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The Influence on CMM Caused by Measuring Bar and Ball

Dec 05, 2016

Measuring bar and ball are key parts connecting three coordinate measuring machine sensor and the surface of the work-piece. If chose inappropriate measuring bar and ball for a specific measurement task, it can lead to the uncertainty of measurement results and the inconsistencies. In order to realize the optimization of CMM precision, in the choice of measuring bar and ball must be thoughtful.

1. The roudness of the measuring ball

Most CMM measuring balls adopt synthetic ruby. Its roundness error may lead to a potential accuracy loss up to 10%.  The manufacturing precision level of ruby ball is to use the grade to define, the level of a measuring ball depends on its relative to the maximum deviation of ideal sphere. Two of the most commonly used measuring levels is grade 5 and 10 (the sphericity error were 0.13 um and 0.25 um). Despite using grade 10 to replace grade 5 can save some cost, but is likely to endanger 5:1 which is the commonly accepted ratio of CMM uncertainty and shape tolerance. 

2. The material of the measuring ball

Although ruby is the most commonly used material of measuring ball, but other materials might be more appropriate to contact scanning.  In scanning measurement, measuring ball slide along the surface of work-piece, it will lead to abrasion. In extreme cases, the long-term exposure can lead to wear of measuring ball(abrasive wear), or the work-piece material is sticking to the measuring ball(adhesion). In either case, will affect the measuring the roundness of the measuring ball.

In scanning measuring surface of some materials such as cast iron, abrasive wear may occur. The remaining small particles may cause slight scratches for measuring ball and work-piece surface as to form a small plane on the measuring ball. For such measurements, zirconia ball which has higher hardness is a good choice. Adhesion wear usually occurs when the ball material and work-piece material which has the chemical affinity, which usually happened when using ruby ball to scan aluminum work-piece. The relatively soft aluminum will transfer from the work-piece to measuring ball, which will generate a layer of coating on the ball, thus reducing the measuring roundness and precision. In this case, the silicon nitride ball is a better choice, because this kind of material with good abrasion resistance, and have no affinity of aluminum.

3. The flexure of the measuring bar

Measuring accuracy reduces with the increase of the length of measuring bar, so the long measuring bar is not suitable for measuring all features (in fact, using a maximum stiffness, the shortest possible length measuring bar was the right thing to do).

Although the steel is suitable to make some short measuring bar, but hard alloy is the best material to make measuring bar. However, due to the density of hard alloy is big, so it should not be used for the long measuring bar. Ceramic measuring bar can usually be used for the measuring task to ask for both good rigidity and light weight. Also, the carbon fiber is usually can be used to make a long measuring bar.

4. The heat stability

Temperature fluctuations can cause a lot of CMM error. To choose the appropriate extension measuring bar material, can get better stability under the condition of the environmental change, and provide better consistency measurement results. 

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