Posts Tagged Sherline

Current Sensing: Powered Iron Toroid

Dell built the GX270 I’m repurposing back in 2004, early on in the capacitor plague years, but only one of the system board caps showed signs of leakage:

Capacitor plague - 2004 Dell Edition

Capacitor plague – 2004 Dell Edition

While I was harvesting some of the connectors, it occurred to me that those powdered iron inductors might make good current sensors, as they’re already wound with heavy gauge copper wires.

I picked an inductor with enough turns and, although slitting didn’t pose much of a problem, the saw did make a mess of the turns adjacent to the cut:

Powdered iron toroid - slitting

Powdered iron toroid – slitting

Iron powder has more magnetic remnance than ferrite, to the extent that iron swarf clogged the gap. After the first pass, I ran the slit toroid through the degausser to shake it clean and see what damage had been done. It looked OK, so I realigned it on the saw blade and continued the mission, with all the dust vanishing into the vacuum cleaner’s snout.

Removing the damaged sections left 22 turns. For comparison, I converted the 56 turn ferrite toroid into a 25 turn model by paralleling two 25 turn sections:

Slit toroids - iron - ferrite

Slit toroids – iron – ferrite

The enamel wire on the iron toroid measures 40 mil diameter, close enough to 18 AWG.

Paralleling two 24 AWG windings on the ferrite toroid produces twice the copper area of a single winding, so the resistance is the same as a single 21 AWG winding (3 AWG steps = factor of two area change). That’s three steps smaller than the 18 AWG on the iron toroid, so the resistance is a factor of two larger than the heavier wire.

The paralleled winding has the advantage of reducing the power dissipation required to produce the same magnetic flux density, without the difficulty of winding heavier wire. That may not actually matter, given the relatively low currents required by the motor in normal operation.

Wedging a Hall sensor into the gaps and stepping the current produced two useful graphs:

Iron and ferrite toroids - Hall sensor output

Iron and ferrite toroids – Hall sensor output

The iron toroid has lower permittivity (less flux density for a given magnetizing force), which means the full-scale range exceeds 3 A and the useful range up to 1 A covers only 300 mV.

The last point on the ferrite curve shows the Hall sensor output saturating just over 4 V, with 1.5 V of range.

The slope, in mV/A

  • Powdered iron: 340
  • Ferrite: 540

Boosting the slope of the powdered iron by 25/22 gives 386 mV/A, so the iron permeability really is 70% of the ferrite. That’s modulo the gap size, of course, which surely differs by enough to throw out all the significant digits.

Obviously, an op amp circuit to remove the offset and rescale the output to 0-5 V will be in order.

The previous graph for the ferrite toroid with the complete 56 turn winding shows, as expected, about twice the output of this 25 turn version:

FT82-43 - 56 turns - 24 AWG

FT82-43 – 56 turns – 24 AWG

The linear part of that line is 1375 mV/A, although I can’t vouch that the data came from the same Hall effect sensor. Scaling it by 25/56 gives 613 mV/A, suggesting it’s not the same sensor.

Having developed an emotional attachment to the ferrite toroid, I’ll use it in the first pass of the current feedback circuit. If the motor need a bit less sensitivity or lower resistance, the powdered iron toroid looks like a winner.

Memo to self: Always degauss iron toroids before slitting!

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Revlon Tweezers: Bad Spot Welds

Mary bought a pair of Revlon tweezers a while ago, picking a Name Brand to avoid hassles with bottom-dollar crap:

Revlon tweezers - bad spot welds

Revlon tweezers – bad spot welds

Well, that didn’t work.

I contend that the only difference between Name Brands and the bottom-dollar crap I tend to buy is a bit of QC and a lot of price. I’ll agree that’s not strictly true, but it does fit a goodly chunk of the observed data.

Anyhow.

I milled a recess into the corner of some scrap plastic to locate the handle end, then arranged a step block to capture the business end:

Revlon tweezers - drilling setup

Revlon tweezers – drilling setup

That setup ensures the holes go into the corresponding spots on both pieces, because I couldn’t figure out how to clamp them together and drill them both at once. I drilled the other piece with its good side up to align the holes; doing it bad side up would offset the holes if they’re not exactly along the center line.

A closer look:

Revlon tweezers - drilling fixture

Revlon tweezers – drilling fixture

Talk about a precarious grip on the workpiece!

I filed the welds flat before drilling, so the pieces lay flat and didn’t distract the drill.

Then:

  • Center-drill
  • Drill 2-56 clearance
  • Scuff up mating surfaces with coarse sandpaper
  • Apply epoxy
  • Insert screws
  • Add Loctite
  • Tighten nuts to a snug fit
  • Align jaws
  • Tighten nuts
  • Fine-tune jaw alignment
  • Apply mild clamping force to hold jaws together
  • Wait overnight
  • Saw screws and file flush
  • Done!

The clamping step:

Revlon tweezers - epoxy curing

Revlon tweezers – epoxy curing

Those nicely aligned and ground-to-fit jaws were the reason Mary bought this thing in the first place.

The screw heads look OK, in a techie sort of way:

Revlon tweezers - fixed - front

Revlon tweezers – fixed – front

The backside won’t win any awards:

Revlon tweezers - fixed - rear

Revlon tweezers – fixed – rear

But it won’t come apart ever again!

There’s surely a Revlon warranty covering manufacturing defects, printed on the long-discarded packaging, that requires mailing the parts with the original receipt back to some random address at our own expense.

Ptui!

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FT82-43 Slit Toroid: Construction

The FT82-43 toroid slit easily enough, using the same diamond-wheel Sherline setup as for the smaller toroids:

FT82-43 toroid - slit

FT82-43 toroid – slit

I’m pretty sure that chip at 1 o’clock happened while it was clamped in the vise between two cardboard sheets, but I haven’t a clue as how it got that much force. In any event, that shouldn’t affect the results very much, right up until it snaps in two.

Although the current will come from a (rectified) 120 VAC source, the winding will support only as much voltage as comes from the IR drop and inductive reactance, which shouldn’t be more than a fraction of a volt. Nevertheless, I wound the core with transformer tape:

FT82-43 toroid - wrapped

FT82-43 toroid – wrapped

That’s 3M 4161-11 electrical tape (apparently out of production, but perhaps equivalent to 3M’s Super 10 tape) cut into half-foot lengths, slit to 100 mils, and wrapped ever so gently.

The thickest offering from the Big Box o’ Specialty Wire was 24 AWG, so that’s what I wound on it:

FT82-43 toroid - wound

FT82-43 toroid – wound

That’s 56 turns, which should convert 2.2 A into 1000 G (enough to max out the Hall effect sensor) and is more in keeping with 24 AWG wire’s 3.5 A current rating.

The insulated core requires just under 1 inch/turn, so figure the length at 56 inch. The wire tables show 26.2 Ω/1000 ft, so the DC winding resistance should be 120 mΩ. My desk meter has 0.1 Ω resolution, which is exactly the difference between shorted probes and probes across the coil: close enough.

The inductance is 170 µH, so the inductive reactance at 120 Hz  = 128 mΩ.

Now, for a bit of armor…

 

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Tour Easy Kickstand Adapter Plate

The venerable Greenfield kickstand on my Tour Easy doesn’t quite match the mounting plate under the frame, with the result that it can pivot just enough to make the bike tippy with a moderate load in the rear panniers. I’ve carried a small block to compensate for sloping ground, but I finally got around to fixing the real problem.

The solution turned out to be a spacer plate that fills the gap between the back of the kickstand casting and the transverse block brazed to the mounting plate:

Tour Easy kickstand adapter plate

Tour Easy kickstand adapter plate

That little lip is 2 mm wide, so it’s not off by much.

The aluminum came from a Z-shaped post that contributed its legs to a previous project. I flycut the stub of one leg flush with the surface, then flycut a slot 2 mm from the edge:

Tour Easy kickstand adapter - flycutting recess

Tour Easy kickstand adapter – flycutting recess

For no reason whatsoever, the width of that slot turned out exactly right.

Bandsaw along the left edge of the slot, bandsaw the plate to length, square the sides, break the edges, mark the actual location of the mounting plate hole, drill, and it’s done!

An identical Greenfield kickstand on Mary’s identical (albeit smaller) Tour Easy (the bikes have consecutive serial numbers) fits perfectly, so I think this is a classic case of tolerance mismatch.

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Installing a Smooth-head Screw

A screw (*) fastens the capacity reduction block to the magazine’s interior floor plate:

Browning Hi-Power magazine - block detail

Browning Hi-Power magazine – block detail

It started as a normal M3x0.5 socket-head cap, but I reduced the diameter and turned off the socket to fit the existing hole in the exterior floor plate:

BHP floor plate screw - disk head

BHP floor plate screw – disk head

The head was just barely too large for the largest of my pin vises. Drat!

The easiest way (for me, anyhow) to install that screw into the epoxy-loaded block started by dropping it into what seems to be a shim-punching tool:

Base screw in alignment block

Base screw in alignment block

It’s in the left hole of the top front row: talk about protective coloration, eh?

Then capture it in one of the Sherline’s drill chucks:

Base screw in Jacobs chuck

Base screw in Jacobs chuck

Which makes it trivially easy to turn right into the nut brazed to the floor plate and the epoxy inside the block. When the epoxy cures, the screw, nut, floor plate, spring, and block become one solid unit.

That punch block came with the lathe tooling, made for some special purpose long lost in history. It comes in handy all the time for other jobs, though, so I think it’s still happy.

(*) The pictures are staged recreations; I was cleaning off the bench and unearthed the spare screws.

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Samsung Vacuum Cleaner Floor Brush: Wheel Retainers

We still haven’t exhausted the never-sufficiently-to-be-damned Samsung Quiet Jet vacuum’s bag supply, so when a wheel fell off the floor brush again, I had to come up with a better fix than a twist of wire. Obviously, those delicate little retaining latches need more persuasion.

Capture the wheel in the Sherline’s 4-jaw chuck on the rotary table and drill four holes just below the end of the latches:

Samsung wheel - drilling

Samsung wheel – drilling

The wheel is 20 mm thick. The holes lie 9 mm back from the open end of the wheel or 11 mm from the closed end at the chuck face. Drill maybe 6 mm down; I did it by eye, jogging slowly downward until the tip of the drill touched the latch.

Tap the holes and install four 8-32 setscrews:

Samsung wheel - setscrews installed

Samsung wheel – setscrews installed

I don’t have a bottoming tap, but an ordinary plug tap was Good Enough; the incomplete threads should hold the setscrews in place.

Reinstall the wheel, tighten the setscrews, and wrap festive silicone tape around the whole affair:

Samsung floor brush - wheel installed

Samsung floor brush – wheel installed

I heroically resisted the temptation to pry the other wheel off for a preemptive repair …

 

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Halogen Desk Lamp: LED Floodlight Retrofit

Quite a while ago, I rebuilt a gooseneck shop lamp with an LED floodlight module, the light from which appears in many pictures of the Sherline mill. That module has a sibling that I just combined with a defunct halogen desk lamp to produce a better task light for the bench; the original 12 VAC 50 W transformer now loafs along at 4 W and ballasts the lamp base against tipping.

My initial idea, of course, was a 3D printed adapter from the existing arm hardware to the LED module, but PLA gets droopy at  normal high-intensity LED heatsink temperatures. That led to doodling a metal bracket around the LED module flange, which led to pondering how annoying that would be to make, which led to the discovery that the screws holding the LED plug to the heatsink were ordinary M2x0.4 Philips head, which suggested I could just screw a bracket to the back of the module, which brought a recently harvested aluminum heatsink to hand, which led to the discovery that the tip of the pivot screw fit perfectly between the fins, which …

Shortly thereafter, I milled off the central fins to fit the shaft of the pivot screw, introduced the heatsink to Mr. Disk Sander to bevel the bottom, sawed the threads off the pivot, press-fit the two together, drilled a 2 mm cross-hole into the pivot, buttered it all up with epoxy, jammed a short M2 screw into the cross hole, and let the whole mess cure:

Desk Lamp LED Adapter - top view

Desk Lamp LED Adapter – top view

The lamp modules were a surplus find, with one pin clipped nearly flush to the insulator. I soldered a pair of the same male pins as in the battery holders, with the matching female pins as a crude connector. The unshrunk heatstink tubing isn’t lovely, but got us to First Light:

Desk Lamp LED Adapter - front view

Desk Lamp LED Adapter – front view

The original counterweight is, of course, much too heavy for the dinky LED module, so I’ll drill the mounting hole for the vertical arm further back on the beam to get another foot of reach. That will require more wire between the transformer to the lamp, soooo the connectors might just become soldered joints.

As you can tell from the background, Mary snatched the lamp from my hands and put it to immediate use in The Quilting Room.

The original doodles bear no resemblance to the final product, but do have some key dimensions that (having discarded the unused hardware) I’ll likely never need again.

The pivot between the arm and the lamp housing, with an idea for the LED holder:

Desk Lamp Bracket Dimensions - doodle

Desk Lamp Bracket Dimensions – doodle

Details of the repurposed heatsink and the pivot bolt, with a block that never got built:

Desk Lamp Heatsink Dimensions - doodle

Desk Lamp Heatsink Dimensions – doodle

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