The tool cutting brass is one I found amongst some cutters at a clock
and watch show
It just suited the job, you can also use commercial thread mills
eg
http://www.kennametal.com/en-US/products_services/metalworking/tapping/thread_mills/thread_mills_products.jhtml

While designed for steel but have a limited range as designed not for
tilting to the correct thread angle
so will produce a concave shape to the thread form if out of design
range (I dont know the maths to show the effect)
but my first attempt did show the error as the tool is large in
diameter compared to thread angle

note this code lacks sensible centerline finding and b axis angle
although #4 is the value it should be based on
and reviewing it some comments are a bit off :)

e&oe

(metric worm cnc prog)
(cnc preamble)
G20 (set inches)
G64 (?)
G90 (set absolute)
G94 (set inches per min)
#<number_of_starts>=1   (#9  number of starts)
(for mod worm calc DP for it)

#<DP>=25 (#10 DP of system)
#<teeth>=60
#<pcd_wheel>=[#<teeth>/#<DP>]     (d ebug, pcd of wheel #14)
#<pcd_worm>=[[1.25-[#<pcd_wheel>/2]]*2]    (de bug, worm pitch dia df
#<pcd_worm>)
#<n_turns>= 6             (#11 number of turns in worm)
#<rotation_angle>=[360 * #<n_turns>]  (#1 set rotation degrees)

#<m>=[1/#<DP>]           (#6 m imperial module)


#<df>=[#<pcd_worm> / #<m>] (deb ug,q diameter factor #<df>)
#<df_od>=[4 / #<m>] (de bug,q diameter factor #<df_od>)

#7=[3.14159625*#<number_of_starts> * #<m>] (D EBUG,Lead =pi*1*#<m> L=#7 )
#8=[#<number_of_starts>/#<df>]
#9=[#<number_of_starts>/#<df_od>]
#2=[atan[#8]/[1] ]     (debug,angle of helix #2 angle to rotate
trunnion from vertical when I get one!!)
#<tp_angle>=[atan[#9]/[1] ]     (deb ug,angle of helix to rotate ar
tip #<tp_angle>)
#3=[#<n_turns>*#7]     (DEB UG,length of cut #3)
#4=[sin[#2] * #3]      (DE BUG,y=#4 offset to move to track worm centerline)
#5=[cos[#2] * #3]      (DE BUG,shortened Z=#5 because tilted)
#20=[#4]               (D EBUB,corrected direction y offset #20)
G0 X0 Y0 Z0 (move cutter centerline lowest point in line with axis and
touch off)
G0 F1 A-5 (rotate blank back to start+1)
G0 F1 A0 (rotate blank to draw center line)

G1 F.1 X-.063 Y0 Z0 A0 (move cutter to depth)
G1 F.05 x-.063 Y[0-#20] Z[0-#5] A[0-#<rotation_angle>] (rotate blank 7 turns)

G1 F1 X0 Y[0-#20] Z[0-#5] A[0-#<rotation_angle>] (move cutter out)

(G10 L2 P1 A0)

(G92 A-2520)
G0 A0 (rotate blank back to start)
G0 X0 Y0 Z0


if email buggers the above see
http://www.archivist.info/cnc/worm2.ngc


Dave Caroline

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