Universal measuring machine
Encyclopedia
Universal measuring machines are measurement devices used for objects in which geometric relationships are the most critical element, with dimensions specified from geometric locations rather than absolute coordinates
. The very first uses for these machines was the inspection
of gauges and parts produced by jig grinding. While bearing some resemblance to a coordinate-measuring machine
(CMM) its usage and accuracy envelope differs significantly.
Originally, universal measuring machines were created to fill a need to continuously measure geometric features in both an absolute and comparative capacity, rather than a point based coordinate measuring system. A CMM provides a rapid method for inspecting absolute points, but geometric relationships must be calculated rather than measured directly. In theory? By using an accurate spindle
with an electronic test indicator
rather than a probe
, a universal measuring machine fills this need. The indicator can be accurately controlled and moved across a part, either along a linear axis or radially around the spindle, to continuously record profile
and geometry. This gives the universal machine a very strong advantage over other measuring methods when profiling radii, contours, and holes, as the detail of the feature can be of any arbitrary resolution. The analog record generated by the machine can be fed directly into a compensator on computer numerical control (CNC) machine tool
s, correcting the machine on a per-job basis.
In practice, the universal measuring machine is a very slow machine that requires a highly skilled and patient operator to use, and the accuracy built into these machines far outstripped the needs of most industries. As a result, the universal measuring machine today is only found as a special-purpose machine in metrology
laboratories. Because the machine can make comparative length measurements without moving linear axes, it is a valuable tool in comparing master gauges and length standards. While universal measuring machines were never a mass-produced item, they are no longer available on a production basis, and are produced on a to-order basis tailored to the needs of the metrology lab purchasing it. Manufacturers that perform work that must be measured on such a machine will frequently opt to subcontract the measurement to a laboratory which specializes in such.
Universal measuring machines placed under corrected interferometric control and using non-contact gauge heads can measure features to millionths of an inch across the entire machine's envelope, where other types of machine are limited either in number of axes or accuracy of the measurement. The accuracy of the machine itself is negligible, as the environment the machine is the limiting factor to effective accuracy. The earlier mechanical machines were built to hold 10 to 20 millionths of an inch accuracy across the entire machine envelope, and due to incredible machine design and forethought, remain as accurate today without computer compensation.
Coordinate system
In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element. The order of the coordinates is significant and they are sometimes identified by their position in an ordered tuple and sometimes by...
. The very first uses for these machines was the inspection
Inspection
An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...
of gauges and parts produced by jig grinding. While bearing some resemblance to a coordinate-measuring machine
Coordinate-measuring machine
A coordinate measuring machine is a device for measuring the physical geometrical characteristics of an object. This machine may be manually controlled by an operator or it may be computer controlled. Measurements are defined by a probe attached to the third moving axis of this machine...
(CMM) its usage and accuracy envelope differs significantly.
Originally, universal measuring machines were created to fill a need to continuously measure geometric features in both an absolute and comparative capacity, rather than a point based coordinate measuring system. A CMM provides a rapid method for inspecting absolute points, but geometric relationships must be calculated rather than measured directly. In theory? By using an accurate spindle
Spindle (tool)
In machine tools, a spindle is a rotating axis of the machine, which often has a shaft at its heart. The shaft itself is called a spindle, but also, in shop-floor practice, the word often is used metonymically to refer to the entire rotary unit, including not only the shaft itself, but its bearings...
with an electronic test indicator
Indicator
Indicator may mean:In chemistry:* pH indicator, a chemical detector for protons in acid-base titrations* Redox indicator, a chemical detector for redox titrations* Complexometric indicator, a chemical detector for metal ions in complexometric titrations...
rather than a probe
Probe
- Science and technology :* Medical equipment** Anal probe** Endoscope** Proctoscope* Atom probe, an atomic-resolution microscope* Hybridization probe or chemical probe, used in molecular biology...
, a universal measuring machine fills this need. The indicator can be accurately controlled and moved across a part, either along a linear axis or radially around the spindle, to continuously record profile
Profile
- Computing and technology :* Profile , a concept in Unified Modeling Language* Apple ProFile, a hard drive* User profile refers to the computer representation of user information...
and geometry. This gives the universal machine a very strong advantage over other measuring methods when profiling radii, contours, and holes, as the detail of the feature can be of any arbitrary resolution. The analog record generated by the machine can be fed directly into a compensator on computer numerical control (CNC) machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, correcting the machine on a per-job basis.
In practice, the universal measuring machine is a very slow machine that requires a highly skilled and patient operator to use, and the accuracy built into these machines far outstripped the needs of most industries. As a result, the universal measuring machine today is only found as a special-purpose machine in metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
laboratories. Because the machine can make comparative length measurements without moving linear axes, it is a valuable tool in comparing master gauges and length standards. While universal measuring machines were never a mass-produced item, they are no longer available on a production basis, and are produced on a to-order basis tailored to the needs of the metrology lab purchasing it. Manufacturers that perform work that must be measured on such a machine will frequently opt to subcontract the measurement to a laboratory which specializes in such.
Universal measuring machines placed under corrected interferometric control and using non-contact gauge heads can measure features to millionths of an inch across the entire machine's envelope, where other types of machine are limited either in number of axes or accuracy of the measurement. The accuracy of the machine itself is negligible, as the environment the machine is the limiting factor to effective accuracy. The earlier mechanical machines were built to hold 10 to 20 millionths of an inch accuracy across the entire machine envelope, and due to incredible machine design and forethought, remain as accurate today without computer compensation.