DIY induction heater

[Willy]

I've been toying with the idea of building a small induction heater for heat treating small parts. The feature that intrigues me the most is the fact that it heats the metal in such a uniform way, something which is almost impossible with a flame. Also since it heats the metal from the inside I would assume that the soak time could be reduced.

If anybody has any plans or experience with this, I could use a little advise and education on the subject. I'm not really sure if it warrants the trouble to undertake such a project.

[Mcgyver]

seach cnczone. there's some stuff there, one of the guys was trying to build an induction furnace for melting metals....I don't think he got there but there was some good content along the way

[Carld]

You may have to turn off everything in your house an shop to power it.

[jdunmyer]

I've used a small ceramic kiln for heat-treating. Got a digital temperature controller off eBay, so I have precise temperature control.

Slow, but seemed to work OK. The real slowness comes when cooling it down for tempering, after doing the initial hardening.

[darryl]

You're not supposed to put metal objects into a microwave oven because they get hot- hmm, induction heater.

[Stepside]

What kind of power?

The Induction melt furnace I was working with had a huge power supply. It ran at 2000 Hertz and enough amperage that the power cables to the furnace were fluid cooled. It was fun though, It would just sit there and hum for awhile then the metal would smoke a bit and then glow red hot. This was big enough that the crucible was about 18 inches in diameter and 24 inches tall. I think I will see what the intranet has for infomation on heat treating by induction.

[darryl]

That sounds like a serious amount of power. I wonder if the water cooling was mostly for the sake of the induction coil itself?

[snowman]

You can do it.

Do a search for electric gas melting furnaces. Lots of guys have done it.

You are much better off doing a simple resistance kiln though.

[rantbot]

A small induction heater doesn't suck up any more power than a small oven. But it also doesn't penetrate metal very far. Induction hardening is much like flame hardening, and if done right allows you to heat-treat the surface only, giving an effect like case hardening - a bit thicker than case hardening, but the same idea, hard on the outside, with a soft chewy center. The big advantage of induction over flame is of course that you don't have an open fire out on the production floor.

[oldtiffie]

That's so

A small induction heater doesn't suck up any more power than a small oven. But it also doesn't penetrate metal very far. Induction hardening is much like flame hardening, and if done right allows you to heat-treat the surface only, giving an effect like case hardening - a bit thicker than case hardening, but the same idea, hard on the outside, with a soft chewy center. The big advantage of induction over flame is of course that you don't have an open fire out on the production floor.

Thanks rb.

Over 50 years ago "induction-hardened and ground" lathe-beds - particularly for heavy or "production" machines (including profile, turret and capstan lathes) was "all the go".

They were nice machines that had the ground surface finish on the lathe bed (and carriage? Not sure about that) with the compromise between oil retention and friction "just right".

IIRC correctly, the depth of the (case?) hardening was not all that deep - didn't need to be.

[J Tiers]

The feature that intrigues me the most is the fact that it heats the metal in such a uniform way, something which is almost impossible with a flame. Also since it heats the metal from the inside I would assume that the soak time could be reduced.

If anybody has any plans or experience with this, I could use a little advise and education on the subject. I'm not really sure if it warrants the trouble to undertake such a project.

That isn't quite so for the hardening type heaters. More so with certain type furnaces.

The heating is at the surface, with a variable amount of penetration. The penetration is dependent on the induction power frequency. The induction hardening systems use that to set the depth of hardening, by varying the frequency.

Depth is due to the 'skin effect", since the externally applied field is attenuated inside any conductor, to a degree depending on the resistivity of the material. iron isn't a great conductor, so penetration is deeper than copper or silver, for instance.

Takes a considerable amount of power to do that.

To make a furnace, the system is different. The material to be melted can be in a pot, for a system more similar to the hardening system, or in a ring forming a shorted turn on a transformer type core.

The latter is a resistance system, and often needs to be "primed" with some molten iron to serve as the initial conductor.

[Willy]

J Tiers, thanks for the excellent info, that's the kind of information that I was looking for.
The application that I had in mind was for a uniformly applied heat source of quick heat to pieces no more than 3/4" in diameter and no more than 3" in length.
Is this feasible or practical while utilizing a standard 15 amp circuit to supply power to the HF power supply?
As you can see I haven't done a lot of research on this yet...just putting out some feelers to those with more experience than I.
Bumped into this video which has been...rightly or wrongly...a source of inspiration.

[TGTool]

One of the most interesting applications I've seen was at a Link Belt plant where they were hardening the splined end of a shaft. These were stuck, hanging end-on to magnets on a conveyor with a coil traveling along with them heating them. The rate and speed was calculated so that as they reached transformation temperature they were hanging over the quench tank. Also, if you remember, steel is no longer magnetic at that temperature so they no longer stuck to the magnet and dropped straight down into the quench. My hat's off to the engineer that thought that one up.

[2ManyHobbies]

If you do anything more than harden small chisels or hobby gears, the issue will be the required power feed. The required frequency is inversely proportional to the cross section of the material to be heated. For example, if you wanted to slag powdered iron, you need a really high frequency, and you could probably heat a piece of 3" bar by wrapping a bunch of 6ga wire around it and plugging it into your favorite 50A outlet (though you might want to include a way to limit current -- and probably an off timer or thermal cutout).

http://www.richieburnett.co.uk/indheat.html has a lot of useful info, and probably the best explanation of how it all happens. Basically you want to build a resonant circuit (coil, material to be heated, and a capacitor) and a pump circuit that will push smaller but steady amounts of energy into the resonant circuit at the right time. Everything should work like a swing set or a bell with a harmonic "push". Be very careful if you do build something more than just a curiosity, you could easily be sliding around a few hundred amps at a few kV at any given time.

If you build something that clears 500W or so, you are really going to want to know about power factor correction, otherwise most of your energy will go into causing brownouts or other interesting phenomena on the local grid. Basically, you have to not try and push or pull when that 60 Hz input is near zero voltage. If you have access to 3-phase, this is easier to deal with, if not, it is a matter of playing with small inductors a current sensor or two and a feedback loop (they make chips that handle most of the thinking there...)

Most people use flexible copper tubing for the output coil because it works well with the skin effect at 1kHz+ frequencies, and it is relatively easy to cool (resistance increases with temperature, so a cool coil can carry higher currents than a hot one).

I've also heard it said that designing an induction heater is much like designing a Tesla coil -- just replace the secondary with a bucket of metal the needs melting.

Please be very careful if you attempt to build something like this on your own, and do not take the energies involved for granted. Letting the magic smoke out can be humorous -- but only if you are still around to laugh about it later. Keep electrical rated extinguishers around and have a kill switch that is easy to hit a reasonable distance away from any experiments. Remember with induced currents that ground might not be where or what you expect it to be, and if you happen to short a turn or two on your induction coil with a piece of metal, it will be more exciting than dropping a wrench across a set of golf cart batteries.

A 50A 220V outlet can source 11kW, or ~15hp. Or in other words it should be able to send 1 oz of iron clean past forging temperature in about a second and a half without tripping the breaker.

[Willy]

2ManyHobbies, thanks for the reply and info, it's given me a lot to think about and consider. The link you included is a wealth of information, I've been studying it for the last several days. Much to take into consideration here. Not quiet the pie in the sky I was hoping for. But still doable.

I guess if it was easy...everybody would be doing it.
The gas furnace is starting to look more practical again.
Although I am still very intrigued with the induction heat concept.