Close

Compared to... what?

A project log for Minamil: a minimal CNC mill - HaDPrize 2021

Minamil: a minimal 3-axis CNC mill that works. Build from laser-cut hardboard with a screwdriver (mostly). Mill fine-pitch PCBs.

Paul McClayPaul McClay 08/20/2021 at 19:540 Comments

tl;dr: Minimal appears able to produce finer PCB features than common low-end generics.


To what existing baseline can we compare Minamil's PCB milling capability?


The "cheapest" CNC

As of August 2021, it looks like the dominant “cheapest” CNC machines for PCB routing[note 1] and other (very) light hobby use are variations of “3018” - a generic design with 30cm x 18cm work area, and typically 4-5cm Z travel. Other “xxyy” designations indicate similar machines with larger or smaller work area. Various sellers offer machines of very similar design built to various levels of cost & robustness. Current examples listed by Amazon include: basic <$150, “pro” ~$200, and “max” ~$300.

3018: lightest, lighter, light
images from linked Amazon seller listings

Smaller machines of similar kind include essentially all the same parts and do not cost very much less.  Construction from shorter lengths of similar linear rods & extrusions may give smaller units greater stiffness.

I have not yet found where this design comes from, or when. My current best guess is that “CNC 3018” was already generic before English speakers wrote much about them on Teh Internetz. Please comment if you know.

Not having one myself, I don’t really know what they can do. After a bit of searching I’ve found several mentions of milling/routing PCBs on these machines, but little indication of use for very fine pitch components or high density of small features.


So it looks like the baseline for PCB milling on "cheapest" desktop CNC machines is mainly around through-hole or 0.1" pitch density, with best case examples of clean SO/0.05" pitch or marginal 0.5mm pitch footprints.


"Minamil"

For the comparison, here is an image crop from a July log entry about milling an SSOP footprint: 

While the SSOP footprint has pads on 0.65mm centers instead of 0.5mm, the horizontal traces crossing each end of the DIP footprint lie on 0.5mm centers. Using a beater old Dremel for a spindle, Minamil[note 2] cut isolation paths slightly narrower than half of that interval, or <0.25mm wide, leaving traces at least wider than 0.25mm if not fully 0.3mm wide for nominal 0.5mm pitch footprint. Cutting the same part using a PROXXON IBS/E for a spindle left traces over 0.3mm wide on 0.5mm centers separated by isolation paths under 0.2mm wide. So far I haven't found an example of a "cheap" 3018-type CNC router matching that.


Advantage: Minamil. For less cash in a smaller footprint.


In the baseline case, it's pretty clear that the 775 motor spindles contribute to spreading out minimal isolation path width by runout and vibration.  Also relatively low speed for standard motors -- 8k to 12krpm -- limits cutting edge speed for tiny tool diameters, and limits feed rate.

Minamil uses a general-purpose rotary tool, which will almost certainly spin up faster than the 8k-12k rpm range for typical "3018" spindles, and may achieve lower runout than most 775-derived spindles.

Because the vertical axis must necessarily have a counterbalance, Minamil can easily accommodate the greater mass of a general-purpose rotary tool.

A general-purpose rotary tool has a cost advantage in that it can earn its keep doing other stuff when not making PCBs. 



Notes

  1. So is it "mill" or "router"?

    I wrote a little about that earlier. I doubt there is a concrete specific answer. My over-simplified rubric goes like this:

    • Router - Relatively compact/light Z axis moving laterally (say, X) on a gantry (that may also move in Y) around a relatively large work area. Typically broader than tall. Penalizes Z mass because axis & spindle accelerate and move in X or XY plane.
    • Mill - Relatively massive fixed Z axis with spindle moving only in Z, maybe ponderously, while relatively small work area moves in XY underneath fixed Z. Typically taller than wide. Z axis and spindle may be much more massive & slow relative to XY table.

    Always exceptions; if your gantry has an operator’s cab, please call it anything you want.

    Also from the earlier log entry: “... And the pretentiousness of calling it a "mill" hides in plain sight the pretentiousness of calling a toy-scale device a tool at all. Kind of like an Easy-Bake® "oven".” But seeing that it works, I don't mind calling it a tool. 

  2. "Minamil" essentially as published but with a modified Z axis that I haven't published yet. I think that won't make a huge difference.

Discussions