Re: [CAD_CAM_EDM_DRO] Re:Lapping

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@...>
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