The DWO feature in the Haas control is a very powerful tool but understanding the limits and the benefits is absolutely critical to success.
The old way of doing things:
A typical multi-face setup requires that the fixture datum be located precisely on the center of rotation of the rotary axis. Then the job is usually programmed with a unique work offset for each face. If the programmer moves one of the offsets then he needs to recalculate and translate each of the other offsets. This is reasonably straight-forward if the part is square and the work offsets are at the 90-degree faces. It gets a lot more complicated when there are features at odd angles – and an order of magnitude worse when there are compound angles involved. If the fixture is removed and reinstalled then the whole setup process starts over again and each work offset gets reset. If the castings aren’t cleaning up and the machinist wants to move the datum face in 0.010” then all of the other work offsets get recalculated and reset.
The DWO functionality makes all of these problems disappear on the right kind of job. Consider the cylinder head pictured to the left. The combustion chamber side of the casting has features machined from eight different angles. The rocker shaft side of the head has features at two more orientations. That’s ten different work offsets to manage.
If this job is programmed and set up using DWO then all features are referenced back to a single work offset. If the head gasket face doesn’t clean up simply move the work offset in 0.010” and all of the other features move along with it; no changes to the CNC program or to other work offsets are required. If the fixture is removed and reinstalled the machinist simply needs to probe the original datum (or probe a known tooling point on the fixture) to re-establish G54 and the job can be back up and running in minutes.
The Haas DWO function relies on a very accurate measurement of the relationship between the B-axis and the C-axis. This relationship is heavily dependent on how precisely the machine is leveled. The UMC is configured with probing as a standard feature and the CNC control has a conversational probing template that walks the user through the process of recalibrating MRZP offsets that drive the DWO calculations. Bear in mind that the process is only as accurate as the results of the probing cycle. The WIPS work probe is extraordinarily accurate, but it is not a laboratory-grade inspection device – nor is the inside of a machine enclosure as clean as a laboratory. It is reasonable to expect a couple tenthousandths of an inch of error one way or the other when probing the included calibration tooling ball. The MRZP calculations probe the ball at many different locations to minimize the effects of these tiny probing errors, but there might be small amounts of error nonetheless.
Recognize also that the B- and C-axes of the machine are remarkably accurate; but like the work probe, they aren’t perfect. No rotary table from any manufacturer is. Understand that an angular positioning error of 20 arc-seconds, while tiny, turns into a linear positioning error of 0.001” at 10” from center and 0.002” at 20” from center.
If the MRZP offsets have 0.0005” of error in the X-axis direction then a hole bored halfway through a part at B90 and then finished from the other side of the part at B-90 would have 0.001” mismatch just from the slight inaccuracies in the MRZP offsets. But B-90 is outside of the travel limits of the UMC. This kind of feature needs to be machined halfway through at B90 and C0, and then finished at B90 and C180. So now the part might have a small true position error along the X-axis from any inaccuracy in the MRZP X-axis and Z-axis offsets combined with a small mismatch from both sides in the Y-axis direction from any inaccuracy in the MRZP y-axis offset. Now add in any small deviation that could come from B axis and C-axis positioning errors and it is easy to see that true positional errors of a few thousandths of an inch aren’t unreasonable when using the machine this way.
If the customer’s job has reasonable milling machine tolerances of +/- a couple thousandths of an inch then DWO is a good choice. If the customer is going to tear his job down and set is back up frequently then DWO is a good choice. If the customer’s job requires exquisite true-positional accuracy then DWO is not a good choice. This job probably requires individual work offsets for each face. If the customer wants to use DWO for this job he still can, but he needs to accept the compromises he will have to make. He might need to manually adjust his MRZP offsets. He might need to adjust some of the programmed points in his CNC program.