Tag: Mini-lathe

While I was cutting threads for the Floor Lamp poles, I tried measuring my progress over wires:

Those are three lengths of music wire, slightly bent from their storage roll, held in place with a precision clamp metric micrometer. Given the crudity of the setup, the uncalibrated wire diameter, and my lack of thread-fu, the results came out both close and unconvincing.

A set of real thread measuring wires being cheap & readily available, I’m prepared for the next time around this block:

The 185 mil “wires” (they’re all allegedly ground rod) will let me cut threads matching things like a Jesus nut; they’re suited for 3 TPI / 8 mm pitch screws. Mostly, wires from the front row will be all I ever need.

Which look like this in action:

The black doodad (the set includes half a dozen for all the wire sizes) fits over the micrometer anvil and holds two wires betwixt anvil and screw, leaving me to manipulate the screw, the third wire, and the micrometer with my remaining hands. Hence the vise holding the micrometer, which is known to be Very Bad Practice.

From the side:

All of the smaller wires measure 0.5 mil too thin, which is likely due to my lack of calibrated measurement equipment:

The few thread pitch diameters I measured also came out slightly too small, again likely due to calibration and screw tolerances.

The LittleMachineShop description of measuring threads over wires seems entirely adequate.

To forestall link rot, a slightly rearranged version of their tables of wire constants:

The lower table has metric thread pitches with the wire sizes in inches.

You measure the distance over the recommended wire (in inches or millimeters, as appropriate), subtract the constant, and get the pitch diameter in the same units. Conversely, add the constant to the desired pitch diameter to get the target over-wire distance, carefully cut the thread until it measures a bit less than that, back up sixty seconds, and cut it spot on.

Verily, it is written: there is no UnDo key (⎌) in machine shop work.

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:

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:

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:

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:

Set up for threading:

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:

The first real pass looked good:

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:

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:

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

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:

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:

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 …

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:

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

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:

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