Tag: Improvements

Making the world a better place, one piece at a time

Cheese Slicer Rebuild

The cheese slicer frame looked much better after sandblasting with 220 aluminum oxide grit:

Cheese slicer – sandblasted

The flower bed outside the Basement Laboratory door seems a bit dusty, though.

Slathering it with JB Weld steel-filled epoxy went reasonably well:

Cheese slicer – JB Weld curing

JB Weld is much much more viscous than the clear XTC-3D I used last year and the final coating, while smoother than what you see here, has too many sags and dents to say “good job”. I didn’t bother coating the upper tips, because the epoxy will wear off from my morning KP.

The aluminum roller turned on those bare stainless steel screws in the tray, with the threads chewing into the roller bore. While the epoxy was curing, I drilled out the roller to remove most of the ridges:

Cheese slicer – drilling roller

Cut a pair of stainless screws slightly longer than the old screws, then turn the threads off to make a shaft:

Cheese slicer – screw reshaping

The lathe spindle runs in reverse, so the cutting force tends to tighten the screw in the nuts. The big old South Bend lathe had a screw-on chuck and didn’t really like turning backwards.

The new screws won’t win any beauty prizes, but they get the job done:

Cheese slicer – screw shafts

Turn a Delrin rod to a press fit in the drilled-out roller:

Cheese slicer – turning Delrin bearing

Part it off, repeat, ram them into the roller, then drill to a loose fit around the smooth-ish screw shafts:

Cheese slicer – drilling Delrin bearing

Reassemble in reverse order:

Cheese slicer – rebuilt

Looks downright industrial, it does.

Stipulated: this makes no economic sense, absent the simple fact we appreciate utensils that just work.

2018-11-05
MPCNC: Drag Knife Holder Spring Constant vs. Stiction

Sliding a drag knife body in a PETG holder, even after boring the plastic to fit, shows plenty of stiction along 2 mm of travel:

MPCNC – Drag Knife Holder – spring constant

Punching the Z axis downward in 0.5 or 1.0 mm steps produced the lower line at 210 g/mm. Dividing by three springs, each one has a 70 g/mm spring constant, which may come in handy later.

The wavy upper line shows the stiction as the Z axis drops in 0.1 mm steps. The line is eyeballometrically fit to be parallel to the “good” line, but it’s obvious you can’t depend on the Z axis value to put a repeatable force on the knife.

I cranked about a turn onto the three screws to preload the springs and ensure the disk with the knife body settles onto the bottom of the holder:

MPCNC – DW660 adapter drag knife holder – spring loaded

The screws are M4×0.7, so one turn should apply about 140 g of preload force to the pen holder. Re-taking a few data points with a 0.5 mm step and more attention to an accurate zero position puts the intercept at 200 g, so the screws may have been slightly tighter than I expected. Close enough, anyway.

The stiction is exquisitely sensitive to the tightness of the two DW660 mount clamp screws (on the black ring), so the orange plastic disk isn’t a rigid body. No surprise there, either.

Loosening the bored slip fit would allow more lateral motion at the tip. Perhaps top-and-bottom Delrin bushings (in a taller mount) would improve the situation? A full-on linear bearing seems excessive, even to me, particularly because I don’t want to bore out a 16 mm shaft for the blade holder.

It’s certainly Good Enough™ as-is for the purpose, as I can set the cut depth to, say, 0.5 mm to apply around 250-ish g of downforce or 1.0 mm for 350-ish g. The key point is having enough Z axis compliance to soak up small table height variations without needing to scan and apply compensation.

2018-10-26
MPCNC: Jogging Keypad for bCNC

The bCNC G-Code sender sends jogging commands to GRBL from an ordinary numeric keypad:

MPCNC – Jogging keypad

Unlike the keypads on my streaming radio players, this one requires no configuration at all, because bCNC regards it as just another keyboard input. The catch: you must select any screen element other than a text entry field to have bCNC recognize the keystrokes as “not text”.

You would get the same results from the numeric keys on the right side of a full-size / 104-key plank. I’m using a small “tenkeyless” keyboard, which means I can put the keypad wherever it’s easiest to reach while tweaking the MPCNC.

The ÷10 and ×10 keys along the top row alter the step size by factors of ten, which is pretty much what you need: jog to within a big step of the target, drop to the next lower decade, jog a few more times, maybe drop another decade, jog once, and you’re as close as you need to be with an MPCNC. The -1 and +1 keys aren’t as useful, at least to me: changing from 5 mm to 4 mm or 6 mm doesn’t make much difference.

Jogging to align the spindle (well, a pen or drag knife) with a target using the video camera works really well:

bCNC – Video align

GRBL and bCNC don’t do smooth jogging and the discrete steps aren’t as nifty as the Joggy Thing with LinuxCNC, but it gets the job done.

2018-10-18
Dutch Reach

I carry a garish scar under my right arm from my collision with a frameless driver door window while commuting from classes at Lehigh U, back in the day, so I’m as bike-aware as any driver you’ll ever meet. After reading several articles describing the Dutch Reach, I put a reminder on the Forester’s driver door handle:

Dutch Reach reminder

The bright yellow block reminds me to peer into the mirror (*) before yanking the handle, regardless of which hand I’m using. Haven’t had any close calls yet, but practice makes perfect.

If you don’t have a label maker, you can hang a tag on the handle.

It’s surprisingly hard to retrain a habit, though …

(*) Update: Yes, I should look over my shoulder, too. At least now I’m aware of the situation and don’t just open the door without thinking. One step at a time.

2018-10-14
MPCNC: Modified Drag Knife Adapter Spring Constant

The bars on the original MPCNC drag knife / plotter pen adapter had a 100 g/mm spring constant:

MPCNC – Plotter pen force test

Making the bars slightly thicker improved their print-ability:

MPCNC knife adapter mods – OpenSCAD model

The reddish tint marks the new bars, with their location carefully tweaked to be coincident with the stock STL.

Shoving the pen into the scale with 0.1 mm steps produces another unnervingly linear plot:

Modified MPCNC pen adapter – Spring Constant data

Real plotter pens want about 20 g of force, so this isn’t the holder you’re looking for.

A bunch of plots at Z=-1.0 mm turned out well with the ballpoint pen insert, though:

MPCNC Modifed pen adapter – first plots

The globs apparently come from plotting too fast for conditions; reducing the speed to 1500 mm/min works better.

2018-10-09

MPCNC: Modified Drag Knife Adapter

A trio of Cutter Cutting Plotter Blade Holders arrived:

Despite the name, they’re not well-suited for drag knife blades, because they’re collets gripping a 2 mm shaft. The blade doesn’t rotate unless the plotter / cutter rotates the entire holder, which is actually a thing.

I got ’em because the snout of a common ball-point pen refill measures about 2 mm:

Collet pen holder - detail — Collet pen holder – detail

The glob around the tip comes from plotting too fast for conditions; about 1500 mm/min works better for continuous lines and 250 mm/min improves text.

The stock MPCNC adapter has a single recess suited for Genuine Plotter Pens, but the knurled lock ring on these cheapies sticks out far enough to make them wobbly. This being an inconvenience up with which I need not put, a few lines of OpenSCAD tweak the stock STL:

MPCNC knife adapter mods - OpenSCAD model — MPCNC knife adapter mods – OpenSCAD model

The original STL is ivory, new cuts are cyan, and additions are reddish.

The two support beams are now 1.6 mm = four thread widths, for improved slicing with a 0.35 mm nozzle and a higher spring constant.

It’s by-and-large indistinguishable from the old adapter:

MPCNC - Pen Holder Detail — MPCNC – Pen Holder Detail

Which I was using upside-down, because the flange fit better.

The MPCNC works reasonably well as a pen plotter with a genuine ballpoint pen:

The OpenSCAD source code as a GitHub Gist:

	// Adding clearance for eBay collet pen holder

	MPCNC_OD = 12.0; // pen holder OD (matches STL curvature)
	MPCNC_Z = 8.9; // Z offset of pen axis

	Pen_OD = 11.5; // actual pen body OD

	ID = 0;
	OD = 1;
	LENGTH = 2;

	Flange = [11.5,15.7,2.2]; // actual pen body flange

	Locknut = [16.0,16.0,2.8]; // knurled locknut
	Locknut_Offset = 4.5; // flange center to locknut

	Wall = [41.0,4 * 0.4,9.0]; // thicker walls for more spring and better fill

	$fn = 32; // default cylinder sides

	difference() {
	translate([-(101.3 + MPCNC_OD/2),-111.9,0]) // put pen axis above Y axis, flange centered on X axis
	import("/mnt/bulkdata/Project Files/Mostly Printed CNC/Accessories/Tool Holders/MPCNC_525_Drag_Knife-1860310.STL",
	convexity=5);

	if (true) // improve holder-to-mount fit if needed
	translate([0,60/2,8.90])
	rotate([90,0,0])
	cylinder(d=MPCNC_OD,h=60);

	translate([0,0,MPCNC_Z]) // improve flange slot clearance
	rotate([90,0,0])
	cylinder(d=Flange[OD] + 1.0,h=Flange[LENGTH] + 0.5,center=true);

	translate([0,Locknut_Offset – 0.5,MPCNC_Z]) // add locknut clearance
	rotate([-90,0,0])
	cylinder(d=Locknut[OD] + 1.5,h=Locknut[LENGTH] + 1.5,center=false);
	}

	# translate([-(27.0 + Wall.x/2),0,0]) { // embiggen walls for higher spring constant
	translate([0,-24.4,0])
	cube(Wall);
	translate([0,20.6 – Wall.y,0])
	cube(Wall);
	}

view raw MPCNC Drag Knife Adapter – Collet Pen Mod.scad hosted with ❤ by GitHub

2018-10-08

Adulting 101

The library runs courses teaching useful skills:

Adulting 101 – Car Maintenance

The classes cover the basics of home finance, cooking, sewing, and suchlike.

I could have used a few Adulting courses, back in the day.

2018-10-06

Tag: Improvements

Cheese Slicer Rebuild

MPCNC: Drag Knife Holder Spring Constant vs. Stiction

MPCNC: Jogging Keypad for bCNC

Dutch Reach

MPCNC: Modified Drag Knife Adapter Spring Constant

MPCNC: Modified Drag Knife Adapter

Adulting 101