Using various prototypes of the bar clamp mounts, here’s the left-side clamp in action:
In round numbers, the (yet to be installed) spindle won’t exert any upward force worth mentioning, so clamping the material in the horizontal plane should hold it firmly enough for my simple needs. A more robust router needs more downward force.
The left-side clamp sits outside the MPCNC’s frame to prevent blocking the leftmost inch or so of the work area:
Although the right-side clamp is inside the frame rails, the gantry’s asymmetry puts the clamp outside of the work area:
Yes, those are nylon bolts; my 1/4-20 bolt stash is greatly depleted. I picked up a small assortment of stainless bolts in useful sizes, but they top out at 1-½ inch.
Fastening the blocks to the bench required a bit of fiddling after squaring the bars against the edge. Transfer-punch the hole location, then drill a 1/16 inch pilot hole:
Gingerly counterbore a t-nut recess in the bottom with a 3/4 inch Forstner bit marked with a suitable depth to completely sink the t-nut:
The shop vac snout keeps the chips out of your face. Works like a champ!
Redrill the pilot hole with a 5/16 inch brad-point bit to fit the 1/4-20 t-nut body:
The t-nut may not be exactly centered in the counterbore, but nobody will ever notice.
Rather than hammering the t-nut into the bench, gently & quietly pull it in place with a bolt atop a pair of washers:
Again, the shop vac collected all the chips from the brad-point bit.
Of course, Harbor Freight bar clamps aren’t intended for this duty, so they’re held together with assemble-only pins and clips. Disassemble the clip with a Dremel cutoff wheel and the pin will fall right out:
I had to through-drill the bar + hardware + 3D printed mount to get a consistent hole, as the overall tolerances aren’t particularly tight and things tend to not fit back together the way they came apart.
The bar clamps started out at 36 inches and stuck out over the far end of the bench. I hacksawed them to a suitable length, cleaned up the cut on the bandsaw, and the cut disappears in the end block:
By complete coincidence, the rear bolt holes turned out to be exactly lined up with the edge of the metal bench frame, so I had to remove eleven of the twelve screws holding the bench to the frame, rotate it slightly, drill the rear holes, install the t-nuts, un-rotate the top, and reinstall all the screws. As it turns out, the four end screws are located in blind parts of the frame where I could remove three of them, but cannot re-install them with any tool at my command. I think I can conjure a modified finger wrench, but …
The bars are made of the softest aluminum known to man in the thinnest cross-section that won’t crumple under a stiff glance, so they’re more flexy than you’d (well, I’d) like. Various comments suggest running a snug-fitting strip of 3/4 inch plywood inside the rail to stiffen it up; we’ll see how they fare against the MPCNC’s actual cutting forces before doing anything rash.
The jaws are also way slicker than I’d like and may need screwed- or glued-on plywood pads for better grip.
Those are all early versions of the mounting blocks, because this happened while printing the final set:
The black smudge on the block in the upper right is what happens when a MAXTEMP error shuts the printer down in mid-stride, leaving the nozzle to cool in the part. Looks like it’s time for a new thermistor …