Portable CMM Calibration ( Centerline Ranger) — Articulating Arm CMM Calibration — Portable Arm CMM Calibration

Sending your Portable Coordinate Measuring Machine out for calibration poses several problems. The first is the danger of loss or damage in shipping. Next, there is a considerable cost involved, for both shipping and for the calibration. In addition, you loose the use of your machine while it is out for calibration. One breakthrough after another has recently brought in house calibration of Portable C.M.M. to an accurate, economical science.

These new developments have so improved the science of portable C.M.M. calibration, that the level of quality is now entirely compatible with the well tested ANSI- B89.4.1-1997 standard for C.M.M. evaluation. The development of the “Centerline Ball Bar Ranger” gives Portable C.M.M. Calibration both a legal and a traceable foundation, and it can all be done on the shop floor. All of this highly technical stuff, is all well and good, but the calibration can also be done very economically.

In addition to providing a complete calibration device for all Portable C.M.M.’s, the “Centerline Ball Bar Ranger”, and mounting system will accurately calibrate a Laser Tracker, an Optical Scanner, Optical Probes and the various other devices in current use and still on the drawing boards.

For use with optical scanners and optical probes, we offer high quality Satin Finished Titanium Balls. The Satin finished Titanium Balls give good sensitivity without any hot spots or spectral reflectivity. These balls are made in the three standard SMR diameters of one half inch ( 1/2", 0.5", 12.7 mm ) part # SFTB-PERM-.5 seven eights inch ( 7/8", 0.875", 22.2 mm ) part # SFTB-PERM-.875 and one and one half inch ( 1.5", 38.1 mm ) part # SFTB-PERM-1.5.

This same device can also be adapted for use as a Scale Bar, by very accurately calibrating the inter sphere dimension. A standard ball bar consists of two very precise balls permanently attached to the opposite ends of a long rigid bar, (See Plate #1.).

Standard Ball Bar
Plate 1, Standard Ball Bar

This centerline Ball Bar Ranger approach to Ball Bar technology uses the same age old principle, that the distance between the two balls of a Ball Bar will remain “exactly” the same, no matter what the geometric location of the Ball Bar. With this absolute knowledge, the actual performance of a Portable C.M.M. can be determined right out in the field.

To very accurately measure the true X-Y-Z position of each of the two balls of a conventional Ball Bar, requires seventeen ( 17 ) hits ( measurements ) each. It is quite difficult and very time consuming to make that many accurate measurements by hand with a portable coordinate measuring machine. The solution for this difficulty is to use the “Centerline Ball Bar Ranger”, instead of a conventional Ball Bar (See Plate #2.).

Centerline Ranger
Plate 2., Centerline Ball Bar Ranger

The “Centerline Ball Bar Ranger” substitutes a magnetically preloaded three ball kinematic nest for each one of the two Ball Bar balls. When a one half inch diameter ball probe is placed in each one of the two three ball Kinematic nests, the distance between these two measurements form the Ball Bar (See Plate #3.). The “Centerline Ball Bar Ranger” is actually a revision of a 15 year old product that was developed to calibrate Laser Trackers. A very important change in the standard version of the “Centerline Ball Bar Ranger” puts the one half inch (12.7mm) diameter balls right on the neutral bending plane of the Ball Bar. This dramatic improvement, reduces the effect of any sag or bending of the bar, to a minute second order, cosine error. This approach assures that the distance between the two kinematic couplings remains constant, when the “Centerline Ball Bar Ranger” is moved to all the various positions required for calibration.

Ball Bar Ranger
Plate 3., Three Ball Mount, with Ball While Calibrating Length

In other calibration devices, even some used by the measuring machine manufacturer, the centerline between the two balls is above or below the neutral bending plane of the bar. In this situation, any minute sag or bending of the bar creates a huge first order error in the distance between the two ball centers.

In performing the Ball Bar evaluation, we recommend a one half inch diameter ( 1.5”, 38.1 mm ) , very high quality, hard stainless steel or tungsten carbide ball, be used as the probe tip on the end of the portable C.M.M. arm. ( See our web page, "Gage Tips" )

The wrist joint of the portable C.M.M. is rotated and articulated to position the one half inch diameter hard stainless steel or tungsten carbide ball, in different attitudes, in the three ball Kinematic nest. These movements also change the position of the elbow and shoulder joints of the Portable C.M.M., so that every parameter, of both the machine and the software get a full workout. Obviously, all of the readings taken in any given position should stay exactly zero and any deviation from zero is either a mechanical or a software error of the Portable Coordinate Measuring Machine.

Next, the one half inch diameter probe ball is moved to the other three ball kinematic coupling on the other end of the Centerline Ball Bar Ranger and the test is repeated.

Next, the “Centerline Ball Bar Ranger” is moved to a different geometric location and measured again. (as outlined by the ANSI B89.4.1 specification.). To make a full machine evaluation there are 21 of these positions, but just a few of these measurements will give you a good interim evaluation of your machine’s performance (See Plate #4.).

Fast CMM Fast CMM Fig 4. Fig. 1
Plate 4: A Few Ball Bar Positions
Plate 5

In addition to its use as a volumetric evaluation tool, (which does not require any length calibration) the “Centerline Ball Bar Ranger” can have the actual ball centerline, to ball centerline dimension, between the two, three ball kinematic couplings, accurately calibrated. This is done by placing two one half inch diameter master balls, in the three ball kinematic nests and then measuring the distance between them. With this extra data you can reach a new level of measurement assurance for your machine. The typical cost for this inter ball calibration is only $150.00 per Centerline Ball Bar Ranger (See Plate #5.).

There are actually two supports required for the “Centerline Ball Bar Ranger”. The “Centerline Ball Bar Ranger” used for calibration of Portable CMM’s are typically 900 mm (35.5 inches) or 1000 mm (39.37 inches) or 1.5 meters (1500 mm) [59 inches], depending on the measuring capacity of the machine. Carbon fiber giant centerline rangers, are available that are capable of calibrating portable arm machines up to ten feet. The ideal length for the Centerline Ball Bar Ranger is one and one half the length of the radius of the machine, but shorter or slightly longer devices may be used. To stop the bending of these long Ball Bars when they are moved to different positions during the calibration, the Centerline Ball Bar Ranger is kinematically supported at each end by an auxiliary support ball. This design is called the Cantilever Ball Bar Support system that is used with the Way Out Ball Bar supports. (See Plate #6.).

Long Way Out Rail
Plate 6., Long Way Out Rail

In order to provide support for the Cantilever Ball Bar Ranger and to connect it to our Tri-mount Collar on one of our Anchors, a contoured aluminum rail, called the Way Out Ball Bar Support is provided. This contoured rail is five times as rigid as the Ball Bar itself (See Plate #6.).

It is also important that the “Centerline Ball Bar Ranger” on the Way Out Ball Bar Support be rigidly held during the probing of the three ball kinematic couplings. The ideal supporting device, for this is our 14” Heavy Duty Trivet (Part # TRI-14-HD). This rugged 14 inch ( 356 mm ) Trivet is a Miniature tripod with a huge 3 inch ( 76 mm ) Diameter post (This post is our “Anchor,” which is our Part# AN-36-0) and a husky Tri-mount Collar, (our part # AN-3) that will resist any movement during probing. This combination will allow universal location of the Centerline Ball Bar Ranger in every possible position (see the “Heavy Duty Trivet,” the “Anchor” and the Tri-mount Collar on this website).

In order to accurately calibrate the rather large capacity of Portable C.M.M.’s, Laser Trackers, Laser Scanners etc, rather long versions of the “Centerline Ball Bar Rangers” are required. This long distance can generate serious problems, due to temperature changes, that occur during calibration. These temperature variations will cause expansion or contraction of ordinary steel Ball Bars. This problem is eliminated by making the tube, used to make the bar, from “Invar”. Invar is a metal that has almost no thermal coefficient of expansion (it is only about one tenth that of steel) so it is temperature insensitive.

A Complete 900 mm Portable C.M.M. calibration system, (our Part # CBBR-900-KIT ) includes a 900 mm ( 35.4 inches ) long, invar “Centerline Ball Bar Ranger” ( Our Part# CBBR-900), a Wayout Ball Bar Support (our Part# WO-900 ), a 14 inch ( 356 mm ) heavy duty Trivet ( our Part# TRI-14-HD ), a 36 inch ( 941 mm ) tall 3 inch diameter Anchor ( our Part #AN-36-0 ) and a Tri-mount Collar ( our Part # AN-3 ).

A complete 1000 mm Portable C.M.M. Calibration system (our Part # CBBR-1000-KIT ) includes a One Meter (1000 mm, 39 inches ) invar “Centerline Ball Bar Ranger” ( our Part# CBBR-1000 ), a Wayout Ball Bar Support (our Part# WO-1000 ), a 14” heavy duty Trivet (our Part# TRI-14-HD), a 36 inch (941mm) tall, 3 inch (76mm) diameter Anchor (our Part #AN-36-0) and a Tri-mount Collar (our Part# AN-3).

A complete 1500 mm Portable C.M.M. Calibration system (our Part # CBBR-1500-KIT) includes a one and one half meter (1500 mm ) [ 59 inches ] long, Invar® “Centerline Ball Bar Ranger” (our Part # CBBR-1500), a Wayout Ball Bar Support ( Our Part# WO-1500 ), a 14 inch ( 356 mm ) heavy duty Trivet ( Our Part# TRI-14-HD ), a 36 inch ( 941 mm ) tall, 3 inch ( 76 mm ) diameter Anchor, ( our Part #AN-36-0 ) and a Tri-mount collar (Our Part# AN-3).

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