Acceptable magnetism from machining

[Nellysdad]

Can anyone tell me an acceptable level of magnetism from the maching operation? ie. grinding.
I am informed that there is some level permissible, but am yet to have that quantified. A Gauss level?
regards

 

[UKpete]

Nellysdad, I may have misunderstood your question but fully magnetized PM material can be ground or machined with no detrimental effect on magnetic properties. It is very messy as the grinding dust sticks to any adjacent metal surfaces; but it is a process carried out particularly by magnet manufacturers (e.g. Magnequench) all the time on rare earth materials such as samarium cobalt and neodymium iron boron. Grinding magnet material is carried out in cases where it is not possible to magnetize in situ after assembly.

With neodymium the magnet has to be recoated after grinding to protect against corrosion.

 

[Nellysdad]

Thanks for the response Pete,
perhaps my inquiry was not very clear. I know that magnetism is induced in material as it being machined. I would like to know what "acceptable" levels of magnetism are induced by processing, before they are magnetized in a formal magnetizing process. Basically we received some magnets that had partial magnetism but they were enough to hold weight. from experience, we have had problems with this condition in achieving our desired rate (Gauss) after our magnetization process, finding that they do not reach the target. I am told it is because we do not know the polarity of the magnets arriving and this effects our end result because we may be "hitting" this material with our machine in the wrong direction(s).
We use neodimium and ceramic ferrite. We are currently seeing this with our Neo.
1. I would like to specify on the drawing a quantity of acceptable gauss after machining (before magnetizing)
2. Is there a simple method that I can inform our vendor to demagnetize completely after machining? (heating is possible I guess....corrosion!)
3. Does this relatively low-level really have the effect on our magnetizing operation or is it something else?

 

[MagMike]

Hello Nellysdad,

Sometimes there is residual magnetism in a part if a magnetic chuck is used to hold the part during grinding (I would not recommend using the term "machining" as it implies that magnets can be worked through conventional machine tools).

I believe this is the source of the residual magnetism you are seeing. If the magnetic material is ceramic then it is pretty easy to demagnetize the part (if necessary). If the part is Neo, it is considerably more difficult, as you noted.

A magnetic chuck would leave a pattern of alternating North-South poles, but only on the face of the magnet that was in contact with the chuck. This pattern would be sufficient to hold a paperclip, maybe even hold the magnet to a steel filing cabinet.

These levels of residual magnetism do not normally cause a problem. As long as your magnetizing equipment is providing a sufficiently strong magnetizing field, it would completely overwrite & erase the residual pattern.

I would not recommend calling out a quantity of acceptable gauss on your drawing, it is requirement that may add unnecessary labor & cost to the part. A proper magnetizing pulse will eliminate the residual pattern.

I would recommend checking your magnetization process. If you are using capacitive discharge magnetizers, be sure to check the capacitors to make sure they are holding enough charge prior to magnetizing. They usually only last a couple of years before needing replacement. I've seen magnetizers lose over 20% of their performance before the capacitors were replaced. If some of the capacitors look swollen, they need replacement immediately.

 

[RHMagster]

Nellysdad,

In general terms, it is acceptable to say NdFeB requires 32 k Oersted to fully saturate (some as low as 18k Oersted ) and as mentioned, can erase any amount of small magentiztion caused by manufacturing processes. However, the magnetic chucks used in this type of processing can nearly saturate (to a given depth) the material NdFeB and most certainly the ceramic materials.
Ceramic, can easily be resaturated with 15 k Oersted.

If you are border line on field strength, but sure that you are able to saturate virgin material, you may try pulsing a few samples twice in the same direction. Measuring with a helmholtz coil and observe any increase in flux measured. If you find you have a increase from 1 pulse to the next (> 2%) I'd suggest you look at your equipment. there you should find the answer.

Heat demagnetization:
Curie temperatures are another factor to deal with. Most NdFeB material can be fully demgnetized by approx 310 degree C, corrosion can be a problem here but precautions can be made to prevent. Ceramic is in the area of 450 degree C, no problems with corrosions here.

To further:
There are test specification for testing of residual magnetization. Dealing with iron filings of a given size. Sample parts are exposed to the filings and if parts attract below a set number of filings then it is said to be ok or not require demagnetization.

 

[EdStainless]

And, to continue MM's comments.
Make sure that your magnetizing pulse is long enough. If you have an assembly that doesn't have much mass, and the coil has few turns, the pulse can be very short.
A little bit of residual magnetization can make re-saturation more difficult, but your system should be able to handle it.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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