Drilling through a magnet?

[DonRotolo]

OK, since I didn't see this yet, I'll state the obvious: The magnet material is harder than the drill material.

The magnet is most likely Ferrite. Google it for it's hardness, and compare that to High Speed Steel, which is likely what the drill bits were made of. Hint: Like using butter to drill through glass.

But you knew that.

A "glass" or Masonry bit might get a hole in it, but in all likelihood the magnet will shatter instead. It is very brittle. If you could just get a BB to move at 1/100 the speed of light it'll make a nice hole for you.

I, for one, am very pleased that you took the radio apart. Find more stuff to take apart, and let your imagination do stuff. (Just be careful, there's toxic materials in most things. And TV tubes are not to be trifled with, you can kill yourself with one. I almost did.)

[Al Skierkiewicz]

Like Don, I am sorry I didn't see this earlier. Speaker magnets can be made out of any of a number of materials. If it looks like steel, i.e. has a nice shiny surface, it is likely a steel alloy using a fair amount of nickel, cobalt, iron or some weird stuff like neodymium, strontium or Alnico. All of these materials are super hard and heat treated during manufacture so that they retain the magnetic field they are required to have. Ceramic magnets are also popular for smaller speakers but are not shiny and usually have a soft texture. More often, ceramic is used in motors with several poles or for straight DC motors like the CIM.
In all cases, drilling of a completed magnet usually results in chipping and cracking. The heat developed and the new hole in the magnet structure will reduce the magnetic field to some extent depending on a variety of factors.
In speakers, remember that the maximum magnetic flux is designed to pass through the gap between the center pole and the support structure. The magnetic field returns to the opposite magnetic pole through that structure. This intense field is what the speaker coil must cut through to become essentially a motor.
Some speakers are designed with very long voice coils so that only a portion of the coil is in the field at any given moment. Other are designed with long magnetic structures so that the entire coil will never leave the field. To produce the least distortion, the coil must ride within a field that has no variation.
If you had taken apart a really old radio, ca. 1930's, you would have found no magnet in the speaker. In that era before rare earth and super magnetizing machines, manufactures performed a little electronic black magic. Since they needed an inductor to smooth power supply ripple, they simply wound the inductor around a piece of nickel/iron and used it for a speaker magnet. This was the high voltage supply in the radio, usually 300 volts or more. And yes, I tried replacing a speaker of that type that had become damaged with a magnetic one, only to find out the radio didn't work at all. I learned about LC low pass filters, and speaker design all in one lesson.
I like taking things apart (it is what I am paid to do) and that is where I have learned some of the most valuable lessons. I also tried drilling a magnet once, an Alnico. That's all it took for me, just the once.

Oh and Andrew who you calling old?!?!

[Jeff Pahl]

We do this at work, it's not that hard if you have a significant investment in the right equipment....

The right equipment being diamond tooling for processing ceramics.....

As someone has noted, the magnet from a radio is most likely a ferrite type material (blackish), which is a high temp fired ceramic. The stuff is hard.

Also, you need to use liquid cooling, magnets do not like to be heated up. They tend to become ceramic paperweights. Google "Curie Temperature".

[ebarker]

1st - what is your definition of "old radio" ?

I am curious about the 'old' definition especially for us 'old' radio types like Don and Al and myself.

2nd - a comment. There is potentially a LOT to be learned by your exercise. First of all the history of magenetic devices in radio speakers. And secondly a lot about properties of materials.

A lot of people don't know it but there are engineering degrees in Material Science Engineering. Those folks spend all their time on making sure engineers have access to "the right stuff".

I think I saw on 60 minutes or somewhere that in some countries in Europe a part of engineering education involves spending time in a lab with a variety of different metals and substances. Basically they file and file and file and cut and cut and drill and drill and cut and file and file. All by hand. It helps the student really get a hands on feel for material properties.

[Tom Line]

Originally Posted by ebarker

Basically they file and file and file and cut and cut and drill and drill and cut and file and file. All by hand. It helps the student really get a hands on feel for material properties.

That sounds remarkably like our 1718 mechanical team. Did they slip a camera crew in when I wasn't paying attention?

[Dick Linn]

If the magnet is shiny like so many in the inexpensive speakers, it is probably Alnico. That's Aluminum-Nickel-Cobalt-Iron. It's hard, somewhere around 45-50 on the Rockwell C scale. Some hard objects are machined, but the cutting geometry of a drill bit isn't suited for it.

[nighterfighter]

The magnet is very hard,and black.

For the age of the radio- We aren't sure, it was lying in a pile of old stuff. I couldn't give you any estimation, other than 10+ years? :/

And the magnet- I'm at school now (my lunch hour) and have made some progress on the magnet. I used a 1/2 bit again, and have managed to make a small indentation. I didn't want to go to much, because the magnet was getting hot (still has it magnetism though!) so I am letting it cool down.

Thanks guys!

[EricH]

Originally Posted by nighterfighter

I used a 1/2 bit again, and have managed to make a small indentation. I didn't want to go to much, because the magnet was getting hot (still has it magnetism though!) so I am letting it cool down.

A word of caution: use cooling oil or cutting fluid, if you aren't already. You're heating and cooling the metal; this can harden it further, making it more difficult to cut. I've seen even a heat-treated sprocket take a lot of time and cutting fluid to drill a hole in it. 3 holes were needed in that one, too, and it's softer than that magnet likely is.

Take your time, use lots of cutting fluid (or cooling oil), and if you aren't sure you can keep going, stop.

And think of how much extra you're learning about magnets...

[nighterfighter]

Thanks for the advice!

Apparently, the mill we are getting (On Monday!!!) will have some special bits (I think diamond tipped) so I am going to wait and use that instead.

Never used a mill before, but one of our mentors has, so I will learn to use a mill WHILE brutally learning about magnets! Haha.

[JesseK]

Interesting, and relevant!

A Blueprint for a Quantum Propulsion Machine
Push on the electromagnetic fields in the quantum vacuum and you should get an equal and opposite force.

[mikelowry]

A Blueprint for a Quantum Propulsion Machine
Push on the electromagnetic fields in the quantum vacuum and you should get an equal and opposite force.

That looks really cool. I like the question at the end:

The question is: who has the balls to try it?

[Dick Linn]

The milling cutters you get with your mill may be quite expensive. Someone may get upset if you ruin one trying to bull on through a hard magnet just to see what happens.

I wouldn't worry so much about the color, as the magnet might not have been ground after sintering. Looks like many of the Samarium Cobalt magnets are black, and still extremely hard.

Grinding or machining anything containing Iron with a diamond isn't a great idea. Iron has a strong affinity for carbon, which it will gather from the diamond. Grinding steel on a diamond wheel is a good way to ruin it.

[Molten]

I just want to add a point about possibly using a plasma cutter. If you get the magnet too hot it will start to lose its magnetism. Think of it as being lots of miniature magnets all facing one direction, but when you heat it up, the particles shift and they won't all be facing the same direction. They will be slightly off parallel and the overall field will be weaker.

I know this doesn't help with cutting. I have no experience with hardness and such, I can just tell you heat is bad for a magnet.

[Rion Atkinson]

Originally Posted by Molten

I know this doesn't help with cutting. I have no experience with hardness and such, I can just tell you heat is bad for a magnet.

Better to know what is bad than nothing at all.

I did indeed ask my physics teacher today what would happen if you guys do indeed get the hole cut. I would tell you what she said but I'll let you figure out for yourselves. Everything is more fun that way.