Time to read: 6 min
Sometimes juggling all the requirements for tolerancing your part can feel like a circus act. As a design engineer, you must choose machining tolerances for fit, form, function, customer requirements, industry specifications,manufacturing capabilities, and don’t forget about making sure it can be inspected and properly defined.哎呀！It’s a struggle to create parts which are cost-effective, repeatable, and manufacturable, so we’ve got some helpful tips to help you leave the circus act behind!
If you’ve utilized theDesign for Manufacturing(DFM) technique, these steps will feel familiar.
Getting the Right Information Prior to Tolerancing
Determine the function of each geometric feature
Your goal is to be specific. Consider each geometric feature and determine its most critical function. Analyze all the交配组件和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!
For example, instead of saying a feature allows for placement of two rotating shafts, specify how two holes may define the minimum required distance between two shafts. Determine which features will be cast, forged, stamped, welded, machined and finished after machining. Consider how theCNC 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 ofGeometric Dimensioning and Tolerancing(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.
Calculate the tolerance values
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.
处理竞争要求
可以容忍零件的大量要求，因此设计工程师的混淆来源是如何优先处理和分类要求。但是您不必担心一次杂耍所有这些，因为要处理以下所示的竞争要求的优先顺序：
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
Because tolerances control the amount of variation on each feature, they are used to control the CNC machining accuracy and precision. The goal of tolerancing is to provide as much allowance in the dimensions as possible while still allowing a part to function properly. And keep in mind that narrower tolerances mean higher machining costs. Simple, single-step operations can be utilized to achieve wider tolerances where several, complex operations may be needed to achieve a narrow tolerance. Simple CNC machining operations coupled with wider tolerance generally leads to fewer inspections and a lower rate of rejects. The more narrow the tolerance and complex the operations, the more numerous the inspections and rejections.
In order to achieve tight tolerances in CNC machining, the equipment must have the capability and be extremely well maintained – which means added costs to the part manufacturer. Typically, buying high precision equipment with the ability to meet strict tolerance requirements requires a much higher capital investment up-front than acquiring lesser machines. Additionally, tooling for precision equipment may cost more and will likely require more frequent maintenance and replacement.
最后，使用更紧密的公差检查功能的方法可能需要更多的前期和重复的成本。CMM机器可以准确至0.0001英寸，这是传统的动手测量技术无法实现的测量。因此，许多制造商使用计算机化的坐标测量机（CMM）来确保具有紧密公差的功能准确测量功能 - 除了硬件的成本外，他们还必须付费才能培训技术人员如何操作它们并解决问题。
Design for Manufacturing
Proper DFM and tolerancing a part requires a cross-functional approach. Input from design and manufacturing staff, inspectors and customers should be gathered and weighed when determining which tolerances to apply to a feature or dimension. It’s also a good idea to use every computer tool at your disposal when designing a part for CNC machining — modern Computer Aided Drafting (CAD) and Computer-Aided Machining (CAM) software is an excellent resource for designers.
选择公差时要考虑的第一件事是您计划使用的CNC流程（铣削，转弯，钻探等）。此选择应由所需的部分几何形状以及您对任何预处理处理（例如铸造，形成，焊接或切割）的理解。
Here’s a breakdown of the most common CNC processes:
| CNC Process | Achievable Accuracy | Achievable Tolerance | 可加工的功能 |
|---|---|---|---|
| Milling | 高的 | Tight | 平面，插槽，轮廓，燕尾尾，弯腰，凹槽，甚至更复杂或不规则的特征 |
| Turning | 高的 | Tight | Commonly utilized to produce shafts or cylinders |
| Drilling | 可能需要多个操作才能达到高精度 | 可能需要几次操作才能实现紧张的容忍度 | 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 2768和ASME Y14.5 – 2018，是找出某些功能的公差的好起点。
Lastly, it’s time for a confab with your cross-functional team to determine if the chosen CNC machining technique can actually provide required tolerances. Whether you’re outsourcing machining or performing it in-house, your manufacturing personnel are the experts on the feasibility of manufacturing parts — so you’re best served by leveraging their knowledge and experience.
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和get an instant quote today!
