What I do in my spare time.

[PHXPhlyer]

I recently saw a TV clip about a program in California, I think, where they have a bunch of guys like myself in a library basement once a week and people can bring small appliances and such to be repaired. Not everything is repairable, on the spot, but a large number of items is.
I am thinking of starting something like that in my small town.

That is a great idea in principle.
The main drawbacks are:
Convincing people who have been living in a "throwaway society".
Having products to repair that have readily available and affordable spare parts.
Products that are not made to be readily repairable, ie, have to be destroyed to gain access to the parts that might be repairable/replaceable.

PP

[llondel]

I've thrown something out in the past because the cost of the spare was greater than the cost of a new appliance. That irritates me no end, but then they're made with economies of scale and often I couldn't buy the parts to make my own for less than I can buy a complete one, simply because the manufacturer gets million-item pricing and I get single item pricing.

[G-CPTN]

But your item would be bespoke.

[Hydromet]

Some days are diamonds.
I've been really busy lately - busier than I want to be - and would like to leave my woodwork as a hobby, rather than the full-time job it has become. To that end, I'd decided that I wouldn't take on any more commissions for the foreseeable future. I still have nine boxes/chests lined up to make.
This morning I had a query asking if I could make something I've never made before - traditional Japanese Ma knife handles. This is a project I'd really like, but i determined to stick to my resolution, so I replied to the enquirer, explained the situation, even offered to put him in touch with someone who could probably do the job better than me, but he really wants me to do them. He's happy to wait, with no time pressure. His hobby is cooking and he wants to put traditional handles on his kitchen knives. He will supply the wood & bone for them, which means I wouldn't have to waste time chasing around the countryside.

I think I just heard the sound of another resolution falling off a cliff.

[Dushan]

After experimenting with several thicknesses of nichrome wire I finally found the right mix of resistance/lenght to use in the toaster I was fixing for the lady down the street.
I took it to her today and she was very happy, especially about the new 3-prong plug that grounded the contraption.
We shared a toasted bagel, and all is good.

IMG_9462.JPG (83.91 KiB) Viewed 341 times

[PHXPhlyer]

After experimenting with several thicknesses of nichrome wire I finally found the right mix of resistance/lenght to use in the toaster I was fixing for the lady down the street.
I took it to her today and she was very happy, especially about the new 3-prong plug that grounded the contraption.
We shared a toasted bagel, and all is good.

IMG_9462.JPG

Well done!
I'm sure a well deserved sense of satisfaction.

PP

[Hydromet]

There a feeling of value in repairing something like that that outweighs the replacement cost you may have saved.
Back when those toasters were in vogue, you could buy replacement elements. It was commonplace to replace them if they failed. Same with electric kettles.

[Fox3WheresMyBanana]

Very nice job.
As a matter of interest, what thickness of nichrome wire did you end up using, for what volts/amps?

[Dushan]

Voltage is standard North American 120 VAC.
Based on the original size of mica plates and filament length I used 2.4 m of 32 gauge wire. It measured about 6 ohms/foot which was in line with the original.

Original filament was not a wire but a thin tape about 2 mm wide and 0.1 mm thick, but when I ordered something similar (what I thought would work) it fried the mica support plates in 10 seconds. It was waaaay too hot. I then tried 24 gauge wire which was also too hot. Then I tried 36 gauge which would not go red at all for the length I needed. It did go red hot at about half the length. That’s when I settled on 32 gauge and it worked.

There are formulas to calculate all that, but I am a practical “hands on” guy, not so much a theoretical formulas guy so I didn’t try to calculate the right answer. Amazon and its return policies is my friend…

Intuition tells me that a thinner wire would heat up faster, but in practice it turns out that the thicker the wire the hotter it runs for the same length and voltage. I am sure F3, that you have a theoretical explanation for that given your knowledge of math and physics.

Interestingly the mica plates I got, from Amazon, are sold as microwave cover plates. Not sure what they do in a microwave oven, but they come in variety of shapes and sizes.

[Fox3WheresMyBanana]

Human intuition in physics is not infrequently wrong.

Voltage is fixed, so Current is inversely proportional to Resistance. Double the resistance and the current halves.
But the power dissipated by that resistance, as heat, depends on Current squared times Resistance. Half squared is a quarter, so doubling the Resistance causes power dissipated to halve.
As the wire gets thinner, it won't heat up so much.

There's a second factor.
As the wire gets thicker (lower gauge number), the power dissipated increases, and the wire heats up. Heating a wire causes its Resistance to increase, which then lowers the power dissipated.
The rapid initial heating when the power is turned on can cause material failures, and does cause a current/power spike relative to the steady operating state that follows.
You will be familiar with this as big power users can trip circuit breakers when they are switched on.
This can be controlled by increasing the current slowly with an extra circuit component, so allowing the material temperature to rise before full current is reached.
Interestingly, I have just bought a couple of such 'soft start' devices for the big saws in my workshop, as they are very distant from my main breaker box and occasionally trip the breakers on start-up.

There's a third factor
How hot the wire gets depends on how rapidly it can dissipate the heat generated. This depends on the temperature and the surface area.
A flat tape has a bigger surface area compared to a round wire, for the same cross-sectional area. It's the cross-sectional area that is important for the gauge.
So a bigger surface area means a lower average temperature, for the same cross-sectional area.

There's a fourth factor - the material affects the resistance, and also the mechanical properties of wire. Nichrome is about the best cheap material for running hot without changing shape or rusting too much.

The mica plates are transparent to microwaves, so act as a window to allow the microwaves from the generator into the food compartment. They also have the properties of being electrical insulators, and are able to stay strong at high temperatures. They are also relatively cheap.

[Dushan]

As I was putting stuff away I realized I used 30 gauge wire, not 32 as I stated above.