Turning Ferrite

There is some useful [but not very encouraging] info. here

MichaelG.

I had a go at turning a couple of small(about 10mm) neodymium magnets down a few years ago to see what was inside the casing and they both burst into flames via some sort of spontaneous combustion

A puff of smoke and the insides were carbonised

Don't know how much was ferrite but it appeared to be a powder which was very heat sensitive on even a slow lathe

Edited By Ady1 on 27/12/2013 09:27:19

Ferrite won't burn Ady, it must have been the casing.

I have successfully shortened some ferrite beads using a diamond disk in a Dremel. It is very brittle and I think it will need grinding rather than turning. Do you have a toolpost grinder?

Russell.

I wish you luck, but I don't think you'll have a ghost of a chance doing anything using HSS tools!

I tried to repair a ferrite magnet on a ride on mower starter motor that had become detached, seems with good reason that motor manufacturers bond them in place when they could order them pre-formed to be bolted or rivetted, the stuff is unbelievably brittle!

Good quality drills won't touch the stuff, and neither would a modified TCT drill, so I don't think you'll have any luck with HSS tooling!
Ended up using a diamond burr with water to get under the surface, then the piece shattered before I'd got through it!

Wierd stuff too, it's like carbon, very smooth and slippery, but unfortunately rock hard!

I gave up in the end and bought a new starter motor!

John.

PS. Tried gluing the stuff too, but it's also near impossible to get a good strong bond even using the likes of Araldite type epoxies, so motor manufacturers must use a very good heat resisting glue on it for motor applications!

Edited By RJW on 27/12/2013 09:35:45

I would certainly not attemp to turn Ferrite MikeW answer seems the best I have always went out and purchased new brushes.
Bob

Its generally accepted that standard moulded ferrites cannot be reliably cut or shaped. If you have no alternative diamond cutting blades or wires run at minimal pressure can work provided the cutting debris is immediately washed away. Not that easy given its magnetic nature. Any build up will jam in the cut and fracture the material. Grinding will probably have heat and pressure problems although I understand that it can work when removing small amounts of material, for example trimming relatively large, well supported, lumps to shape.

I've successfully glued ferrites to metal using one of the Loctite structural adhesives. Something over 20 years ago and, so far as I'm aware, its still holding. Sorry, no record of the type number but it was a very special order item so I had to wait. The Loctite tech rep was very specific as to the cleaning, priming and application procedure needed for a full strength job.

Clive

Ferrite is a nightmare for almost every ordinary manufacturing process except sintering, which is how ferrite cores are generally made. It's too hard to turn. If you get a tool point to bite, the ferrite will shatter. If you grind ferrite it will quickly clog your grinding wheel.

The shape of an inductor is often an important consideration, especially if you want to focus magnetic flux to increase flux density. For this reason, complex shaped sintered ferrite coil formers are very popular. There are some frequency response reasons too, but I think the ease of manufacture for complex shapes by sintering is the commercial driver for ferrite use.

The frequency response stuff is important because you might need to control the relative permeability of the material at specific frequencies, to improve the Q factor of inductors for filter circuits. Because sintering is based on powder metallurgy, it is easy to accurately introduce impurities which support frequency selectivity.

For ordinary turning, filing, and general working, soft iron (Fe - 0% carbon) is an excellent magnetic material. It is often better than ferrite, in the sense that it often has a higher relative permeability.

Don't use steel, or cast iron. Both are poor magnetic materials, unless you want a permanent magnet. The carbon impurities and other alloying elements lock the magnetic domains down. You can magnetise steel and cast iron electrically, but it has remnance and a non-linear B/H curve. You either have to heat treat it to release the remnance, or electrically demagnetise it.

If you actually want a permanent magnet, then you can use ordinary carbon steels (like silver steel). All you have to do is to figure out how to harden it (as hard as you can get) whilst it is under the influence of an external magnetic field. As the material falls below the curie point the field is frozen into the object. You can use this scheme to produce odd shaped magnets, but odd shaped magnetic fields.

Generally the harder the material the better the magnet. Obviously modern rare earth magnets use exotic materials.

Edited By Andy Ash on 27/12/2013 15:01:21

Ferrites are essentially ceramics as far as machining goes. They are machinable but the correct process to apply is grinding or abrasive cutting. Imagine you are machining a lump of glass and you won't go far wrong. Parts for volume production are sintered and fired, during which time they shrink a bit, then any critical faces are machined flat. For prototype parts, samples can be machined up from solids but it's not as simple as turning or milling.

However, if you are thinking of producing magnets rather than magnetic components (transformers, inductors etc), steel and similar magnetic alloys would be a lot more convenient to machine, as noted. Generally, magnets are magnetised after they have been physically made, whether ferrite, neodymium, steel etc. This requires some pretty challenging equipment. Apart from the difficulties machining magnetic material (swarf clearance?!), the magnetism may suffer during machining, especially if they get hot. And the original alignment of the magnetic field may not be exactly what you are hoping for in the final part.

If you are thinking of forming a custom magnetic part, why not buy some std neodymium magnets such as buttons or cubes and then machine up some magnetic pole pieces to sandwich around them to direct the flux where you want it? Although the maximum flux density of steel is not as impressive as the likes of neodymium, you can still develop a fairly decent field with steel as a conductive medium.

I bought a selection of impressive magnets from here **LINK** some years back although there must be many more suppliers, not least ebay.

Merry