Re:Lapping

I believe hydraulic control valves and diesel fuel injectors are
lapped with "parts in place" resulting in matched assemblies. The
lapping compound is not the same as valve lapping compound, but the
result is similar. There is a time and place for lapping. Is it
usable here, or will I ruin my machine?
Randy

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Marcus and Eva"
<implmex@a...> wrote:

> Hi All:
> I've been following this thread with interest; here's my take

on it:

> The points about abrasive embedment and not being able to create

accurate

> reference surfaces are well made, but there are other things to

consider

> too.
> The first is that excellent geometry is hard to achieve and hard to

maintain

> on poorly designed and poorly stress relieved castings: many of the

Asian

> imports suffer from this and cannot be made to perform to the

standard of

> precision machine tools no matter what is done to them.
> Second, these machines still do work after a fashion and can be

improved

> significantly over their "out of the box" state, with some simple
> techniques, but they take reasonable diagnostic skills to determine

what is

> wrong in the first place, and the solution always depends on the

root cause

> of the symptoms.
> For example, a hard spot at the end of an axis travel might be due

to lead

> screw misalignment...no amount of scraping of the ways will fix

this, and

> may actually make it worse.
> On the other hand, non parallelism of the dovetails will also make

for a

> hard spot in the travel, and scraping or lapping will improve this

fault

> significantly, even if the surfaces are still otherwise

geometrically

> deplorable!!
> Lapping is a perfectly reasonable way to reduce non parallelism, or
> localized bulging of way surfaces, but you must be sure that this

is the

> actual problem.
> This brings me to the practical problems of how an assessment can be
> realistically done.
> Most hobbyists who buy this class of machine don't have surface

plates and

> dovetail straight edges and 0.0005" in 12" spirit levels, so it's

kind of

> pointless to invoke this sort of gear as a requirement to achieve
> improvement in a hobby type machine.
> Blueing, however is cheap as dirt, and everyone who's in this trade

has

> mikes and DTI's, or should.
> It's amazing what a stripdown and a light coating of blueing on one

side of

> the ways will reveal.
> Similarly, simply miking a pair of dowel pins across the dovetails

in

> several places will tell you a lot.
> So will the act of removing bits and assessing the

consequences...for

> example, the hard spot at the ends of an axis travel can be

diagnosed as to

> cause far more easily if you remove the leadscrew and slide the

table by

> hand.
> Removing the gib and sliding the table with a clock against its

side can

> also be very revealing.
> Putting a clock on each end and pushing with your thumb to detect

rocking is

> another simple and very worthwhile trick.
> Filling the coolant trough with oil and miking the table surface

down to the

> oil level at either end of the travel can discriminate twist in the

ways

> without having to spend more than a couple of bucks for the oil.
> The point I'm making so long windedly, is that you can do a lot

with a bit

> of ingenuity, and the aim of the game here is to make it the best

it can be,

> not to make a silk purse etc etc etc.
> So, sure, lap it if it will help...but figure out first if it's

actually

> going to do you any good.
> If you do decide to lap, make a pair of lead or soft copper strips

that you

> can slip between the ways and then charge those with abrasive and

lap the

> long ways (usually the male ones), making sure you hit the high

spots while

> roughing to consistent feel, and then run the whole length with each
> finishing stroke.
> WHen you're done, toss the strips into the trash.
> Just piling in a load of grit and waggling back and forth till you

get bored

> won't do it here; as Ron points out, you'll just wear out the

machine

> faster.
> Cheers
>
> Marcus
>
>
> ----- Original Message -----
> From: "Ron Ginger" <ronginger@a...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Thursday, March 24, 2005 5:29 AM
> Subject: [CAD_CAM_EDM_DRO] Re:Lapping
>
>
> >
> > 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