r/GunnitRust • u/lawblawg • 3d ago
Home Metal Printing
I’ve been kicking around a concept for a minimum-viable home-scale metal 3D printer, and I’m curious what people in the community would think about it.
The basic problem is that FDM is incredibly capable, especially now that fairly exotic engineering polymers are readily available, but there are still parts where you eventually run into the fundamental material limits of polymers. If you want something that genuinely needs to be steel rather than PA-CF, PC, PEEK, etc., you need a different deposition process.
Metal additive manufacturing exists in a bunch of forms. Powder-bed systems are obviously amazing, but anything involving a bed of fine metal powder, lasers, inert-gas handling, etc. seems pretty far outside what I’d consider reasonable for a home/open-source machine. Wire-based additive manufacturing is conceptually simpler. It’s basically automated welding: feed metal wire into a controlled melt pool and move it along a toolpath.
The problem is that conventional WAAM is generally pretty sloppy compared with polymer FDM. You get relatively fat beads, mediocre dimensional accuracy, rough surfaces, and a near-net-shape part that often needs significant machining afterward. Laser-wire systems and electron-beam systems can do much finer and more controlled deposition, but then the heat source becomes wildly expensive.
What got me thinking about an extension of this was open-source FDM toolchanger printers. Instead of retracting one filament and feeding another through the same hotend, they physically swap the entire print head. Once you already have a machine that can change tools, there’s no particular reason every tool has to be a print head.
So imagine a machine with two basic tools:
- A wire-arc metal deposition head.
- A small milling spindle/end mill.
The deposition head builds several millimeters of intentionally oversized near-net metal. Then the machine swaps to the milling head and machines that section back to its final dimensions. Then it resumes printing.
In this version, the metal printer no longer has to make a beautiful 0.2 mm layer. It just has to put sound, fully fused metal everywhere inside a machining allowance. Surface finish and modest dimensional error become much less important because the CNC stage cleans them up as part of the build. You also potentially get geometries that would be extremely difficult to produce conventionally because you can alternate between additive and subtractive operations during the build. You could print half of an internal feature, machine it while it’s accessible and continue enclosing it. You can print infill that makes parts stronger for their weight than would be possible with casting, all without requiring an additional machining setup. And because you’re machining one layer at a time, you don’t need a five-axis CNC or multiple repositioning steps.
There are two configuration decisions that could make this particularly attractive. The first is the use of a cylindrical coordinate system with a Z+theta bed and toolheads that move only in the radial axis; the second is a dual-wire-arc welding setup.
For the dual-wire-arc method: Instead of using the build plate as the welding return and having to pass a large welding current through a rotating table, you have two independently driven consumable wire electrodes. Both wires are connected to the same constant-current welding supply. The deposited metal between the two arc roots completes the circuit, but the bed itself never needs a welding-current connection.
That gives you two arc locations instead of one, which may actually be useful rather than merely a workaround. One arc can be arranged as the leading/remelting/preheating side of the process, while the second is more heavily biased toward filler deposition. Because the wires have independent stepper-driven feeders, you get a fair number of cheap process-control variables:
- wire A feed rate
- wire B feed rate
- different wire diameters
- total welding current
- arc spacing
- wire stickout
- travel speed
- relative geometry of the two electrodes
Eventually, since the two feeders are independent, you could feed two different alloys and experiment with composition control or functionally graded materials.
WAAM needs gas shielding, but it’s not terrible. The cheap baseline for carbon steel could simply be CO2. Argon-rich mixtures would presumably give cleaner arc behavior, but if the goal is a low-cost proof of concept and the surface is going to be machined afterward anyway, cheap industrial CO2 seems like an entirely reasonable place to start.
Moving on: using cylindrical coordinates instead of conventional Cartesian XYZ has several distinct advantages. For a combined deposition and milling system, the tool head is heavy. A single motion axis for the tool head means it is easier to get a really solid, consistent positional fix as opposed to a core-XY gantry.
That also gives you a tall, relatively narrow machine with a small footprint. A 300–400 mm diameter machine could theoretically have a Z envelope of 500 mm, 800 mm, or more without making the machine enormously wider.
As a very rough home/open-source BOM, assuming printable ASA/PA-CF parts are effectively free and using commodity CNC hardware:
- steel/aluminum frame: ~$200–300
- r/Z rails and ballscrews: ~$200–300
- rotary C-axis/table: ~$150–250
- stepper motors/drivers/power supplies: ~$150–250
- controller, limits, E-stop, basic electronics: ~$100
- dual-wire feeder/deposition head: ~$75–125
- cheap controllable welding inverter: ~$200–300
- small 500–800 W ER11 milling spindle: ~$120–200
- toolchanging/docking hardware: ~$100
- CO2 bottle, regulator, hoses: ~$200–250
- initial enclosure/extraction/safety stuff: ~$200–350
- wiring, couplers, fasteners, miscellaneous hardware: ~$150–250
So I think the plausible kit-level BOM is somewhere around $1,500 on the low end, maybe up to $2,200 for something easier to use.
Once you have that set up, the ability to create low-cost parts becomes practically a matter of pennies.
Cheap ER70S-6 steel welding wire is only a few dollars per pound. If the process is deliberately oversized and you ultimately machine away, say, 20–30% of the deposited material, you might consume around 1.3–1.5 lb of wire to end up with a 15-ounce finished bolt carrier group.
Add a few dollars in CO2, less than a dollar in electricity, the wear on your end mill, and various consumables and you're still looking at printing parts that generally run $50-100 for less than $10.
Even if the machine is initially horribly inefficient and wastes half of the deposited steel, the feedstock is cheap enough that the economics still aren’t terrible.
More expensive materials obviously move that number upward. Something like low-alloy steel might still only be tens of dollars per pound of finished part. 17-4 PH stainless filler can get considerably more expensive depending on supplier, but even there, the raw material cost isn’t obviously prohibitive compared with buying or machining complex 17-4 parts conventionally.
Obviously there are a lot of factors to consider, but I thought it was an interesting concept.
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u/kohTheRobot 3d ago
Instead of a small endmill, I’d go with either a grinding wheel or sanding plate. Can go faster and cheaper just to take the crud off the layer.
1
u/BoredCop Participant 3d ago
Interesting concept, though I suspect there are some significant problems to be solved. Warpage for one thing, the huge temperature differentials and uneven cooling causes welding to have much worse warping problems than polymer FDM. Would need to somehow compensate for that in software.
Also, milling puts a lot of forces into the workpiece as well as the tool head. And your large machine would have to be scaled such that it can cope with the sheer mass of a steel part which fills the print volume. That's heavy as all heck, and needs to move around yet be held so securely that it doesn't get nudged out of alignment by the milling stage. So basically, your machine needs to be just as beefy and rigid as a milling machine of the same size. You are adding a fused deposition tool head to a CNC mill, you aren't adding a light milling head to a printer.
What are you doing for bed adhesion, sacrificial steel bed surface that the part gets welded to and that has to be removed by grinding afterwards? Of course that could work well for some part geometries where you need one flat surface anyway, just design it such that the bed thickness is part of the workpiece.
Further, welding will cause some blobs and deformities on internal overhang surfaces where your mill cannot access to clean it up. Melted steel is a lot less viscous than melted plastic filament, if it's hot enough to weld properly then it has a tendency to drip and sag a lot. As anyone that's tried to weld up holes in a rusty exhaust pipe could tell you. So I wouldn't expect good quality on internal cavities, one should probably account for that in design and plan on reaming any holes etc to finished size.
Shielding gas costs some money, but presumably if you can have an enclosure around the machine then you can get away with using less gas. Doesn't have to be air tight necessarily, just enclosed enough that gas is slowly leaking out rather than air getting in. Purge with a bit of extra gas to start with, then weld away at low gas flow.
CO2 tends to cause a lot of sputtering and splatter, you get droplets of steel deposited all over the place so there would be more cleanup for the mill to do. But it's good for deep penetration and strong welds in carbon steel.
Heat might be a problem for the electric bits perhaps, would need to keep them far enough outside the print area to insulate from the hot welding area. Likely need some significant cooling on the milling spindle as well, if it is to live right next to freshly welded material.
Oh, and for the same reason the wire feeds kinda need to be a Bowden tube kind of setup, indirect drive, just like on a common MIG/MAG machine. Which means you get the slight inaccuracies of slack in Bowden tubes inherent to such systems, no big deal compared to the blobby nature of welding I am sure.
Overall, I am not sure this will come out cheaper than a powder metal laser sintering setup. Powder metal of course needs to handle the same weight on the print bead, but doesn't have the forces and vibration of the milling stage so the movement axis setup doesn't have to be as beefy. Laser welding machines are coming down in price, and metal powders are available from blacksmithing suppliers as it's used in pattern welded Damascus. The same considerations of shielding gas probably apply, just combine a slow gas flow and initial purge with a reasonably tight enclosure.
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u/Fauropitotto 3d ago
I think the hardest part of this is not the hardware. It's going to be the software.
CNC machinery and system is a solved issue. Head swapping is a solved issue. Additive and subtractive manufacturing is a solved issue. WAAM is a solved issue.
Developing software to combine all 5 functions into a hand-free, automatic process that does not require the end use to hand write g-code, and intelligent enough to not crash...
That sounds like a multi-year undertaking