How to Determine Optimal CMM Size

A CMM should be large enough to measure your most important parts or components. Let's look closely at one example: If you are measuring a component that is roughly 500mm in the X axis, 500mm in the Y axis, and 500mm in the Z axis (a cube), the machine should be at least as large as the component. However, because most CMMs have an indexable probe head, and you might need to get into all four sides of that component, it is then necessary to add a stand-off distance of approximately 200mm from each face. Effectively, you would need a machine that measured 900 x 900 x 900mm to measure that particular cube-shaped component.

What if the most essential part you will measure is a flat one? Perhaps it is 1m x 1m, and it is flat, such as a sheet metal component. Then, you would need a machine the size of the part itself, because you are only measuring in one plane.

There's more to optimal CMM size than just the measured component size. It is not just the part to be measured that must be taken into account when determining the size of the machine. On every CMM, there is a component called a reference sphere, which must be added to the volume of any coordinate measuring machine. So in the last example above, for instance, you would need to add to the 1m in one dimension (or axis) an additional 200mm distance in order to reach the reference sphere. The reference sphere enables you to datum the probe in any direction. 

When it comes to CMMs, size does matter. If the size of the CMM machine is not going to fit into the room where you intend to use it, this can present a problem. If the room is not large enough, and ceiling height is usually the governing factor, you must either think about putting the CMM onto the shop floor or consider constructing a special room where it will fit properly.

To determine what is the best type of CMM, here are some questions to answer:

• What is the accuracy required for measurement of your part?
• Will the machine fit into the allocated space?

If you do not have space in the lab for a CMM, and most of your components are better checked on the shop floor, it is advisable to contemplate the purchase of a shop floor coordinate measuring machine. Shop floor CMMs come in two essential categories:

1. Hard-bearing machines, which are by nature shop hardened and do not require factory air
2. An air-bearing machine that has covered ways and temperature compensation

If you have important components to measure, which are over 20 feet in length, for instance, but you don't have space for a large CMM, there are other options and methods of measuring to consider:

• A laser tracker. This is a coordinate measuring device that uses laser light and a target. It takes up very little space and has similar accuracies to a large CMM, but it is portable. A laser tracker is often more expensive than some CMMs and inspection times are longer because it is a manually operated device, rather than an automatic CMM.
• A portable arm. This can measure large components by employing a leapfrog technique. An arbitrary datum is selected and the arm is physically and sequentially moved to each new location to continue the measurement.