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Cimatron Auto-Drill procedure supports, among other things, a special thread-mill cycle, which is not supported by the legacy Drill procedure. This cycle contains several dedicated parameters, and is expected to be executed by the machine itself, like other drill cycles.
However, many machines and controllers do not include built-in thread mill cycles. In such cases, the cycle has to be executed using standard milling operations (linear and circular motions). The cycle needs to be “exploded” to multiple milling motions. This used to be done inside the post-processor (GPP or GPP2) program, and was usually limited to the basic thread mill behavior. Options like multiple passes (at different depth) or tapered threads were hard to implement.
The feature in GPP2 does all that automatically, breaking the thread mill cycle into multiple milling motions (if so desired). Every standard PP can handle thread milling with no modifications needed in the PP at all.
See below for information on how to use the new feature, how to invoke it selectively even if the machine does support thread mill cycles, and the various GPP2 variables that control execution.
GPP2 will invoke the thread mill process (breaking the cycle into multiple milling motions) when the toolpath contains a thread-mill cycle (cycle type 14, variable CYCLE_SUB is 14) and the PP does not contain any block to handle this cycle (no CYCLE block with qualifier “specialon” or “special”). This indicates to GPP2 that the PP has no capability to handle thread-mill cycles through the machine, and the cycle is exploded.
Another way to invoke the cycle breakup process is available to the PP developer, from within the CYCLE block (any cycle type). For instance, suppose the machine can handle simple thread-mill cycles, but cannot handle tapered thread-mill cycles. In that case, the PP developer can check for tapered angle (see variables later), and if non-zero, raise a new GPP2 system flag (TRD_BREAK_MOTIONS) to TRUE_ (and not send anything to the machine). This instructs GPP2 to start the whole exploded thread-mill process.
See relevant variables in Block: CYCLE.
Note that not all parameters are actually set by the user as part of the Auto-Drill procedure. Some are internal to GPP2 and can be modified inside the EX2 program if the default values are not suitable. Others are received from the NC file.
Notes:
Auto-drill does not automatically support external threads, because it only identifies holes, not cylinders. However, it can be used for external thread milling, by manually selecting the points in the center of the cylinder top.
If the stock width does not divide perfectly to the side step, then the following algorithm is applied (similar to Profile procedure): the first passes differ by full side step, and the last pass removes the remainder. This way, a “finish” pass can be programmed.
The approach radius parameter is not always adhered to. If the gap between the tool and the hole is too small – a smaller approach radius will be used. The maximum radius used is half the difference between the thread radius and the tool radius.
When helical approach and retract are used, note that the tooltip will slightly descend below the hole defined depth (either in the approach in bottom-up mode, or in the retract in top-down mode). The maximum additional depth is equal to 1/8 of the thread pitch.
In tapered threads, the nominal diameter is always taken to represent the diameter at the top of the thread, both for internal and external threads. The taper direction is “inside” for internal threads (diameter becomes smaller with depth) and “outside” for external threads (diameter grows with depth). That way, undercuts are avoided. Milling direction (top-down or bottom up) has no effect on diameters or the taper direction.
A work-around can be applied for using GPP2 thread milling inside legacy drill. It requires marking the procedure in some way (For example, special comment, or just choosing one other cycle type to represent thread-mill). Inside the PP, the EX2 program must recognize the special marking and set the TRD_BREAK_MOTIONS flag.
Caution: Special care must be given to the parameters, such as nominal diameter and pitch, since they are not provided by the legacy drill procedure. Such parameters can be given through user-defined parameters and must be copied by the EX2 program to the proper thread-mill parameters before invoking the TRD_BREAK_MOTIONS flag.
The GPP2 thread-mill procedure also supports “dumb” rotary head machines, where a work-plane cannot be defined. When the thread axis is not aligned with the machine Z axis, GPP2 explodes the thread mill cycle to full 3-axes linear motions (no arcs are generated). In addition, 2D cutter compensation is disabled in such cases.
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