How to achieve lightweight automotive check fixtures
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1 Introduction
In automotive development and production, automotive check fixture is an efficient special tool for quality control. Automobiles are made up of thousands of parts, produced by an industry chain consisting of automotive plants and parts suppliers respectively, and quality issues are bound to arise during the development and production process. When a quality problem arises, a gage is a good quality judgment standard to help the analysis of the problem. This may result in the need for parts suppliers to transport the gauges to the automotive plant for inspection and certification. The gauges also need to be periodically rechecked for accuracy at the CMM. In the early stages of product development, the product needs to be loaded onto the gage and moved to the CMM for full dimensional measurement. All these will cause the check fixture to be moved and handled within the automotive factory or parts suppliers. Therefore, quality personnel are very concerned about the weight of the gauges, requiring light weight and ease of handling. In this paper, we discuss and analyze the demand and development trend of lightweight gauges in terms of gauges material and structure process.
2 Basic introduction of check fixtures
The check fixture is a special tool used to check the hole position, peripheral shape, segment difference, access and other dimensions of complex-shaped parts. Automotive check fixtures are mainly divided into metal parts check fixtures and plastic parts check fixtures according to the material of the inspected parts. According to the structure of the check fixture is mainly divided into clamping mechanism, positioning pin, inspection pin, inspection block, base, sliding or flip inspection mechanism, handheld inspection gauge, calibration mechanism and support column. The base of the check fixture mainly has two types of frame base and plate base.
3 Lightweight check fixture materials
Traditional check fixtures generally use 45 steel as the main material, although its high strength, durable, but along with the bulky, poor mobility. With the popularization of the concept of lightweight and the continuous progress of lightweight technology, many new materials are developed and put into the manufacture of check fixtures, for example, aluminum, synthetic resin, bakelite and carbon fiber, etc.. The new materials are much less dense than structural steel and can realize the lightweight of the gage.
From the structural classification of the gage, the inspection block, support column and base occupy most of the weight of the gage, so the gage lightweighting mainly from these three pieces. Inspection block is only to detect the size of the product, when the product is clamped on the gage, the product and the inspection block is a gap (usually 3mm gap), and the inspection block does not need to bear the weight of other parts, so the inspection block strength requirements are not high, do not need to do too big. But the product in the check fixture assembly and disassembly process may encounter the detection block, so for different types of products and factory environmental conditions, the detection block strength and hardness requirements are different, mainly in the following two cases.
First, for metal products, the factory environment is poor, the product in the process of assembly and disassembly may easily touch the check fixture, it is recommended that the inspection block choose structural steel, to ensure the strength and wear resistance. If there are lightweight requirements, the factory environment conditions are better, the detection block can choose aluminum alloy weight reduction. If there is a requirement for the wear resistance of the inspection block, the aluminum alloy can be anodized and hardened. If the requirements for lightweight are very high and the factory is equipped with a special gage measuring room, you can choose to substitute wood materials. Commonly used wood substitute material grades are 460 and 5166, 460 wood substitute is relatively light, hardness is not high, suitable for temporary check fixture use; mass production of formal check fixture recommended to choose a higher density, higher hardness of 5166 wood substitute.
Second, for plastic products, due to its own soft, testing type block can choose aluminum or wood to achieve light weight. Similarly, if the factory environment is poor, it is recommended to choose aluminum alloy for the inspection block; if the requirement of light weight is higher and the factory environment is better, the inspection block can choose wood substitute for further weight reduction.
The support column mainly plays a connecting role between the positioning block, testing block, clamping and other mechanisms and the base plate. The support column needs to bear the weight of the positioning block, inspection block and clamping mechanism attached to it, so there are certain requirements for the strength of the support column, which needs to ensure sufficient strength. If the mechanism above the support column is larger and heavier, then it is recommended to choose structural steel material for the support column. In general, the support column can choose aluminum alloy to reduce weight.
The base of the check fixture is to support all parts and products on the check fixture, and to ensure that the flatness of the four corners of the base plate measurement reference in 0.1mm under self-weight, so the strength of the base has very high requirements, need to ensure sufficient tensile strength and flexural strength. The base generally choose structural steel, aluminum, bakelite and carbon fiber which are four kinds of high strength materials.
The first case, the gage detection type block material for structural steel, support column material for steel or aluminum alloy, the base needs to bear the weight is very large, then the gage base must choose structural steel.
The second case, the gage detection block and support column material are aluminum alloy, the base needs to bear the weight is not very large, you can choose aluminum alloy material to reduce the weight.
The third case, the check fixture detection block and support column directly using the substitute wood, the base to bear the weight is lighter, you can choose aluminum alloy material. If the check fixture is a temporary check fixture, there are high requirements for lightweight, the base can use bakelite material.
The fourth case, the extreme requirements for the lightweight of the check fixture, you can choose high strength and low density of carbon fiber material to do the frame base, the detection type block selected to replace the wood material, through the paste resin will be the detection type block bonded to the carbon fiber frame base. For example, the front window frame inspection has a frame base made of carbon fiber material, bonded to the wood substitute by paste resin, machined into the inspection block and support seat, the support seat can be installed positioning blocks and other institutions. This method of making the front window frame check fixture has the ultimate light weight and high strength to ensure the high accuracy of the check fixture. The carbon fiber check fixture is certainly first-class in terms of light weight, but the cost also rises. If there are certain requirements for lightweight, but also to consider the cost, it is recommended that metal products inspection block, support column and base are selected aluminum materials; and plastic products inspection block and support column are selected for wood materials, base selection of aluminum materials.
4 Lightweight check fixture structure
In addition to the use of lightweight materials to reduce weight, the check fixture can also be optimized through the structure to reduce weight. As the detection block, support column and base occupy most of the weight of the check fixture, structural optimization will be analyzed from these three blocks. First, the inspection block is only used to measure the size of the product reference, can mainly bear its own weight, so the design is as small as possible. In some cases, the detection block is directly connected to the base plate, without the support column, when the detection block will be larger, can be hollowed out in the middle of the detection block, as shown in Figure 2. Second, the support column needs to support the weight of the positioning block, inspection block and clamping mechanism, so it needs to ensure its own strength. When a support column supports more or larger parts, the volume of the support column will be large, and if made solid, it will be very heavy. Two structural optimization methods are described here.
(1) The support column is welded with a flat plate, and reinforcement bars are welded at the lap of the plate to ensure the rigidity and strength of the support column, as shown in Figure 3.
(2) For the use of one-piece support column, when the volume is large, you can drill holes from the bottom of the support column to reduce weight, as shown in Figure 4. Third, the base of the check fixture is to support all parts and products on the check fixture, the strength of the base has very high requirements. If the parts on the base are not dense, leaving empty space, then you can ensure the base strength and rigidity premise, the base hollowing weight reduction.
The above describes how to achieve the lightweight of the gage from the selection of gage materials and gage structure optimization, basically can meet the requirements of most companies on the lightweight of the gage. Lightweight gage certainly has many benefits, but the choice of high strength, low density materials are more expensive, structural optimization of weight reduction will also increase the cost of machining. The greater the lightweight of the check fixture, the higher the corresponding cost will be, the fish and the bear’s paw can not be both. In reality, the automotive industry is very competitive, the cost control is very strict, so you can not just pursue the maximum lightweight of the check fixture, regardless of the cost to make the check fixture, but should be based on the company’s own situation and demand to choose the comprehensive check fixture materials, to find a balance between lightweight and cost.