how is magnetism induced without an electric field 2

[cwins]

Hi
I am involved in a manufacturing procses utilizing stainlees steel needles (dia approx. 0.3mm, parts are partially shotblasted to a depth of approx. 5 microns). The needles are contained in a metal hopper and moved back and forth over a steel plate with slots machined in to them. The needles are supposed to drop into the slot but we are having a lot of problems with magnetism getting induced into the base of the hopper, this creates problems with the needles dropping into the slotted steel plate.

The magnetism is very localised in the edges of the hopper and as far as we can determine there is no electrical field to induce the magnetism.

I am hoping someone can help me discover where the magnetism is coming from and maybe how we can prevent it building up. We can remove the magnetism with a demagger but it causes a lot of down time removing the apporpriate parts and we are desperate for production at present so all help gratefully received,

Clive

 

[NickE]

In my experience any ferro magnetic items (parts or parts of the machine) that move around develop residual magnetism (RM).

We commonly take steel that has no RM and stamp it into flat cutout parts using mostly carbide tooling. After a good run most of the tool has developed RM and almost all the pieces coming out the other end do to.


we de-mag parts several times throughout our processing. They often like to stick together and that can be rather annoying.

 

[btrueblood]

An old boy scout manual describes how to take a nail and make it magnetic: hold the nail so it's pointing north, and smack the head with a hammer about 40-50 times. It will become magnetic due to the presence of earth's mag. field during the banging (cold working?). If your shot peening is banging on the steel hopper as well, then yes, it will become magnetic, same reason.

 

[EdStainless]

There is not only the earths field, but hte fact that your parts are not really totaly non-magnetic to start with. The motion and rubbing will cause fields to build. the reason that you see it on the edges is that the field will be concntrated at discontinuities.
You might consider putting an insert plate in the bottom of your feeder. I would suggest a stainless alloy that does not become magnetic when work hardened.
I suppose that you need this plate hard to prevent wear. There are a number of stainless grade avaialble as cold rolled hard sheet that are completely non-magnetic.

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