Different Types of CMM Machines - How to choose a CMM Machine

How to choose a CMM Machine – Different types of CMM Machines and Their Application

There are four basic types of coordinate measuring machines: bridge, cantilever, gantry and horizontal arm. Each one provides unique advantages based on the components being measured.

Bridge. The bridge is the most popular style of coordinate measuring machine.

The description found on Wikipedia is as follows:

The typical 3D "bridge" CMM machine allows probe movement along three axes, X, Y and Z, which are orthogonal to each other in a three-dimensional Cartesian coordinate system. Each axis has a sensor that monitors the position of the probe on that axis, typically with micrometer precision. When the probe contacts (or otherwise detects a particular location) on the object, the machine samples the three position sensors, thus measuring the location of one point on the object's surface. This process is repeated as necessary, moving the probe each time, to produce a "point cloud" which describes the surface areas of interest.

Due to their basic structure and simplicity of build, bridge machines have become extremely popular. Bridge machines have a low cost to build and the ability to maintain accuracy and repeatability over the long term. Most bridge machines are based on a precision granite plate with two legs supporting the X-axis carriage. Generally, only one side of the bridge is driven; the other side is the slave side and is allowed to float freely. 95% of all bridge machines run on air bearings to allow friction-free movement and minimize mechanical interaction.

Machines are built in sizes ranging from 300×300×300 mm XYZ to 2000 mm x 5000 mm x1500mm, with some exceptions. Bridge machines are the workhorses of CMMs. However, there are both pros and cons associated with their function. For instance, accessibility to the part being measured is sometimes quite difficult because of the uprights holding the X-axis beam. Heavy components have to be lifted onto the plate, necessitating a crane or a lift truck and potentially causing a collision with the machine. The accuracy of most bridge systems is usually better than other types of coordinate measuring machines, and certainly, when considering a system for machined parts with higher tolerances, a bridge is hard to beat.

Cantilever. The cantilever CMM machine was the initial design of Ferranti in Scotland in the 1970s, and today they are manufactured in small numbers, usually as shop floor hard bearing machines. Generally used for measuring relatively small parts, they provide open access to the operator on three sides. The X-axis measuring beam is attached on the side of a rigid structure housing the Y axis. This limits the size of the X beam, because of inherent rigidity, making the machine suitable only for smaller components. As a shop floor CMM, the cantilever machine excels, because it lends itself well to automatic loading and unloading.

Gantry. Gantry CMM machines are used predominantly for very large or heavy parts that require the high precision of a bridge machine. Most gantry machines are mounted directly to the floor and therefore must have a substantial foundation. This requirement is specified by the manufacturer and should not be ignored.

Smaller gantry machines have four upright columns supporting large Y-axis beams, usually 1.5 to 2 meters in height. The X-axis carriage runs along the two supported beams of the Y axis. Larger gantry machines have six or eight columns, or more, depending on the length of the Y axis. The Z axis, located on the X-axis carriage, can be as long as 4 meters, but normally they are between 1.2 to 2.0 meters in depth.

The measuring range of gantry CMMs can vary from 1 x 2 x 1m XYZ to 4 x 10 x 3m XYZ, and even larger, specially built units can be purchased.

Gantry coordinate measuring machines provide significant advantages over conventional bridges:

1. Heavy parts and large components can be loaded directly on the floor (in the measuring volume) this is a tremendous safety feature, and enables the programmer to easily walk up to the part during programming of certain features.
2. Programming a massive part is also much easier, as access is unrestricted through 360°, enabling smaller features that are not easily visible to be carefully measured and programmed.
3. Surface plates for smaller components are easily inserted into the gantry, making measurement simpler and less tedious for the operator. This enables the smaller components to be within easy reach during programming.
4. All scale and drive systems are well clear of lift trucks and cranes during the loading process.

Disadvantages of the gantry CMM include:

1. Significant cost of building a foundation
2. Complete lack of any kind of portability. Bridge machines can be moved relatively easily within the plant either by forklift trucks or riggers and can be ready for use after calibration. Gantry machines can be moved but need significant disassembly and reassembly.
3. Gantry CMMs take up a tremendous amount of real estate.

Horizontal Arm. Steifelmayer in Germany first pioneered horizontal arm CMMs. Previously, horizontal arm systems were strictly scribing tools and were operated manually. Companies such as Bendix (now Sheffield) under the Portage brand produced the machines in the USA. Vernier scales were used to measure components and scribers were utilised as probes.

The configuration of a horizontal arm coordinate measuring machine is quite different from the three other types in that it consists of a vertical column (X axis) and a horizontal arm (Z) mounted on a saddle that runs vertically up and down the X axis. The Y axis is then the length of a surface plate or a long beam.

There are 2 basic types of horizontal arm CMMs:

1. Plate mounted. Here the column is mounted on a large surface plate, usually made of steel. A small percentage of the machines run on top of the plate (top mounted) and are generally quite inexpensive and less accurate than side mounted systems. Unfortunately with this configuration part of the plate cannot be utilized, as it is taken up by the guideways for the vertical column. The far more popular side mounted system allows the whole surface area of the plate to be utilized for measurement. It also provides a far more stable and accurate platform for measurement, as the tipping moment is minimized because the two sides of the plate are utilized as guideways.
2. Two runway mounted. As the name implies, this CMM runs on a dedicated beam; there is no integral plate. This top mounted design provides the utmost in flexibility, because it can be used singularly or as a dual arm machine for measuring large components. Usually automotive bodies and trim are measured on these machines. The dual arm machines allow measurement of both sides of the component simultaneously. Parts can be wheeled or pushed into the measuring volume, or an independent plate can be added.

Horizontal arm coordinate measuring machines are the least accurate of the four types of CMMs mentioned. However, they have some decided advantages: they can measure large thin-walled/sheet-metal type components mounted in fixtures that represent car body position, such as doors, body sides, grills, dashboards, etc.

Accuracy is not the strong point of the horizontal arm CMM. Most machines perform to volumetric tolerances of 30 µm and larger; however, when measuring large components that require open tolerances they are by far the most cost-effective solution.

Measuring sizes of horizontal arm machines vary from 2 x 2 x 1m XYZ to 4 x 10 x 3m XYZ.