In the previous article, we had discussed the basics of setting up a true position evaluation with Zeiss Calypso. In that we touched base on selecting the proper datum reference frame (DRF) and explained the difference between a base alignment and a proper datum reference frame. In this issue, we will delve deeper into a true position evaluation with the application of the positional modifier tools. 

The Division of Geodesy invites you to workshop

Manufacturing Excellence Control Using CMM or Scanner

Date: Friday September 10 2010, 9-16 o'clock

Place: KTH, Drottning Kristinas väg 30, Stockholm

Content: Practical and theoretical introduction to flatness, straightness and roundness. Determination of surface irregularities from scanned data using inner reference technology. The new technology yields a "push the button" interface related to AIDM (Artificial Intelligent Decision Making). The theoretical background will be presented.

There are no direct datum measurement commands contained in the DMIS language so this article will build on the previous ones by using features we would have already measured or constructed to create them. The reason we create datums, sometimes referred to as coordinate systems, is so that we can report valid feature positions from a known origin in up to three dimensions. The orientation of the coordinate system will also have a direct effect on the shape of some features when analyzed. DMIS’ various datum statements allow us to define the orientation and position of the axes and planes that make up the coordinate system. When you create a datum you use features to define the direction and position of three scales that are square to each other. Each scale comprises an axis that will lie along the direction of the datum feature and a plane that is square to that scale, passing through the scales zero point. Because of their close relationship with CAD systems, DMIS datums are always right handed.

A cylinder has a total of three surfaces: a top, bottom, and middle. The top and bottom, which are circles, are easy to visualize. The third is actually the curved wall of the cylinder. It is a surface that has measurable volume.

What is it?

Circles are simply closed curves. Where points along the curve converge to close the curve and the results are a circle.

With all instruments used for scanning, i.e. roundness machines and surface roughness gages, when scanning with a CMM the raw data must be filtered. When using a CMM to scan features on a part there are data filtering options that can affect the scanning results.

The software default selections may not always be the best option. As CMM programmers, when software messes with our data we better have a good understanding of what is going on with that data. Understanding filtering is key when faced with the multiple filtering options available.

Surface Normals

There are several issues that arise when bringing a CAD model into your CAD based programming software. One of these issues has to do with surface normals (surface vectors). You can bring in a model and the entire model or a portion of that model is visible, but dark (Figure1), or certain sections are not visible at all. This problem arises from the surface vectors pointing in the opposite direction than your CAD system views them. You are looking at the back side of the surface. You must reverse the surface normal (Figure 2). If your software has this capability, you are looking for something similar to ‘reverse surface normals’. This will flip the surface so the front faces in the correct orientation for your software to view the surface.

 In our on going series of covering the CMM basics we need to cover the all too familiar DFR, or the Datum Reference Frame. We see this DFR associated with a feature on the part but what does it mean? and how does it change my approach to programming this part?  We will attempt to answer these questions. This won’t be an attempt to address the overall make up of the DFR but we will look at the datum order and how that affects our programming