Additive vs Subtractive fabrication

Hello all,

I've been interested in the reprap project and other Von Neumann machines for some time but recently become more determined to actually build something. My first question is: why do you use an additive fabrication method rather than subtractive?

I assume there are good reasons but it would seem easier to me to build a milling head rather than an extruder. Obviously there is a lot of wasted material, but if a thermoplastic is used, can the waste material be gathered up and melted into another billet? When I first tried programming a CNC mill, we used a rigid type of wax that could be remelted repeatedly to practise on.

Apologies if this has been discussed before on this forum - I have been browsing for a while and didn't see anything similar.

One reason - a mill (or router) has to be quite stiff to resist all the vibration from the cutting head. For this reason, it would be difficult to make a decent mill mostly out of machinable wax or thermoplastic. If we were to make it out of steel or aluminum, it would essentially duplicate commercially available tools and not be particularly revolutionary.

Secondly, fabricating a conventional mill generally requires tooling like a bandsaw, drill press or mill, lathe, etc. You'll probably need a decently set up workshop to do it effeciently and well. I'm using weasel words like generally and probably. While one can bootstrap all this stuff gingery-tools style, or make clever use of hand tools, it's somewhat difficult.

Thirdly, you need to be a decent amateur machinist to do all this.

Fourthly, you'll want a 4 DOF machine if you want to make "just about anything". Perhaps more difficulty, you'll need software to determine the toolpaths. This is non-trivial.

If you compare this with RepRap, you need a 3D printer, some hand tools, and maybe a drill press.

By the way, you may get a kick out of the granite+epoxy thread:
[forums.reprap.org]

... and a major point for additive fabrication is the ability to mix the materials in the fabbing process:

So with a two- or three-head-reprap (extruder and paste-dispenser) you can start with plastic and build the bottom of a housing, then print some conductive trays and landing-pads, insert some IC's and discrete parts in the wet paste, cover with plastic, embed some elastic bulbs as buttons and cover with the top housing for an personal calculater (for example )

Try to make this with a mill

And with interchangeable toolheads your selection of materials isn't limited - you can use wax, thermoplastics, silicone-rubber, 2K-epoxies and such.

And with additive curing (heat, UV, laser) you can process pastes from glass, ceramics or metall, stack lasercutted foils and much, much more ...

I want to go a slight different way: i'm converting a fully functionable CNC-mill with some additional toolheads into a milling-repstrap-combination, so it's my free decision, if the fabbing is subtractive, additive or a combination of both ...

Viktor

One trick you can do with a milling head on a 3D printer is to deposit plastic and then machine it a little bit.

Say you want a precisely sized block with a cm through a precisely sized through hole, but there's a little bit of slop in each layer from the plastic spreading and moving before cooling. So you use the milling head after depositing each layer to clean it up a bit.

... yes, here: [forums.reprap.org] - and here: [forums.reprap.org] - are examples and some basics of a milling/fabbing combination.

A very helpful aspect is the fact, that by milling/finishing the fabbed layers with a mill you only have to mill one sheet at once, but you can build very big objects by stacking many sheets.

So you have the overall precision of a CNC-mill and could process any geometry and size your robot can handle, but the mill didn't have to be so rigid, as you only process one sheet deep - e.g. 0,1 to 0,5mm per sheet ...

Viktor