Passes
The number of passes that must be cut to make your thread is very important. Take too few passes, and surface finish is apt to be poor and you might even break your threading tool by forcing it to work too hard. Take too many passes and you’re going to waste a lot of time.
You can’t change most of the information relating to the thread’s specifications, so your primary tools for controlling the number of passes include:
– Start Position: Turn things down as I describe above to minimize the work the threading tool must do.
– First Pass Depth: Pick the largest pass you can. G-Wizard Calculator will give you a good recommendation here.
– Minimum Pass Depth: Try to avoid using this parameter too much and set it to your Finish Allowance.
– Finish Allowance: A smaller finish allowance can mean larger roughing passes remove most of the material. Just remember, too small an allowance will force your cutter to rub.
– Spring Passes: You shouldn’t need more than 2 passes and 1 may suffice. Experiment with your particular situation to see if you can get by with 1 or perhaps even no extra passes.
Your next challenge will be in determining how many passes the cycle will actually make. This is not easy as G76 threading cycle will dynamically change the depth of each pass after the first to equalize the amount of material removed. You have to do quite a lot of calculation to figure out exactly how many passes will be made.
But there if you have a GCode Simulator, it may be able to help out. Take a look at this screen shot of G-Wizard Editor:
G-Wizard Editor will tell you in the hint below the backplot how many passes the G76 threading cycle will take…
G-Wizard Editor will tell you in the hint below the backplot how many passes the G76 threading cycle will take. You can use it to help tune your G76 threading cycle so it doesn’t have an excessive number of passes. Note the WARNING message given that tells you the finish allowance will not be used due to the Minimum Cut Depth being larger. That’s also helpful when setting all this up.
Don’t Feed Too Fast
Many lathes have problems synchronizing the spindle if fed too quickly. If your threading passes are not synchronizing, try slowing down until it improves. Typically, this means slowing the RPM, which determines how fast you’ll be feeding based on your thread pitch.
Code Dialects for G76 Threading Cycle
Hopefully you’ve gathered up the values for all the parameters described above. Perhaps you can use a spreadsheet to make that process easier and more complete. Now you’re ready to plug the parameters into the particular GCode dialect used by your cnc control:
Fanuc Double Line G76 Threading Cycle
Here is the syntax for the Fanuc CNC Control:
G76 P(m) (r) (a) Q(dmin) R(d)
G76 X(U) Z(W) R(i) P(k) Q(d) F(L)
P Word: The P-word has 6 digits consisting of three 2-digit clusters for m, r, and a.
m: Repetitive finishing count (1 to 99)–spring passes.
r: Chamfering amount (1 to 99)
a: Angle of Tool Nose. Select 80, 60, 55, 30, 29 or 0 degrees.
Q Word: dmin is the Minimum Cutting Depth. If the depth of either a roughing or finish pass is less than this, it is clamped to be at least this much.
R Word: d is the finish allowance.
X/Z/U/W words (2nd line): Specify the coordinates of the end point. X, Z use the current mode (absolute or relative) while U, W can be used to specify a relative position.
R Word (2nd line): i is the taper amount when cutting tapered threads.
P Word (2nd line): k is the thread height (thread depth) expressed as a radius (not diameter) value.
Q Word (2nd line): d is the depth of the first cut.
F Word (2nd line): L is the lead of the thread.
Example: Fanuc Controls 2 line G76 threading cycle cutting a tapered pipe thread:
G-Wizard Editor will tell you in the hint below the backplot what all the G76 threading cycle parameters are doing…
Fanuc Single Line G76 Threading Cycle
G76 X.. Z.. I.. K.. D.. F.. A.. P..
X = Diameter of last threading pass
Z = Position of the thread end
I = Taper over total length
K = Thread Depth: Single depth of the thread – positive
D = Depth of first threading pass – positive
A = Included angle of the insert – positive
P = Infeed method (one of 4)
Haas G76 Threading Cycle
G76 D.. K.. X.. Z.. U.. W.. I.. P.. F.. A..
D = Initial cut depth
K = Thread height (ala Thread Depth)
X* = X-axis absolute ending location
Z* = Z-axis absolute ending location. Determines thread length.
U* = X-axis incremental distance to end. May be used instead of X.
W* = Z-axis incremental distance to end. May be used instead of Z.
I* = Thread taper amount (radius measure).
P* = Subsequent pass positioning method (1-4)
Q* = Thread Start Angle (do not use a decimal point)
F* = Feedrate
A* = Tool nose angle (0 -120 degrees. 0 assumed if not specified)
LinuxCNC / PathPilot G76 Threading Cycle
G76 P.. Z.. I.. J.. R.. K.. Q.. H.. E.. L..
P = Thread pitch in distance per revolution
Z = Final position of threads
I = Thread Peak offset. Negative for external, positive for internal.
J = Initial cut depth
K = Full thread depth
R = Depth digression (optional). R = 1 is constant depth, R =2 is constant areas.
Q = Compound slide angle (optional)
H = Spring passes (optional)
E = Distance along drive line for taper
L = Which end of the thread gets tapered. L0 = no taper. L1 = entry taper. L2 = exit taper. L3 = entry and exit taper.
Mach 3 G76 Threading Cycle
G76 X.. Z.. Q.. P.. H.. I.. R.. K.. L.. C.. B.. T.. J..
X = X end
Z = Z end
Q = Spring passes (optional)
P = Pitch
H = Depth of first pass
I = Infeed angle
R = X Start (optional)
K = Z Start (optional)
L = chamfer (optional)
C = X Clearance
B = Depth Last Pass (optional)
T = Taper (optional)
J = Minimum depth per pass (optional)
