The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

Toner Transfer PCBs: Alignment Accuracy
Here’s an example of the dimensional accuracy you can get from toner-transfer PCBs in real life.

I drill the holes with a CNC-ed Sherline mill, so they’re pretty much spot on. Drilling the holes by hand simply isn’t practical: there’s no way to get both global alignment and local accuracy.

The toner transfer sheet, printed on a laser printer, gets aligned to the existing holes atop a light table. The paper stretches & shrinks and moves around while printing, but I can generally average out the errors so that the 24-mil holes (the smallest I generally use) across the board have no more than a few mils of error: the pads don’t show more than that inside the drilled holes. In the picture below, you can see a dark rim around the corner alignment hole that looks worse than it really is due to the perspective.

I put the toner transfer sheet on the light table, toner-side up, lay the PCB atop the paper, and adjust for best overall alignment. I then tape them together along one edge with strips of laser-printer address labels: guaranteed to hold up to high temperatures, which is more than you can say for most tapes.

PCB alignment and taping

Here’s the board after etching both sides, with the black toner and green sealant film still in place. The toner & film are slightly smeared from the solvent I used to clean off the other side before etching it. The brownish dabs on the green areas come from a brown Sharpie that works fine as a touch-up etching resist.

WWVB Simulator – Top surface toner mask

The narrowest traces are 16 mils, most of the others are 32 mils, and the fat ones down the middle of the chip are 40 mils. Click on the images for bigger versions; you’ll get some JPG compression artifacts, but the resolution is good enough to see what’s going on.

Here’s the same area with the toner removed and a touch of silver plating applied to make it pretty and more easily solderable. The colors aren’t particularly reliable; in real life, it’s a lot more silvery.

Top surface copper

Fairly obviously, the alignment isn’t nearly as good as you’d expect from the initial taping. In round numbers, the pads to the left side seem offset by about the diameter of the holes; call it 25 mils. The holes in the DIP pads are off by perhaps 10 mils.

The bottom surface looks pretty much the same, with similar alignment issues.

Bottom surface copper

The misalignments are not uniform, as you’d expect if the toner transfer sheet moved across the board during fusing. The sheet deforms during the fusing process in a completely unpredictable way, despite my trying all of the usual tricks:
- Pre-shrinking the transfer paper by running it through the printer with a pure-white image (so no toner gets applied)
- Fusing quickly after printing to prevent moisture absorption (there’s a limit to how fast I can work)
- Taping more than one edge to lock the paper in place
It’s fair to say you (well, I) can get within 25 mils of a board hole for sure, less than that most of the time, and be spot on over much of the board. I use large pads and vias for anything I have control over, as witness the pads surrounding the DIP, and avoid very fine features near holes.

Anyhow, it’s good enough for what I do, but you shouldn’t get your hopes up that toner-transfer circuit boards come anywhere close to commercial quality. If you’re doing a lot of pure surface-mount work, it’ll probably be good enough because there’s no need for global alignment to holes in the underlying board. Obviously, the smaller the board, the better off you’ll be.

I etched this board by rubbing ferric chloride on it with a sponge (wearing forearm-length rubber gloves and a shop apron!), renewing the solution as it turned black and gooey. Works like a charm, gives good control of the process, doesn’t erode the Sharpie masking, doesn’t over-etch the traces (much, anyway), and uses less etchant than soaking the board in a bath.

I have other posts describing the process in more detail. Search for PCB, toner-transfer, and other keywords to unearth those entries.
2009-10-15
Extended Sewing Machine Quilting Surface

Extended quilting surface

Mary has been quilting up a storm lately and wanted a larger surface to handle a bed-sized quilt. A table in the basement was big enough, but she wanted a larger flat surface around the sewing machine adjacent to the table.

I converted the typing return (*) from her upstairs desk into a table, then cut a piece of aluminum-clad 1-inch foam insulation board to fit. It’s 4 feet long, a convenient length to cut from the 4×8-foot insulation board, and slightly narrower than the typing return. Cutting it required a long X-Acto knife blade, but a really sharp utility knife would work as well.

Some stainless-steel tape finished off the edges. The tape itself is lethally sharp-edged, but it’s perfectly harmless if you do a good job of smoothing it against the foam board…

A pair of closed-cell rigid foam blocks held one end of the board at the proper height around the sewing machine, while a pair of cutoffs from the wood pile were just the right thickness & length to extend under the other end. It turns out that precise height isn’t nearly as vital as we expected; close enough is fine.

I cannibalized a pair of table-saw feed roller stands for this project; they had just the right height adjustment and shape to support the typing return and the foam board.

The end result aligns the surface of the sewing machine with both the top of the table and the surface of the foam board. The quilt slides easily over the whole affair and doesn’t bunch up like it did before. Success!

Foam support blocks

(*) A “typing return” is the little table that sticks out from a desk, upon which you put a typewriter, back in the day when typewriters ruled the land. Nowadays, she uses it for her sewing machine, which normally lives at her desk, because there’s no practical way to type at right angles to one’s desk.

That’s the sort of item you can’t do web searches for, because all the terms are so heavily overloaded. Give it a try; you’ll find one or two useful hits. There’s a difference between syntax and semantics; we’re not in the semantic web yet by long yardage.

2009-10-11
Sherline Z-axis Backlash: Check the Bearing Preload Nut!

Loose bearing nut

I don’t do any fancy 3D milling, so it takes a lot of Z-axis backlash to get my attention. While setting up for some circuit-board drilling, I finally noticed that the backlash far exceeded even my slovenly specs: something like 20 mils.

The Z-axis backlash adjusting nut on the saddle was as snug as it usually is. Heaving on the saddle, though, pulled it up & down and moved the handwheel on the top of the Z-axis motor.

Ah-ha! That says the leadscrew itself is moving, which shouldn’t be possible because it’s captured at the bearings in the stepper motor mount.

Some tedious disassembly later, the top picture shows the Z-axis leadscrew and motor mount, with the nut obviously too far away from the lower ball bearing housing. The nut was finger-loose and I moved it while extracting the leadscrew; it’s supposed to be snug against the bearing in normal operation.

The solution is a drop of Loctite, which should be applied to the canonical “clean and dry” threads. Hosing this part of the leadscrew down with solvents isn’t a good idea, because you don’t want any inside the lower bearing in the motor mount, so I spent some Quality Shop Time spinning the threads against a (dry) rag, running the nut to the other end (all of a few millimeters), and repeating until most of the oil was gone.

Properly adjusted nut

Sherline documents how to assemble & install the motor mounts, so there’s not much mystery involved. I loosened the preload nut until the housing spun freely on the shaft, then tightened it a teensy bit; the housing still spun freely and there’s no detectable end play.

Reinstallation requires putting the motor mount at the same spot on the Z-axis column as before. I moved the saddle to the top of the column, ran the leadscrew into the saddle nut, and then tightened the motor mount screws. That allows the mount to move to suit the saddle nut’s position, rather than going through the tedious saddle alignment process I mentioned as part of the gib adjustment.

It’s all good… call it 3 mils of backlash on all three axes.

Memo to Self: It’s possible to run the Z-axis backlash adjusting nut off the top of the leadscrew thread, then re-engage it without removing the motor mount. The trick is to hold the anti-backlash nut firmly against the saddle nut while turning the leadscrew to engage the thread. Remember that it’s a left-hand thread…

2009-10-10
Clothes Rack Dowel Splicing
Clothes Rack Dowel Glue

Mary picked up a rather well-used wooden-dowel clothes drying rack at a tag sale for essentially nothing; one of the dowels was missing. That’s easy enough to fix, as I have a stash of dowels from what seems to be another rack of the same type on my wood stockpile…

Of course, those dowels are just an inch or two shorter than needed.

So…
- Turn down the ends of two dowels to 0.29″ x 3/4″ to fit the holes in the support struts
- Sand a small taper on the ends
- Pull the staples, insert the longer dowel and mash the staple back in place
- Eyeball the length of the other dowel, hacksaw to fit, install similarly
- Find a length of brass tubing that slips over the dowels
- Cut some heat stink shrink tubing to fit
Spliced dowels

I used urethane adhesive, because it expands as it cures and will fill the gaps inside the brass tubing. The heat stink tubing is just for nice… although it does make for a rather stunning contrast to the aged wood dowels, I’ll agree.

And it’s all good!

(Use it up, wear it out, repair it, wear it out again, then save the pieces because they’ll come in handy for something else.)
2009-10-09
Bike Lighting: Automotive Specs

Having recently taken a thorough drubbing on the ‘Bentrider forums for having a rear-facing white light on my bike, I should accelerate my plans for a red / amber taillight.

This Philips LumiLED app note gives some specs on automotive lighting. The one we bikies all tend to ignore is the surface area: greater than 37.5 square centimeters for rear combination stop-turn fixtures. Call it a scant 4 inches in diameter. You’ve never seen a bike light that large, have you?

LED combo tail stop light

Maybe the right thing to do is start with a street-legal truck light and build some electronics around it. This is a 4 inch diameter, 44 LED rear light with both taillight and brake light terminals. At 12 V, the taillight draws 10 mA and the brake light is 250 mA. Got it from Gemplers with a recent order, but they’re certainly not the optimum supplier if that’s all you’re buying.

Obviously, it’s unreasonable to run a 3 watt taillight on a bike, as the most recent crop of single-LED killer headlights are merely a watt or three. Battery life remains a problem.

At 10% duty cycle the brake LEDs would average 300 mW. That might be roughly comparable to the running lights on some cars these days.

With the taillight constantly energized and the brake flashing at 4 Hz, it’d be 120 + 0.5 * 300 = 270 mW.

That’s more reasonable. With a 50% efficient upconverter to 12 V, that’s half a watt. Start with 4 AA cells, triple the voltage, draw 100 mA, runtime is 1500 / 100 = 15 hours. Good enough.

And it ought to be attention-getting enough for anybody! The only trouble will be fitting the damn thing on the back of the bike; fortunately, ‘bents have plenty of room behind the seat, so maybe attaching it below the top seat rail will work.

Memo to Self: The rear reflector must be something like 3 inches in diameter, too. We ignore that spec, too.

2009-09-27
Why You Need a 6-Point Socket to Remove a Water Heater Anode Rod

Anode rod head with sockets

As mentioned there, removing a water heater anode rod generally requires considerable, umm, persuasion. I used a 12-point socket wrench, as I didn’t have a 1-1/16″ impact wrench on hand. Now I do…

The first pic shows the head in front of the two sockets; the 6-point socket on the right will do a much better job of not ruining the anode rod bolt head because it grips along the entire length of all six sides.

Now, in general, you don’t care about ruining the head, because the rod’s pretty much not going to be there by the time you remember to check it. What you do not want: the wrench rips the corners off the head before loosening the thread.

Goobered anode rod head

Goobered anode rod head – side view

The thread on this anode rod was in great shape (I’d wrapped it in Teflon tape the last time it was out), but it was still firmly jammed in place. These pix show what the 12-point socket did to the bolt head during the beatdown.

Bottom line: right now, while you’re thinking about it, buy yourself the nice 6-point 1-1/16-inch impact socket you’ll need to extract the anode rod from your water heater. If you don’t already have a honkin’ big breaker bar, get one of those, too; this is no job for a sissy 3/4″-drive ratchet wrench.

The real problem is holding the water heater in place while you beat on the breaker bar. I have yet to see a good solution.

Offset Tank – 2009

That husky 6-point socket isn’t going to fit into the stupidly offset hole in the top of the water heater, even after applying the nibbling tool to get the 12-point socket in place, but that’s in the nature of fine tuning…

2009-09-24
Improvised Water Heater Element Installation Wrench

Lower heating element access

Removing a water heater element is no big deal: apply the appropriate socket (1-1/2 inch for this heater) to the hex head and turn it out. The trouble comes during installation, when you must hold that long rod exactly horizontal inside the tank, gripping the electrical fittings inside a narrow access port amid all the insulation.

My fingers can’t hold the element horizontal and twist it at the same time, so I made a tool: cross-threading the heating element and goobering the threads in the tank port is not an option!

Improvised heating element installation tool

A 32 mm socket just cleared the square blue electrical insulation block and butted against the 1-1/2 inch hex head. Because the block is square and the socket is hex, it was a pretty loose fit, but this was the right general idea.

I put a layer of masking tape on the inside of the socket and covered the electrical connections on the element.

Then I mixed up a batch of Bondo auto-body repair epoxy, buttered up the end of the heating element, and gooshed it into the socket. The Bondo filled in the gaps between hex and square, turning the wrench into a custom-fit tool that firmly gripped the heating element.

Reinstalled heating element

A brief pause for Bondo curing, pop an extension into the socket to use as a handle, return to the water heater, and screw that sucker right in place. Worked like a charm!

There’s a flexible gasket sealing the element to the tank port and I gave the element a few degrees more twist when I tightened it up, so the insulation block isn’t neatly aligned.

Getting the socket off wasn’t too difficult: twist to the side, pull, and the Bondo pops off the masking tape. Peel the tape off the element and it looks pretty much like it did before. The Bondo fell out of the socket when the element came out, so that was easy enough.

I was busy getting the water heat back in action and didn’t take any detailed pix, but I think you get the idea…

2009-09-23