticky particle" SLS creates weak parts suitable for metal sculptures stc.. and "Fully melted" SLS has the possibility to create full strength metal parts. The term sintering refers to a generic label for all these processes of converting powder in to metal parts, whether the powder grains are just made sticky against each other, so parts are weak and only good for sculptures, or the powder is fully melted so you can have the possibility of creating a full strength part . Confusion around this means that machines are called sinteres which actually fully melt the powder thus demanding a laser above 150W while other cheaper machines also called sinteres with smaller lasers only make weak porous parts suitable for metal sculptures are mistakenly assumed to be the same as the high powered expensive machines with large lasers..[3] 35Secs in they explain this.

SLS vs EBM

SLS and EBM have some similarities: both put down layers of powder, and both scan a beam over each layer of powder to get the powder to stick together in the right places. High power laser SLS can be a "fully melt" process or be a "sticky grain" weak part production process. They are both low force not needing the built part to be specially secured in the machine and therefore low skill.

EBM has some advantages over SLS

• EBM may be able to reach finished parts IT grade 07 ,SLS cannot produce finished parts unless very expensive laser pointing systems are employed. So SLS lacking these highly expensive pointing devices always need further machining, so the 3D printer is not a replacement at all, as you still need all the normal machines ie cnc grinding milling..

• electron beam melting produces parts that are void-free, and therefore fully dense and full strength, In a SLS machine a laser of over 150 W is needed to produce "Fully melted" SLS , ie full strength parts.

• EBM does not have the limit of the one expensive bought in item ie "fully melt" SLS ( ie the laser 100K euro Yag 400W)

• The parts that generate an electron beam are simpler/possible to replicate than the parts to generate a gas based laser beam, (Solid state lasers are extremely difficult) ( Adrian Bowyer" we may never reprap a laser").

• in principle, it seems possible to EBM materials that are highly transparent to laser beams, (reflectivity/ oxide coating of aluminum for example means a laser has major problems(3% absorption), but other metals for example steel takes 40% of laser beams energy in an effective manner(YAG Q pulse).

• Electron beam machines do not need any permit from Darpa or its European equivalent, SLS machines which produce full strength parts through fully melting the powder need powerful lasers over 150W which need a permit form Darpa or its European equivalent.

• Wall socket efficiency for EBM is 85%, for higher power lasers they require up to 10 times more electricity , C02 lasers and ND YAG lasers 10%-30% wall socket to beam efficiency ( for fibre lasers and Diode lasers similar to EBM 85%)

• ... other advantages ...

(See Below for advantages of SLS over EBM). ( both ebm and "full melting" SLS can vaporize metal subtractively or fully melt powder for additive manufacture.)

Discussion questions:

Q: The EBM machines I've seen photographs of generate the electron beam at the top, then focus and steer the beam to strike the appropriate spot on the powder at the bottom. That requires high vacuum -- otherwise the electron beam is completely absorbed by the air or other gas. is it possible to generate the electron beam on a moving toolhead a short distance above the appropriate spot on the powder?

A: Yes but beam spreads out and looses power, but the main problem is your metallurgy suffers so they are no longer equivalent to wrought iron milled parts, which is only possible in a vacuum 10-3Torr or better. Welding is traditionally a poor metallurgical join so weakness is allowed for in the join design, but this weakness through out a part often makes the part too inferior compared with milled parts. see (Bureau of Mines 1993) below). Also 1mm lead or equivalent metal shielding will be required around the whole machine for xrays produced as the electrons strike the surface.

Q: How short does the distance traveled by the electrons through air need to be, in order that at least half the energy of the beam makes it all the way through to melt the powder, rather than heating the air/nitrogen/argon atmosphere? See the artist's conception of a potential "Cartesian robot gantry" for non-vacuum electron beam welding[4]. Wikipedia: electron beam welding#In-air welding

A: ?

SLS has some advantages over EBM.

• SLS does not require a high vacuum. It works at normal air pressure but in this case also leaves the metal unprotected so metallurgy suffers leading to weak parts suitable for sculptures e.t.c.. So everyone uses argon barrier so get a protective enviroment

• Most plastics, ceramics and lignin can be sintered open-air. very very locally high temp means oxygen react with material. (Most metals require an inert Argon or Nitrogen atmosphere, which is still an advantage over high vacuum in simplicity but not metallurgy eg.see (Tisza 2001) below Hydrogen metal cracking. ). In comercial polymer welding single mode diode lasers 8 W are sometimes used.

• "Sticky powder" SLS demands less development than EBM as lower power laser can be on cartesian axis, but creates weak metal parts suitable for sculptures strength of metal resembles cast iron. etc. the process requires less heat and only makes the powder stick against each other (using lasers less than 150 W where no permit is required).

• SLS that has a protective environment for metal, and fully melts powder grains together has the potential to produce full strength metal parts, this melting process requires more powerful lasers above 150W, ( Having a externally heated build chamber, which is preheated to 50 degrees below melting point, ( Fiber laser at 100 Watts ). degrees below melt which also require a permit in Europe and USA from Darpa or the Local equivalent agency. This "fully melting" SLS process is characterized by bringing all the metal powder up to 50 degrees below melting point and then apply a powerful laser to finish the melting ( above 150W typically 200W to 400W) .

• Laser process does not produce X-rays like an electron beam process, The amount of Xray produced is proportional to the voltage applied to the apparatus, So for your your old TV it was at 30KV but as long as you keep the electrons in a box that was no problem. But If you do like those crazy people in hospitals fire them all round the place precautions need to be taken, for a 60KV xray machine 1mm of lead (or equivalent 3mm of stainless steal ) is suggested screening for some one who works with it every day. The Vacuum chamber in a electron beam machine has to be over 8mm thick stainless steal to resist the pressure .

... other advantages ...

EDM vs SLS

(Electric discharge machining Selective laser sintering)

EDM has sub micron tolerance

SLS requires very expensive pointing method to achieve micron level tolerance.

other Advantages Disadvantages....

General guidance on comparison with the production of metal parts.

For Metal parts the main issues when comparing alternative processes with conventionally machined parts need to include;

1. Metallurgy; including; Resultant grain structure from cooling profile and metal alloy type (ingredients) , heat damage/distortion/residual stresses internal compressive or internal tensile forces from localized heating and thermal expansion in the heat-affected zone of the process, loss of alloy additions giving change in alloy constituent quantities/proportions due to out-gassing, stress/strain/fracture characteristics and hardness.

2. IT grade dimensional tolerance achievable ,

3 Difficulty/special requirement of securing part in machine/skill required

4 Metal - gas chemical reactions at temperature.

5.Roughness R<math>q</math> (units nm) is a route mean square of many absolute height measurements, measured in nm, For example a polish surface R<math>q</math> of 3 nm is typical A typical powder printed finish R<math>q</math> of 25 nm

6. porosity

7. limitation on size of manufacturable parts

8. other

Info

Free 60 page summary of the important metal processes, ( beyond the well know milling grinding etc ) chapter 5-3 to 5-31. Bureau of Mines. "New Materials Society, Challenges and Opportunities: New Materials Science and Technology". 1993. [5]

As a Metal alloy cools at different rates, the different metal types ( eg Nickel & Chromium) form different types of crystals, some hard , some ductile, etc.. . This defines the metals quality/usefulness, in any manufacturing process the rates of change of temperature at any particular part of the product are therefore critical, these diagrams allow you to predict the metal products final qualities. Phase Diagrams are explained in this book: Miklós Tisza. "Physical metallurgy for engineers". 2001. [6]

Existing self replicating finished parts IT grade 7 or better machines

List your successful; largely self replicating/ finished parts producing machines!

Combined CNC and EDM machine have produced finished parts IT grade 0 , Self replicating, Amazing Yes!! unfortunately many years (15 yrs) experience is needed to operate it.

Scenario's

metal SLS “ Full melt “ Scenario

30 W multi mode Laser 500 euros, beam diameter 20/ 30 µ

optic collimator , lens 250 Euro,

commercial point 2000 Euro ( 900m/s mechanical , absolute position 1-30µ) ( scanning mirror Hack hard drive arms glue on two mirrors )

Preheated bed to 50 degrees below melt by external heater coils, So laser only come up last bit,

Build speed very slow scan laser at 25mm / sec ( Could this be much quiker ? Recalculate needed) ( beam diameter 20/ 30 µ area 7.5MW /M2)

Example B build

Steal stainless 300*300*20mm 90% part 100µ layer thickness 400µ finished part tolerances

Total area 300x300x200 =18000000 mm2 divided by small area in one second

Given scan rate 25mm/S * .03mm wide path area scanned in one second is .75mm2

24million seconds / 3600 *24 = 277 days but only 10% part so 27 days Too slow heat

calculation for 30µx30µx100µ heat required to melt from 50 degrees below melting given power input Calculate duration.

Further reading

S. Narayanan. "CAD/CAM Robotics and Factories of the Future: 22nd International Conference". 2007. [7]