Permanaent Magnetic Chuck-- Changing Flatness

Hello Guest,

I am wondering why you need to keep checking the piston/piston rings like this?

Are these pieces you make machined by hand, and, if so why not set an automatic milling/turning/etc machine to work these pieces, then each should be the same? When you lap something unless you are very careful you can end up with a slight 'bevel' rather that a dead flat surface.

I have read your post several times and do not understand why you have anything mounted on Granite? Is this to allow the chuck only, to be magnetized and to prevent the rest of the machine from getting magnetized? [p[] How do you check for accuracy? Do you take one in a hundred and check it, or check every one piece?

I would use a non magnetic metal (Aluminium, tungsten, Copper, all are non ferrous) as a base plate to mount the magnetized metal chuck on. This is in answer to another question near the end of your post.

You say there is scratches on part which have been measured? So these parts are no good. The reason for the problem removing the piece from the demagnetized chuck, may be a slight temperature differential between the chuck and the piece you are checking? Or tiny metal pieces wedging the piece you are checking into the chuck? If the chuck is entirely demagnetized it cannot be to do with any magnetic force I presume?

Keep in touch.....

bb

Three possibilitiez I can thing uv.

1. The chuck iz fine. The dial gage haz steel parts that are being influenced by the magnetizm.

2. The chuck was damaged by cutting such a deep slot in it.

3. The chuck iz cheap junk.

Your discription uv your set up iznt clear, so I'm not sure exactly wut you are trying to measure or how. Wut duz 'w.r.t.' mean?

'w.r.t.' = with respect to

I can't say I fully understand your problem, however, it seems to be related to the use of magnetic fields. I suggest you replace the magnetic chuck with a vacuum chuck. These chucks are usually made from a sintered metal, ground flat, with a vacuum supplied to one side. The chuck will remain flat,won't be affected by residual magnetism, and won't affect your dial indicator.

If the ring is difficult to remove from the chuck, that would suggest the chuck is not working properly. When demagnetized, magnetism should not be present at the surface. Why is there a slit in the chuck face? It seems a slit would disturb the magnetic field.

Hello Udayan, Not a good idea to post your EM address on line (attracts spam). If you register (free and easy) people can send you private messages via CR4.

Any chance of a quick diagram.

Is the chuck electromagnetic.

You might get on better with a magswitch, these are effectively permanent magnets which can be switched on and off.

It is highly inadvisable to post an e-mail address in an international forum. E-mail addresses are easily harvested by spambots. They quickly bombard it with advertisements for non-prescription pharmaceuticals, scam requests to update on-line banking details, links to matters of an intimate nature and scams purporting to be about getting large sums of money out of Nigeria, rendering the address practically useless.

It sounds to me like you have residual magnetism that is making it difficult to remove the ring from the chuck. This is very common and I don't know of a good solution for it. If your magnetic chuck is electromagnetic, they usually have a switch that temporarily reverses the magnetic field. With practice, you can flip these switches to remove the part quite easily.

The scratches you are getting are indirectly caused by this residual magnetism. You are scratching the face of the ring as you drag it across the chuck face trying to remove it.

The flatness problem with your chuck can probably be fixed by a good machinist on a surface grinder by repeatedly flipping the chuck (it may take two or three tries) and grinding a small amount off each side each time you flip it.

Your idea of a brass face plate will help to reduce the magnetic force reaching the ring, and will likely reduce scratching because the ring will be held with less force and the brass will likely be softer than your piston ring.

Alternatively, you may be able to skip the magnetic chuck completely, and just butt the ring up against your square plate with a somewhat heavy block that presses against the ring near the center of the ring. A light coating of a (very) thin oil is also helpful in making sure the part stays tight against the square.

Good luck.

I am also unable to visualise your inspection fixture, a photograph or a sketch will be useful.

What is the material of the Piton RIng? Did you check whether it is having retained magnetism? We have faced this problem a lot of times.

What we prefer to do is not to clamp the pieces on the magnetic chuck. It may be better to keep the job horizontally on the granite bed and then make your dial or whatever to move across in a horizontal guide a la CMM.

Also check up whether your dial gauge itself is being affetced by the magnetism.

When you use a magnetic chuck, you will observe lot of small iron particles (powders) getting attracted to its face. These are the most probable scratchers for you. And depending upon your scratch sizes, and also due to the likely absence of background colour contrast between the chuck and iron particles, you will not be able to see them and also the normal wiping will not be effective. And when the piston ring is stuck, of course you will try to slide it out and that will scratch it.

Better to use the vaccum grip or often you will see just a smearing of oil on granite face will be able to hold the test pice in position and then, the granite plate is easy to clean.

Re-grind the magnetic chuck with magnet on. You can eliminate your flatness and magnetism issues by using a vacuum chuck instead, or make your own vacuum plate, which is rather easy to do.