How to install the workpiece
In order to process a surface that meets the specified technical requirements on a certain part of the workpiece, the workpiece must occupy a correct position relative to the tool on the machine tool before machining. This process is often referred to as "positioning" of the workpiece. After the workpiece is positioned, due to the action of cutting force, gravity, etc. during processing, a certain mechanism should be used to "clamp" the workpiece so that the determined position remains unchanged. The process of getting the workpiece in the correct position on the machine and clamping the workpiece is called "setup".
The quality of workpiece installation is an important issue in machining. It not only directly affects the machining accuracy, the speed and stability of workpiece installation, but also affects the level of productivity. In order to ensure the relative positional accuracy between the machined surface and its design datum, the workpiece should be installed so that the design datum of the machined surface occupies a correct position relative to the machine tool. For example, in the process of finishing ring grooves, in order to ensure the requirements of the circular runout of the bottom diameter of the ring groove and the axis of the skirt, the workpiece must be installed so that its design datum coincides with the axis of the machine tool spindle.
When machining parts on a variety of different machine tools, there are various installation methods. The installation methods can be classified into three types: direct alignment method, scribe alignment method and fixture installation method.
1) Direct alignment method When using this method, the correct position that the workpiece should occupy on the machine tool is obtained through a series of attempts. The specific method is to use the dial indicator or the scribing needle on the scribing plate to correct the correct position of the workpiece by visual inspection after the workpiece is directly mounted on the machine tool, until it meets the requirements.
The positioning accuracy and the speed of the direct alignment method depend on the alignment accuracy, alignment method, alignment tools and the technical level of the workers. Its disadvantage is that it takes a lot of time, low productivity, and it needs to be operated by experience, and it requires high skills for workers, so it is only used in single-piece and small-batch production. For example, reliance on imitating body alignment is a direct alignment method.
2) Scribing alignment method This method is to use a scribing needle on the machine tool to align the workpiece according to the line drawn on the blank or semi-finished product, so that it can obtain the correct position. Obviously, this method requires one more scribing process. The drawn line itself has a certain width, and there is a scribing error when scribing, and there is an observation error when correcting the position of the workpiece. Therefore, this method is mostly used for small production batches, low blank accuracy, and large workpieces. Fixtures should not be used. in rough machining. For example, the position of the pin hole of the two-stroke product is determined by using the marking method of the indexing head.
3) Using the fixture installation method: the process equipment used to clamp the workpiece and make it occupy the correct position is called the machine tool fixture. The fixture is an additional device of the machine tool. Its position relative to the tool on the machine tool has been adjusted in advance before the workpiece is installed, so it is not necessary to align the positioning one by one when processing a batch of workpieces, which can ensure the technical requirements of processing. It is an efficient positioning method that saves labor and trouble, and is widely used in batch and mass production. Our current piston processing is the fixture installation method used.
①. After the workpiece is positioned, the operation of keeping the positioning position unchanged during the machining process is called clamping. The device in the fixture that keeps the workpiece in the same position during processing is called the clamping device.
②. The clamping device should meet the following requirements: when clamping, the positioning of the workpiece should not be damaged; after clamping, the position of the workpiece during processing should not change, and the clamping should be accurate, safe and reliable; clamping The action is fast, the operation is convenient and labor-saving; the structure is simple and the manufacture is easy.
③. Precautions when clamping: the clamping force should be appropriate. If it is too large, the workpiece will be deformed. If it is too small, the workpiece will be displaced during the processing and will damage the positioning of the workpiece.