yes turning the taper on first.
Hello nissan300ztt,
The G76 cycle isn't perfect and there are compromises. but with a bit of trickery. most issues can be resolved.
Whatever value is set for the First DOC, each successive DOC will be diminished, relative to the First DOC by the control using the following algorithm:
DOC = SQR(N) x P
where:
N = the Nth number of threading pass - 1st, 2nd, 3rd and so on.
P = Specified First DOC
This sequence continues until the difference between the next DOC and the previous DOC is less than the Minimum DOC set in the first G76 Block by the "Q" address. From that point on, until the value of the specified "X" in the second G76 Block is reached, the specified Minimum DOC will be used. Without setting a minimum DOC, the DOC would continue to be set via the above algorithm where the DOC would eventually be equal to the minimum programmable increment of the control; usually 0.0001" for an Imperial configured machine. This very small DOC has two major affects:
1. It increase the number of Threading Passes extraordinarily and therefore, the cycle time.
and
2. Numerous, infinitesimally small DOCs will have an adverse affect on the Threading Insert.
As can be gleaned from the above, the smaller the First DOC, the smaller will be all successive DOC and the specified minimum DOC will be reachedearlier. Accordingly, my advice to my clients, is to use a First DOC as great as the cutting tool and the work-piece set up will tolerate.
Irrespective of how large the Thread Height may be, the major limit on the First DOC will be how much DOC the insert can consistently handle. For thread up to circa 8TPI (or 3mm pitch) the system works well. However, with very coarse threads with considerable Thread Profile height, it would be a benefit if the First DOC could be set at a value that the insert could handle, but the value used in the algorithm for calculating the next DOC could be larger. You can achieve this by specifying a greater Thread Height than actual and I believe this is what Ox was referring to, but using the "P" address in the wrong G76 Block.
With a male thread, the control calculates where the Major Diameter is by adding 2 x Thread Heights to the specified "X". The first and subsequent DOC are applied to this calculated Major Diameter. In both your Thread Cycle examples, you have specified a very small First DOC; 0.003" in one and 0.005" in the other. These values are what I would use as Minimum DOCs, not a First DOC and it will take only one threading pass for the next DOC in your first program example to be less that the specified Minimum DOC.
On just about all threads cut using the G76 Cycle, I start with a First DOC of 0.020” (0.5mm) and work up from there. This DOC is one that most inserts will handle easily on a wide range of materials.
So lets say that the you’re cutting a very coarse thread and therefore, a thread with a reasonably large Thread Height. You have found that a First DOC of 0.5mm is about all that the Threading Insert will handle consistently, but because of the large Thread Height, the minimum DOC will be reached with considerable Thread Depth still to cut and the minimum DOC value has kicked in. The way to get the Cycle to only take the 0.5mm DOC you’re comfortable with, yet take larger successive DOCs than would be the case if 0.5 is used in the calculation for subsequent DOC, you can do the following:
1. Set the Thread Height at 0.5mm larger than actual.
2. Set the First DOC at 1.0mm
With the above values set, the control will calculate the Major Diameter as being 1.0mm (diameter value – 0.5mm in radius) larger than it actually is. The First DOC of 1.0mm (radial value) will be applied to the diameter that is 0.5mm in radius larger than actual and therefore, 0.5mm of the 1.0mm First DOC will be lost in fresh air and only a 0.5mm DOC actually cut on the work-piece. However, the 1.0mm First DOC specified will be used in the calculation of successive DOCs and the minimum DOC won’t be reached so quickly.
If you're experiencing a very large First DOC, when you have 0.003" and 0.005" First DOC specified in your program, then something else is going on that's not being shown in the program snippet you have Posted. Accordingly, I suggest that you Post the whole of your program here for the Forum Members to see, including the finishing cut on the tapered thread blank.
Regards,
Bill
