Granite as a CMM material

One of the great things about Wenzel CMM's is that the moving axis all ride on air bearings. The bearing surfaces are all granite. Some companies use a variety of materials on their different CMM's. In fact you can see arguments for granite and against granite all over the internet, sometimes written by the same person. Let's talk about the granite mass of the bridge.

First generation scanning CMM

One of the main reasons people give against using granite on the bridge is the elevated mass. In the picture to the left, it gives a great example of why this was a reason some time ago. Moving a solid piece of tall granite which is supporting a massive steel tower, which in turn carries a massive Z ram would clearly illustrate an advantage for using lighter materials on the X bridge, Z carriage, Z ram.

In the case of this particular machine type, I was part of a group at my last company that would retrofit these frames with the newest and latest scanning probe heads. These machines were
incredibly accurate because the use of granite lapped bearing surfaces, round air bearings and CAA. The speed penalty was mitigated by the increased accuracy and the fact that when scanning the probeheads  operated best at 5-100mm/sec.

You can also see the size of the Z ram. It had to carry the massive scanning measuring probehead. This machine was stoutly built, to accommodate scanning which provided great accuracy and data confidence.

What about today?

Wenzel controls its granite supply and still retains the capability to process it's granite, in house. This means there is a significant advantage to controlling costs and building machines to custom sizes.

You can also see the comparison between the Z rams. I used the same size allen key to demonstrate the difference in the size. The difference in size of the scanning head on both machines is also significant. While not on display here. If you look at the difference between a Measuring probe head and a SP25 it is substantial.

The argument that using granite in the X bridge and Z ram because of the increase mass slowing the machine down, just doesn't hold water.  A quick check of various manufacturing (Wenzel, Zeiss, Hexagon, Mitutoyo)  spec's shows comparative machine speeds 500mm/sec - 550mm/sec. 

To see Wenzel turning granite into CMM's check out this video.