Mini-lathe – The Smell of Molten Projects in the Morning

Floor Lamp: Threaded Fittings

The reshaped copper elbow on the floor lamp now has the right angle, but lacks threaded connections to the tubes. The OEM tube threads are close to M15×1, thus prompting the change gear exercise persuading Tiny Lathe™ to cut metric threads.

Chuck up a length of 5/8 inch aluminum tube, clean up the end, and poke a thread runout slot into it:

Floor Lamp - tube fitting - thread runout — Floor Lamp – tube fitting – thread runout

Turn the soon-to-be-thread OD to 14.7 mm, well under the minimum 14.794 mm major thread diameter. I figure it’s better to match the existing not-quite-standard tube threads than to get all fussy about tolerances:

Floor Lamp - tube fitting - thread OD — Floor Lamp – tube fitting – thread OD

Drill out the tube to 27/64 inch = 0.422 inch = 10.7 mm, a bit larger than the OEM fittings, to easily pass the JST-SM connector I added so I could take the lamp apart:

Floor Lamp - tube fitting - drilling bore — Floor Lamp – tube fitting – drilling bore

Yeah, you’re not supposed to let the swarf build up like that, but it’s hard to stop when you’re getting good chip.

Break the sharp edges:

Floor Lamp - tube fitting - ready for threading — Floor Lamp – tube fitting – ready for threading

Set up for threading:

Floor Lamp - tube fitting - external threading setup — Floor Lamp – tube fitting – external threading setup

That’s a really nice Warner laydown threader I won as a Cabin Fever door prize quite some years ago.

A comprehensive discussion of threading may be handy.

The compound is at 90° to the cross slide, because the DRO housing doesn’t let the compound swivel to the proper angle for thread cutting. I’m just ramming the threader straight into the tube, taking sissy cuts, and hoping for the best.

Kiss the OD with the cutter, set the cross slide DRO to zero, position the cutter just off the end of the tube, close the split nuts around the leadscrew, engage the threading dial at a conspicuous mark:

Mini-Lathe Threading Dial - aligned — Mini-Lathe Threading Dial – aligned

The first real pass looked good:

Floor Lamp - tube fitting - first thread pass — Floor Lamp – tube fitting – first thread pass

The runout slot is 1/16 inch = 1.6 mm wide and I’m running the lathe dead slow, so there’s plenty of time to punch the STOP button as the cutter enters the slot and let the spindle coast down. Flip the switch to REVERSE, crank the cross slide out a turn (1 mm with 0.3 mm of crank backlash), run the cutter back to the starting point, crank the cross slide in, and iterate until the fitting screws into one of the OEM lamp tubes:

Floor Lamp - tube fitting - final thread — Floor Lamp – tube fitting – final thread

The 5/8 inch tube is just a smidge too small for the copper fitting, so knurl the fitting to enlarge the OD slightly more than a smidge:

Floor Lamp - tube fitting - knurled — Floor Lamp – tube fitting – knurled

Break the knurl edges, part off the fitting, clean up the new end, and do it all over again:

Floor Lamp - tube fitting - threaded adapters — Floor Lamp – tube fitting – threaded adapters

The knurls got filed down to an exact slip fit in the copper elbow and will eventually be epoxied in place.

The cut-off tube on the lamp head also needs internal threads, so bore out the interior to flatten the weld seam:

Floor Lamp - tube fitting - cleaning tube bore — Floor Lamp – tube fitting – cleaning tube bore

No pix of the threading, but you have the general idea; the tube wall is a scant 0.6 mm thick, so this isn’t the place for full-spec threads. I stopped when the OEM tube screwed in place.

Apart from the hideous solder job, it came together pretty well:

Floor Lamp - tube fitting - unpainted — Floor Lamp – tube fitting – unpainted

It’s much more stable than Kapton-wrapped tubes jammed into a bare copper fitting, although that’s not saying much.

A rattle-can finish seems appropriate …

Machine Shop

Mini-Lathe: Threading Dial Alignment

As received, the mini-lathe’s threading dial was misaligned by about 1/4 division, which is nearly halfway to the next engagement point midway between the divisions:

Mini-Lathe Threading Dial - as received - colorized — Mini-Lathe Threading Dial – as received – colorized

I added the red lacquer crayon while contemplating what to do, because I thought the dial was swaged onto the shaft. It turns out to be threaded, so I marked where the dial should be, grabbed the shaft in the (soft-jawed) bench vise, and twisted the dial with a Vise-Grip until it lined up:

Well, it’s closer than it was, OK?

There’s about that much slop on either side of the index line coming from the loose gear engaging the leadscrew, so that’s as good as it gets.

Machine Shop

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 4-50-42-60 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 …

Machine Shop, Software

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:

Machine Shop, Oddities

Monthly Image: Wappinger Dam Gears

A walk around Wappinger Lake brought me to the old penstock controls:

Wappinger Dam - old penstock gearing - E view — Wappinger Dam – old penstock gearing – E view

Still meshed after all these years:

Wappinger Dam - old penstock gearing - SE view — Wappinger Dam – old penstock gearing – SE view

The “new” penstock intake control, a pair of utterly practical and totally non-photogenic screw drives, sits just to the left of these relics.

A night view of the penstock from some years ago:

Wappingers Falls Bridge - Pixel XL HDR - 2017-09-22 — Wappingers Falls Bridge – Pixel XL HDR – 2017-09-22

It still carries water to the recently refurbished power plant downstream on the right.

Those gears will remain meshed after everything else rots away …

Machine Shop, Software

Mini-Lathe Metric Threading: 42 Tooth Gear

Going from a 21 too th 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:

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 — 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):

Save the outline as an SVG:

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:

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 …

Machine Shop

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 — 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 — 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 — 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 — 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 …

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