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有时杂耍为您的公差可以部分感觉像一个马戏团的行为的所有要求。作为一名设计工程师，您必须选择配合，外形，功能，客户要求，行业规范，制造能力加工公差，并且不要忘记确保它可以检查和正确定义。beplay客户端下载Whew!这是创建作为高性价比的，可重复和可制造零部件斗争，所以我们一定要帮助你留下的马戏表演一些有用的技巧！
If you’ve utilized the可制造性设计(DFM) technique, these steps will feel familiar.
Getting the Right Information Prior to Tolerancing
Determine the function of each geometric feature
你的目标是要具体。考虑每个几何特征，并确定其最重要的功能。分析所有的mating componentsand determine how the part you’re designing interfaces with them. Document the most specific function of each geometric feature. The more specific you can be, the better!
例如，而不是说一个特征允许两个旋转轴的位置，指定两个孔可以如何定义两个轴之间的最小所需距离。确定哪些功能将铸造，锻造，冲压，焊接，机加工和加工后成品。考虑如何CNC machiningprocess will be utilized to produce each feature. In general, machined features which don’t have an important function can have looser tolerances.
Determine the most important function of the part
Review all the functions defined in step one. Consider the intent of the part’s design which will include the physical functions and features of the part. Choose the most important function of the part. Is one function critical to safety? Does one portion of the part bear the load for another critical to safety component? Utilize your experience and ask other designers if you need further input. Ideally, one function of the design should maintain priority. If the most important function is not one which is CNC machined, that’s okay! You just need to determine how the most important function relates to the features that are CNC machined.
Choose a datum reference framework and type of geometric control for each feature
Utilize your company guidelines and/or industry specifications to perform the first steps of几何尺寸和公差(GD&T). Select a datum reference frame, which can be anywhere from one to three planes. The datums will be utilized as references to perform GD&T and may also be utilized as a machining datum for CNC work. Correctly identifying this reference frame enables lower manufacturing reject rates because of a decrease in defects and builds the system for which geometric controls and tolerances may be based on.
Next, determine the type of geometric control for each geometric feature. Start with the control that has the lowest impact and highest amount of variability allowed — a method called “zero-based budgeting”. Sequentially move up in complexity, impact and restrictiveness. More complex or important geometric features should have more specific geometric controls applied to them. Geometric controls are important because they tell manufacturers how to manufacture your geometric features. They may be used to define a coordinate system when setting up a part for CNC machining or they may be utilized to determine any subsequent operations needed. Correct geometric controls can also help with the repeatability of manufacturing and interchangeability. Excessive geometric controls could result in increased manufacturing costs, without any added value.
计算公差值
Although you may want to rush to determine theoptimal tolerancesfor the part, those determinations should be the last step in the design process. This is because tolerancing depends on the datums and geometric controls you set forth previously. In addition, there are many sources for tolerance requirements. You may find requirements for blanket tolerances or a combination of feature specific and blanket tolerances. You may find a formula based on the manufacturing processes or from your customer that dictates achievable tolerances.
In addition, a system of parts — when two or more mate together — should be considered when determining tolerances. And it’s always a good idea to lean on manufacturing or CNC experts to help guide the tolerancing process.
Handling Competing Requirements
There can be a vast array of requirements for tolerancing a part, so a common source of confusion for design engineers is about how to handle and sort requirements by priority. But you don’t need to worry about juggling all of them at once, because there’s an order of precedence for handling competing requirements illustrated below:
In general, customer requirements will supersede all other requirements. At the end of the day, your customer is paying you to provide them with products and ensuring you have read through all of their supporting documentation and correctly implemented their stipulations is key. The customer requirements are more than likely more specific than your company or industry standard recommendations or stipulations. But, if a conflict amongst customer, company or industry requirements exists, the customer requirements should be met, as long as they are more restrictive. In addition, your company may have certification from a regulatory body like AS, Nadcap, or ISO. The requirements of your certifying body tend to be more broad than that of your company, industry standards or customers but they should also be met. Consult with your company’s quality representative for assistance in understanding how to handle all your specific, competing requirements.
The Cost of Tighter Tolerances
因为公差控制上的每个特征的变化量，它们被用来控制数控加工准确度和精度。公差的目标是提供在尺寸尽可能多的余量，同时仍然允许一部分功能正常。而且要记住，窄公差意味着更高的加工成本。简单的单步骤操作可以被利用来实现，其中，可能需要几个，复杂的操作来实现窄容限更宽的公差。简单CNC机加工操作加上更宽的公差通常导致更少的检查和不合格品的较低速率。更拉近了宽容和复杂的操作，更多了检查和拒绝。
为了实现数控加工严格的公差，该设备必须具备的能力，是非常良好的维护 - 这意味着增加成本，部件制造商。通常情况下，购买高精密设备与满足严格的公差要求的能力要求更高的资本投资的前期比获得更小的机器。此外，模具精密仪器可能成本更高，可能会需要更频繁的维护和更换。
最后,该方法与ti检查功能ghter tolerances may require more up-front and recurring costs. CMM machines can be accurate to 0.0001 inches, a measurement that cannot be achieved with traditional hands-on measuring techniques. So, many manufacturers employ computerized Coordinate Measuring Machines (CMMs) to ensure accurate measuring of features with tight tolerances — and in addition to the cost of the hardware, they’ve got to pay to train technicians on how to operate them and troubleshoot issues.
可制造性设计
适当的DFM和公差一个部分需要跨功能的方法。从设计，制造人员，检验员和客户的意见应该收集并确定哪些公差适用于一个特征或尺寸时称重。这也是一个好主意，在您的处置设计的数控加工零件时使用的每一台计算机工具 - 辅助绘图（CAD）和计算机辅助加工（CAM）软件，现代计算机是设计师的优秀资源。
The first thing to consider when choosing your tolerances is the CNC process you plan to use (milling, turning, drilling, etc.). This choice should be determined by the required part geometry and your understanding of any pre-machining processing such as casting, forming, welding or cutting.
Here’s a breakdown of the most common CNC processes:
| CNC加工 | 达到的精度 | 可实现公差 | Machinable Features |
|---|---|---|---|
| Milling | High | 紧的 | Flat surfaces, slots, contours, dovetails, chamfers, grooves, and even more complex or irregular features |
| Turning | High | 紧的 | Commonly utilized to produce shafts or cylinders |
| 钻孔 | May require several operations to achieve high accuracy | May require several operations to achieve tight tolerance | Holes or slots |
Next, you need to obtain a good understanding of the tolerances achievable with your chosen CNC machining method. Standards for GD&T, such asISO 2768andASME Y14.5 – 2018, are a good starting place to figure out tolerances for certain features.
最后，它的时间与你的跨职能团队confab，以确定所选择的CNC数控加工技术，实际上可以提供所需的公差。无论您是外包加工或内部执行它，你的制造人员都在零件制造的可行性专家 - 所以你最好通过利用自己的知识和经验提供服务。
Conclusion
Now you know the main considerations for choosing the tolerances for your next CNC machining project. Simply follow these steps and do your homework before tolerancing and you’ll wind up with a cost-effective design that’s simple to manufacture.And regardless of the complexity of your design, Fictiv is the manufacturing partner who can meet your needs. Our platform provides instant quotes, free DFM feedback and our CNC machining services can deliver parts in as little as 2 days with tolerances as tight as +/- 0.0002 in.Sign up for a free account今天获得即时报价！
