Clever 14-axis water jet cutter cuts costs

A specialised water jet cutting application whose main purpose is to cut heavy plate steel with four independently spaced heads needed "tangential following" for tooling control.

A specialised water jet cutting application whose main purpose is to cut heavy plate steel with four independently spaced heads needed "tangential following" for tooling control.

Due to the plate material being from 19 to 32mm thickness and up to 3.5 x 3.5m, surface mapping was a must to control stand-off of tools automatically and independently for each head.

The system also had to remove the waste or cutouts automatically as the program was run to minimise time between plates as there could be over 2000 circle cutouts of scrap per plate.

The basic design of the machine was a large gantry and the design challenges were related mostly to the scrap removal system mechanics.

Therefore the real project challenges were related to controller capabilities as they applied to the independent control of the four heads with regard to surface mapping, tangential following, custom G and M codes, total number of axes etc The principle feature for the PMAC was the ability to control 32 axes which of course more than covered the 16 channels needed.

Next, the ability due to tangential following through the use of the inverse kinematics buffering, along with the I5186 dual federate control, to maximise and control the C1-C4 axes speeds.

Then there is the ability to slave to the commanded position Vs the actual position, which allowed the use of axis, commands on top of the slaved positions.

This allows incremental adjustments to each of the head assemblies on the fly while the program is running.

Another important feature was the ability to implement 2D-planer compensation tables for each of the four head assemblies as a means of surface mapping of the plate.

The PMAC allowed setting up user G and M code definitions and permitting the decimal values for the G and M codes such as M11.1, G60.12 etc.

Several of these types of definitions were used to control the scrap removal, automatic head spacing, rotary tangential control for example.

Finally, the facility to interrupt a running program, control motion of the system and return to the interrupt point and continue.

(jog retract and program retrace features) This was used to control part of the scrap removal system.

The scrap removal was implemented as a combination of M-codes and PLC control with each head having the ability to pick up scrap based on the x-y value passed with an M-code.

Each head capacity has approximately 40 pieces of the cut scrap depending on sise.

Battery backed parameters and counters are set-up on the DIAG (pages dat) to display and override control of this system.

These parameters would keep track of the number of scrap pieces achieved as part of the program and would stop and jog the X bridge to the end of travel where trap doors would open to dump the scrap from each head into a large hopper.

The doors then would close and the X bridge would return to the pre-jog position and resume the program.

Bram parameters were very beneficial for this purpose by maintaining the counters even when the machine was shut down Inverse kinematics buffering was used when the tangential control was part of the program where the implementation of this routine was a combination of G code (G43.2 and G49.2), M code (M70) and PLC.

The G43.2 would set parameters for the kinematics (tangential) and set a flag for the PLC to re-address the co-ordinate system and the PLC would resume program execution.

The M70 was used as a synchronous method of control and incremental tangential offset for drag control.

This is one of the most unique features that have been implemented for this application.

The C axes were un-fraxed in this mode so that I5186 would optimise the speed of the rotary at all times.

Surface mapping was probably the most time consuming of all as after the first implementation it was found that the plate surface changed dramatically as the part was cut.

This required a new technique through use of the 2D-planer comp table definitions and so a separate 2D table was defined for the entire work envelope of each of the four heads.

Then the memory address of each entry in the table was determined and associated with M-variable definitions.

Through the use of an M code and an x value, basically the probes would store data for one hole at a time, one row of holes at a time, or the entire plate.

Once data are collected the bridge moves to the machine zero position to prevent any axes from jumping due to a step change in position.

Then the calculated data is written directly into the comp table memory locations and this is done with all I52 active.

Once the entries are written out and the axis moves the new values then take effect.

Program retrace combined with jog retract has become a very powerful feature in the Turbo-PMAC series especially when joined with the extended memory option (ex Opt-5C).

An operator now presses a button while the part program is running and the program is reversed until the operator lets go of the button which shuts the heads off and places the machine in feed hold until forward motion is resumed with a cycle start.

The distance that the retrace can backup through a program is based on "look ahead" definition compared with the setting of I5120.

With the extended memory option, the program can backup a long way usually to the last dwell or rapid move.

Most controls do not allow this and the ones that do only allow two to three blocks of retrace.

With Turbo-PMAC only memory size and the type of blocks being run limit the implementation.

Jog retract becomes important when a nozzle blows out or abrasive lines clog.

Then the operator can program retrace to the desired location followed by jogging the axes to wherever convenient to correct the problem.

The next cycle start will first go to the pre-jog position before turning the heads back on and continuing through the part program.

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