TABLE 3-28. THREAD CONSTANT FOR VARIOUS STANDARD THREAD FORMS.
PERCENT OF FULL THREAD DESIRED
American Std Course Series C =
British Assn Std C=
Amer Std 60 Stub C=
Amer Std Sq C=
Amer Std 10 modified Sq Sq C=
3-195. TAPPING. The taps used for threading aluminum alloys should be of the spiral fluted type for best results.
Straight fluted taps can be used but have a tendency to clog and tear the threads during the tapping operation. Spiral
fluted taps for cutting right-handed threads should have a right-hand spiral of about 400 angle with a generous back off
taper and highly polished flutes.
3-196. Spiral Pointed or "Gun Taps" (straight fluted except they have a short spiral on the starting end) cut aluminum
more freely than the other types. With this type tap the major portion of cutting occurs at the spiral end and curls ahead
of the tap. The use of the "Gun Tap" is therefore limited to tapping holes which have room for the cuttings ahead of the
tool. This spiral pointed tap should not be used for cutting tapered thread or for bottoming taps.
3-197. The following procedures and tools are recommended for tapping aluminum alloys:
Cutting Speed: 40 to 130 feet/minute use lower speed for hard alloys and higher speed for soft alloys.
Tap Type Selection: For blind holes and bottoming use spiral fluted; for semi-blind use spiral pointed (gun taps);
and for hole through work use spiral pointed (gun taps).
Thread Type: Rounded or flattened (turn coated) thread contour for general use.
Tool Angles: Spiral flute-grind a lead spiral extending one full thread beyond chamfer on straight fluted tap. To
make gun tap and spiral flute tap should be 28 to 40; cutting angel 40 to 45; top rake 45 to 50; back rake 4-8;
cutter area (included angles); 2 flute 36° to 72 and 3 flutes 24 to 48.
Tapping Allowance: Drill diameter for general tapping should be from 0.005 to 0.006 inches per inch larger than
standard for the same thread in steel or in accordance with the following.
Drill diameter (1.005 X tap diameter)-thread per inch
C = Thread constant for various thread forms and percentages of thread depth required as given in Table 3-28.
Lubrication: For high speed tapping use lard oil/mineral oil and for hand tapping a more viscous lubricant is
recommended such as heavy grease/oil, white lead, etc.
3-199. Hand files of the single cut type having milled teeth usually give the best results for filing aluminum. The main
consideration in file design/ selection for aluminum is to provide ample chip space clearance. The cuttings generated are
large and have a tendency to powder, pack and clog between file teeth. To overcome clogging problem chip space is
increased, grooves are cut deeper and teeth are cut with generous side and top rake.
3-200. For finish filing a long angle millfile (single) (cut) with tooth spacing of 14-24 teeth per inch with side rake angle of
45 to 55° is recommended. In absence of the preferred file the same effect can be obtained using standard mill cut files
by adjusting angle of filing incidence to the metal worked. The file is often adjusted until force or motion applied is
parallel to the work piece for best results. A good general purpose file is the curved tooth type (often called "vixen")
having about ten deeply cut teeth per inch. It can be used for heavy and finish cuts. Lightly double cut files having tooth
spacing of 14 20 per inch can be used for light duty rough cutting and finishing when working the harder alloys.