tailieunhanh - Dimensioning and Tolerancing Handbook Episode 3 Part 4

Tham khảo tài liệu 'dimensioning and tolerancing handbook episode 3 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 20-16 Chapter Twenty Once the machine stabilized about 2 hours the largest drift within any two hour segment in a single axis was approximately pm with individual spikes of pm over a 30-minute time frame. Two additional 24-hour versions of this test were run with the same level of results. It is critical to note that the charts clearly display a direct correlation between temperature change and displacement very close to a linear relationship. Tolerances on Tooling Components and Assemblies What needs to be kept in mind on this issue is that the enhanced-accuracy CMM was justified principally to measure critical features on tooling components and assemblies. In addition we were clearly aware up front that this CMM or any CMM was not capable of measuring every feature we considered critical to process or function. For example one of the restrictions on a contact CMM is probe diameter. The smallest standard probe tip available is mm which restricts measurements on an inside radii or diameter. A large percentage of the features of size have tolerances of pm to pm with feature location tolerances of 5 pm. I believe I would be conservative in saying that greater than 50 of the features that are measured on this CMM are 5 pm. These are current tolerances defined on tooling drawings at this time. If we look back at one of the original assumptions 1. pm is accurate enough to tell us what effects the tool shapes have on the forming process this was a worst-case statement which included accuracy and repeatability of the measurement system. What has been discussed so far has been only repeatability. Miscellaneous Feature-Based Measurement Tests It is essential that the results from the thermal drift test are understood to be based on a simple measurement within a small known envelope of 25 mm so accuracy and repeatability are at their best. Where it starts becoming more difficult is in measuring other types of geometric features within a larger .