the disk-seat relationship from time to time so the
disk will be moved gradually, in increments,
through several rotations. During the grinding
process, the grinding compound will gradually be
displaced from between the seat and disk surfaces;
therefore, you must stop every minute or so to
replenish the compound. When you do this, wipe
both the seat and the disk clean before applying
the new compound to the disk face.
When you are satisfied that the irregularities
have been removed, spot-in the disk to the seat
in the manner previously described.
Grinding-in is also used to follow up all
machining work on valve seats or disks. When the
valve seat and disk are first spotted-in after they
have been machined, the seat contact will be very
narrow and will be located close to the bore.
Grinding-in, using finer and finer compounds as
the work progresses, causes the seat contact to
become broader. The contact area should be a
perfect ring covering about one-third of the
seating surface.
Be careful to avoid overgrinding a valve seat
or disk. Overgrinding will produce a groove in the
seating surface of the disk; it will also round
off the straight, angular surface of the disk.
Machining is the only process by which over-
grinding can be corrected.
Lapping Valves
When a valve seat contains irregularities that
are slightly larger than can be satisfactorily
removed by grinding-in, the irregularities can be
removed by lapping. A cast-iron tool (lap) of
exactly the same size and shape as the valve disk
is used to true the valve seat surface. The
following are some precautions you should follow
when lapping valves:
l Do not bear heavily on the handle of the
lap.
l Do not bear sideways on the handle of the
lap.
l Change the relationship between the lap
and the valve seat occasionally so that the
lap will gradually and slowly rotate around
the entire seat circle.
l Keep a check on the working surface of
the lap. If a groove develops, have the lap
refaced.
l Always use clean compound for lapping.
. Replace the compound frequently.
. Spread the compound evenly and lightly.
l Do not lap more than is necessary to
produce a smooth even seat.
. Always use a fine grinding compound to
finish the lapping job.
. Upon completion of the lapping job, spot-in
and grind-in the disk to the seat.
You should use only approved abrasive
compounds for reconditioning valve seats and
disks. Compounds for lapping valve disks and
seats are supplied in various grades. Use a
coarse grade compound when you find extensive
corrosion or deep cuts and scratches on the disks
and seats. Use a medium grade compound as a
follow-up to the coarse grade; you may also use
it to start the reconditioning process on valves that
are not too severely damaged. Use a fine grade
compound when the reconditioning process nears
completion. Use a microscopic-fine grade for
finish lapping and for all grinding-in.
Refacing Valves
Badly scored valve seats must be refaced in
a lathe, with a power grinder, or with a valve
reseating machine. However, the lathe, rather
than the reseating machine, should be used for
refacing all valve disks and all hard-surfaced valve
seats. Work that must be done on a lathe or with
a power grinder should be turned over to shop
personnel.
Repacking Valves
If the stem and packing of a valve are in good
condition, you can normally stop packing gland
leaks by tightening up on the packing. You must
be careful, however, to avoid excessive thread
engagement of the packing gland studs (if used)
and to avoid tightening old, hardened packing,
which will cause the valve to seize. Subsequent
operation of such a valve may score or bend the
stem.
Coils, rings, and corrugated ribbon are the
common forms of packing used in valves. The
form of packing to be used in repacking a
particular valve will depend on the valve
size, application, and type. Packing materials
will be discussed in more detail later in this
chapter.
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