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Archive for June, 2019

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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Monthly Image: Turkey Hen and Chicks

Mary saw a fox trotting behind the garden, gripping a (dead) turkey chick in its jaws, with the hen in hot pursuit. The fox dropped the chick, circled the pine grove, picked up the chick, and departed stage right. The hen eventually led her remaining chicks into the yard, but gathered them underneath while watching for danger:

Turkey hen with chicks - alert
Turkey hen with chicks – alert

She settled down for a few minutes:

Turkey hen with chicks - resting
Turkey hen with chicks – resting

With the fox safely departed, she released the chicks:

Turkey hen with chicks - emerging
Turkey hen with chicks – emerging

Then they returned to foraging, with one chick trying out its wings:

Turkey hen with chicks - dispersing
Turkey hen with chicks – dispersing

Two days earlier, she led nine chicks through the yard; we think the fox picked off a chick a day. She lost two more during the next four days, suggesting they rapidly improve their ability to scamper out of harm’s way.

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Monthly Science: Water Bottle Refill History

This year’s Northern Star Quilt Guild show in the O’Neill Center at WCSU gave me an opportunity to update last year’s data points, although not quite in the way I expected.

The overview:

Gym water bottle refill station B - overview
Gym water bottle refill station B – overview

The left display is rotting out:

Gym water bottle refill station B - left
Gym water bottle refill station B – left

The center display seems undamaged:

Gym water bottle refill station B - center
Gym water bottle refill station B – center

The right display took a direct hit:

Gym water bottle refill station B - right
Gym water bottle refill station B – right

So the middle station refilled 3025 = 10460 – 7435 bottles, roughly eight bottles a day, every day, for a year. Seems like a lot of refilling, doesn’t it?

Unfortunately, I didn’t take pictures of the other watering hole last year, but here’s what it looks like now:

Overview:

Gym water bottle refill station A - overview
Gym water bottle refill station A – overview

Left:

Gym water bottle refill station A - left
Gym water bottle refill station A – left

Center:

Gym water bottle refill station A - center
Gym water bottle refill station A – center

Right:

Now, it’s entirely possible I have the two stations reversed, in which case I have numbers for all three displays:

  • Left = 242 = 4758 – 4516
  • Center = 633 = 8068 – 7435
  • Right = 800 = 9689 – 8889

Does a bottle or two a day, every day, for a year, seem more reasonable? Hard to say, so, with a bit of luck, we’ll have more data next year.

For science!

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