Category: Machine Shop

Mechanical widgetry

Craftsman Hedge Trimmer: Switch Repair
Original Switch

So Mary was going to apply the long-disused Sears Craftsman electric hedge trimmer to the decorative grasses she’d planted on either side of the (equally disused) front entry, but when I deployed the thing it didn’t run. A quick walk through the debugging tree: GFI green, extension cord OK, so it must be the trimmer.

Off to the Basement Laboratory Repair Wing…

Two tricks to getting it apart, after removing all the obvious screws:
- The handle comes out of the sockets after great persuasion
- Remove two of the three hex-head-with-lockwasher screws on the bottom and the case pops apart. The third screw holds the motor plate into that half of the case.
The switch is, of course, not intended to be repairable, but that’s something of a motivator around here. It uses those awful poke-and-pray spring clamps, which you could, in principle, release with a small screwdriver, but I cut the wires on the motor side of the switch, leaving plenty of room to graft connectors onto them.

Next time, I’ll be able to release the wires more easily.

Congealed grease on switch contacts

A rivet holds the switch together, but attacking it with a drill removed enough of the head that I could whack the rest of the body out with a drift punch. A 2-56 machine screw fits neatly into the hole and there’s enough clearance on both sides for the screw head and a nut; hack the screw to length with a Dremel abrasive cutoff wheel.

Notice that the switch trigger button visible from outside the case acts on a push rod that slides the movable contacts (in the top part in the picture) back-and-forth atop the copper contacts (with the wires). A pair of springs loads the movable contacts against the copper strips.

The problem turned out to be, as expected, congealed grease inside the switch. The black gunk on the right halves of the copper contacts was essentially solid; you can see that it formed a nice insulating layer. I cleaned that out, polished up the moving contacts, reassembled it, and … the switch still didn’t work.

At least I discovered that with an ohmmeter, before reassembling the entire trimmer!

Switch contact slider

The movable switch contacts have a small ramp, just about in the middle, that rides up on the black hump between the copper strips when the trigger button is released. That mechanically breaks the connection, but also allowed the grease to congeal in the air gap. The grease also formed a lump that prevented the movable contacts from pressing firmly against the copper strips, despite the springs.

I gnawed out that crud with a small screwdriver, dabbed on more contact oxidation prevention grease, buttoned it up again, and now the switch works perfectly again.

New switch wires

I spliced in somewhat longer lengths of hookup wire with butt-splice connectors I’ve had for years and it’s all good.

The post with the screw hole just below the wires matches another in the opposite half of the case; the post actually fits inside the ring you see here, so it doesn’t crunch the wire. However, the wire must be pushed in far enough to avoid interfering with the switch action rod.

Trimmer assembly is in reverse order …
2010-04-08

Making a Clock Colon: Post Milling

I used a pair of blue LEDs for the colon in the Totally Featureless Clock. Each one has a brass tube to define the dot and a white plastic diffuser to eliminate hotspots.

Some rummaging in the brass cutoff assortment produced a pair of tubes with a 0.300 inch ID that closely matched the width of the LED segment bars. The catch is that I don’t have a core drill that spits out 0.300 inch slugs…

So I taped a chunk of translucent acrylic to some plywood scrap and milled the dots. Helix milling on the lesser of a 4% slope or 1/5 of the cutter diameter, 15 inches/min, no cooling, maybe 1500 rpm.

The resulting disks were snug slip fits into the tubes, although I added a dot of cyanoacrylate to ensure they didn’t get any ideas about perpetrating an escape.

It took two disks to remove all the hotspots, which reduced the light intensity to the point where I had to increase the LED current, which really heated up the linear regulator driving the dots. Fooey! In retrospect, I think frosting the LED lens would eliminate the need for a second diffuser without decreasing the intensity much at all.

The code is available as an OpenOffice file there, too.

(Post milling)
(Ed Nisley KE4ZNU - Feb 2010)
(Origin = center of post at surface)
(Double-stick tape holding acrylic sheet to sacrificial plate)

(-- Dimensions)

#<_PostDia>	= 0.300				(post OD)
#<_PostRad>	= [#<_PostDia> / 2]

#<_Thickness>	= 0.120			(sheet thickness)

#<_MillDia>	= 0.250				(cutter diameter)
#<_MillSpeed>	= 15				(cutting speed)

#<_MaxCutDepth>	= [#<_MillDia> / 5]	(max cutting depth)
#<_MaxCutSlope>	= 0.04			(max cutting slope)

#<_TraverseZ>	= 0.300				(safe travel height)
#<_TraverseSpeed> = 25			(safe traverse speed)

G20					(inches!)

(-- Figure cut depth per helix pass)

#<_PassCut> = [#<_MaxCutSlope> * 3.142 * [#<_PostDia> + #<_MillDia>]]		(limit max cut for each pass)

O9000 IF [#<_PassCut> GT #<_MaxCutDepth>]
#<_PassCut> = #<_MaxCutDepth>		(limit max cut for each pass)
O9000 ENDIF

(-- Set up cutter comp)

G0 Z#<_TraverseZ>

G0 X[0 - 3 * #<_PostRad>] Y0		(get to entry point)

G42.1 D#<_MillDia>
G2 X[0 - #<_PostRad>] I#<_PostRad> F#<_TraverseSpeed>

(-- cut down through sheet)

#<CurrentZ> = 0.0

G0 Z#<CurrentZ>

F#<_MillSpeed>

O1000 DO

#<NextZ> = [#<CurrentZ> - #<_PassCut>]	(figure ending level)

G3 I#<_PostRad> Z#<NextZ>		(once around)

#<CurrentZ> = #<NextZ>

O1000 WHILE [#<CurrentZ> GT [0 - #<_Thickness>]]

G3 I#<_PostRad>					(clear final ramp)

G40			(comp off)		

G0 Z#<_TraverseZ>
G0 X0 Y0

M2

2010-04-05

Tea Ball Revival
Defunct tea-ball rivet

The latch closing my tea ball consists of a nice stainless steel dingus held on by a grotty rivet of unknown provenance that I’ve repeatedly staked over the years. It finally came undone this morning, so I had a few minutes of Quality Shop Time right after breakfast.

My tiny-screw box (left over from the long-gone Leichtung Workshops) has some stainless 0-80 screws that I found somewhere, but only brass nuts. Ah, well, we used to use brass water fixtures and lead pipe, so an 0-80 nut in hot water isn’t going to kill me.

The ball rim has a recess for the rivet head, but the screw head was slightly larger. I braced the rim of the ball across the vise jaws and give the recess a few shots with a fat punch to enlarge it.

Stainless screw and brass nut

Then…
- A dot of Loctite on the threads
- Assemble everything
- Take it apart to put the latch on the correct side of the rim
- Reassemble
- Attempt to close
- Gently bend the rim to flatten it out
- Close
- Attempt to latch
- Brace closed rim on vise opening with screw head up
- A few shots with a drift punch to settle recess around screw head
- Success!
It seems I ain’t worth a damn in the morning without a hot cuppa. The rituals must be preserved.

I tossed the ball in the dishwasher and opted for a tea bag today…
2010-04-04
Replacement NP-FS11 Li-Ion Battery Pack: Plan B

Slitting the case

Just for curiosity’s sake, I applied a slitting saw to the oldest defunct generic NP-FS11 battery pack, cutting carefully along the bonded joint between the two parts.

No coolant, 1000 rpm, 200 mm/min, the saw is 22 mm diameter. Much slower than you’d use if you were in production, but I’m not.

First cut all the way around at 0.5 mm inside the case, then another pass at 1.0 mm. The second cut went ting as it passed the tabs at the base of the cells, so I knew the halves were released.

Inside we find a pair of 14430 Li-Ion cells, wired in parallel, with a little protection circuit board just jam-packed with teeny parts. One may reasonably assume the circuit controls over-charge and over-discharge, as well as current limiting.

Pack opened

So a reasonable (or, perhaps, amusing) thing to do would be to buy raw cells from a nominally reputable supplier, do a heart transplant, and see if that improves the situation.

Protection Circuit – Outboard

Photos of the protection PCB, showing the cell connections. Positive end of the cells is toward the PCB. I think there’s enough clearance in the camera’s battery compartment to allow a wrap of tape around the case in lieu of re-bonding the plastic together.

Protection circuit – inboard

2010-04-02
Tailstock Center: Laser Alignment Thereof

Laser spot on tailstock center

I finally made a test bar to line up the (vertically mounted) rotary table and tailstock on the Sherline milling machine. It’s a ground-and-polished 0.500-inch rod from a defunct HP2000C inkjet printer; the print head zipped back and forth along the rod while printing, so you know it’s pretty smooth. You could probably salvage something similar from any dead inkjet printer.

Making the bar is simple: saw off a suitable length, stick it in the lathe, face off the end, chamfer the edge, poke a center drill into it, and it’s all good.

If you’re a tool-and-die jig-boring high-precision kind of machinist, you better stop reading right about now before you catch a heart attack.

Lining the bar up is almost trivially easy with a laser spot coming down the spindle bore. Move the table so the spot grazes the side of the bar and casts a shadow on the table, jog X to the other end of the bar, and tweak the angle for the same picture on the table.

Repeat until satisfied.

The trouble comes at the tailstock end, where the ram extends about 1.5 inches, tops. That’s good enough for the Sherline, but it also means the test bar must be pretty close to the length of whatever you’ll be machining, rather than as long as possible to get the best alignment.

However, after you get Sherline tailstock aligned to the end of the bar, vertically, horizontally, and angularly, the magic happens

The ram is quite stable, with very little radial play, so the point moves along the X axis (assuming you did a good job aligning the tailstock). Retract the ram a bit, jog X and Y to put the laser spot on the tip of the center (which should correspond to the Y axis coordinate of the center of the bar), and you’ll see a defocused spot on the table (I put a white card on the table to improve the contrast). Jog Z until there’s a nice triangular image of the dead center’s point in that bright round spot.

It turns out that the laser beam in the top picture is about 10 mils wide at the dead center axis, so you can easily see a difference of 1 mil in the Y coordinate. That’s perfectly accurate for the sort of work I do.

Now, remove the test bar, unclamp the tailstock, move it to wherever you need it for the actual thing-to-be-made, snug it down, and jog the table in X (only!) to move the spot over there, too. Move the tailstock around to align the image of the center point in the middle of the laser spot again and you can be sure it’s aligned to the same Y coordinate. Verify that the tailstock has the same angular alignment. Mine is consistent with the T-nuts pressed against the front of the table slots and it’s easy to slide it carefully along the Y axis to get the point in the spot.

Because the bar was parallel to the X axis to start with, the point is now aligned with the axis of the rotary table.

Laser spot focused on tip

The minimum spot size depends on the beam width and the lens, but it turns out that for my setup, twiddling the Z position of the lens can shrink the spot down to essentially the width of the dead center point. As nearly as I can tell, the beam width is 3 mils and the point pretty much occludes the beam when it’s properly aligned.

The picture shows that situation; the spot is half-occluded because the point now looks like the side of a barn. It’s difficult to tell, but the lens (on the brass snout in the endmill holder) is lower in this picture.

All that jogging, particularly creeping up on the proper alignment, goes much easier with a joggy thing!

2010-03-29
Bicycle Reflector Adaptor Bushing

Reflector on bushing

After replacing the seat strut screws, I found a Round Tuit lying there on the workbench, right next to the rear reflectors I’ve been meaning to install for a truly embarrassing period.

Recumbents don’t have the usual assortment of standard-sized tubing in the usual road-bike places, making common items like reflectors difficult to attach. The ideal spot on our bikes is at the base of the VHF/UHF antennas, right next to the white blinky LEDs, but, alas, that’s 20 mm in diameter and the reflector clamp barely shrinks down to a bit under 28.

Turns out that a chunk of 1.5 inch PVC pipe has a 4 mm wall thickness, so wrapping a layer of that around the antenna base will do the trick. I whacked off a length of pipe, faced off both ends in the lathe, and put a shallow recess around the middle of the ring to capture the reflector clamp.

By another rare coincidence, 1.5 inch PVC pipe has an ID of exactly 40 mm… so cutting the ring exactly along a diameter produces the right length. The catch is that the pipe isn’t flexible at all, but brandishing a heat gun in a threatening manner solves that problem.

Reshaped bushing on mandrel

A random hunk of 3/4-inch aluminum rod is about 19 mm in diameter, so I chucked that in the lathe and shaped the saggy strip around it… wearing thick leather gloves.

It springs out to 20 mm with no problem, slides right on, and grips reasonably well. I may add a strip of tapeless sticky (think double-sided tape without the tape: just the adhesive!) under the bushing if it wants to walk away.

I made two of ’em, of course, and put a reflector on Mary’s bike while I was at it. Our young lady’s bike already has a reflector, although I should upgrade that bushing as well… it’s a layer of self-vulcanizing rubber tape that works perfectly, so this may take a while.

I suppose I should buy a length of gray or black PVC pipe, but that’s in the nature of fine tuning.

2010-03-28
Fractured Tour Easy Seat Strut Screw

Broken bolt

Straight up: this is about a stainless steel socket head cap screw I installed eight years ago, not the original Easy Racers screw, so this is not their problem.

I rode out for milk-and-eggs at the corner store, a flat one-mile ride, and stopped at the traffic signal. Light goes green, line of cars accelerates, so do I… and there’s a snap and the left side of the seat sags backwards. I am not a powerhouse rider and it’s March, so I’m not doing leg presses while getting up to cruising speed.

I continued the mission by sitting slightly to the right on the seat and pedaling gingerly, then diagnosed the problem in the corner store’s parking lot. If I’d been further away, I’d have done the repair right there, but I figured it’d hold together until I got home. It did.

The problem turned out to be a broken screw holding the left-side seat strut to the threaded eyelet on the rear dropout. The top picture shows the way I have it set up: seat strut clamp outboard, rack strut inboard, with a socket head cap screw extending all the way through, and secured with a pair of stainless nuts that went missing along with the broken screw end.

$Screw fracture closeup$
Screw fracture closeup

Here’s the fracture across the end of the screw, which shows no evidence of foul play. As nearly as I can tell, the whole thing snapped off in one event, with none of the crud that would indicate a progressive crack. Compared with that wheel stud, this is in pristine condition.

So it’s time to replace the right-side screw, as well, which means a trip to the Bike Repair Wing of the Basement Laboratory. While I had the bike up in the repair stand, I decided to reshape the head on the right-side screw for better chain clearance.

As nearly as I can tell, the usual practice puts both the seat strut and the rack strut outboard of the threaded eyelet on the dropout, but that seems wrong to me. The seat strut puts a tremendous amount of stress on the screw, so you really want that lever arm as short as possible: put the clamp against the eyelet. While the rack isn’t as heavily loaded, cantilevering it outboard of the clamp just doesn’t look right.

But putting the rack strut inboard of the eyelet means the screw head sticks out rather more than I’d like. Very rarely, the chain will snick against the head and even more rarely it jams between the head and the freewheel. Nothing much happens (it’s a freewheel, after all), but I think reducing the head thickness ought to help.

Reshaped socket head cap screw

So I chucked the screw in the lathe, shortened the socket by about half, and put a taper on the head. If I had a stock of round-head cap screws, one of those would be even better.

The shortened socket makes it a bit tricky to get enough bite with the hex key, but this isn’t something that requires much attention after it’s installed… and I get to do all that in the shop.

Dabs of Loctite in the eyelet and nuts, for sure!

By a truly rare coincidence, a standard 1-1/2 inch cap screw is exactly the right length.

Right-side mount

Here’s a view of the installed right-side screw, looking rearward along the upper rear triangle tube. Seat strut to the outside, rack strut to the inside, and reshaped head above the cluster.

Took the bike out for a 16 mile spin today and it’s all good.

A note for the weight weenies in the crowd: a rack on the back of the seat adds a redundant support structure. Without that, a failed seat strut can be a real showstopper. Even if you don’t use your bike as a pack mule, maybe you should add a rack.

Memo to Self: add more nuts to the tool kit!

2010-03-27