4-9. GENERAL MAINTENANCE PROCEDURES-Continued
Scattered short sharp bursts are seen. Bursts are caused by metals working at temperatures that weaken
and break the material. Bursts are not usually seen until the metal is cut through to the burst area.
Separate short wavy lines in the same general direction are seen. Flakes are caused by improper
cooling. They are not usually seen until the metal is cut through to the flake area.
Fine, sharp, tightly packed lines are seen (grinding cracks). These are usually caused by a glazed
grinding wheel. The wheel, instead of cutting the material, rubs and overheats the material. The lines are
thermal cracks similar to heat treat and hardening cracks.
j. Fluorescent Penetrant Inspection. This type of inspection can be carried out on any metal but is usually
reserved for non-magnetic material (e.g. aluminum, stainless steel). The parts shall be warm prior to applying
penetrant. The penetrant may be applied by dipping, painting or spraying. All surfaces to be examined shall
be completely covered. Penetrant time for various metals is as follows:
Aluminum Alloy - no less than 20 minutes.
Magnesium Alloy - no less than 20 minutes.
Brass or Bronze - no less than 30 minutes.
Ferrous Alloys - no less than 30 minutes.
After applying penetrant, the surfaces will be cleaned with lukewarm water - less than 120 deg. F
(49°C). (Cool water may be used, but cleaning time will be longer.) Pressurized water spray may be
used to shorten the washing cycle. The following methods may be used to develop the parts.
Wet Developer Method. Parts shall be completely covered in developer by spraying or dipping. Parts
shall be dried and developed in a recirculating hot air drier for one half the penetration time.
Dry Developer Method. Parts must first be dried. The dry developer shall be spread on all surfaces by
dusting or by dipping the part. Parts shall be developed for one-half the penetration time.
No Developer Method. When no developer is used, drying and developing time shall be at least equal to
penetration time to allow sufficient bleeding of penetrant from defects. If additional clarity is required
during inspection dry developer may be applied by a hand powder bulb.
Following development, the part shall be inspected under black light. Any evidence of cracks is cause for
rejection. After inspection the parts shall be cleaned with dry-cleaning solvent (item 3, Appendix E) and
covered with rust preventive oil.
k. Lapping Procedures.
Clean the part with dry-cleaning solvent (item 3, Appendix E) and dry with compressed air. Clean the
lapping blocks with compressed air. Do not use a cloth or any other material for this purpose.
(2) Spread a good quality 600 grit dry lapping powder on one of the lapping blocks. Place the part to be
lapped flat on the block and, using a figure eight motion, move it back and forth across the block. Do not
press on the part, but use just enough pressure to keep the part flat on the block. It is important that the
part be kept flat on the block at all times.