Mostly Printed CNC: Mechanical Build

A Mostly Printed CNC machine from Vicious1 provides an easily configured platform for low-force CNC activities like plotting, vinyl cutting, PCB milling, and maybe wood / plastic / wax routing with a suitable dust vacuum / downdraft table / enclosure. Despite many videos, the notion of open-air laser cutting remains a non-starter around here.

I opted for the Parts Bundle (all the “vitamins” required, from RAMPS controller to locknuts, to assemble the machine) and the Printed Parts Bundle (all the printed components), then picked up four 10 foot lengths of 3/4 inch ID = 23.5 mm OD galvanized steel conduit locally. Yes, I have a 3D printer, but the notion of feeding two spools of plastic through it over the course of 100++ printing hours, plus figuring out how to get the tolerances right, convinced me to regard this as a kit project, not a design-and-build project.

The first trial assembly atop a new workbench went reasonably smoothly:

I missed the step where you must put the high rails parallel to the X axis, which I want along the length of the table, and had to disassemble and rebuilt the frame to rotate the Middle Assembly a quarter turn clockwise. It’s always easier the second (or third) time and, if you regard the first few passes as dry runs / learning experiences, the process can be soothing, rather than annoying.

A laser rangefinder dramatically simplifies squaring and de-skewing the rails:

I wanted 24+ inches along the X axis and  18+ inches along the Y, so as to handle stock sizes with hard-inch measurements.The current MPCNC design adds about 11 inches to each axis outside the work area, which makes the footprint 35 × 29 (-ish) inches overall. The bench measures 30 inches front-to-back, I allowed an inch along the front to recess the moving parts, and the final frame measures 37+ × 30+ inches to the outside of all the gadgetry.

Within that footprint, the laser says the rails are 845  × 674 mm = 33 × 26+ inch apart, giving a work area of 640 × 475 mm = 25+ × 19- inch.

After some careful surveying, I marked / punched / drilled holes for each mounting foot, then counterdrilled brass inserts on the bottom for that nice clean look:

The screws came out flush when mounted atop washers:

My “careful surveying” produced a 1 mm error over the internal 1 meter diagonal, but a bit of judicious hole filing let me squash the long diagonal and stretch the short one by Just Enough to make the answer come out right, at least according to the laser rangefinder.

Setting the rail height goes more easily with a height gauge:

Stipulated: the absurdity of a height gauge on a plywood tabletop. On the other paw, the corner posts rest on that same plywood, so it actually works pretty well. I slowly pried the three lowest caps upward with the Big Screwdriver, levered on a wood block, to set all the rails to the same height as the highest one.

The X axis rails may need mid-rail supports, although I don’t see any meaningful deflection right now.

One could mount a T-nut atop the table inside each foot (and the center brace, as needed), with a long-ish bolt (head below the table) pushing the corner joint upward, which might be more stable than the current plastic-on-steel compression grip.

The steppers mount on rollers gripping the rail with six bearings, plus two more bending the GT2 drive belt (not installed yet) upward to the motor drive pulley:

I devoted a few quiet hours to threading four-wire cables through 6 mm PET braided sleeves, in hope of protecting the PVC insulation from the usual abrasion & bending stresses. I have some drag chains which may come in handy, although they seem overly klunky for the purpose.

I’m not entirely convinced a PLA stepper mount is a Good Thing, given the warmth of steppers and PLA’s 60 °C glass transition temperature. We’ll see how it goes; obviously, one should not leave PLA parts in one’s car during a hot summer afternoon, either.

The neatly sheathed stepper cable vanishes into the center rail held firmly by the stepper mount. An identical stepper mount grips the other end of the rail, with the motors wired in series. The conduits provide a tidy way to pass wires along the length and width of the frame.

After you install and tension the belts, tweak the pulley location so the 6 mm belt tracks more-or-less in the middle of the 9 mm tooth width:

The hulking Middle Assembly grips the X and Y cross bars:

It has six printed parts, three each in two matching pairs, 24 bearings in eight triples, and plenty of 5/16 inch bolts + locknuts holding it together: all the metal bits make it weigh a lot more than you’d expect.

The Z axis rails fit into the two pairs of three bearings facing you:

You’ll note the correct Middle Assembly orientation, after I rearranged the frame with the high rails along the X axis. Home switches will eventually fit neatly on the untraveled rail sections near the front-left corner post.

I built the Z using the default 12 inch rails called out by the Cut Calculator (metric version), which left an inch of the leadscrew sticking out beyond the bottom of the rails:

I left the work height at the default 4 inches, which specifies a minimum 7 inch = 175 mm leadscrew. The actual leadscrew is 300 mm = 8 inch, which completely explains the situation. I’ll rebuilt the Z axis with longer rails, but this suffices for now.

The concave silvery part joining the Z axis struts is the tool mount, to which you screw a tool holder:

That’s the Official Drag Knife / Pen Holder, generally seen with a Sharpie ziptied in place, but I have Real Plotter Pens, dammit, and I’m going to use them! The holder has one hole dangling to put the pen nib below the end of the leadscrew.

All in all, I like it …