Archive for February 7th, 2010

Farberware Blender Switch Repair

I use a blender to mix up the pancake batter every few days. Over the last week or so, the rotary switch Pulse position wasn’t returning to Off all by itself. After having replaced the impeller bearings, I couldn’t just ditch the mumble thing without at least trying to fix it…

A search for replacement parts reveals that Farberware kitchen appliances are disposable crap: they’re so cheap nobody stocks repair parts. IIRC, this blender was maybe ten or twenty bucks after rebate, which gets you through the shipping charge for the repair part. I would love to believe that paying more for kitchen appliances actually bought better quality.

Switch wire connections

Switch wire connections

As you’d expect, the four silicone rubber feet pop off to reveal machine screws that hold the plastic base to the metal body. This picture shows the wire connections to the switch:

  • L = brown
  • 1 = orange
  • 2 = no connection
  • 3 = red

I couldn’t pull the switch knob off the shaft, so I dismantled enough of the motor mount to ease it to one side, apply a right-angle screwdriver to the switch body screws, and loosen the switch. That gave me enough room to jam a screwdriver between the switch and the mounting bracket to pry the knob off. It’s a plastic-on-plastic friction fit.

After the fact, it turns out that two screws behind the knob secure the mounting bracket to the bezel. Remove those screws, the bracket comes off, and it’s trivially easy to remove the switch screws.

The wires attach through those horrible spring-loaded push-and-pray connections: jam the wires in, pull back, and it’s supposed to be a gas-tight joint forever. I don’t believe a word of it. Remove the wires by poking a small screwdriver into the opening and forcing the brass tab away from the wire. Yuch!

Opening switch with slitting saw

Opening switch with slitting saw

The switch body parts are, of course, bonded firmly together: no user serviceable parts inside. I deployed a slitting saw on the Sherline mill, grabbed the switch in the vise, and sliced 2.5 mm deep along the line between the two body parts.

The switch is some sort of engineering plastic, so I ran the saw at about 2000 rpm, cut at 100 mm/min, and dribbled water on the blade to keep it cool. You can see the grayish-brown residue under the switch.

The thing came apart easily enough after that…

Switch Guts

Switch Guts

These pics show the switch components. Note how the spring fits in the body and the four cunningly folded brass strips that simultaneously attach the wires, make the switch contacts, and spring-load the rotary detents.

I took the liberty of bending the strips to restore the clamping force on the wires; poking the tabs with a screwdriver tends to bend them a bit.

So it goes.

There wasn’t anything obviously wrong inside, but after a bit of puzzling, I discovered the problem residing in the coil spring that returns the switch to Off

Cracked spring

Cracked spring

The spring wire is 1 mm diameter. A bit of rummaging in Small Spring Box Number Two disgorged a bag of spring-clip thingies with the proper wire size and just about the right coil diameter, too.

The right way to make a spring is to start with straight music wire, anneal it, make a mandrel, bend up a spring, then heat-treat the spring to make it just the right hardness and toughness for the job.

Spring iterations

Spring iterations

I deployed my wire-bending pliers, made a few trial runs (well, OK, they weren’t trial runs when I started…), and got close enough by the third attempt (lower right).

Yup, cold-bending spring steel. It is to shudder, huh?

I bent the wire just off straight and worked my way around the coil about 0.5 mm per bend to produce a rather lumpy coil spring. This is definitely the wrong way to go, because the wire’s much too hard for that treatment: it wants to stay straight and doesn’t like those right-angle bends to form the end tabs. I think this will work well enough for long enough, though.

The spring’s chirality turns out to be important; the coil wants to tighten around the shaft when the knob’s in the Pulse position. The spring-clip thing has two ends; only one produces the correct result, which is perfectly obvious in retrospect.

Spring on switch rotor

Spring on switch rotor

The spring fits on the rotor like this, but with a whole lot more preload tension than you’d expect. The end result was a somewhat smaller coil diameter than I started with; I shrank the coil, re-bent a new tab on one end, chopped off about 4 mm of wire, and it was all good.

I also backed off the ramp on the notches that engage the brass contacts in the Pulse position so the switch wasn’t so prone to hang up. That was what motivated me to fix the thing: one morning I manged to leave the switch in Pulse because it didn’t quite snap back to Off, took the lid off the bowl, and the blender started up again. Fortunately, the batter is too thick to jump out of the bowl, but it was a near thing.

Here are the four switch positions and their contacts, in order from Pulse (most counterclockwise) to Speed 2 (most clockwise). You could, I suppose, conjure up a replacement switch if you puzzled out the connections; all the rotor tabs are connected together.

Switch contacts - Pulse

Switch contacts - Pulse

Switch contacts - Off

Switch contacts - Off

Notice that, although switch contact 2 is unused, it is connected when the switch is in the Off position.

Switch contacts - Speed 1

Switch contacts - Speed 1

Switch contacts - Speed 2

Switch contacts - Speed 2

The back of the switch body takes pressure on the switch knob, as well as engaging the end of the rotor to hold it in the middle of the body. I wasn’t comfortable just gluing the body together again, because I suspect none of my adhesives will actually bond to the plastic.

So I chopped off a length of aluminum U-channel, poked two holes it in, shortened a pair of salvaged screws, and made a clamp for the switch body’s back. The body has three locating pins, so the two parts aren’t shifting with respect to each other, and the clamp holds the back firmly in position.

Repaired switch with back clamp

Repaired switch with back clamp

Reassembly is in reverse order, paying a bit of attention to securing the wires in those crappy push-and-pray contacts and keeping everything away from the cooling fan as the bottom snaps into place.

Done!

The economics of this sort of repair make absolutely no sense at all, but I hate throwing stuff away just because some cheap part failed. In this case, I’d be happy to replace the switch… let me know where you can find one with the requisite contacts and spring arrangement!

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