Tag: Improvements

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

Mini-Lathe: Metric Change Gear Tables

Running my assortment of custom 3D printed change gears through the LittleMachineShop calculator and copying the results into a spreadsheet for E-Z formatting produces a useful table:

Mini-Lathe Change Gear Tables – Metric Download

The same table in text-ish format, minus the colored highlights marking the custom gears:

Pitch	A	B	C	D	Actual	Error	In 10 pitches

0.10
	20	80	20	80	0.099	0.786%	0.008
	20	79	20	80	0.100	0.468%	0.005
	20	80	20	79	0.100	0.468%	0.005

0.20
	20	79	40	80	0.201	0.473%	0.009
	20	80	40	79	0.201	0.473%	0.009
	40	79	20	80	0.201	0.473%	0.009
	40	80	20	79	0.201	0.473%	0.009

0.25
	20	55	35	81	0.249	0.225%	0.006
	20	81	35	55	0.249	0.225%	0.006
	35	55	20	81	0.249	0.225%	0.006

0.30
	20	57	35	65	0.300	0.023%	0.001
	20	65	35	57	0.300	0.023%	0.001

0.40
	20	55	45	65	0.400	0.088%	0.004
	20	65	45	55	0.400	0.088%	0.004

0.50
	21	50	45	60	0.500	0.012%	0.001
	21	60	45	50	0.500	0.012%	0.001
	21	80	60	50	0.500	0.012%	0.001
	42	35	21	80	0.500	0.012%	0.001
	60	50	21	80	0.500	0.012%	0.001

0.60
	35	57	40	65	0.600	0.022%	0.001
	35	65	40	57	0.600	0.022%	0.001
	40	57	35	65	0.600	0.022%	0.001
	40	65	35	57	0.600	0.022%	0.001

0.70
	35	55	45	65	0.699	0.087%	0.006
	35	65	45	55	0.699	0.087%	0.006
	45	55	35	65	0.699	0.087%	0.006
	45	65	35	55	0.699	0.087%	0.006
	50	77	55	81	0.700	0.006%	0.000
	50	81	55	77	0.700	0.006%	0.000
	55	77	50	81	0.700	0.006%	0.000

0.75
	42	50	45	80	0.750	0.012%	0.001
	42	80	45	50	0.750	0.012%	0.001
	45	50	42	80	0.750	0.012%	0.001
	45	80	42	50	0.750	0.012%	0.001

0.80
	40	55	45	65	0.799	0.088%	0.007
	40	65	45	55	0.799	0.088%	0.007
	45	55	40	65	0.799	0.088%	0.007
	45	65	40	55	0.799	0.088%	0.007
	20	55	79	57	0.800	0.010%	0.001
	20	57	79	55	0.800	0.010%	0.001

1.00
	21	50	60	40	1.000	0.012%	0.001
	42	50	45	60	1.000	0.012%	0.001
	42	60	45	50	1.000	0.012%	0.001
	42	80	60	50	1.000	0.012%	0.001
	45	50	42	60	1.000	0.012%	0.001
	45	60	42	50	1.000	0.012%	0.001
	60	50	42	80	1.000	0.012%	0.001

1.25
	21	50	60	32	1.250	0.013%	0.002
	35	40	45	50	1.250	0.013%	0.002
	42	40	45	60	1.250	0.013%	0.002
	42	80	60	40	1.250	0.013%	0.002
	45	40	42	60	1.250	0.013%	0.002
	60	20	21	80	1.250	0.013%	0.002
	60	40	42	80	1.250	0.013%	0.002

1.50
	42	40	45	50	1.500	0.013%	0.002
	45	40	42	50	1.500	0.013%	0.002

1.75
	65	42	57	80	1.751	0.029%	0.005

2.00
	42	40	60	50	2.000	0.012%	0.003
	42	50	60	40	2.000	0.012%	0.003
	60	40	42	50	2.000	0.012%	0.003

2.50
	42	32	60	50	2.500	0.012%	0.003
	42	40	60	40	2.500	0.012%	0.003
	42	50	60	32	2.500	0.012%	0.003
	60	20	42	80	2.500	0.012%	0.003
	60	32	42	50	2.500	0.012%	0.003

3.00
	65	50	80	55	3.002	0.061%	0.018
	65	55	80	50	3.002	0.061%	0.018
	80	50	65	55	3.002	0.061%	0.018

3.50
	57	40	65	42	3.501	0.029%	0.010
	57	42	65	40	3.501	0.029%	0.010
	65	21	57	80	3.501	0.029%	0.010
	65	40	57	42	3.501	0.029%	0.010
	65	42	57	40	3.501	0.029%	0.010

4.00
	60	20	42	50	4.000	0.012%	0.005

5.00
	55	21	60	50	4.989	0.215%	0.107
	55	35	80	40	4.989	0.215%	0.107
	55	40	80	35	4.989	0.215%	0.107
	60	21	55	50	4.989	0.215%	0.107
	80	35	55	40	4.989	0.215%	0.107
	80	40	55	35	4.989	0.215%	0.107
	35	45	81	20	5.001	0.012%	0.006
	45	55	77	20	5.001	0.012%	0.006
	77	20	45	55	5.001	0.012%	0.006

Mini-Lathe Metric Change Gear Trains

The basic formulas:

TPI = 16 / ((A/B) x (C/D))
Pitch = 25.4 / TPI = 1.5875 x ((A/B) x (C/D))

So, for example, a 45-50-42-60 train will produce a 1 mm thread pitch with 120 ppm error adding up to a mere 1 micron in 10 pitches:

Mini-Lathe change gears - stacked 50-42 - installed — Mini-Lathe change gears – stacked 50-42 – installed

Overall, the errors are so low as to not matter, even without using the custom gears, but it’s the principle of the thing …

2020-05-19

Mini-lathe Metric Threading: Stackable Change Gear Generator

Although OpenSCAD’s MCAD library includes a gear generator, I don’t profess to understand the relations between reality and its myriad parameters, plus I vaguely recall it has a peculiar definition for Diametral Pitch (or some such). Rather than fiddle with all that, I start with an SVG outline from Inkscape’s Gears extension and go all 3D on it.

So, the “gear blank” looks like this after extruding the SVG:

Mini-lathe change gear - 42 tooth - SVG import — Mini-lathe change gear – 42 tooth – SVG import

Producing this is a lot easier in OpenSCAD than in real life:

Mini-lathe change gear - 42 tooth - solid model — Mini-lathe change gear – 42 tooth – solid model

OpenSCAD centers the blank’s bounding box at XY=0, which won’t be exactly on the bore centerline for gears with an odd number of teeth. One tooth sits at 0° and two teeth bracket 180°, so the bounding box will be a little short on one side

A reference for gear nomenclature & calculations will come in handy.

For a 21 tooth module 1 gear, which should be pretty close to the worst case in terms of offset:

Pitch dia = d = 21 × 1 = 21 mm
Tip dia = da = d + 2m = 23 mm
Tip radius = da/2 = 11.5 mm
Tooth-to-tooth angle = 360/21 = 17.143°
Radius to tangent across adjacent teeth = 11.5 × cos 17.143°/2 = 11.372 mm

An actual metal 21 tooth gear measures 22.87 mm across a diameter, dead on what those numbers predict: 11.5 + 11.372 = 22.872 mm.

So the bounding box will be 11.5 mm toward the tooth at 0° and 11.372 mm toward the gap at 180°. The offset will be half that, with the tooth at 0° sitting 0.063 mm too close to the origin. Gears with more teeth will have smaller errors.

Given that we’re dealing with a gear “machined” from plastic goo, that’s definitely close enough:

Mini-Lathe change gears - 1 mm - 45-50-45-60 — Mini-Lathe change gears – 1 mm – 45-50-45-60

That’s an earlier version with the debossed legend.

The code can also generate stacked gears for the BC shaft in the middle:

Mini-lathe change gear - 42-55 tooth stacked - solid model — Mini-lathe change gear – 42-55 tooth stacked – solid model

In principle, the key locking the gears together isn’t needed and the bore could fit the inner shaft, rather than the keyed bushing, but then you’d (well, I’d) be at risk of losing the bushing + key in one easy operation.

So it’s better to go with the bushing:

Now, to cut some threads!

The OpenSCAD source code as a GitHub Gist:

	// LMS Mini-Lathe
	// Change gears with stacking
	// Generate SVG outlines with Inkscape's Gear extension
	// Ed Nisley – KE4ZNU
	// 2020-05

	/* [Gears] */

	TopGear = 0; // zero for single gear
	BottomGear = 42;

	/* [Hidden] */

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	Protrusion = 0.1; // make holes end cleanly

	Dir = ""; // empty string for current directory

	/* [Dimensions] */

	ShaftOD = 12.0;
	ShaftSides = 4*3;

	GearThick = 7.75;

	Keyway = [3.5 + HoleWindage,3.0 + HoleWindage,3*GearThick]; // x on radius, y on perim, z on axis

	LegendThick = 2*ThreadThick;
	LegendZ = (TopGear ? 2*GearThick : GearThick) – LegendThick;

	LegendRecess = [10,7,LegendThick];

	LegendEnable = (TopGear == 0 && BottomGear > 41) \|\| (TopGear > 41);


	//———————-
	// Useful routines

	// Enlarge holes to prevent geometric shrinkage
	// based on nophead's polyholes
	// http://hydraraptor.blogspot.com/2011/02/polyholes.html
	// http://www.thingiverse.com/thing:6118

	module PolyCyl(Dia,Height,ForceSides=0) {

	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

	FixDia = Dia / cos(180/Sides);

	cylinder(r=(FixDia + HoleWindage)/2,
	h=Height,
	$fn=Sides);
	}

	//———————–
	// Build it!

	union() {

	difference() {

	union() {
	linear_extrude(GearThick,center=false,convexity=5)
	import(file=str(Dir,"Change Gear – ",BottomGear," teeth.svg"),
	center=true);

	if (TopGear)
	translate([0,0,GearThick])
	linear_extrude(GearThick,center=false,convexity=5)
	import(file=str(Dir,"Change Gear – ",TopGear," teeth.svg"),
	center=true);
	}

	rotate(180/ShaftSides)
	translate([0,0,-Protrusion])
	PolyCyl(ShaftOD,3*GearThick);

	translate([ShaftOD/2,0,Keyway.z/2 – Protrusion])
	cube(Keyway,center=true);

	if (LegendEnable) {
	translate([0,1.1*ShaftOD,LegendZ + LegendRecess.z/2])
	cube(LegendRecess + [0,0,Protrusion],center=true);

	if (TopGear) {
	translate([0,-1.1*ShaftOD,LegendZ + LegendRecess.z/2])
	cube(LegendRecess + [0,0,Protrusion],center=true);
	}
	}

	}

	if (LegendEnable)
	translate([0,0,LegendZ – Protrusion])
	linear_extrude(height=LegendThick + Protrusion,convexity=10) {
	translate([-02.5,1.1ShaftOD])
	rotate(-0*90)
	text(text=str(BottomGear),size=5,font="Arial:style:Bold",halign="center",valign="center");
	if (TopGear)
	translate([-02.5,-1.1ShaftOD])
	rotate(-0*90)
	text(text=str(TopGear),size=5,font="Arial:style:Bold",halign="center",valign="center");
	}

	}

view raw Mini-lathe stacked change gears.scad hosted with ❤ by GitHub

2020-05-18

Pride Lift Chair: Fuzzy Felt Feet

By the Universal Principle of the Conservation of Perversity, the base of the floor lamp just barely doesn’t fit under the edge of the Comfy Chair:

Floor Lamp - copper 70° elbow - installed — Floor Lamp – copper 70° elbow – installed

Well, I can fix that!

Lift Chair Foot - installed — Lift Chair Foot – installed

The feet descend from the fuzzy felt feet on the plant shelves, with the hex head socket transmogrified into a circle to match the chair feet. The support structure grew a flat plate to ensure it doesn’t pull loose from the platform:

Lift Chair Feet - solid model - support view — Lift Chair Feet – solid model – support view

Print ’em out, stick the felt in place:

Lift Chair Feet - assembly — Lift Chair Feet – assembly

Lift the chair (maybe with a small prybar atop some plywood to protect the floor), position the feet, lower gently: done!

While the M2 was warm, I ran off another set for the other Comfy Chair, just for symmetry.

The OpenSCAD source code as a GitHub Gist:

	// Feet for Pride lift chair
	// Ed Nisley KE4ZNU 2020-05

	Layout = "Build"; // [Show, Build]

	Support = true;

	//- Extrusion parameters must match reality!
	// Print with 2 shells and 3 solid layers

	/* [Hidden] */

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	Protrusion = 0.1; // make holes end cleanly

	inch = 25.4;

	//———————-
	// Dimensions

	/* [Dimensions] */

	ChairFootOD = 33.0;
	ChairFootRecess = 5.0;

	FeltOD = 39.0;
	FeltRecess = 2.5;

	FootPlate = 6*ThreadThick;
	FootWall = 4*ThreadWidth;

	FootOD = 2*FootWall + max(ChairFootOD,FeltOD);
	echo(str("Foot OD: ",FootOD));

	FootTall = ChairFootRecess + FootPlate + FeltRecess;
	echo(str(" … height: "),FootTall);

	NumSides = 9*4;


	//———————-
	// Useful routines

	module FootPad() {

	difference() {

	cylinder(r=FootOD/2,h=FootTall,$fn=NumSides);

	translate([0,0,FeltRecess + FootPlate])
	cylinder(d=ChairFootOD,h=2*ChairFootRecess,$fn=NumSides);

	translate([0,0,-Protrusion])
	cylinder(d=FeltOD,h=(FeltRecess + Protrusion),$fn=NumSides);

	}
	}


	//——————-
	// Build it…


	if (Layout == "Show")
	FootPad();

	if (Layout == "Build") {
	translate([0,0,FootTall])
	rotate([180,0,0])
	FootPad();
	if (Support)
	color("Yellow") {
	for (Seg=[0:5])
	rotate(30 + 360*Seg/6)
	translate([0,0,(ChairFootRecess – ThreadThick)/2])
	cube([(ChairFootOD – 3*ThreadWidth),
	2*ThreadWidth,
	(ChairFootRecess – ThreadThick)],
	center=true);
	rotate(180/6)
	cylinder(d=0.5*ChairFootOD,h=ThreadThick,$fn=6);
	}
	}

view raw Lift Chair Feet.scad hosted with ❤ by GitHub

2020-05-17

Mini-Lathe Metric Threading: 42 Tooth Gear
Going from a 21 tooth gear to a 42 tooth gear means you must reduce the remaining train ratio by a factor of two for a given thread pitch. Here’s a 42-50-45-60 train, with the same -125 ppm error as the 21-50-60-40 train and no screw / washer clearance issues between the A screw and the C gear:

Mini-Lathe change gears – 1 mm – 45-50-45-60

The original 60-40 CD pair has a 3:2 ratio, the 45-60 CD pair is 3:4, so that’s where the factor-of-two reduction happens.

The first pass at the solid model included a debossed legend:

Mini-lathe 42 tooth change gear – Slic3r

With a printed gear in hand, I realized the legend must be embossed below the surface, so as not to rub against an adjacent gear; better modeling is in order.

The general idea is to set Inkscape’s (known-good) gear generator to the correct gear parameters (module 1 → 3.14 mm circular pitch, 20° pressure angle):

Inkscape Gear Generator dialog

Save the outline as an SVG:

Inkscape Gear Generator result

If you do like I did and neatly position the gear at the bottom-left origin, all SVG viewers will show only the Quadrant I arc, probably because Inkscape sets the SVG file to display it that way. I’ve made that mistake before and maybe, someday, I’ll remember.

Load the SVG into OpenSCAD, which will find the entire gear, no matter where it falls in the coordinate space, and spike it at the origin:
```
linear_extrude(8.0,center=false,convexity=5) 
 import(file="/the-source-directory/Mini-Lathe/Change Gear - 42 teeth.svg",center=true);
```
The linear_extrude( … center=false … ) keeps the bottom of the blank at Z=0. The import( … center=true … ) puts the 2D shape at the XY origin. Because OpenSCAD centers the bounding box, gears with an odd number of teeth will be ever so slightly off-center, which would matter a whole lot more in a fancier machine tool than a mini-lathe.

All of which produces a tidy 3D gear blank:

Mini-lathe change gear – 42 tooth – SVG import

OpenSCAD ignores SVG holes, which isn’t a problem for me, because I’d rather punch the bore, keyway, and so forth programatically.

But that’s another story …
2020-05-14
Mini-Lathe Metric Threading: 32 Tooth Gear

While not strictly necessary for metric threading on a USA-ian mini-lathe, a 32 tooth gear can produce reasonable approximations, so I printed a pair from a Thingiverse collection:

Mini-lathe 32 tooth change gear – Slic3r

The model was designed for a slightly different mini-lathe, as it includes a short boss and thinner plate, but it did fit on the shaft:

Mini-Lathe change gears – 1 mm – bad 32 60 65 55

The gear mesh seemed odd, though, and comparing it with a standard 30 tooth gear and a different printed 32 tooth gear (about which, more later) showed it was definitely not compatible:

Mini-lathe change gears – 32 30 odd 32

Yes, the 32 tooth Thingiverse gear on the right is slightly smaller than the stock 30 tooth gear in the middle.

The larger 32 tooth gear (on the left, above) meshes better:

Mini-Lathe change gears – 1 mm – 65 55 32 60

Both of those trains have a 600 ppm error, so they’re definitely suboptimal compared to the results with a 21 tooth gear in the train.

The real reason you need a 32 tooth gear is for exact 25, 50, and 100 TPI threads with a 1/16 inch leadscrew. I don’t foresee much need for those around here, but you can never have too many change gears …

2020-05-13
Garden Rake Cross Bolt

Mary’s long-suffering garden rake pulled apart while we were flattening a section of what will become something like a lawn next to the garden:

Garden rake – shank and ferrule

For whatever reason, there’s no cross bolt holding the shank into the ferrule, like there should be on any tool subject to pulling force.

After marking the wide spot on the shank, a couple of good shots with a two pound hammer flattened the ferrule around it well enough to start a hole with a 3/16 step drill:

Garden rake – cross drilling

Go through the far side with a 13/16 inch drill for a generous 5 mm fit, drop a bolt into the hole while it can’t get away, tighten the nyloc nut, and it’s all good:

Garden rake – cross bolt

In fact, it’s better than it ever was, because now the shank can’t pull out until the ferrule falls off the handle. Which could happen, but I’m not averse to another bolt.

Admittedly, it’s not a stainless steel socket head cap screw, because that’d just about double the value of the rake. The handle is in such bad shape that the bolt will probably outlast the wood …

Done!

Update: The consensus says I totally missed the Ritual Invocation of the Epoxy, so:

Garden rake – epoxy fill

Now all is right with the world …

2020-05-11
Floor Lamp Height vs. Reach: Plumbing Fitting

Update: Welcome Adafruit! The reshaped elbow shown here eventually got threaded adapters for the tubing and an awful paint job.

The floor lamp with the invisible / non-tactile controls moved to a different chair, where it didn’t have quite enough reach and too much height. Knowing what was about to happen, I spliced a JST-SM connector into the wire inside the tube:

Floor Lamp – base wiring JST-SM connector

After trimming off all the extraneous bits, the larger half of the connector (male pins) fits through the tubing and the smaller half (female sockets) barely fits through the bottom bushings.

It turns out half-inch copper pipe fittings (ID = 15.9 mm) almost exactly fit the tubing (OD = 15.7 mm):

Floor Lamp – copper 45° elbow

A quick test showed the 45° (actually, it’s 135°, but we’re deep into plumbing nomenclature) positioned the lamp head too high and with too much reach:

Floor Lamp – gooseneck exercise

So shorten the tube attached to the head and deburr the cut:

Floor Lamp – tube deburring

The 45° fitting is too high and a 90° fitting is obviously too low, so cut a 20° slice out of a 90° fitting:

Floor Lamp – copper 90° elbow – 20° cutout

Cut a snippet of brass tubing to fit, bash to fit, file to hide, buff everything to a high shine, silver-solder it in place, and buff everything again:

Floor Lamp – copper 90° elbow – 20° fill strip

The 5/8 inch aluminum rods serve to stiffen the fitting, smooth out the torch heating, and generally keep things under control.

Wrap the obligatory Kapton tape around the butt ends of the tubes to fill the fitting’s oversize hole, put everything together, and it’s just about perfect:

Floor Lamp – copper 70° elbow – installed

I immobilized the fitting with black Gorilla tape, but it really needs something a bit more permanent. One of these days, maybe, a pair of setscrews will happen.

The additional reach required a little more counterweight on the far side for security, so I added the broken stub of a truck leaf spring. It should be secured firmly to the base plate, but no tool I own can put a dent in those three pounds of spring steel. Maybe it’ll merit a fancy enclosure wrapped around the base?

2020-05-09