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All Probe Characterization Spheres
Probe Characterizing Spheres (aka probe calibration spheres, probe character sphere or datum spheres) are used to evaluate and determine compensation needed for errors in C.M.M. measuring probes. Most errors and problems on C.M.M.s are in the probe. This should be the first measuring device purchased to check the C.M.M.
How to Choose the Best Probe Calibration Sphere for Your Application
Sphere Diameter
The first consideration is to determine what diameter should the master calibration sphere be?
On most modern C.M.M.'s the probe fitting software will accept almost any diameter spherical master. The most common master spheres have been one inch, ( 1.00", 25.4 mm ), and three quarters of an inch, ( 3/4", 0.75", 19.05 mm ); but 28 mm (1.102 inches) and 30 mm (1.181 inches) diameter have become more popular as C.M.M. technology has taken on a more European focus.
The more popular sizes of master calibration spheres are less expensive. Changing the master calibration sphere diameter will not compromise the C.M.M.'s performance in any way. When using very small diameter spherical contacts on the measuring probe, choose much smaller diameter master calibration spheres to accurately characterize these small diameter contacts. The three most popular diameters for the master calibration spheres for smaller diameter probes are 4 mm ( 0.157 inch ), 6 mm (0.236 inch ) and 10 mm ( 0.3937 inch ). The smaller diameter master spheres are used to calibrate the smallest probe spheres and so on with the larger diameters. The problem of serious wear of the small diameter master spheres, due to the high contact force, makes it almost mandatory that these small diameter spheres be made of tungsten carbide. This is a necessary compromise because there will be some built in error due to the high stiffness of the T.C. material.
Sphere Material
The choice of material for the master calibration sphere can be of great importance, depending on the contact force of the measuring probe and its spherical diameter. The important factor in the choice of the material for the master calibration sphere is its stiffness. When the probe contacts the master calibration sphere, there is considerable Hertzian elastic deformation of both the master calibration sphere and the spherical contacts of the probe. The amount of these deformations depends mainly on the probe sphere diameter and the contact force of the measuring probe. The smaller the probe ball, and the higher the measuring force, the more elastic deformation will occur. If we use a very stiff material like ceramic or tungsten carbide for the master calibration sphere, and then measure ordinary materials such as aluminum or steel; we will loose appreciable accuracy. This built in error will be greater with the high measuring force typical of many of the modern scanning probes and will be higher for small diameter probe spheres than for the larger ones. The nearer the stiffness of the master calibration sphere matches the stiffness of the part being measured, the less error will result. For this reason, steel calibration spheres have been the order of the day until recently.
If the master calibration spheres are replaced in kind, all standard diameters of ceramic and tungsten carbide master calibration spheres are available.
Mounting the Ball
The next variable is choosing the design for the post used to hold the master calibration sphere. The one important word here is the rigidity of the post. What is needed, is a robust stiff structure that will still allow full access of the master calibration sphere by the measuring probe. When simple vertical or horizontal probes are being calibrated, the standard very rugged "PB" series calibration spheres on a very stiff post are the most accurate and the least expensive choice.
Slim Calibration Sphere
When calibrating more complex articulating probes and compound star probes, more complete access to the master probe calibration sphere is required. The first and least expensive of the complex probe calibration devices is the "Slim Probe" Calibration system. It consists of one or more master calibration spheres mounted on very slender, extremely high stiffness cermet rods. These one eighth of an inch 0.125" (3.2mm) diameter, extremely rigid cermets rods allow almost complete access to the periphery of the master calibration sphere or spheres. This more complex design and more expensive components cost about a 20% premium, over the standard PB series probe calibration spheres.
Star and Tree Probes
When the fastest most accurate calibration of complex star and tree probes is required, the device to specify is the "Star Probe Calibration Device". This arrangement uses five exactly matched master calibration spheres. One sphere is positioned vertically, and the other four are arranged horizontally at ninety-degree increments (see details of this device in the technical data sheet under C.M.M. products). The very rigid three quarter inch, 0.75" (19mm), mounting posts that hold the five master calibration spheres are securely fastened to a robust two inch, 2" (50.8 mm), diameter pole, that is in turn fastened to a four inch, 4" (101.6 mm), diameter ultra stable platform. The significance of this design is that the area of one master calibration sphere that is covered up by a support post is exactly opposite another matching calibration sphere, where that area is completely exposed. Subtract the fixed distance between the two spheres to get a 100% calibration of even the most complex probes.
Small Probe Calibration
Calibrating measuring probes with very small diameter contact tips requires small diameter master calibration spheres mounted on small diameter posts.
These parameters become even more critical when small diameter complex probes are used in the scanning mode. For these applications, a petite version of the star probe calibration device is available (see details under C.M.M. products).
Optical Probe Calibration
Optical probes form a very broad spectrum of devices, but they respond well to rather limited range of master calibration devices. The one "no no" for optical calibration devices is bright shiny artifacts. The least expensive calibration sphere is the satin finished stainless steel ball. Some optical probes respond very well to satin finished titanium, which has a very flat gray surface.
The most popular master calibration sphere for optical probes is satin finished aluminum oxide ceramic, which is very white but very dull. All of the metal balls suffer from the tendency to get burnished stripes on the surface that affect the calibration. This burnishing is caused by almost any physical contact with hard materials. This does not happen to the aluminum oxide ceramic. The very white surface of the ceramic ball gets dirty very quickly, but it can be easily cleaned with coarse hand soap. Very short Ball Bars (Dumbbells), with satin finished ceramic balls are a very popular calibration device. The post next to the balls is black oxide coated so they don't effect
To facilitate the holding of these very short Ball Bar (Dumbbell)s, we have developed the "Hammer." (See the technical data sheet under C.M.M. products). The Hammer has a right-angled post that is used to hold the Ball Bar (Dumbbell) in our standard clamping hardware. The characterization spheres are often ordered with and used with risers ( extensions ) and Dual Threaded Adaptor Screws.
Repair
We will remove a worn or damaged probe calibration ball and replace it with a brand new ball of the highest quality at a very reasonable price.
Custom
We will be pleased to quote you custom manufactured probe calibration spheres. These devices can be to your design, or we can replicate an existing product.
Calibration
We will inspect your probe calibration spheres for size and roundness. We will provide you with a long form inspection report that is traceable to N.I.S.T. for $65.00 each.
Using the Probe Characterizing or Calibration Sphere
This Probe Characterizing technique uses computer software and an extremely high quality spherical artifact of a well known diameter to calibrate the radius of the contact tip. It will characterize geometry errors of the measuring probe (probe lobbing), it will compensate for bending of the probe stem and a broad range of other elastic deflections or bending moments throughout the machine. The sphere is very rigidly fixed to the C.M.M. table for this test. In effect, the C.M.M.'s computer is told that it is measuring a perfect sphere of a specific diameter and that any departure from this ideal form should be corrected in all future measurements. Some C.M.M. software is written around a specific master sphere diameter while other software is open to selection by the user.
The most widely used sphere diameter is one inch (25.4mm) but 3/4 inch (19.05mm) is also widely used. In our experience down to about 10mm (.3937 inch) diameter, the smaller the master sphere used, the more accurately probe lobbing will be compensated for. This correction ends up being a simple adjustment in the apparent radius of the contact tip of the measuring probe.
Repeatability Test
This same spherical artifact makes an excellent Coordinate Measuring Machine repeatability test device. Its position on the table and its diameter are simply measured a considerable number of times (usually ten to twelve), and the results are compared. Each measurement consists of a small number of well distributed hits. Any variations in the measurements reflect a lack of machine repeatability. Valuable information about specific machine performance can be learned by looking at the end positions of individual axii. Note that this test must be performed as quickly as possible to avoid the influence of temperature drift.
The 2" By 2" Block
We offer the 2 inch by 2 inch universal mounting block, Part Number BLK-2, as an accessory for use with the probe calibration sphere hardware. The four threaded tie down holes are M10-1.5 thread. A through mounting hole with clearance for 10mm or 3/8 inch tie down bolt is provided. This approach to tie down allows the two inch square by two inch high block to be clamped down in any angular position.
The simplest application is to use it as a 2 inch riser to hold any of our probe calibration spheres. The 2" by 2" block can be used to mount any of our probe calibration spheres at right angles to the C.M.M. table. Mounting up to five of our standard probe calibration spheres creates an excellent star probe calibration device. Leaving it mounted on the CMM table with several of the calibration spheres attached provides a monument that will quickly provide proof of performance as an interim evaluation of your C.M.M.
Five sphere calibration spheres mounted on the 2" by 2" block, creates ten inter-sphere dimensions that are, in effect, ten unique length Ball Bars. The assembly of these 5 devices is a "pretzel" that exercises every function of your Coordinate Measuring Machine. Having the several extra probe calibration spheres available will provide you with the in house hardware to perform frequent temperature drift evaluations. Our entire family of probe calibration hardware is designed to be compatible with the 2" by 2" blocks.