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Posts Tagged M2

MPCNC Drag Knife: Ground Shaft in LM12UU Bearing

The 12 mm drag knife holder on the left slides nicely in an LM12UU bearing:

Drag Knife holders - detail
Drag Knife holders – detail

However, its aluminum body isn’t really intended as a bearing surface and it extends only halfway through the LM12UU, so I finally got around to modifying the 11.5 mm body on the right to fit into a section of 12 mm ground shaft:

Drag Knife - turning 11.5 mm body to 10 mm
Drag Knife – turning 11.5 mm body to 10 mm

The general idea is to turn the body down to 10 mm OD; the picture shows the first pass over the nose after turning the far end down and removing the flange in the process. Exact concentricity of both ends isn’t important (it gets epoxied into a 10 mm hole through the 12 mm ground shaft), but it came out rather pretty:

Drag Knife - 11.5 mm body - turned to 10 mm
Drag Knife – 11.5 mm body – turned to 10 mm

The ground shaft started as a pen holder:

DW660 Pen Holder - ground shaft
DW660 Pen Holder – ground shaft

I knocked off the ring and bored the interior to fit the 10 mm knife body. The large end of the existing bore came from a 25/64 inch = 9.92 mm drill, so it was just shy of 10.0 mm, and I drilled the small end upward from 0.33 inch = 8.4 mm.

The smallest trio of a new set of cheap carbide boring bars allegedly went into a 5/16 inch = 7.9 mm bore, but I had to file the bar body down and diamond-file more end relief into the carbide for clearance inside the drilled hole:

Modified boring bar vs original
Modified boring bar vs original

I blued the bit, kissed it against the drilled bore, filed off whatever wasn’t blued, and iterated until the carbide edge started cutting. Sissy cuts all the way, with no pix to show for all the flailing around.

Epoxying the turned-down drag knife body into the shaft: anticlimactic.

The solid model features a stylin’ tapered snout:

Drag Knife LM12UU holder - tapered end
Drag Knife LM12UU holder – tapered end

Which gets an LM12UU bearing rammed into place:

Drag Knife - LM12UU holder - inserting bearing
Drag Knife – LM12UU holder – inserting bearing

The steel block leaves the bearing flush with the plastic surface, rather than having it continue onward and indent itself into the wood; I can learn from my mistakes.

The new idea: a single spring pressing the knife holder downward, reacting against a fixed plastic plate:

Drag Knife - LM12UU ground shaft - assembled
Drag Knife – LM12UU ground shaft – assembled

Unlike the previous design, the upper plate doesn’t move, so there’s no problem caused by sliding along the screw threads. I should run nylock nuts up against the plate to keep it in place, stiffen the structure, and provide some friction to keep the screws from loosening.

The top of the knife holder now has a boss anchoring the spring:

Drag Knife - turning spring recess
Drag Knife – turning spring recess

As you’d expect, the ground shaft slides wonderfully in the bearing, because that’s what it’s designed to do, and the knife has essentially zero stiction and friction at any point along the bearing, which is exactly what I wanted.

The spring, from the same assortment as all the others, has a 48 g/mm rate.

The OpenSCAD source code as a GitHub Gist:

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MPCNC Diamond Engraver: LM3UU Bearings, First Pass

Gripping a diamond engraver in a collet chuck worked well enough, but the MPCNC’s pen holder lacks sufficient downforce and lateral stiffness. The bit has a chrome-ish plated 3 mm shank, so I tinkered up a mount for a pair of LM3UU linear bearings from the LM12UU drag knife holder:

Diamond Scribe - LM3UU - Rev 1 - point view
Diamond Scribe – LM3UU – Rev 1 – point view

The shank isn’t exactly a precision part, but a few licks with a diamond file knocked off enough of the high spots so it slides reasonably well through the bearings. The bearing alignment is more critical than a simple 3D printed plastic part can provide, so a real version may need bearings in a metal shaft press-fit into the plastic; brute-forcing the bearings into alignment sufficed for now.

The butt end of the shank press-fits into a disk held down with three springs, similar to the LM12UU mount:

Diamond Scribe - LM3UU - Rev 1 - top view
Diamond Scribe – LM3UU – Rev 1 – top view

It draws Guilloché patterns just fine:

Diamond Scribe - LM3UU - Rev 1 - first light
Diamond Scribe – LM3UU – Rev 1 – first light

I don’t like how the spring-around-screw motion works, even if it’s OK for small excursions.

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MPCNC Drag Knife: LM12UU Linear Bearing

The anodized body of the drag knife on the left measures exactly 12.0 mm OD:

Drag Knife holders - detail
Drag Knife holders – detail

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:

Drag Knife - LM12UU holder - solid model
Drag Knife – LM12UU holder – solid model

The 12 mm section extends about halfway through the bearing, with barely 3 mm extending out the far end:

Drag Knife - LM12UU - knife blade detail
Drag Knife – LM12UU – knife blade detail

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:

Drag Knife - spring plate test fit
Drag Knife – spring plate test fit

This time, I tried faking stripper bolts by filling the threads of ordinary socket head cap screws with epoxy:

Ersatz stripper bolts - epoxy fill
Ersatz stripper bolts – epoxy fill

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:

Drag Knife - LM12UU holder - assembled
Drag Knife – LM12UU holder – assembled

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:

Drag Knife - applique templates
Drag Knife – applique templates

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:

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MPCNC Drag Knife: PETG Linear Bearing

Having reasonable success using a 12 mm hole bored in a 3D printed mount for the nice drag knife holder on the left, I thought I’d try the same trick for the raw aluminum holder on the right side:

Drag Knife holders - detail
Drag Knife holders – detail

The 11.5 mm body is long enough to justify making a longer holder with more bearing surface:

Drag Knife Holder - 11.5 mm body - Slic3r preview
Drag Knife Holder – 11.5 mm body – Slic3r preview

Slicing with four perimeter threads lays down enough reasonably solid plastic to bore the central hole to a nice sliding fit:

Drag Knife - 11.5 mm body - boring
Drag Knife – 11.5 mm body – boring

The top disk gets bored to a snug press fit around the flange and upper body:

Drag Knife - 11.5 mm body - flange boring
Drag Knife – 11.5 mm body – flange boring

Assemble with springs and it pretty much works:

Drag Knife - hexagon depth setting
Drag Knife – hexagon depth setting

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 …

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Desk Lamp Conversion: Round 2

A bit of rummaging produced a desk lamp arm, minus whatever lamp it originally held, ready to hold the second photo lamp, after a bit of epoxy on one locking knob:

Lamp arm clamp screw rework
Lamp arm clamp screw rework

The flanged nut will seat on the wrecked part of the knob, with the epoxy holding it in place and somewhat reinforcing the perimeter. I’m not sure this will last forever, but it’ll be a start.

Printing a second cold shoe, though, worked perfectly, and everything fit:

Photo Lamp - right arm installed
Photo Lamp – right arm installed

I love it when a plan comes together!

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Seam Ripper Cover

The cover for Mary’s favorite seam ripper cracked long ago, has been repaired several times, and now needs a replacement:

Seam Ripper cover - overview
Seam Ripper cover – overview

The first pass (at the top) matched the interior and exterior shapes, but was entirely too rigid. Unlike the Clover seam ripper, the handle has too much taper for a thick-walled piece of plastic.

The flexy thinwall cover on the ripper comes from a model of the interior shape:

Seam Ripper Cover - handle model
Seam Ripper Cover – handle model

It’s not conspicuously tapered, but OpenSCAD’s perspective view makes the taper hard to see. The wedge on top helps the slicer bridge the opening; it’s not perfect, just close enough to work.

A similar model of the outer surface is one thread width wider on all sides, so subtracting the handle model from the interior produces a single-thread shell with a wedge-shaped interior invisible in this Slic3r preview:

Seam Ripper Cover - exterior - Slic3r preview
Seam Ripper Cover – exterior – Slic3r preview

The brim around the bottom improves platform griptivity. The rounded top (because pretty) precludes building it upside-down, but if you could tolerate a square-ish top, that’s the way to go.

Both models consist of hulls around eight strategically placed spheres, with the wedge on the top of the handle due to the intersection of the hull and a suitable cube. This view shows the situation without the hull:

Seam Ripper Cover - handle model - cube intersection
Seam Ripper Cover – handle model – cube intersection

The spheres overlap, with the top set barely distinguishable, to produce the proper taper. I measured the handle and cover’s wall thicknesses, then guesstimated the cover’s interior dimensions from its outer size.

The handle’s spheres have a radius matching its curvature. The cover’s spheres have a radius exactly one thread width larger, so the difference produces the one-thread-wide shell.

Came out pretty nicely, if I do say so myself: the cover seats fully with an easy push-on fit and stays firmly in place. Best of all, should it get lost (despite the retina-burn orange PETG plastic), I can make another with nearly zero effort.

The Basement Laboratory remains winter-cool, so I taped a paper shield over the platform as insulation from the fan cooling the PETG:

Seam Ripper Cover - platform insulation
Seam Ripper Cover – platform insulation

The shield goes on after the nozzle finishes the first layer. The masking tape adhesive turned into loathesome goo and required acetone to get it off the platform; fortunately, the borosilicate glass didn’t mind.

The OpenSCAD source code as a GitHub Gist:

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Desk Lamp Conversion: Photo Light Cold Shoe

Having recently acquired a pair of photo lights and desirous of eliminating some desktop clutter, I decided this ancient incandescent (!) magnifying desk lamp had outlived its usefulness:

Desk Lamp - original magnifiying head
Desk Lamp – original magnifiying head

The styrene plastic shell isn’t quite so yellowed in real life, but it’s close.

Stripping off the frippery reveals the tilt stem on the arm:

Desk Lamp - OEM mount arm
Desk Lamp – OEM mount arm

The photo lights have a tilt-pan mount intended for a camera’s cold (or hot) shoe, so I conjured an adapter from the vasty digital deep:

Photo Light Bracket for Desk Lamp Arm - solid model
Photo Light Bracket for Desk Lamp Arm – solid model

Printing with a brim improved platform griptivity:

Photo Light Bracket for Desk Lamp Arm - Slic3r preview
Photo Light Bracket for Desk Lamp Arm – Slic3r preview

Fortunately, the photo lights aren’t very heavy and shouldn’t apply too much stress to the layers across the joint between the stem and the cold shoe. Enlarging the stem perpendicular to the shoe probably didn’t make much difference, but it was easy enough.

Of course, you (well, I) always forget a detail in the first solid model, so I had to mill recesses around the screw hole to clear the centering bosses in the metal arm plates:

Photo Lamp - bracket recess milling
Photo Lamp – bracket recess milling

Which let it fit perfectly into the arm:

Desk Lamp - photo lamp mount installed
Desk Lamp – photo lamp mount installed

The grody threads on the upper surface around the end of the slot came from poor bridging across a hexagon, so the new version has a simple and tity flat end. The slot is mostly invisible with the tilt-pan adapter in place, anyway.

There being no need for a quick-disconnect fitting, a 1/4-20 button head screw locks the adapter in place:

Photo Lamp - screw detail
Photo Lamp – screw detail

I stripped the line cord from inside the arm struts and zip-tied the photo lamp’s wall wart cable to the outside:

Photo Lamp - installed
Photo Lamp – installed

And then It Just Works™:

Photo Lamp - test image
Photo Lamp – test image

The lens and its retaining clips now live in the Big Box o’ Optical parts, where it may come in handy some day.

The OpenSCAD source code as a GitHub Gist:

The original dimension doodles, made before I removed the stem and discovered the recesses around the screw hole:

Photo Light - Desk Lamp Arm Dimensions
Photo Light – Desk Lamp Arm Dimensions

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