Dennis Storzek wrote: Let's not get too hung up on the three surface deal. Yes, when one has no access to other equipment, lapping three surfaces to each other will generate flat surfaces, but it's a slow go. Much faster if you can introduce a pre-made reference flat into the mix, such as a cast iron lapping plate, or just 3M wet-or-dry paper on a granite flat. Small Chinese surface plates are so cheap these days that everyone should have one, and brass cuts fast with wet-or-dry paper.
Likewise the cast iron lap used on the valve body seat can be repeatedly reconditioned on a surface grinder, rather than laboriously lapping it against a third surface to remove the wear imparted by the valve body.
Let's put the effort into lapping the valve and seat flat, not trying to flatten the laps by lapping.
I second the sentiment. Although this work process on air valves has historically been called "lapping"
there are some important differences with respect to the "textbook" definition of a lapping process:
True lapping must be done with a lap that is softer than the target material, so the lapping compound
embeds in the lap. Lapping a hole that must be a precision fit to a matching pin in a fixture, for
example, would generally done with a copper lap against the steel of the fixture. On air brake work,
the lap is cast iron and the lapped surface is brass, which violates this principle.
Lapping falls into a series of machining processes going from milling to grinding to lapping to polishing,
in decreasing order of metal removal rate and surface roughness index. What is actually needed for
practical air brake work is more towards the grinding side of the spectrum and certainly not polishing.
An air valve surface which has a mirror finish may actually perform worse than one which has a dull,
frosted appearance, if the former is not flat. The fluid viscosity of air and the actual leakage rates
which are acceptable in practical air brake work do not call out a mirror finish. The device will perform
fine with a lesser finish. Flatness and particularly little high spots are the biggest issue. Even small
scratches will have no measurable effect. Large scratches (that you can feel with your fingernail)
that lead directly to a port may cause leakage.
Optical flats and monochromatic light sources are great in the lab, or in optics work, but they are too
high of a standard for this work and they will be very, very difficult to apply to a slide valve seat. You
will have great difficulty getting the surfaces sufficiently clean and oil-free to the point where you'd be
able to get the flat down. You'd also have to make an optical flat that is ground to the precise dimension
so that it fits into the slide valve chamber.
A toolroom grade granite or steel surface plate makes an excellent reference surface for lapping the
slide valve and the lap. As I said before, I prefer abrasive paper because it does not wear down
the reference surface. You'll generally get the brass slide valve done with one sheet of coarse
wet-n-dry SiC abrasive paper, about a 200 grit, followed by one sheet of 400-600 grit. Finish with
a 1000-1200 grit if mirror finish makes your happy. Skip the coarse pass if the valve is only slightly
out of true and there isn't that much metal to remove.
Doing the seat is much more tedious. The hardest part is holding the paper to the lap. I've experiemented
with a number of approaches. Sticky tape can work -- anchor the strip of paper to the top side of
the lap, pull it around fairly taut and then stick it down on the other end. If you are using cutting oil,
which is helpful when working at the coarser grades, it quickly impedes the tape's stickiness. Another
approach is to mill a couple of slots on the top of the lap, get two little strips of metal, drill and tap
so the strip can be dropped into the slot and held down with a thumbscrew. You'll also need a precise
paper trimmer to make strips of paper that are exactly as wide as the lap: too wide and they'll curl
and bind on the sides, too narrow and they may not cover the entire seat surface.
The paper will load up much faster when working the seat this way vs working the valve against the
surface plate. I often pull the lap out every 10 strokes or so and blow down the paper and the seat
with an air nozzle.
There is an investment in time making the lap. Protect it. Don't ever stamp the lap to ID it. Light
scribing is acceptable, or paint marker on the top. Keep it in a cardboard box when not in use.
Record the critical dimensions and the valve or valves that it is intended for and also write this
information on the box.
Always interested in other's experiences with this tedious task.
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