I apologize for sending this to everyone again, since it's already been
posted on the list twice. But when I dug it up to send to Mitch, I saw
dozens of places where I wanted to add things I've learned since I wrote
it.

Mike Taglieri -- miket-nyc@...

"Fundamentally the marksman aims at himself."
- from Zen In the Art of Archery


--------- Forwarded message ----------
From: Michael C Taglieri <miket-nyc@...>
To: 7X10minilathe@yahoogroups.com
Date: Fri, 15 Mar 2002 23:12:39 -0500
Subject: [7x10minilathe]
Message-ID: <20020315.231247.-94325.0.miket-nyc@...>

Over the past week or so, I've been lapping the various ways on my 7x10
to increase the contact area and minimize play. This has improved the
lathe tremendously. For example, before this modification, the compound
had significant slop unless I kept the gib screws so tight that it was
hard to move. Now, it slides easier than before, but it has zero slop
both side-to-side and up-and-down, as measured on a test indicator. The
process is tedious, but not hard. Here's how I did it:

The Compound

This is the easiest one to start with because it's small and you can
adjust the gib by setscrews. I lapped it (and everything else) with
Mothers Mag & Aluminum Polish. This is a metal polish (recommended for
all metals, not just aluminum) that's inexpensive and readily available.
Simichrome is similar but more expensive. These polishes are much finer
than valve-grinding compound and other coarse abrasives. (And even if it
turns
out that the job would have gone faster with coarser abrasives, it would
also be
easier to screw up the lathe ways with one if you did something wrong).

To lap the compound, take off the compound leadscrew so you can move the
compound slide back and forth on its base by hand. Remove the locknuts
and make sure all the gib screws can tighten enough to move the gib. (One
of my screws was distorted on the end and couldn't move the gib, so I had
to file the end). Loosen the gib screws enough so the slide goes on
easily, and use some heavy grease to hold the gib in place in the
dovetail. (You'll be taking the slide off over and over, so this will
keep the gib from falling off each time).

Now, put gobs of polish on the bottom of the slide and the dovetails, and
reinstall the slide, then tighten the gib screws until there is slight
resistance and start sliding the compound back and forth. I slid mine
each way enough to show the socket head screws underneath. As the high
spots wear down, you will have to keep tightening the gib screws.

After several dozen strokes, take the compound off and look at the
surfaces. As the first photo shows, the areas where the polish is
grinding will be almost black, but the center of the dovetail on the base
should not be black because the slide does not touch here (also, the
underside of the gib shouldn't be black because it should not be low
enough to touch the base). Wipe the polish off, put on new polish, and
do
it all again. I used toilet paper, then a soft cloth, for wiping, which
is
convenient because you can get most of the polish off on the toilet paper

and throw it away each time).

When you examine the surfaces you're working on, you'll see shiny places
(and
probably not many at first). These are the only places where there is
contact, and the goal is to grind until there are shiny places all over
the sliding surfaces. You want a fair amount of the surface in contact,
but not total contact, since the pockets where the surfaces don't touch
act as reservoirs for oil.

You keep repeating this process -- renew polish, grind, wipe and inspect
-- until you're satisfied with the results. After awhile, it gets hard
to tighten the gib screws without locking the slide completely, because
more and more of the sliding area is coming into contact. (At that point
I
found it helped to tighten just one gib screw at a time, grind until that
area
slid easily, then tighten the next gib screw, etc. This let me grind
pretty hard without locking up the slide (or wearing myself out).

As you continue, you may see small areas that are much shinier than the
rest (especially on the underside of the slide). These are areas that
were originally way too tighter than the rest because they were distorted
during manufacture. You can remove metal from these places with a
scraper, the end of a file, or a bit of sandpaper on a the end of a stick
so the
rest of the surface can catch up. (If you do it with sandpaper, clean
the
surface thoroughly before doing any more lapping, because you may have
added bits of a much coarser abrasive).

Every once in awhile, wipe the polish off totally, wipe a few more times
with oil-covered toilet paper to get any remaining polish, then assemble
and lube. This is so you can check how well the slide works with the
degree
of lapping you've done. Eventually, you should get easy sliding with no
play until the slide is moved way out. When the slide is lapped as far
as you want, disassemble and clean the polish everywhere, (including
pushing rags through the tapped hole in the base, in case some polish got
in there). You can always do more in the future if you get more
critical.

The second photo shows my final result on the compound. Only about 40%
of the surface is now in contact so I could have gone further, but the
results are incredibly better than before, and I can always do more
someday.

The Cross-slide

The cross-slide is lapped just like the compound, only it's larger and
you have to remove the splashguard. Also, because of the much larger
area and thin cross-section of the cross slide, it's more likely to warp
after
being machined (and the Chinese don't let their castings sit for a year
before final grinding like the prestige machine-tool makers). So you
may
have to use coarse abrasive or sanding just to get the cross-slide as
flat as
the compound started out.

The Carriage

The carriage is very different, because of the way the gibs are attached,
with
capscrews holding the gibs on and setscrews pushing them away. This
method is extremely flawed because the gibs can never be solidly
attached,
so I decided to get rid of the setscrews and shim the gibs instead.

I have some .005" brass shimstock, so I used that, but the exact
thickness doesn't matter. I cut 1/2" by 4" shims and made holes by
clamping the gib on the shims (on a piece of scrapwood), and punching
holes with a 1/4" hollow punch sold as a paper punch. (You can make one
by filing a bevel on the inside of a piece of 1/4" tubing). Before and
after punching the holes, I annealed the brass by heating it dull red.
By pressing hard and twisting with the sharpened punch, I found I could
make the holes in three shims at once. The only trick about shimming is
not to let the shims stick out too far on the inside, or they'll rub on
the bed themselves and make you think the gibs are tighter than they
really are. Also, if you're using thin stock like mine, you have to be
careful they don't get creased when putting them on.

Unlike the compound and cross-slide, I lapped the carriage one side at a
time, doing the rear first (which was arbitrary -- the reverse might have
been better). To lap the carriage onto the bed, first remove the apron.
Then shim the gib on the side you're lapping until it's a bit loose with
the capscrews not completely tight. Then you can tighten the screws as
you lap, and eventually you remove a shim and do it all again. Keep the
screws loose on the side you're NOT lapping, so all the tightness you
feel comes from the side being lapping. However, the V-way determines
the parallelism of the carriage, so keep some pressure (and oil) on the
top of the front V-way when lapping the rear.

On my lathe, the underside of the bed (especially in back) was very
crudely finished, so I took off the high spots with a scraper, finding
the high spots by feeling for where the carriage dragged. (To try to get

these gross irregularities off with lapping would take forever, so I
recommend
doing it this way if the underside of your bed is also very rough. You
can
make a scraper by grinding the teeth off the end of a file, then grinding
a
90-degree angle on the end. The top of the bed is precision ground and
pretty
well finished, so I did almost all the scraping and lapping on the
underside.
However, as the job progressed and the fit got closer, I had to scrape a
lot
where the serial number was stamped on the top rear corner, because the
edges of the stamping stuck up enough to make it impossible to get the
carriage on and off.

After lapping, I wound up needing 5 shims (approx. .025") in back. In
front, 4 shims was just a bit too tight, so I made another shim from
.002" plastic (from a bag of pasta -- it was clear and dimensionally
stable). Now, with the capscrews tight, I have free movement of the
carriage, zero vertical play in back, and about .001" vertical play in
front. This isn't perfect, but it's much better than before, and I can
always do it more someday.

By the way, I noticed that removing and replacing the gibs always made
them tight for a few passes even when there was no change in shimming.
This is probably because they never go on in exactly the same position
twice, so there are always microscopic tight spots. The real solution is
alignment pins in the (now empty) setscrew holes, but for now I just did
the cleaning of the carriage and gibs after the final lapping without
loosening the screws, so the final position of the gibs didn't shift.

The Apron

The halfnut mechanism on the apron can also be lapped a bit, but this is
very crude compared to the other ways and it doesn't matter very much.
On
mine, the gib had to be set looser than it should be or the halfnuts
wouldn't
close properly on the leadscrew. Precision here is not really needed.

Finally, I discovered that when the halfnuts were closed on the
leadscrew, they were slightly higher than the leadscrew and pulled it up,
resulting in a lot of drag. There's no adjustment for this and the
leadscrew couldn't move enough to correct it, so I put .020" of shims
above the right side of the apron, lowering it enough to match the
leadscrew without affecting the carriage-transport mechanism on the left.
The apron touches the carriage only on the outside edges, so "L" shaped
shims are the easiest here, because they let you adjust the vertical
alignment of the right side of the apron without loosening the left
capscrew enough to disturb the front-to-back alignment.

Mike Taglieri -- miket-nyc@...