MPCNC: Bar Clamps

Using various prototypes of the bar clamp mounts, here’s the left-side clamp in action:

MPCNC - bar clamp trial installation
MPCNC – bar clamp trial installation

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:

MPCNC - bar clamp left
MPCNC – bar clamp left

Although the right-side clamp is inside the frame rails, the gantry’s asymmetry puts the clamp outside of the work area:

MPCNC - bar clamp right
MPCNC – bar clamp right

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:

MPCNC - t-nut counterbore
MPCNC – t-nut counterbore

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:

MPCNC - t-nut in counterbore
MPCNC – t-nut in counterbore

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:

MPCNC - bolt for t-nut installation
MPCNC – bolt for t-nut installation

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:

Bar Clamp - pin removal
Bar Clamp – pin removal

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:

MPCNC - bar clamp end block
MPCNC – bar clamp 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:

MPCNC - failed bar clamp mounts
MPCNC – failed bar clamp mounts

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 …

8 thoughts on “MPCNC: Bar Clamps

  1. Check your pull-up resistor while you’re at it – 1k is quite a bit more suitable for stable hi-temp operation then stock 4k7 provided by most printer boards. I had up to 10C deviations @250C with stock resistor, but it’s rock stable with 1k. Just don’t forget to reconfigure thermistor table in Marlin.

    1. The M2’s temperatures remain within a few degrees of the setpoints, which suggests tweaking the circuitry wouldn’t make much difference. In any event, the resistors are SMD parts on the RAMBo board inside the case, giving me a powerful motivation to leave them untouched!

  2. If I find myself going back to get more of a given size of bolt, I start looking for the50 or 100piece boxes. Not having to wait several days for the market day trip makes the expense worth it.

    1. Absolutely!

      Sometimes the added expense turns out negative. I needed four long 5/16-18 bolts, came up dry at the local big-box store websites, then unearthed a bag of 50 on Amazon for a bit over $7 delivered in two days. I cannot explain why 50 bolts cost much more than four and less than 25 (were they in stock locally!), but they look no worse than any of the other zinc-plated steel bolts around here.

      So now I have four dozen bolts on the shelf … [sigh]

      1. In the neolithic era, HP was building 3″ wafers. I scrounged a few dozen of the empty boxes and use them for smaller fasteners. They fit nicely in a CD/DVD bookshelf, with added shelves.

        Julie gave up on steel breadpans, so I have a source for longer parts bins. [grin]

        Silicone breadpans FTW!

  3. Thanks for the pics. That’s much clearer. I didn’t realize the clamps were made with C-channel.

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