Need fast setups or 100% inspection of critical features? Do it with on-machine probing
By Jim Destefani
Engineers have debated the merits of using the machine tool as an inspection platform since the early 1970s--in other words, for as long as spindle probes have been around.
Benefits of the technology include consistent inspection of key features on every part, reduced scrap and rework rates, immediate detection of any machining errors, reduced post-process gaging requirements, and automatic compensation for tool wear and other changing process variables. And, inspecting parts while they're still on the machine dovetails nicely with current "done-in-one" manufacturing philosophies.
Perhaps the biggest potential gain from on-machine probing is fast and accurate setup. Machining programs can be referenced to part features rather than to one feature on the stationary fixture, cutting setup times to just a few minutes.
But many have been reluctant to apply on-machine probing for part setup and inspection. Simply putting a probe in the spindle does not turn a tired machine tool into a CMM. The probe will only be as precise and repeatable as the machine on which it's applied.
But the biggest objection to probing is generally based on time--time needed to program the measuring routines, and time out of the cut for the probe to perform its tasks. Complex programming also limited probe use in low-volume machining operations.
Accordingly, probe suppliers and software developers have put considerable effort into making the devices easier and faster to program and use. Here's a look at the current state of spindle-probe technology, as well as the story of one company that applied on-machine tool checking and inspection to achieve multiple benefits.
Advances in software for programming probing routines are one of the main drivers simplifying probe use. Packages for easier, faster programming of measuring routines are being developed by a variety of vendors, including probe suppliers, CAD/CAM developers, and suppliers of inspection software.
Marposs Corp. (Auburn Hills, MI), for example, launched the Instant Productivity Cycles (IPC) software package for use with its Mida touch probes at the recent IMTS in Chicago.
Machine operators simply jog the spindle probe to the approximate location of the part origin, enter a line of code, and start the cycle. The package determines offsets for all three axes automatically and enters them into the machine CNC. Marposs says the system allows setups to be completed in less than three minutes.
| PC-DMIS NC streamlines programming of probing routines using an interactive CAD model of the part and a computer-animated probe.
CAD/CAM software developer Gibbs and Associates (Moorpark, CA) worked with probe supplier Renishaw to produce a probing plug-in for the GibbsCAM package that lets users program probing routines for process control and inspection at the same time as they generate machining toolpaths. The Productivity+ plug-in will be jointly marketed by both companies worldwide.
The developers say the capability makes on-machine probing a more natural part of the engineering process, allowing use of the probe just like any other tool in the magazine. The plug-in supports probing routines to set tool length and diameter, perform workpiece identification and setup, in-process tool offset control, and part inspection. The entire process can be fully simulated before machine prove-out.
Renishaw also has developed Productivity+ plug-ins written specifically for various machine CNCs. The probing programs are processed within the controller at run time.
Yet another entrant in the probing software arena is Wilcox Associates Inc. (Elgin, IL), the developer of PC-DMIS control and data acquisition software for coordinate measuring machines. The company says its new PC-DMIS NC package can monitor CNC machines and generate performance trend reports in a format that can provide real-time process control.
The package's interactive graphical programming uses a CAD model of the part as a template to guide users through programming steps. Wilcox says this simplifies creation and implementation of automated setup measurement and in-process dimensional checking routines that are integrated with the machine's metal-removal operations.
Routines are translated into the appropriate controller code and integrated with the NC program via PC-DMIS' NC Server capability. NC Server also collects measurement results from the machine control, and organizes them in its database for analysis in parallel with the manufacturing process. The system is said to work with various combinations of serial or Ethernet machine tool interfaces, as well as with Fanuc, Siemens, Mazak, and other machine CNCs.
Probes themselves, meanwhile, keep getting more compact and more powerful. The TS 440 3-D touch-trigger probe from Heidenhain (Schaumburg, IL), for example, is suitable for repeated setup and inspection procedures on machines with tight work envelopes. The infrared unit can be as small as 0.49 X 63 mm, and offers 360º IR transmission.
Another Heidenhain probe offering, the TS 640, features an integrated blast unit consisting of three nozzles at the bottom of the touch probe. The nozzles use a blast of air or coolant to remove coarse contaminants from the area being probed, facilitating automated measuring. The unit's area of transmission is 7 m, making it suitable for use on larger machines without additional receiver units.
Both probes are supported by canned measuring cycles on the company's TNC controls and other CNCs. Automatic measurement cycles are available for such common geometries as bore holes, rectangular and circular pockets, slots, studs, bolt hole circles, and planes.
For users looking for a simple way to get into probing and speed setups, Command Tooling Systems (Ramsey, MN) offers its Zero Master 3-D edge locator. The device allows operators to quickly detect workpiece edges from the spindle centerline, eliminating errors and providing fast workpiece location.
In use, operators clamp the unit into the machine spindle and adjust TIR to zero. A digital readout indicates the distance between the spindle axis and the workpiece edge.
The battery-operated, splash-proof, and dust-proof device can be stored in the machine ATC, and is available in inch or metric versions. Command says the unit measures accurately and repeatably down to 0.0004" (0.01 mm). It can also be used to inspect the machine bed or measure part flatness.
One company that has benefited from application of spindle probes for setup and inspection is Tech Machine (Colorado Springs, CO). The 25-year-old outfit combines probing, toolsetting, calibration, and pallet fixturing technologies to achieve fast setups and automated machining of complex-geometry parts for medical applications.
As the shop's volume of intricate medical work increased, scrap rates climbed--sometimes as high as the 50% range. Owner David Wiggans eventually decided to go into on-machine probing to solve the scrap problem, but not without doing his homework.
"We conducted a six-month study and attended every seminar we could find," Wiggans recalls. "Quite honestly, when we started the study, I thought we had a personnel problem. But I came to see we had a management problem. We made the decision to build our operation around the use of probing, and made the commitment to master the technology."
Wiggans heard the standard "out-of-cut time" objection to probing. "But if a probing routine that takes 30 sec saves me from ruining one part in a family fixture, it is well worth it," he says. "The probing time is immaterial--if you have a 20-min cycle time but scrap the part, it will take you 40 min to make one good part."
Each of Tech Machine's 13 VMCs now operates with an automation system based on a pallet fixturing system coupled with machine-tool probes. "Our machines won't make a chip until the probing system verifies the batched parts are properly fixtured and all necessary offsets are calculated," Wiggans explains. "As a result, our scrap rate has gone from 50% on some parts to virtually zero on everything."
Tech Machine runs two 10-hr shifts, six days a week, with only three operators needed to keep the 13 machining centers running. The machines can run untended into the third shift to gain extra productivity. "Using the probes, we can now leave the machines running every night and get at least one cycle out of them," Wiggans says.
The stainless and titanium parts Tech Machine produces have complex contours and geometries, and often require tolerances to 0.0001" (0.003 mm) and super-smooth surface finishes. "Flowing contours made the parts difficult to accurately fixture," Wiggans says. "Probing ensures the machine knows exactly where the part is located on the fixture."
Tech Machine's probing process begins by having a Renishaw probe identify the multiple-part pallet loaded onto the machine bed. The probe then identifies the type and size of parts, and their locations in the fixture.
A family of medical parts may have only slight differences, such as number of holes or other features. This allowed Tech Machine to create one master program for an entire part family, in place of individual programs for each part. Probing identifies the part numbers--as many as 14 different part numbers on each fixture - and their respective position in the fixture. The control then knows which parts to run from the family machining program.
The probe aligns the program to the part and checks its thickness, then finds the end of the part and resets the geometry. Prior to probing, 20% of the parts being run were scrapped because of improper offsets being entered by operators. The probing software now calculates and applies all offsets automatically.
The system has essentially reduced setup time to zero, according to Wiggans. "We don't have any setup downtime," he says. "Parts are fixtured offline on a pallet while the machine is cutting other parts. We just slide out the finished pallet and slide in the next batch for probing and machining. Now our spindles run all the time. This is key to meeting our JIT production schedules."
Probing also eliminates the use of expensive workholding fixtures, he says. "We no longer need high-dollar fixtures that are perfectly positioned and aligned with everything in precisely the same plane. We get by with inexpensive fixtures because the probe locates everything and compensates before machining."
Wiggans redesigned all fixtures around the use of the probe, eliminating locating pins, and wrote probe macros to find the fixture and each part automatically. Tech Machine went from 200 part machining programs to five macro programs, and one employee can run as many as three families of parts.
Probing is also used for tool presetting and broken tool detection. With the probes inspecting setups and checking tools, plus routine machine calibration, the VMCs are virtually uncrashable, according to Wiggans. "For example, if an operator installs a pallet without clamping it down, the probe will detect that condition right away and provide an error message," he noted. "If the machine ran with an unclamped pallet, every tool in the machine, the fixture, and parts could be ruined. I can't think of a machine that's crashed in our shop since we implemented the probing system."
This article was first published in the November 2004 edition of Manufacturing Engineering magazine.