One of the problems faced when locating on jigs and fixtures is the presence of burrs and foreign material on the locating surfaces. These instances occur because manufacturing processes are imperfect and produce chips and rough/raised surfaces. Burrs are usually raised edges or leftover material from machining processes that can interfere with the accuracy of the locating process. They occur on machined surfaces due to the elongating nature of metals. Images of burrs can be seen in Figure 1. To avoid this there are a number of methods to reduce their negative effects. There are 2 principles that can help negate the effects of burrs and other foreign matter on a locating surface:
- Use locating features that are easy to clean
- Incorporate a self-cleaning design so that the action of locating cleans the datum surface
Techniques to avoid burrs
As we can see above, steps can be taken to avoid the problems associated with burrs. The first method to avoid burrs is to ensure the surface is easy to clean so that accuracy can be maintained while locating. The optimum way to achieve this is with the use of wear resistant locating pins. This would ensure that the locating datum surface is high above the top surface of the fixture so that any chips produced from machining operations performed on the workpiece drop to the fixture surface instead of building up on the locating datum surface. Also having fixture design with easy to see parts and spaces would help in troubleshooting to help identify and rectify issues. Figure 2 shows these principles in action.
The second way to avoid burrs is to incorporate designs that negate their effects entirely. This can be done by providing reliefs in the fixture’s design. This can be seen easily where corners lack a chamfer. On machined workpieces these locations are where burrs are most prevalent. Figure 3 shows the design of a typical fixture and a fixture designed to properly negate burrs.
Burrs, shavings, and debris can be a nuisance. With slight forethought, it can be easily avoidable. Implementing these features in your next design can save you from any inconveniences. In our final article, we will touch on considering external forces when beginning your fixture design.
In Part 4 of our Workholding Technology series, we will examine the considerations needed for locating of workpieces with external forces applied.