Posts Tagged CNC
Drilling a pair of holes into a length of ground steel shaft turned it into a holder for a Sakura Micron pen:
The aluminum ring epoxied to the top keeps it from falling completely through the linear bearing.
The hole sizes are the nearest inch drills matching the pen’s hard metric sizes:
While I was at the lathe, I turned another layer of epoxy on the printed holder down to a consistent 11.95+ OD. It fits the bearing nearly as well as the steel shaft, although it’s not quite as smooth.
The steel version weighs about 20 g with the pen, so it applies about the same downforce on the pen nib as the HP 7475A plotter. The force varies from about 19 g as the Z axis moves upward to 23 g as it move downward, so the stiction amounts to less than 10% of the weight:
However, the more I ponder this setup, the less I like it.
When the Z-axis moves downward and the nib hits the paper, it must decelerate the weight of the pen + holder + ballast within a fraction of a millimeter, without crushing the nib. If the pen moves downward at 3000 mm/min = 50 mm/s, stopping in 0.3 mm requires an acceleration of 4.2 m/s² and a 20 g = 2/3 oz mass will apply 0.08 N = 0.3 oz to the nib. Seems survivable, but smashing the tip a few hundred times while drawing the legends can’t possibly be good for it.
Also, the tool length probe switch trips at 60 (-ish) g, which means the pen can’t activate the switch. Adding a manual latch seems absurd, but you can get used to anything if you do it enough.
Adding a lock screw to the camera mount stabilized the camera-to-spindle offset enough to make calibration meaningful. Mark the spot directly under the camera:
Then mark the spot directly under the spindle, perhaps by poking a small cutter into the tape, measure the XY distances between the two center points, and use bCNC’s camera registration process to set the camera offset.
With those numbers in place, switching to the tool view (the green button with the end mill to the right in the ribbon bar) puts the camera at the spindle location:
The view from outside shows the relation between those two pieces of tape:
Now I can align the camera view to a fixture position and be (reasonably) sure the spindle will automagically align to the same XY coordinate when I switch to the “tool” view. Seems to work well in preliminary tests, anyhow.
Inside, it uses the same pushbutton and pogo pin as the pen holder design, with a similar brass tube around the pogo pin.
There’s a conspicuous lack of good wire management; we all know where those wires will snap. In practice, you’d secure it to the DW660 power cord, way up on top, to eliminate most of the flexing. Still, it wants better strain relief than its gets from those heatstink tubes.
The solid model looks like a weaving shuttle:
It’s sitting upside-down in a 5 mm brim for more platform adhesion.
The next one will have a 1/8 inch stud to fit the DW660’s other collet and shorten the top by 3/8 inch, because I want the rod inserted three diameters for stability. The bottom can’t get much shorter, because the pogo pin determines the switch-to-tip distance. Maybe a simple membrane switch will work well enough?
You can see the depression in the glass sheet pretty clearly in a bCNC Autolevel scan on 30 mm centers (clicky for more dots):
The OpenSCAD source code as a GitHub Gist:
They’re supporting the snippets produced by trimming the clamp extrusions to fit across the bench under the MPCNC; I figure they ought to come in handy for something.
Both extrusions carry a warning sticker giving the bar’s serial number:
I could be persuaded the number applies to a given production batch, although I’d be unsurprised to learn it’s a batch of labels, not clamps.
They don’t look much different than the previous versions:
The main change was to raise the bars by another 2 mm to give one of the clamp shoes more clearance. As you might expect, the top and bottom halves of the clamp castings aren’t quite symmetric.
The plastic mounts come in mirror-image sets due to that off-center bolt hole.
Yes, the threaded casting is slightly angled from the screw clamping force.
All in all, the mounts look pretty good, in a bright-orange sort of way.
Rather than attach a spoil board directly to the bench top under the MPCNC, one can grab it in bar clamps anchored to the bench, which requires suitable mounts. Because bar clamps are all the same, one must be flipped over to point the other way, soooo the mounts come in mirror-image sets.
Holding the clamp on the left side of the table:
For the right-side clamp:
The chunky clamp prints on its end, with its bottom surface facing away from you, to let the block in the middle print without support. In that orientation, the bar slides in from the top.
The fancy rounded corners happened while I iterated on getting the dimensions right.
Actually printing and installing the things turned out to be separate challenges.
The OpenSCAD source code as a GitHub Gist:
The original doodles, with initial dimensions & some bad ideas:
The MPCNC uses a DW660 Cutout tool as a low-cost spindle for tools with 1/8 and 1/4 inch shanks. It features a tool-free “collet grip” to twist the collet nut against the shaft lock, which is convenient for a hand tool and not so much for a CNC spindle: I find it difficult to get two hands into the MPCNC setup with the proper orientation to push-and-hold two locking buttons, while applying enough torque to twist the collet nut:
Fortunately, it’s easy enough to remove the collet grip. Remove the collet nut, unscrew the four screws holding the yellow snout in place, then pull the snout straight off to reveal the spindle lock plate:
Capture the spring, slide the spindle lock plate out to expose the snap ring (a.k.a. Jesus clip) holding the collet grip in place:
Remove the snap ring, make the appropriate remark, pull the collet grip out of the snout, reassemble the snout in its One Correct Orientation, and you’re done:
The retroreflective tape snippet let my laser tachometer report a top speed over 29 k rpm, pretty close to the advertised 30 k rpm.
If one were fussy, one would 3D print a thing to cover the snout’s open end:
The original snap ring holds it in place and the fancy pattern comes from octogram spiral infill on the bottom.
The collet nut fits either a 5/8 inch or 16 mm wrench, both of which stick out to the side far enough for a convenient hold while pressing the shaft lock button.
The first height map looks like a mountain sproinged right up through the glass:
More red-ish means increasing height, more blue-ish means increasing depth, although you can only see the negative signs along the left edge.
The Z axis leadscrew produces 400 step/mm for a “resolution” of 0.0025 mm. The bCNC map rounds to three places, which makes perfect sense to me; I doubt the absolute accuracy is any better than 0.1 mm on a good day with fair skies and a tailwind.
The peak of the mountain rises 0.35 mm above the terrain around it, so it barely counts as a minor distortion in the glass sheet. Overall, however, there’s a 0.6 mm difference from peak to valley, which would be enough to mess up a rigidly held pen tip pretty badly if you assumed the glass was perfectly flat and precisely aligned.
Rotating the glass around the X axis shows a matching, albeit shallower, dent on the other side:
For all its crudity, the probe seems to be returning reasonable results.
The obvious question: does it return consistent results?