lapping

Dear a7yvm109gf...:
As relates to forming a more uniform surface (aka. flatter) after a a machining operation?
Not too many people still knock around here.
What is your question?
David A. Smith

Something along those lines.
Well, I suppose the most important question would be where else can I ask these questions? Any good forums to recommend?
See, even the definition of lapping is still not clear to me. 3M makes lapping films I can buy on eBay. What's the relationship to the rouge the other poster mentionned? Particle size?
I wish to machine some ferrite transformer cores so they can slide apart but still complete a magnetic circuit when simply pressed together with light finger-type pressure.
This already exists so it's definitely doable.
However, I wish to do this at home with a minimum of expense.
I figure a 5.25 full height hard disk has the necessary flatness on the platters for a good lapping surface. I'm not looking for the ultimate polished finish like a gage block. Just good enough.
I figure putting the lapping film on the platter and making a jog to hold the work piece steady ought to do it.
I need to know what to do! How to ensure dust doesn't mar the finish. etc
Textbooks about these processes are quite expensive. Any recommendations though?
I'm going to the library later.
Thanks!

Dear a7yvm109gf...:
OK.
As we get into your need, I think it will be clearer what the newsgroups might be.
Bonded to the surface.
Should be discoverable, once you have 3M's part number.
Unless you are doing really high frequency stuff, you really do not need to worry about "jeweler's precision" in fitting this stuff up. Planning on "interleaving" is a really good idea too.
With hand lapping, you have difficulty maintaining any sort of parallelism, making the faces smooth is silly if you end up with large gaps.
The disk will warp, and you'll have corner gaps. Get a granite counter top.
Doubtful.
Plenty of water (contraindicated with ferrite materials). Do only one pass on the "tape". Or simply don't do it.
College library.
Recommend magnetics or electomagnetics newsgroups. They might be able to give you a handle on how-to / if you really need to do this.
David A. Smith

Thanks. I was wondering about the granite cutting slabs you see in kitchen supply stores. They're all shiny and stuff, but are they really "flat"? (I don't mean an entire counter top, just a cutting board-sized thing.)
I guess there's a difference between "locally" flat and overall flatness.
As for the frequency range, it's DC to 50MHz. I already have parts to do this, the Hall sensor is a thin film somehow bonded to ferrite parts.
As for the accuracy I need, I was surprised to see the existing parts were actually pretty sloppy. One of the pieces actually rocked back and forth (barely so, but still perceptible) on my HD platter.
I didn't even need my micrometer to measure it either. One end of the thing is 1 mil lower than the other, over a total of 176 mils.
So if I manually lap on a granite slab with a 3M lapping film, well, how do I insure the work piece is not going to "rock" back and forth as I move it?
A LONG time ago, I assembled FC fiber optic terminations. Once the epoxy set, you put the FC connector in a steel puck and just rubbed the FC/puck combo over a lapping film.
I would need suck a puck that fits my parts.
Is there a name for such a thing? The bottom of the puck was very smooth.

"such" a puck. "suck a puck"? Ouch.

... Probably close enough on the scale of a few inches.
Several different definitions.
Then you might find actual noise problems showing up if you get things too tight.
Might talk to either a rock and gem shop, or a jeweler.
You've tapped as much as I can help. Sorry. I'd try the parts raw myself.
David A. Smith

Dear brian whatcott:
... Truer words were never spoken. It is really common to establish air gaps in transformer cores, for specific functions. He'd be better off finding out if "lapping" were necessary first...
David A. Smith

Thanks. I've put down the espressos and started to relax a bit.
I went to a kitchen supply store. No granite, but I bought a marble, or something claiming to be marble, cheese cutting block. It's large enough, and at any rate it was only 7$.
It certainly seems flat enough, I placed two of them shiny face together and got that fun "vacuum cement" effect or whatever. There was an airtight seal between the two.
I already have 600 grit sandpaper from my model rocket days.
We'll see as I go along what happens...