Re:Lapping

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Ron Ginger <ronginger@a...>
wrote:

> Maybe I should have made a clearer statement-
>
> Lapping, where you use a tool called a lap, either a purchased one

or a

> purpose made one, is a very good and usefull process. You can buy

flat

> laps and clyndrical ones and you can make them from lots of

materials.

> But you use a lap against a part, not two parts.
>
> If you simply put an abrasive between two machined parts and wear

away

> some of each part, you are NOT lapping, just wearing out parts.

They

> might get smoother and shinny but you are not improving their fit

or

> usefulness. They will not become straighter or flatter. In fact,
> telescope makers make their mirrors by grinding two surfaces

together to

> make them curved.
>
> ron ginger

I agree that there are different technologies for lapping, and the
most common in machine tools is as you say.

I cannot find any reference that labels the parts, not states that
lapping cannot be done between two parts.

I'll assume you are correct, but final fitting of two parts is not
uncommon. I would assume there is a word for that. Barring a
specific word, the description of the process is being used by many
who already do this. There are quite a few websites that call the
fitting 'lapping'

As for irregular wear, I had mentioned that if you just run a pair of
surfaces back and forth, the ends will get half the abrasion or
removal of the center. This would be due to the fact that the center
never leaves the part. Lense grinding is a great example.

I know internal compression engines have their valves seated, valve
ground against the seat using some sort of compound. That is part
against part. I know it as valve seating. Maybe we are way
seating ? I thought that procedure was called lapping ?

Making of telescope lenses is not similar as since the intention is
to make a convex surface, the lense will wear more in the center by
the intention of the lense maker, it is not possible to make two
convex parts at the same time by grinding two lenses together.

I can see why people have used the term lapping for the procedure.
primarily due to lack of a more appropriate term.

Since lapping is intended only for removal of tenths and not
thousandths, and since there have been a great many people who have
increased the fit and accuracy of their machines with the procedure,
I would not rule it out. I think more damage can be done with a file
or hone or other tool in the hands of a novice.

The use of a lap would assume that the parts are flat and parallel.
Something I fear we cannot assume for the Chinese iron we are talking
about.

I'll accept that the procedure is a bastard child, but buying the
hand scraping book, video and a scraper can cost $150.00. An X/Y
table might be $90.00. Since the parts are not interchangeable,
having the two axes mating, even with bowed surfaces (like a piston
in a cylinder ) is better than a poor factory finish, and not the
sway-back of a worn out axis.

Since you claim it does not improve fit, I would assume you have
never tried it. I find the idea similar to breaking-in an engine.
Maybe breaking-in is a better term.

If you have had a part that had irregular movement, had an axis rock,
had the lock snag in one direction and become loose in another, then
you know that the machine is in such poor condition that something
needs to be done. This breaking-in has vastly improved some machines.

Since it can be done, has been done, and has gotten great results,
then it is a process or procedure looking for a name. I
submit 'premature wear' is just not the right one.

Dave