Posts Tagged Sherline
The original Kenmore Model 158 sewing machine used a two-wire line cord:
In light of my modifications, grounding the frame seems prudent. The heap produced a long IEC extension cord with screw-mounting ears on the socket end that fit neatly into the GX270’s rear panel area occupied by two PCI cover plates, so a bit of Quality Shop Time seemed in order.
The GX270’s carcass yielded a complex bit of sheet metal that held the hard drive and a few other odds & ends, with some clean right-angle bends in exactly the right places:
Some bandsaw work removed the gimcrackery:
More bandsawing produced a rough outline:
Sawing to length, removing the small flanges, and squaring the sides:
I traced the existing PCI cover tabs, bandsawed the outlines, and filed to suit:
They look a bit ragged, but fit well enough:
From the outside, it looks like it grew there:
The divider between the PCI slots succumbed to tin snips and a bit of filing. The tabs climbing over the bottom edge come from the internal EMI shield covering the entire back surface.
A bit of coordinate drilling and manual milling produced the IEC socket outline
Which looks pretty good from the inside:
And great from the outside, if I do say so myself:
Match-drilling a #6 clearance hole below the hole in the clamp arm, then ramming a self-tapping case screw into it, provides a convenient grounding point for the sewing machine cord:
The chassis lid has two matching holes for those screw heads, which would ordinarily hold the two PCI cards in place. I could remove the clamp arm, but it doesn’t get in the way of anything.
I loves me some Sherline mill work…
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:
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:
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:
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:
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!
Mary bought a pair of Revlon tweezers a while ago, picking a Name Brand to avoid hassles with bottom-dollar crap:
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.
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:
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:
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.
- 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
The clamping step:
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:
The backside won’t win any awards:
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.
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:
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:
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…
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:
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:
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.
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:
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:
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:
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.
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:
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:
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:
I heroically resisted the temptation to pry the other wheel off for a preemptive repair …