Posts Tagged MPCNC
The anodized body of the drag knife on the left measures exactly 12.0 mm OD:
Which happy fact suggested I might be able to use a standard LM12UU linear bearing, despite the obvious stupidity of running an aluminum “shaft” in a steel-ball bearing race:
The 12 mm section extends about halfway through the bearing, with barely 3 mm extending out the far end:
Because the knife body isn’t touching the bearing for the lower half of its length, it’ll probably deflect too much in the XY plane, but it’s simple enough to try out.
As before, the knife body’s flange is a snug fit in the hole bored in the upper disk:
This time, I tried faking stripper bolts by filling the threads of ordinary socket head cap screws with epoxy:
Turning the filled section to match the thread OD showed this just wasn’t going to work at all, so I turned the gunked section of the threads down to about 3.5 mm and continued the mission:
Next time, I’ll try mounting the disk on telescoping brass tubing nested around the screws. The motivation for the epoxy nonsense came from the discovery that real stainless steel stripper bolts run five bucks each, which means I’m just not stocking up on the things.
It slide surprisingly well on the cut-down screws, though:
Those appliqué templates came from patterns for a block in one of Mary’s current quilting projects, so perhaps I can be of some use whenever she next needs intricate cutouts.
The OpenSCAD source code as a GitHub Gist:
The 11.5 mm body is long enough to justify making a longer holder with more bearing surface:
Slicing with four perimeter threads lays down enough reasonably solid plastic to bore the central hole to a nice sliding fit:
The top disk gets bored to a snug press fit around the flange and upper body:
Assemble with springs and it pretty much works:
Unfortunately, it doesn’t work particularly well, because the two screws tightening the MPCNC’s DW660 tool holder (the black band) can apply enough force to deform the PETG mount and lock the drag knife body in the bore, while not being quite tight enough to prevent the mount from moving.
I think the holder for the black knife (on the left) worked better, because:
- The anodized surface is much smoother & slipperier
- The body is shorter, so less friction
In any event, I reached a sufficiently happy compromise for some heavy paper / light cardboard test shapes, but a PETG bearing won’t suffice for dependable drag knife cuttery.
Back to the laboratory …
Flushed with success from engraving a hard drive platter for the 21HB5A tube, I bandsawed an acrylic square from a scrap sheet and unleashed the diamond drag bit on it:
That’s side-lit against a dark blue background. The long scratch and assorted dirt come from its protracted stay in the scrap pile.
If you look closely, you’ll see a few slightly wider loops, which came from a false start at Z=-0.1 mm.
Engraving at -0.5 mm looked pretty good:
Despite an angular resolution of 2°, the curves came out entirely smooth enough. The gritty scratchiness resulted in a pile of chaff covering the engraved area; perhaps some oil or lube or whatever would help.
Rescaling the pattern to fit a CD platter worked fine, too:
Polycarbonate seems to deform slightly, rather than scratch, leaving the final product with no chaff at all:
In this case, the doubled lines come from the reflection off the aluminized lower surface holding all the data.
That CD should be unreadable by now …
The three collet pen holders I got a while ago came with ink cartridges:
So I bought three bucks worth of a dozen pens to get pretty colors, whereupon I discovered they didn’t fit into the collet. Turns out the locating flanges aren’t in the same place along the cartridges:
The flanges on the top cartridge have been shaved down perilously close to the ink, but it now fits into the collet.
Bonus: a dozen fairly stiff springs that are sure to come in handy for something!
That’s with a 0.1 mm cut depth, sidelit with an LED flashlight.
Feeding those nine digits into the Guilloché pattern generator script should get you the same pattern; set the paper size to 109 mm and use Pen=0 to suppress the legend.
The same pattern at 0.3 mm cut depth looks about the same:
It’s slightly more prominent in real life, but not by enough to make a big difference. I should try a graduated series of tests, of course, which will require harvesting a few more platters from dead drives.
Either side will look great under a 21HB5A tube, although the disks are fingerprint and dust magnets beyond compare.
Drawn at Z=-0.1 mm on scrap acrylic with the diamond engraver in the modified collet holder:
The badly rounded corner comes from a Z touch off in facepalm mode; the poor diamond must have been trying to dig a 2 mm trench through the acrylic.
Then again at Z=-0.5 mm:
At half a millimeter, the holder applies well over 100 g of downforce. There’s no way to know how much lateral force the tip applies to the holder, but it’s obvious the parallel beams on the MPCNC drag knife adapter lack lateral stability:
Bending beams still seem much better than a linear bearing, though.
Of course, the diamond engraving points have a 3 mm shaft that doesn’t fit in the 2.5 mm Collet Pen Holder, but making a hole bigger isn’t much of a problem …
Commence by drilling out the collet closer nut:
The hole didn’t start out on center and I didn’t improve it in the least. A touch of the lathe bit and a little file work eased off the razor edge around the snout.
The knurled ridges at the top are larger than the threaded body, which requires a shim around the threads to fit them into the lathe chuck. Start by cutting a slightly larger ID brass tube to the length of the threaded section:
I finally got a Round Tuit and ground opposing angles on the cutoff tool ends, so I can choose which side of the cut goes through first. In this case, the left side cuts cleanly and the scrap end carries the thinned slot into the chip tray.
Grab the tube in a pair of machinist vises and hacksaw a slot:
Apply a nibbler to embiggen the slot enough to leave an opening when it’s squashed around the threads:
Put a nut on the collet threads in an attempt to keep them neatly lined up while drilling:
Drill the hole to a bit over 3 mm in small steps, because it’s not the most stable setup you’ve ever used. Eventually, the diamond point just slips right in:
Reassemble in reverse order and It Just Works:
Now, to scratch up some acrylic!