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Journal of Materials Processing Technology 179 (2006) 117123 Development of a low cost Touch Trigger Probe for CNC Lathes Marcelo Del Guerra a, , Reginaldo a Escola de Engenharia de Sao Carlos-EESC-USP, Av. Trabalhador b Escola de Engenharia de Sao Carlos-EESC-USP, Av. Trabalhador Abstract a w the Howe routines lo a discussing ne is proposed, CNC a repeatability characteristics K On-machine 1. f control part-to-part using to ments and calibrations based on projects specifications, closed- loop process control, dimensional control of parts manufactured, tool compensation, fast setup, process statistical data acquisition and more. It is very important in current manufacturing systems trends to inspection errors a parts starts nologies paper rtcoelhosc.usp.br Probe, type studies. 2. cific and revolutionize the use of coordinate measuring machines (CMMs) for post-process inspection of manufactured compo- nents. In the end of 1970s, almost all the major CMM manufac- turers in the world were using Renishaws TM probes. Later, due to CNC advances including measurement func- 0924-0136/$ doi: make the use of these functions, making possible a part-to-part control if necessary, which assures that dimensional are immediately detected, reducing scraps, avoiding that full batch of pieces are scraped and reducing the costs due to reworks. To assist dimensional control on turning operations, this work introducing the Touch Trigger Probes characteristics, tech- used to generate the “trigger signal”. After that, this presents a simple and low cost wireless Touch Trigger Corresponding author. E-mail addresses: mdguerrasc.usp.br (M. Del Guerra), (R.T. Coelho). tions and linear encoder on its axes, the use of such devices spread to machine tools, where they became a vital component on automated production systems, allowing companies, from automotive to aerospace, to deliver precise components, reduc- ing scraps and maximizing productivity. David McMurtry developed the Touch Trigger Probes tech- nology in 1972. The problem arose at Rolls-Royce plc, where he was Assistant Chief of Engine Design. At that time, he was fac- ing the problem of measuring some complex pipe runs, of only 6 mm diameter, for the Olympus engines used on Concorde. The pipes had to fit accurately between solid mountings, but the diffi- culty came in measuring them once they had been manufactured. The solution found was the Touch Trigger Probe (TTP), which uses a simple principle to identify the contact between the probe see front matter 2006 Elsevier B.V. All rights reserved. 10.1016/j.jmatprotec.2006.03.093 The use of Touch Trigger Probes for CNC Machine Tools has become ability to promote workpiece accuracy on-machine measurements. programming difficulties, high costs of these equipments and the review on the applications of Touch Trigger Probes on shop floor, w Touch Trigger Probe model based on a simple electrical contact Lathes. The tests of the probe developed in the laboratory shown seem to be highly comparable to most needs for lathe applications 2006 Elsevier B.V. All rights reserved. eywords: Dimensional quality monitoring and control; Touch Trigger Probes; Introduction One of the, nowadays, most difficult challenges on manu- acturing system developments is to achieve total dimensional of parts produced, creating statistical data analyses and control. This kind of integration may be reached by devices commonly called Touch Trigger Probes attached CNC Machine Tools. These equipments are used for measure- Teixeira Coelho b Sancarlense 400, CEP 13566-590, Sao Carlos, SP, Brazil Sancarlense 400, CEP 13566-590, Sao Carlos, SP, Brazil orld standard, mainly due to the reduction of machine setup time and ver, some barriers still need to be transposed, like the measurement w number of technical literature about this subject. This work presents nowadays technologies used to generate the trigger signal. A with the needed characteristics and low cost, to be used for of 0.003 mm (3 s or 99.73% confidence interval). Those in industries. measuring systems; CNC Lathes based on a direct electrical contact principle. A proto- is presented together with the repeatability and accuracy Touch Trigger Probes earlies The first Touch Trigger Probe was invented to solve a spe- inspection requirement, but went on to transform the design 118 M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123 and tered pro turers the gies to 3. on moment signal, con the sa e interf munication wire for vides interferences proof contact, sion radio 2 which 4. sionals setting in vie ha in To make probing effective and more affordable to a wide range of users, some of the main leading CNCs manufactures, such as Fanuc, Heidenhain, Okuma, Cincinnati Milacron and Siemens, are including, in its CNCs software, graphical user- friendly interfaces specially developed for probing. Heidenhain TNC control series, for example, are able to automatically or manually enables accurac w identifying f be bility addition, to enable tools. an tronic lar ters services sources machines able. bility CNC between 5. nal probe probe quantity signal old the tra it ate light piezoelectric Fig. 1. A measurement system attached to a CNC Machine Tool. the part to be measured. In 1973, David McMurtry regis- a company called Renishaw that early become the leading vider of metrology and spectroscopy solutions to manufac- and researchers around the world (Renishaw 1). Due to patented mechanism used by the company, other technolo- needed to be developed by other companies that intended join on this market. Touch Trigger Probes main characteristics Independently of the technology used, the principle is based the generation (or interruption) of an electrical signal at the the probes tip touches the part to be measured. This generally called trigger signal, is sent to the interface that verts it to be read by a CNC or a CMM. At the moment that machine receives the trigger signal, it instantly freezes and ve the axis values for calculations and reports generation (see xample in Fig. 1). The trigger signal transmission from the TTP to the probe ace can be established using basically three different com- technologies: infrared, radio frequencies or a simple cable. Infrared communication is used for conventional applications standard lathes and milling machines; this technology pro- high reliability, high transmission speed rates, low noise and low implementation costs. It is close to a fool- system, except when probe and receiver are out of visual as when the probe is deep within a bore or other depres- within the workpiece. For these cases and applications, a frequency system is recommended, (Tooling & Production ). Wire cable communication is used for tool setter probes are mounted and fixed on the machine tables. Probing cycles and programming According to Modern Machine Shop On Line 3, few profes- will dispute the merits of probing for speeding part setup, tool offsets and performing in-process inspection, at least theory. In practice, however, probe systems often have been wed as difficult to use and expensive to buy. As a result, they ve become tools used primarily by experienced programmers high-production environments. 5.1. widespread. olutionized nism on carry through three-dimensional measurements and check functions, such as checking the dimensional y of holes, determining feeds for finishing, checking orkpiece geometry and identifying deviations from tolerances, workpieces prior to machining and scanning 3D sur- aces. Also, probing cycles are being widely developed and can activated by a single line of information (Zhou et al. 4). Todays standard machine tools deliver accuracy and repeata- approaching levels formerly available only on CMMs. In technology advances are making these machines easier maintain. Test and calibration technology are now available to shops to ensure the accuracy and health of their machine Telescoping ballbars are readily affordable by virtually y shop. A well-stocked toolbox should also contain either elec- levels or a good set of precision machine levels. Plants and ge shops increasingly maintain their own laser interferome- and electronic levels, while rental equipment and diagnostics are commercially available to small shops from various (Modern Machine Shop On Line 5). Also, according to Modern Machine Shop On Line 6, CNC are, in general, very accurate and extremely repeat- They compare favorably with the accuracy and repeata- of CMMs of similar size. With some care during the inspection process, the factors that lead to interdependence measurement error and cutting error can be minimized. Technologies used to generate the trigger signal According to Shen and Moon 7, there is no trigger sig- generated when the probe tip touches the workpiece. The will continue to move toward and the force between the tip and the workpiece will increase, causing a physical (e.g., resistance) to reach a threshold setting. A trigger is generated when the physical quantity exceeds a thresh- limit in the sensing system. The travel distance between touch instant and the trigger instant is known as probe pre- vel. Mainly caused by bending deflection of the stylus shaft, accounts for the majority of probe errors. Basically there are four different principles used to gener- the trigger signal: electrical contact (kinematic contact), (laser interferometry), deformation (strain-gauges) and the effect. Kinematic contact Currently, this is the most used Touch Trigger Probe Developed by David McMurtry, this principle rev- the use of the coordinate measurement machines. According to Fig. 2, it is a kinematic re-seating mecha- based on three small cylindrical pieces that stay seated six other cylinders/spheres by the pressure caused by the M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123 119 Fig. tion spring is normally tip contacts detected measurement piece, stable 5.1.1. distinguished: electro-mechanical 5.1.2. are contacts. is to acteristic 10 mechanism causing v 11 to v models manuf Cumulative errors happen when using stylus longer than 30 mm due to the fact that the touching force necessary to gen- erate the “trigger signal” bends the stylus and this deflection is proportional to its length. The acquisition cost of these equipments is relatively high if compared to other systems that provide similar benefits. This equipment them 5.2. deri the electrical limits 5.2.1. pre-tra because the rate ( lo able 5.2.2. high it erature 5.3. system it 2. Kinematic Touch Trigger Probes working principle (Tooling & Produc- 2). action, restricting its degrees of freedom until the stylus brought into contact with the workpiece. An electrical current flows through the three seats in series. When the probe moves away from its stable position, one or more of the seat breaks and the increase in resistance is immediately electronically (Mayer et al. 8 and Reid 9). After the is done, when the probe tip moves back from the the probe spring forces the mechanism back to the initial position, re-establishing the standard resistance. System advantages From the main advantages presented by this system, it is the simplicity of the system, robustness of the assembly, widespread and studied principle. System disadvantages Due to the electro-mechanical assembly, kinematic probes susceptible to the consuming and corrosion of its electric Besides, the main typical characteristic of this kind of probe related to the pre-travel variation. Since stylus bending prior trigger is the major constituent of probe pre-travel, this char- varies according to changes in touch directions (Reid ). This behavior happens due to the fact that the internal seats in three “V-Blocks” separated by a 120 angle, the force necessary to generate the “trigger signal” to ary according to the approaching direction (Kim and Chung ). Miguel et al. 12 also say that the dynamic force necessary open one of the electric contacts depends on the approaching elocity. According to Shen and Moon 7, same Touch Trigger Probes may show different pre-travel variation behavior due to acture and small changes during the assembly process. dif Heidenhain 5.3.1. to pro 5.3.2. the first measured ilarly the tems is very well known and the brand that manufactures is associated with very high quality. Strain-gauges Strain-gauges probes deals with analogical information ving from the electrical resistance variation produced when stylus touch the part to be measured. To use this technique as a digital switch, it is necessary an signal conditioning and treatment, establishing the from which the “trigger signal” is generated. Advantages One of the main advantages of this technique is that the vel variation is much lower than on kinematic probes the stylus deformation itself is responsible for trigging event. For this reason, this kind of probe is extremely accu- and makes possible the use of stylus as long as 200 mm Fig. 3). The probe uses solid state electronics, providing long life and w wear. Based on analog signal calibration, these systems are to detect contact forces of 0.2 N. Disadvantages The disadvantages associated with these probes rely on the acquisition cost, extremely advanced electronics that makes very delicate equipment and the fact that there are very few lit- and information specifically approaching these systems. Laser These probes operate with an optical switch as sensor. A lens collimates the light generated from an LED and focuses onto a differential photocell. When the stylus is deflected, the ferential photocell produces a trigger signal, (Dr. Johannes 13). Advantages One of the main advantages of these equipments is related its cost, relatively low if compared to other equipments that vide similar performance. Disadvantages To have a change on the differential photocell point where collimated light happens and generates the “trigger signal”, of all, a force between the stylus tip and the part to be will be needed to deflect the internal mechanism, sim- to what happens with “kinematic probes” regarding to opening of its electrical contacts. For this reason, both sys- present similar pre-travel variation characteristics, showing 120 M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123 probes (TP20) with strain-gauge probes (TP800) (Renishaw 1). accurac sured 30 5.4. electric of principle, the moment 5.4.1. strain-gauge kinematic sensiti 5.4.2. lisions, machine 6. to T Lathes. bly used 6.1. closing the “trigger signal” is generated and sent by radio (wireless) to the probes interface that will be responsible for sending a 24 V dc to inertia a its ger system trigger properly after will de to according 6.2. INDEX-TRA Fig. 3. Pre-travel variation: comparing the performance of kinematic y dependence on direction approach to the part to be mea- and both systems are not indicate to use stylus longer than mm. Piezoelectric sensor Piezoelectric probes are based on the principle for which an potential difference is generated between the two faces a piezoelectric crystal when it is pressed. Using this physical it was possible to design a probe able to generate “trigger signal” from the electric potential generated at the the probe stylus reaches the part. Advantages They are extremely accurate and repetitive systems. Like probes, pre-travel variation is much lower than on probes and it is possible to calibrate the system to be ve to forces in the order of 0.2 N. Disadvantages They may be sensitive to some low frequency noises like col- start of motors, high spindle accelerations/decelerations, vibration, etc. Proposal of a model based on a simple electric contact This work aims to use basic and well-known technologies design and manufacture a low cost, robust and accurate ouch Trigger Probe able to measure external diameters on CNC For that, precise mechanical machined parts and assem- were designed and applied to develop a system able to be on the major Touch Trigger Probes applications. The system The working principle of the proposed equipment consists on an electrical contact between the stylus (grounded) and internal target (Positive Pole). At this particular moment, the Fig. process. an appropriate input of the CNC. Due to the fact that the carriage will continue to move by after the “trigger signal” happens, the target should have drawback mechanism able to adsorb this movement, assuring surface integrity (Fig. 4). Consequently, the repeatability of the developed Touch Trig- Probe is directly associated with the ability of the drawback to precisely seats back to its original position after a event occurs. If the drawback system does not work , there will be a change on the probes length preset a measurement and the diameter of the next part measured be showed as different than the real. For this reason, the velopment of the drawback system was done very carefully guarantee a good performance for the system. The probe was designed to be used on VDI Toolholders, to DIN 69880 and VDI 3425 standards. System repeatability and reproducibility (R&R) The CNC Lathe used to test the Touch Trigger Probe was an UB, model GU600, 22 kW power. The CNC was 4. Dynamic behavior of the drawback mechanism during the measurement M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123 121 Fig. 5. Illustration of the experiment procedure used to determine the probe R&R and a photo of the probe inspecting the part. a Siemens 810D, able to provide the measurement functions needed for the experiment. To determine the total system R&R (probe + machine tool), the experiment was accomplished taking a part of which diam- eter was previously machined and normalized. After that, a turning operation was made using a carbide cut- ting tool VBMT160404UC6010, cutting 0.010