AN INVESTIGATION INTO THE EFFICIENCY AND IMPROVEMENT OF INDUSTRIAL LAAG FIXTURE

Open Access
Author:
Janarthanam, Santhosh Kumar
Graduate Program:
Industrial Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Edward Demeter, Thesis Advisor
  • El Amine Lehtihet, Thesis Advisor
Keywords:
  • LAAG
  • Advanced fixturing
  • Photocurable adhesives
  • Workholding
Abstract:
A Work-holding fixture is one of many important factors in machining a component with accuracy and affecting the rate of throughout. The setup time involved in machining a component and workability of tool is dependent on the type of fixture design. The use of traditional fixturing techniques such as jaw chucks and vise while machining components in machining centers has many disadvantages such as clamping induced errors, reduced workability for tools and increased setup time. A novel approach to workholding and an alternative to the use of mechanical, pneumatic and hydraulic mechanism is Light Activated Adhesive Gripper (LAAG) technology. LAAG utilizes photocurable acrylic adhesives as the main component of the system. The preferred adhesive for the technology is CTECH® 14-90-2. The patented technology utilizes a sapphire pin housed in a metallic casing to allow transmission of UV light to the adhesive which cures and adheres itself to the workpiece. The number of adhesive joints needed, size of sapphire pin and orientation of the pin can be designed using FEA software. The preferred light source is a metal halide UV spot curing lamp. The LAAG system was proven to exhibit high holding strength, maximum dynamic rigidity and increased workability for tool. As a case study, the LAAG technology was applied to study its relative benefits and cost efficiency in comparison to conventional fixturing mechanism used by Precision Grinding and Machining (PGM), where a LAAG fixture was conceptualized and developed to hold an Aluminum bracket during its machining operation. The case study revealed that the use of LAAG workholding could significantly lower the total machining cycle time and cost associated, while maintaining good part quality. However the case study also raised a number of questions. Specifically the major issues were the existence of a large secondary cure zone, effect of leaving trimmed adhesive layer over the surface of gripper pins and workpiece coating. Previous research with Dymax 602 Rev A under smaller gap thickness showed no secondary curing effect. The effect of secondary cure zone on the adhesive joint strength and the factors influencing the formation of secondary cure zone are little known. It is little known whether the practice of leaving trimmed adhesive layer on the gripper will affect the adhesive joint strength. Also it is not known whether the adhesive layer will attenuate the light reaching the uncured adhesive layer during re-bonding cycle. The effect of different trimming methods on adhesive joint strength and attenuation are also to be studied. As the life of the UV spot cure lamp ages, the intensity of the light output decreases. An investigation to achieve the same adhesive strength as that obtained by a new lamp just by increasing the exposure time is also to be evaluated. A detailed analysis of bonding on to a painted surface with and without residual adhesive on it is also to be studied. More specifically, techniques to remove residual adhesive from the surface of workpiece effectively so that further operations on it such as painting etc is not affected by the underlying layer of residual adhesive. To begin the investigation, the presence and contribution of secondary curing effect in CTECH® 14-90-2 was evaluated. The results were compared with Dymax 602 Rev A. Secondary curing was previously observed with Dymax 602 Rev A, even with smaller joint thickness. However, the diameter was observed to be small, and it was never crazed. Consequently it was assumed to have negligible impact on adhesive joint strength. In the cases in which Dymax 602 was mixed with carbon black, a secondary cure zone never appeared. An investigation where the strength proportional to gripper pin diameter and secondary cure diameter with CTECH® 14-90-2 was also studied. This investigation revealed that secondary curing does contribute significantly to the final adhesive strength of the adhesive, however no proportionality effect was revealed. Statistical comparison was done to compare the means of CTECH® 14-90-2 with Dymax 602 with 15mm chassis diameter and different diameter of gripper pins (0.132”, 0.1875” and 0.25”) and it was proven that the means were statistically different. The experiments were carried out with 15mm chassis diameter with pin diameters (0.132”,0.1875” and 0.25”) consistently produced much larger secondary cure zones and much higher average axial load strength for all three pin diameters. Further investigation into the effect and contribution of secondary cured adhesive layer was executed by reducing the diameter of chassis to choke the formation of secondary cured adhesive layer. Reduced diameter steel chassis was taken, 6mm and 8mm and bonded with Aluminum anvils by using Dymax 602 and CTECH® 14-90-2 and the comparison of strength was made. With 6mm chassis diameter the formation of secondary cured adhesive layer was greatly reduced and the mean strength observed with CTECH® 14-90-2 and Dymax 602 was not statistically different. However, when the chassis diameter was increased to 8mm, sufficient area for formation of secondary cured adhesive layer was present. The mean strength observed with CTECH® 14-90-2 was 1217 N as against to 623 N with Dymax 602. The means were statistically different. It is evident that with enough room for formation of secondary cured adhesive layer, the strength of the adhesive bond increases suggesting that secondary cured adhesive contributes significantly to adhesive strength. The relationship between secondary cure zone diameter to the diameter of gripper pin used was studied. Varying diameters of gripper pin was taken such as 0.132”, 0.1875” and 0.25” and the secondary cured adhesive layer diameter was measured in each case. However, no correlation was found between them. From this study, it was found that the secondary cure zone extends in diameter range from 7mm to 15mm. An investigation to study whether the adhesive strength increases with increase in chassis diameter, a reduced chassis diameter experiment was done with CTECH® 14-90-2 as the test adhesive. Steel chassis with 0.132” diameter gripper pin was taken with chassis diameters - 6mm, 8mm, 11mm, 12mm and 12.7mm. No trend of increase in strength with increase in chassis diameter was noticed. Secondary cured adhesive layer does contribute significantly to the strength of the final adhesive bond, however no trend was noticed. Chassis with 6mm diameter yielded an average strength that was substantially and statistically lower than the averages for 8mm, 11mm, 12mm and 12.7mm. Likewise, the chassis (8mm, 11mm, 12mm and 12.7mm) yielded average strength that were statistically equivalent. A 15mm chassis yielded an average strength that was significantly higher, which was attributed to the randomness due to inhomogeneous nature of the adhesive. The next investigation was to study whether the type of chassis material determined adhesive strength. Steel and Aluminum chassis with different diameter of gripper pin was taken. Aluminum roughly has twice the reflectivity of steel in UVA and Blue spectrum. Since it is believed that secondary curing is driven by light bleeding into the secondary cure zone, it would seem logical that a chassis made of aluminum could reflect more curing light photons into the secondary cure zone than steel. Another way in which the use of aluminum could affect the light reaching the adhesive joint is its possible affect on the number of photons passing through the gripper pin. In theory the gripper pin surrounded by adhesive, acts as a perfect wave guide and does not leak light. However, in case it did, an aluminum chassis would have a greater capacity to reflect the light back into the gripper pin than a steel chassis. From the experiments, it was noticed that chassis made from aluminum yielded greater average joint strength than steel chassis for identical experimental conditions. However, the differences were moderate, and can not be proven statistically. Also the relationship between chassis material and secondary cure zone diameter was also not established. Both material types produced the same range of secondary cure zone diameter under similar testing parameters. The results of light transmission experiments indicate that light transmission through the aluminum chassis was slightly better than through the steel chassis for both the UVA spectrum and blue spectrum. There is evidence that the greater reflectivity provided by aluminum over steel does influence the overall strength of an adhesive joint formed from CTECH® 14-90-2. But the influence is not significant. The next investigation was to study the combination of gripper pin diameter and light guide diameter that produces the maximum adhesive strength and also be economical in usage. The different gripper pin diameter 0.132”, 0.1875” and 0.25” was used with both 3mm fiber core light guide and 5mm liquid core light guide and the strength was observed. It was found that under similar parameters, a 3mm fiber core light guide performs better with 0.132” diameter gripper pin and the results were proven statistically. This was attributed to loss of light intensity in larger diameter (0.1875” and 0.25”) of gripper pins due to diffusion. A 5mm liquid core light guide yields better result with 0.25” diameter gripper pin. The mean adhesive joint strength using 0.1875” gripper pin was 1329N (at 50% lamp power) and 2296N (at 100% lamp power) as against to using a 0.25” gripper pin which produced 2205N (at 50% lamp power) and 2904N (at 100% lamp power). The huge difference in mean strength at 50% lamp power was due to randomness in adhesive nature. However, the increase in strength by using a 0.25” diameter gripper pin to a 0.1875” diameter gripper pin also comes with increase in cost of operation. The effect of residual adhesive on adhesive strength during re-bonding was studied. An appropriate trimming method for reducing the cured adhesive layer after de-bonding the workpiece from the fixture was analyzed. The different trimming methods used were facing, Facing & buffing and Milling. Different trimming processes were used to study whether the tool marks left by facing versus milling influenced the adhesive joint strength. Likewise, buffing the adhesive layers was done to smooth out the tool marks left during facing operation and thereby increase the translucence. All the trimming methods produced similar adhesive strength during re-bonding; milling was selected as the best method as the fixture need not be removed from the machining center and helps in reducing the cycle time involved. The UV light transmission through residual adhesive layer was also studied. It was found that the presence of residual adhesive layer on gripper pin reduces the available UV light intensity by about 68% when compared to gripper pin that are stripped clean. Another investigation that was performed was to study the impact of leaving the cured adhesive layer on the gripper pin with no trimming or cleaning cycle. The variation of adhesive strength under re-bonding was studied. It was found that the adhesive strength varies randomly but the variation falls within the range of adhesive strength that one would notice with a clean chassis. The drawback to not trimming the adhesive is that the failure of the adhesive joint cannot be predicted. As the old adhesive layer is being exposed to extended UV light curing, the adhesive layer becomes brittle over time and fails suddenly. This was observed with sections of residual adhesive layer missing generally over the gripper pin on successive re-bonding cycles. As the life of the curing lamp ages, the light intensity output reduces. An investigation was carried out to determine whether an increase in exposure time at lower irradiance would match the same strength as that achieved by using maximum irradiance was studied. Logically an adhesive joint that has been exposed for the longest period of time (80 seconds) and at maximum power level (100% lamp power) would have greater adhesive joint strength. It is critical to find ranges of exposure time and power level that will yield averages that are statistically the same or better than that achieved by using 80 seconds and 100% lamp power. The standard parameter for bonding a sample was 0.02” bond thickness, 40 second exposure time and 100% lamp power. In this study, the exposure time was increased to 60 seconds and 80 seconds and at different irradiance value (7.24 W/cm2 – 50% lamp power, 9.71 W/cm2 – 67% lamp power, 11.6 W/cm2 - 84% lamp power and 13.6 W/cm2 – 100% lamp power), the strength was analyzed. A 3mm fiber core light guide was used and steel chassis with 0.132” diameter gripper pin with aluminum anvil was used as test samples. At 60 seconds exposure time, the mean strength achieved at 50%, 67%, 84% and 100% lamp power were statistically not different. At 80 seconds exposure time, the mean strength at 50%, 67% and 100% were not statistically significant. This clearly indicates that by increasing the exposure time to 60 seconds and at reduced irradiance level, the mean strength can be matched with that of maximum irradiance under standard testing parameters. Even if the life of the bulb gets older, equivalent results can be achieved. The results of the previous experiment were extended to verify at larger bond thickness. Samples were bonded at 0.1” and by increasing the exposure time, it was found that the strength of the adhesive bonded at 50% lamp power with 60 seconds and 80 seconds exposure time matched the mean strength of adhesive bonded at 100% lamp power with 40 seconds curing time. Any exposure time greater than 40 seconds lead to this conclusion, except at 84% lamp power and 60 seconds exposure time, which is an aberration that was attributed to randomness in the adhesive nature. The bonding of adhesive to painted surface and techniques to remove cured adhesive layer from the surface of the workpiece after bonding effectively were also studied. Samples were painted similar to the process by which the aluminum brackets were painted in industry. The adhesive strength was observed to have dropped significantly when bonded to painted samples in comparison to unpainted samples. It is to be noted that the paint was failing and not the adhesive. The strength of adhesive when bonded to paint was 391 N as against to 1217 N when bonded to unpainted sample. The impact of cleaning the cured adhesive layer by using a hot air gun and studying the effect of residual adhesives on paint adhesion was also studied. Samples were cleaned using hot air gun and the sample surface was visibly clean from any adhesive remains. It was painted and subjected to axial load test. Aluminum anvils with turned/bead blasted surface and steel anvils with turned and bead blasted surface was taken and bonded with 8mm steel chassis diameter with 0.132” diameter gripper pin. Steel anvils with bead blasted surface had the maximum effect from residual adhesive as it recorded 3 runs with 0N axial strength. The presence of residual adhesive weakens the bonding of paint to the substrate and the usage of hot air gun does not effective remove the adhesive. Ultrasonic cleaner with hot coolant bath and adhesive remover technique was used to clean the residual adhesive before painting the sample. Statistical analysis was done to determine whether the means for the two treatments were different from each other and also to identify which was the best treatment for adhesive removal. It was found that the means were not statistically different. The two treatments provided similar results and the means were compared with anvils painted with no adhesive remains. It was found that both the treatments provided results similar to using a painted anvil with no adhesive remains which signifies that these methods effectively removed residual adhesive from the surface of workpiece.