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GCode is Complicated
G-Wizard Makes it Easy

New Feature: G-Wizard Editor Makes G76 Threading Cycles Easier

Apr 26, 2016   //   by Bob Warfield   //   Blog, GCode, Products, Software  //  No Comments

The G76 threading cycle is probably the most common g-code for cutting threads used on CNC lathes.  BTW, that link will take you to the chapter on programming G76 cycles that is part of our Free GCode Tutorial.  It’s chock full of cool GCode Programming tips and information, so whether you’re learning or just trying to get better, it’ll help!

While G76 is very commonly used, it can be complex and mastering it isn’t easy.  There are a variety of different syntaxes, depending on which G-Code dialect your CNC Controller uses.  For example, Fanuc has offered both 2 line and 1 line versions, on their controls.  The 2 line Fanuc G76 works something like this:

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)

    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 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.

Phew! That’s a lot of parameters needed to specify how to cut a thread.  And they can be a bit tricky.  For example, the Minimum Depth (Q Word) can override the roughing and the finish allowance, which might not be what you want.  Or consider that the P and Q words have no decimal point and are divided by 1000 to determine measurement.  If we have a thread of height 0.938″, it is expressed as P938, not P.938.  That’s the kind of detail that’s important to get right.

One of G-Wizard Editor’s most useful features is its ability to simplify G-Code understanding by telling you what each line of code means in simple English.  Let’s say we want to do a tapered pipe thread with a G76.  In GW Editor, it might look like this:

Fanuc2LineG76

Fanuc 2 line G76 cutting a Tapered Pipe Thread…

Notice the Hint below the backplot.  G-Wizard has unpacked a ton of useful information to help you decode what the rather cryptic G76 is doing:

  • You get the thread starting and ending coordinates.
  • You get Thread Pitch and Threads Per Unit, in this case it’s 8 TPI.
  • You get the Thread Height
  • First Pass Depth and Finish Allowance are Called Out
  • The Infeed Angle is specified
  • The Thread Taper is there
  • We can see there will be 2 Spring Passes
  • The Chamfer Pull Off Angle will be 10 degrees
  • Most interesting is a calculation of the number of passes that will be made (37), how many are clamped by the min cut depth (25), and a warning that the finish allowance will be ignored because of the clamping.  That means the thread is not finished and some sort of subsequent pass will be needed or we’ll need to reduce the min cut depth to be less than the finish allowance.  Min Cut Depth, BTW, was specified in the first line and is listed on that hint.

That’s a lot of very helpful information that is otherwise pretty hard to come by.  It can really help when hand programming G76’s or when you want to really understand what a G76 is doing.  For example, 37 passes seems like a lot.  Perhaps you want to try to tune this up to use fewer passes, and to make sure that the finish pass is taken.

If you haven’t tried G-Wizard Editor, it’s full of these kinds of handy helpful tools and features.  Check out a free 30-day trial today.

 

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