GB2305502A - Detection of tool failure - Google Patents

Abstract

Detecting breakage or wear of the tip of a machining tool (2) involves measuring the temperature of at least part of the tip after a machining operation. Too high a temperature indicates a worn tip, and too low a temperature indicates a broken tip. An alarm may be sounded or the machining halted on detection of breakage or wear.

Description

DETECTION OF TOOL FAILURE The present invention relates to a method and apparatus for detecting breakage or unacceptable wear of the tip of a machining tool.

A machining tool is typically changed upon completion of a pre-determined number of cycles. This can result in machining cycles using worn tooling, or premature removal of a good tool.

It is therefore desirable to be able to detect when a machining tool tip is broken or worn to a point where the tool cannot perform the machining operation with sufficient accuracy or precision. Failure to detect such a worn or broken tip will result in the production of machined parts which do not meet the required quality standards.

Detection of tip damage by eye is unreliable and labour intensive, so it has been proposed to monitor tool wear by comparing the average power consumption during a machining cycle with an average power consumption value taken from a sample of a number of machining cycles. The idea is that the power consumption will change as the tool tip becomes blunt or broken. A problem with this method is that for a tool with multiple heads or multiple machining tips, the measurement does not indicate which particular tip or tips are defective.

Furthermore a broken tip (reduced friction) will use less power than an unbroken tip, and a worn tip (increased friction) will use more power than an unworn tip. The effects of a broken tip and a worn tip on a tool with multiple machining tips may therefore to some extent cancel each other out and escape detection.

We have now found that by measuring the temperature of the tip of the tool soon after completion of the machining operation, it is possible quickly to determine whether the tip is unacceptably worn or broken.

According to a first aspect of the present invention there is provided a method for detecting breakage or wear of a tip of a machining tool, the method comprising the steps of measuring the temperature of at least part of the tip by means of an infrared detector after completion of a machining operation and before the tip cools to ambient temperature, and determining whether that temperature falls within a predetermined acceptable temperature range.

The method allows the quick identification of a tool tip which is broken or unacceptably worn.

If the tip is broken, a low temperature will be measured, because some or all of the tip is missing and therefore is not radiating.

An unacceptably worn tip will radiate more infrared radiation than an acceptable tip because of greater friction between the leading edge of the tip and the workpiece which is being machined. The measured temperature will therefore be greater.

The term "temperature" is used herein for convenience. It is not essential that a temperature measurement be made in degrees Celsius, Kelvin, or the like. The rate of energy emission (ie power) could be measured, which can be related to temperature by application of Stefan's Law.

All that is required is to know what temperature band is measured for an acceptable tip, under similar conditions as the test measurements are made. The limits for the band may be determined by carrying out measurements on tools with known acceptable tips, and on tools known to have unacceptably worn or broken tips. The limits may then be set accordingly.

The acceptable limits will vary according to the nature of the machining operation, and the tolerance requirement for the job. The broader the band, the greater the range of acceptable tool tips. The limits may readily be determined empirically by a skilled person in the light of the above operational standards.

The comparison of the measured value with the stored values is preferably carried out by means of a suitably programmed computer or dedicated microprocessor unit.

However the comparison could also be carried out-manually by an operator.

For convenience the term "computer" will be used herein to refer to any electrical or electronic means for carrying out the comparison step.

Where different tool tips are used and/or tips are constructed of different materials, the computer is preferably programmed with suitable data for each different type of tool tip. The computer could automatically sense which type of tip is being used, and adjust its program accordingly, or the operator could input the relevant information about the tool tip and select the required program for that tip.

Where a computer is used, it may be provided with means for automatically halting further machining operations, and for removing a worn or broken tool tip, or optionally the whole tool, and replacing the tip or tool with an equivalent undamaged part.

Alternatively, or additionally, the computer could be programmed to halt the machining operation and alert an operator to replace the damaged part manually. Halting the machining operation as soon as a damaged part is detected would reduce the number of unacceptable machined parts made by faulty machining with the worn or broken tool.

Any convenient component part of the tool which carries the tip could also be removed and replaced, automatically or manually, if desired.

The measured value may be calculated by reference to the temperature of the tool tip just befdre it begins the machining operation, for example by subtracting the "before" value from the "after" value, and the reference values could be obtained in a similar manner.

For a tool with multiple tips and/or heads, for example a milling cutter, the tool may be rotated in front of the sensor so as to bring each tip to the same predetermined position and orientation, and the temperature may be measured for each tip. Measurement of the first tip will indicate whether any of the tips is likely to be too worn, and a missing infrared pulse will indicate whether each other tip is missing or broken.

The computer may be preprogrammed with the expected number of tips in the tool, and to take suitable action if one is found to be missing. Suitable action may include halting the machining operation, replacing the tool or the missing tip, or alerting an operator to intervene manually.

Typically the operator would change the tool, and remove the tool with the worn or broken tip to a tool setting room to be maintained and re-set.

The tool tip will remain hot for several seconds after the machining operation has been completed, and the temperature measurement may be made at any time when the tip is still significantly hotter than its environment.

However more accurate results will be obtained if the measurement is taken as soon after completion of the machining operation as is practicable. Preferably the measurement is taken within about two seconds of completion of the machining operation. It is particularly preferred that the measurement is made within about one second of completion of the machining operation.

For consistency of results, and for simplicity, it is desirable that all measurements should be made at the same time interval after completion of machining. If different time intervals were to be used, it would be necessary to introduce weightings or other calculations to relate the different measurements to one another.

A single point infra-red sensor may be used for the temperature measurement. However it is envisaged that a multi-point sensor or thermal imaging system may be used if it is desired to measure a temperature gradient across the tool tip and so gauge the extent of tool breakage or wear more precisely.

Because the environment around the machining operation may an undesirable one for the location of sensing equipment, it is preferred that the sensor is remote from the machining operation, and gathers temperature information via a fibre optic cable or bundle.

According to a second aspect of the invention, there is provided an instrument for use in the method, for detecting breakage or wear of a tip of a machining tool after completion of a machining operation and before the tip cools to ambient temperature, the instrument comprising means for measuring the temperature of at least part of the tip and means for determining whether that temperature falls within a stored predetermined acceptable range.

The instrument may include an optical element, for example a lens, which collects the infrared radiation and focuses this into a suitable thermal sensor (bolometer).

The invention will now be further described, by way of example, with reference to the following drawing in which: Figure 1 is schematic view of a machining arrangement and a sensor for carrying out the present invention; and Figure 2 is a block diagram of an arrangement in accordance with the invention, for detecting breakage or wear of a tip of the machining tool shown in Figure 1.

A machining tool 26 comprises a milling cutter 8 which is turned by a motor 14 via a gear box 16. The milling cutter 8 has 20 ceramic tips for milling a component 4.

The tool 26 is mounted on a motorised slide 18 which is movable to slide the tool 26 between an operating position in which the tips of the milling cutter 8 act on the component 4, and a diagnostic position (shown in Figure 1) in which the tips of the milling cutter are in a predetermined position and orientation relative to a bolometer 10.

As best shown in Figure 2, infrared radiation 8 which is emitted from a tip of the milling cutter 2 is collected by the bolometer 10. A signal 28, which is related to the temperature of the part of the tip being measured is sent from the sensor 10 to an electronic interface unit 20.

The unit 20 is programmed to compare the measured temperature (or power) of detected radiation, and to compare this value with a stored reference range of acceptable temperature values.

The reference range of temperature values was determined by measurement under similar conditions, for a tip which is not unacceptably broken or worn. The breadth of the range was determined by comparison with similarly obtained values for tips which were unacceptably worn and broken.

If the measured temperature is within the programmed acceptable range, the tip is deemed acceptable. This information may be displayed on a monitor 22 and/or may be sent to a PC or other computer via a network or other suitable connection.

For a tool with a single tip, this one check is sufficient. However for the milling cutter 2, with 20 tips, each tip should be checked. This is achieved by means of a drive 12, connected to a controller 6 which may be linked to the electronic interface unit 20 directly or indirectly via a network. The drive 12 controls the rotation and speed of the tips of the milling cutter 2, and indexes each tip around a spindle so as to present each tip in turn to the sensor 10 at the correct position and orientation. Each tip produces an infrared pulse unless the tip is broken. A broken tip will produce either no pulse (no emission) or an attenuated pulse (some emission, where the break is in the region of the area of the tip being monitored.

Each tip may also be assessed for wear as described above, making due allowance for extra elapsed time between completion of the machining operation and the measurement.

However it is envisaged that for many machining operations it will not be necessary to carry out wear measurement of each tip, provided that each tip on the tool is fitted new at the same time, so that each tip would be expected to wear at the same rate. Measurement of wear for the first tip would then provide sufficient indication of wear for all the tips.

If a tip is found to be unacceptably broken or worn, the monitor 22 may display this information and sound an alarm so that an operator can shut down the machining operation while the damaged part is replaced. Alternatively, or additionally, automatic action may be taken by means of the computer link 24, for example halting production.

The invention therefore provides a fast and simple method of detecting unacceptable breakage or wear of a machining tool tip.

Claims (13)

1. A method for detecting breakage or wear of a tip of a machining tool, the method comprising the steps of measuring the temperature of at least part of the tip by means of an infrared detector after completion of a machining operation and before the tip cools to ambient temperature, and determining whether that temperature falls within a predetermined acceptable temperature range.

2. A method as claimed in Claim 1, including the step of halting further machining operations if the measured temperature falls outside the acceptable range.

3. A method as claimed in Claim 2, including the step of replacing an item selected from the tip, a part carrying the tip, or the tool.

4. A method as claimed in any one of the preceding claims, wherein the tool carries a plurality of tips and the method is carried out sequentially on each tip.

5. A method as claimed in any one of the preceding claims, carried out under computer control.

6. A method as claimed in any one of the preceding claims, carried out within about two seconds of completion of the machining operation.

7. A method as claimed in any one of the preceding claims, carried out within about one second of completion of the machining operation.

8. A method as claimed in any one of the preceding claims, which is calibrated to allow for the temperature of the tip immediately before starting the machining operation.

9. A method as claimed in any one of the preceding claims, wherein the temperature is measured by a plurality of sensors so that a temperature gradient of the tool tip may be measured.

10. An instrument for use in the method as claimed in any one of the preceding claims, for detecting breakage or wear of a tip of a machining tool after completion of a machining operation and before the tip cools to ambient temperature, the instrument comprising means for measuring the temperature of at least part of the tip and means for determining whether that temperature falls within a stored predetermined acceptable range.

11. An instrument as claimed in Claim 10, which is provided with means for halting further machining operations when the measured temperature falls outside the acceptable range, corresponding with an unacceptable tip.

12. An instrument substantially as herein described with reference to and as shown in any Figure of the accompanying drawing.

13. A method substantially as herein described with reference to any Figure of the accompanying drawing.