What's the Condition of that Tool?
It's worth a closer look
By Robert B. Aronson
A missing, broken, worn, or wrong tool can cause deadly downtime or generate costly trash. And because many jobs are bid on the basis of how long the cutting tools will last, performance data on edge wear and coating condition are critical. Plus, the ever increasing precision requirements of the medical, aerospace, and auto industries have brought fresh challenges to cutting tool metrology. That's why it's usually worthwhile to take advantage of a cutting-tool monitoring instrument.
Tool-checking instruments use either a contact or a noncontact probe. In the contact type, data are generated when the probe finger touches the tool or part. The noncontact type uses a beam, usually a laser. Reflection of the beam off the part or tool to a receiver generates the data.
Several manufacturers of laser-probe-based instruments protect them from the operating environment with its flying chips and coolant. The laser is housed in a closed container. When making a measurement, a container window opens. The laser makes its scan while a stream of air keeps the chips and fluid away. After the scan, the container door closes.
The contact probe is adequate for many applications, but has its limitations.
- Some require that the spindle be stopped to take a measurement, so dynamic testing is limited,
- They can only sense a limited number of features, chiefly tool dimensions,
- Fixturing has to be considered, so the probe is not prevented from touching the part,
- They can't be used on very delicate parts, and
- If the tool is broken they work well, but if the error is something like one flute on a tap, then the fault might be missed.
Probes can be powered by external cables or batteries. The batteries are an advantage where it's difficult to run wire to the probe or there is a hostile environment. With these units, it is important that ambient signals not interfere with the signals sent by the tool sensor.
For tool length and diameter measurement only, M&H Inprocess USA (Double Oak, TX) offers the 35.40-TS toolsetter. It's mounted in a docking station fitted to the machine tool's worktable. To make a tool-length measurement, the machine's spindle picks up the toolsetter and positions it near the tool. A probe scans the tool and sends dimension data to a receiver.
The tool measuring and inspection instruments from Parlec (Fairport, NY) carry out initial inspections of the tool. A high-resolution video camera scans the part. The light intensity and direction are adjusted to suit the tool being checked. These units can also check for tool wear or broken tools during the machining process.
"The 50x magnification measures and inspects tools as small as 0.0001" [0.0025 mm]," says Chris Nuccitelli, product manager for the company's tool measurement and inspection systems.
The camera sends images to a computer or a technician's screen for evaluation. The cameras are also programmed to measure and report certain critical dimensions such as tool length and diameter.
"Visual inspection includes checks for tool geometry, wear, and cracks," says Nuccitelli. "In addition, certain data, such as wear compensation or tool offsets, are sent directly to the machine's control. The operating program then compensates as needed."
Renishaw offers several instruments that can be used for tool evaluation.
They include the compact NC4 used for noncontact tool setting and tool-breakage detection. It's available in a separate transmitter/receiver version with units that measure a 30 x 35-mm area and tool diameters down to 0.2 mm. The unit detects breakage of tools as small as 0.1 mm.
The NC1 is available with transmitter and receiver premounted as a single unit, or as individual modules with beam length up to 2 m. The NC3 measures 135 x 26 x 77 mm for use on small machining centers and high-speed cutting machines.
The TRS2 broken-tool detection system combines the laser source and detection electronics in a single unit measuring 83 x 38 x 73 mm. The unit detects solid-center tools, including drills, taps, reamers, slot drills, standard and ball-nose end mills, and gundrills as small as 0.2 mm. The laser system offers a 0.3–2.0-m detection range.
With Toolwise programming, the TRS2 distinguishes good tools from broken tools. It analyzes reflected light patterns from the rotating tool as it enters the beam, rejecting random light patterns created by coolant or chips. Tool edges generate a regular light pattern as the tool spins, while chips and coolant give random reflections. A repeating pattern represents an intact tool while a nonrepeating pattern indicates a broken tool.
The cable-free Twin Probe System combines part setup and measurement with tool setting and broken-tool detection. The cableless toolsetter is particularly suited to machines with twin pallets or rotary tables. Its optical transmission system is designed to resist light interference.
The Renishaw probes are carried in the tool magazine and programmed into the part-processing routine. Transferred to the spindle by the automated toolchanger, a probe is used to locate the part on set up, to perform in-process measurements and compensations, and to automatically reset machine coordinates for changing processes, axes, and part faces. The 360o infrared optical transmission system lets the probe operate in any spindle orientation.
For milling machines, Mida, a division of Marposs Corp. (Auburn Hills, MI) offers both contact (touch setter) and noncontact (laser) tool measuring devices. "Contact-type touch setters are limited in that they can only measure tool length and tool breakage, and give a rough evaluation of tool diameter, because the tool is measured at a max of 400 rpm," says Sharad Mundra, product manager for the Mida division of Marposs. "The measuring cycle is slower because the control has to compare a reading against a reference. But these systems are lower cost than the noncontact laser system and accurate to 1 µm in repeatability for length measurement. And there are no problems with flowing coolant or flying chips."
Mida offers a wireless system that enables the installation of both tool setting and part-inspection probes using a single wireless interface. The Mida Optical Twin Probe System has a compact receiver interface (ORI). This design simplifies probing, especially where connecting cables and line of sight proves difficult, such as installations on machines with rotary tables, twin pallets, and multiaxis machines. The probe system produces data on tool length and diameter, detects breakage, sets the part up before cutting, and then inspects the part after cutting to verify the cutting operation.
"The noncontact laser-probe system, such as the Mida Laser 75P, is faster, takes dynamic measurements, and checks runout, and tool wear," explains Mundra. "Because the unit detects points as small as 20 µm, it can evaluate the edge wear on cutting tools. Advanced laser systems have programs in the software that can evaluate scan readings, distinguish between refraction from the coolant in the cutting atmosphere and reflections from the actual tool, and filter out false triggers and readings generated by coolant splash, mist, and residual coolant drops."
Longer machining cycle time is a trend, particularly with mold and die makers. A Mida unit can monitor wear during the operating cycle, and when necessary change the offset to compensate for tool wear. When a worn tool is found, machining can continue untended when redundant tooling is used.
A separate type of instrument is needed for turning operations, chiefly because the tool is not rotating. The probes are touch-type and available in both manual and automatic versions. In an effort to achieve the goal of "first part/good part," the "cut and reset" setup method is not convenient. Instead, before the cycle begins the instrument/toolsetter measures the tools to be used, checks their position against the programmed ideal, and presets tool positions accordingly. During the operating cycle the system modifies tool positions to compensate for wear and thermal drift, as well as monitoring offset.
The Marposs Mida 3-D Shape Inspector software is a CAD-based measurement routine that turns the machine tool into a virtual CMM. It is used with probe sensors to check both parts and tools. A user works offline and generates the G code for all the points that must be touched. The generated G code is entered into the machine control and runs the probing routine. The touch data from the probe is then transferred back into the 3-D Shape Inspector program to generate a report or to make the tool and work offset adjustments as required.
Blum Laser Measurement Technology Inc. (Erlanger, KY) offers a variety of contact and noncontact instruments, but laser noncontact units used for dynamic testing are their specialty.
The laser is a better instrument for production work. "Lasers measure tool lengths and diameters, as well as tell if a tool is broken or missing," explains Blum's General Manager, Paul D. Meinhardt. "This instrument can also check the tool edge for quality and wear. This capability is particularly important for mold and die milling and unmanned operation. For these operations, knowledge about tool wear is critical, because the tool may run a long time untended."
The units are normally built into the machine tool, often as OEM equipment, and therefore must have a high resistance to coolant and chip intrusion.
"It is important to check tool runout before cutting begins," says Meinhardt. "For example, if even a single chip gets into the toolholder during a tool change, a reamer becomes a boring bar."
Each system has its own calibration tool, which is in a shrink-fit toolholder.
The calibration takes only 15 sec, and is done when needed. System accuracy depends on laser focus. Absolute (tool to tool) accuracy depends on laser beam focus. The smaller the focus diameter the better the absolute accuracy. Blum Laser can measure tools as small as 0.005 mm. Some machines require a split laser system with up to 203 cm distance, with a special focus beam, tools as small as 1- mm diameter can be measured.
Some of the Blum instruments are used in conjunction with Caron Engineering (Wells, ME) units on production machinery. They are mounted on the machine tool and sense changes in the power drawn by the spindle or drive motors. Any change may indicate some problem. When draw is higher than normal, it might mean the tool is wearing. If it drops, possibly the tool is gone. The monitor then signals the machine to take corrective action before tools or parts are destroyed. Aberrations in power to positioning motors are also detected.
M&H, Probing Systems (Berlington, Ontario), a Hexagon company, offers three types of onboard tool setters.
- A manual mount version measures lengths or diameters.
- A "pick up" version stored in the tool magazine, and placed when needed, measures the tool's length and diameter. It is mounted off the side of the pallet where it isn't in the way, or put away entirely when not needed.
- A "length-only" version.
A battery-powered switch-based touch probe does the measuring. Data are sent from the probe to a receiver. On three-axis machines, or where there is a clear line of sight from the problem to the receiver, infrared can be the data link. If there is no line of sight, an RF signal carries the data.
Tool-Probing System Jobs
- Tool measurement
- Machine accuracy verification
- Setup checks
- Offset check
- Some probes supply data for part inspection.
- Setup check before the process begins.
- Check tolerances during the process.
- Verify part dimensions after the process.
Reducing setup time and clamping accuracy is a benefit claimed by some Instrument manufacturers. The workpiece is clamped without requiring a lot of measuring. Then, when the probe is activated, it registers contact with the workpiece. From that data point, the control calculates the exact position of the part relative to the cutting head and begins the work cycle.
Here are some questions to ask when shopping for a probe.
- Do you want contact or noncontact?
- Is it temperature sensitive?
- How is it calibrated, and what does it cost?
- What's the smallest dimension that can be detected?
- Do you want a wireless system?
- Is it used on or off the machine?
- What tool conditions should it check (diameter, length, edge wear, cracks)?
- Is it to be used preprocess, in-process, or/and postprocess?
- Can it be carried in a conventional tool magazine?
This article was first published in the January 2009 edition of Manufacturing Engineering magazine.