A top range CMM is a big investment for many companies. Their benefits are often well-documented and well-understood. For a QC department, such marque acquisitions may even be considered a crowning glory for them. While CMMs can no doubt improve the quality of a company’s products, accessories like extension bars and stylus do play an important role too. It is therefore important to address this issue to make sure the CMM fulfils its true potential.

“Our accessories increase the productivity of your CMM,” said Andreas Holowitz, global business development manager of Zeiss when he spoke at the company’s user meeting in Singapore.

Focusing on the stylus extension bars, he said the materials used to make those bars must be considered thoroughly. He added that there are different extension bars available in the market. They are typically made of steel, aluminium, titanium and carbon fibre. The first thing to note according to him is that extension bars should be lightweight, bending resistant and thermally stable.

Materials Matter

“Steel is stiff and stable but it is also very heavy. If you have a long extension, it is not ideal. (You) need counter weight, which is again not ideal for the whole balance of the system,” he said.

Aluminium extension bars, while described as “very popular”, may yet prove to be difficult to manage due to its reaction to heat. After all, it is consistency and reliability that most QC engineers are going after.

“You don’t want materials (in the measuring system) to be any different than it was one hour ago. You want a stable process,” he said. Using aluminium as an illustration, he added: “Aluminium has a high temperature coefficient of 23.8. That means 1 m of aluminium is expanding by 23.8 µm with 1 degree of temperature change. If you have a 100 mm extension, 1 degree temperature change would see 2.5 µm (of expansion). Most measuring rooms I see have at least one degree temperature change, some have two, three or four degrees of temperature change in the course of a day.”

According to him, it is possible to lose up to 4 µm of accuracy with one extension. This loss could be multiplied with complicated systems because it is made up of more than one extension. However, he conceded that if the cycle time is short, aluminium extension bars are reliable because there is not enough time for the effects of heat to kick in.

Finally, titanium has fewer problems with temperature but the wall of the extension was described as “paper thin”, making it susceptible to instability when large bending force is experienced. 

Higher Speed Better Savings

The issue of materials is related to the scanning speed and ultimately the productivity of the whole measuring process.

At a very low scanning speed of 2 mm/s, there is not much need to choose between extension bars made out of various materials. However, at higher scanning speeds, the carbon fibre extension bar displays the least form deviation. In other words, if an operator uses a different material, he/she has to measure at a slower speed in order to achieve the same level of accuracy and repeatability.

Linking scanning speed to operating cost, Mr Holowitz said: “If the cycle and measuring time is 20 minutes and 10 percent reduction can be gained by using the right materials (extension bars), it works out to two minutes per part and six minutes in an hour. After three hours, you can measure one part more and after 10 hours, you have gained an hour in productivity.”

This point is particularly pertinent given that the hourly operating cost of CMMs typically fall in the region of US$100 to US$200. One hour saved can potentially be quantified as US$150 (on average) of productivity gained.

Using the bicycle as an analogy to sum up the issue of materials, he said a bike frame made of steel is cheap; an aluminium frame represents an upgrade as it is lighter. However, if someone is going to race, he would need a carbon fibre frame.

Interface — When Less Is Better

A stylus system is made up of various accessories like adapter plates, extension bars and joints. Although this modular system or approach offers flexibility, the interface where two accessories meet is an area of weakness.

“An interface may come a little loose over time, becoming a potential weak point in your stylus (system),” said Mr Holowitz. Based on his argument, reducing the number of interfaces would theoretically make the system more reliable and accurate.

This forms the basis for the ThermoFit technology, which according to the developer, saves operators from the problems that come with connecting interfaces. In addition, the system based on this technology is some 25 percent lighter and has nearly double the stiffness of the stylus system.

At the end of the day, modern metrology is all about getting the tiny deflections right. High-end CMMs promise unrivaled accuracy but having a correct mindset and paying attention to details like using the right accessories ensure reliable results and higher productivity. 

Other Tips For More Reliable Measurements

Diamond — Metrology’s Best Friend?

Another important part of the stylus system is the point that is in contact with the part. When scanning soft parts made out of aluminium, there is a possibility of material build-up at the ruby tip. According to Mr Holowitz, this is a problem because operators have to spend time cleaning up the ruby tip and recalibrating the system. He said using a diamond tip can solve this problem as it apparently does not pick up materials from soft parts even though it is a very hard material. Allaying fears that diamond, given its hardness, may scratch parts, he said the developers have done testing on copper and the diamond tip did not deform the copper part.

Be Mindful Of Heat Sources Around Your CMM

“You bought a wonderful machine and there are things you need to do to keep it wonderful,” said Mr Holowitz. According to him, some measuring rooms are not sensitive to the conditions that CMMs require in order to do what it is designed to do properly.

For instance, he said thin walls and windows of the measuring room can potentially be sources of heat and that can in turn affect the performance of the CMM. He also discouraged having non-essential personnel in the measuring room and placing a CMM too close to a door. As minute as the issue may seem, activities like that could affect air flow, resulting in fluctuating temperature. He recommends keeping track of the temperature on an hourly basis or investing in a temperature tracking system that can provide live monitoring of temperatures.