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From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Making CRS concentric
Date: Tue, 23 Nov 1999 03:54:44 GMT

Perhaps I should elaborate a little!  8^)

A lap, whether internal for a hole or external for a rod or cylinder, does not
need to be as accurately round or straight as the surface it is generating.

Indeed most laps are far from round..a toolmaker's "micro-lap" looks like the
eye end of a large needle and expansion is achieved by wedging the "eye" open
further with a special tool (or a screwdriver blade)

The accuracy of lapping is achieved by rotation and the even stroking of lap and
the hole or rod to be lapped.  Material removal is slow and tightspots can be
felt and the lapping stroke concentrated until an even "feel" is obtained over
the full length.

For best accuracy the lap should be as LONG as possible.


Robert Bastow wrote:

> No..Nowhere close!!
> teenut
> Ed French wrote:
> >
> > Thanks for all the answers. Just one more question regarding this.   Does
> > the split lap have to have tolerances equal or better than the desired
> > tolerances on the finished piece?

From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Materials used for lapping
Date: Tue, 23 Nov 1999 16:52:38 GMT

Linus Sherrill wrote:

> I understand the common wisdom that when lapping, the lap
> material is softer than the material being lapped (so the
> abrasive embeds in the lapper not the lappee)
> Recently I read an article that described using a steel lap to
> lap aluminum scope rings. I have to believe that this works
> (i've seen the pictures) so I must be missing something.

The steel is soft enough to "take up" abrasive particles..I don't know what
technique the person used but I would "charge" the soft steel lap by smearing it
with abrasive paste and then rolling it on a hard surface
with as much pressure as I could generate.  The charged lap would then be washed
clean with solvent.

Lapping scope rings should involve removing only a tiny amount of material..and
the charged steel lap, used gently with a thin lubricant, will achieve this.
Some abrasive WILL be left behind..embedded in turn in the even softer
aluminum.  This probably isn't a problem..maybe even an advantage if it enhances
grip under heavy recoil!!

However, if the scope rings were some kind of running bearing..this would be a
formula for disaster!!

> Similarly, lapping with a glass plate. The glass is harder than
> the steel, so why doesn't the abrasive become embedded in the
> steel and grind away the glass?

It will!

Lapping is a "mutually destructive process" both "lapper" and "lappee" suffer
material removal..especially when loose abrasive is present.  Lapping a piece of
steel on a glass surface would be a "Bass Ackwards" approach!  Yes, some steel
will be removed..but the steel would rapidly become the lap and grind away the

However, in the case of useing a glass plate as a TRUING surface (this is NOT
true lapping)..this is usually advocated for trueing water or oil stones..using
loose abrasive.  In this case the abrasive grains "roll and catch" frequently
enough to dress the stone by a combination of abrasion..and (I suspect) crushing
of the bond holding the stone together.  The Glass, being harder, is not ground
away quite as quickly..though the surface will become rapidly "frosted".

> I know I must be missing part of the theory. Can anybody help me
> out here?
> --

Hope this helps.


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