Re: Improving accuracy on mini-mill? (lapping?)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Abby Katt" <cnc@a...> wrote:

> Roger,
>
> But I wasn't really thinking of using say.. 60 grit..
>
> I was thinking more of using something fine (ie 600 to 1200 grit),

to smooth

> out the bearing surface and gibs, so that I could reduce friction,

allowing

> me to tighten them more and gain better rigitity. If I used a small

enough

> grit size, wouldn't it just remove the current irregyularities from

the

> surface and even it out more, so that they still more or less

defined the

> same plane, but moved with less friction?
>
> I've heard of a few people using lapping.. Does anyone else have any
> experience here?
> A worthy-try, or a just plain bad idea?
>
> Thanks!
> Abby
>

You're right on the money.

Rough chineese iron has flycutter marks you can feel with your finger.

Quality scraping is not rough to the touch.

But, high quality is not perfectly smooth either. somewere around
40% is open to hold the oils.

If the surface was 99% flat, the oils would squeeze out and you would
have other problems.

If you take a pint of Dye-chem and blue up everything, let it dry ,
then just slide the surfaces once, you'll get a feel for how they
mate and how much is free.

less than 20% and you need to do something, over 70% and you need to
do something !

As for lapping. the biggest problem is that you double lap the center
and half lap the outsides.

Since the center is rarely open, it is rarely in free air, so it gets
laped on both strokes.

Then ends only get lapped half as much.

One of the ways to solve that is to make a stage where the parts are
allowed to fully pass over one another.

The neat thing is that if you do it right to begin with you are way
ahead of the game. if you mess up, you still have a way to save it.
the correct way is to buy the book and video on scraping and do it
right.

But, this is for rough finishes, not anyting that looks scraped or
ground. My X/Y table is horrible, has huge ridges from the fly
cutter. perfect candidate.

An old Bridgeport ? not a chance. That almost requires the care of a
professional.

Dave