Machine tools abstract
The invention relates to a low-cost, low-weight, highly stable
reference object, easy to handle, for the measurement of the errors
of geometry of coordinate measuring machines and machine tools.
The reference object consists of an array of probing elements in
the form of rings, spheres, or flats, each representing reference
points by means of their surface geometry. The probing elements
are glued between two panes of a geometrically stable material,
establishing a firm connection between these panes. Holes in the
panes enable the measuring probe of the machine under inspection
to probe the reference elements' surfaces. The resulting box-type
structure of the reference object gives it the said advantages of
low weight, low price, and high stability.
Machine tools claims
Having described the reference object in detail, I claim:
1. Reference object made of a plurality of parallel panes, firmly
connected to each other with a spacing in-between and with a plurality
of probing elements located in the spacing between these panes,
said probing elements being firmly connected to the panes and thus
establishing an array of reference points defined by the probing
elements' surfaces that are probed by a machine under inspection
through holes in the panes.
2. Reference object according to claim 1 characterised in that
the probing elements themselves serve to fix the panes to one another,
thereby establishing the desired spacing between the panes.
3. Reference object according to claim 1 wherein the probing elements
are spheres which are flattened at the poles to facilitate their
stable fixing to the panes.
Machine tools description
A patent application for this invention entitled "Prufkorper
fur Koordinatenme.beta.agerate und Werkzeugmaschinen" was filed
in Germany under the number 195 07 805.5 on Mar. 6 1995.
FURTHER LITERATURE
[1] VDI/VDE 2617 part 3 and part 5
[2] E. Trapet, F. Waldele: A Reference Object Based Method to Determine
the Parametric Error Components of Coordinate Measuring Machines
and Machine Tools, Measurement Vol. 9 No 1 January-March 1991
[3] Koordinatenme.beta.lgerate rationell uberwachen, QZ 37 (1992)
5
[4] Prufkorper, German Patent Application DE 4036181 A1
BACKGROUND OF THE INVENTION
Two-dimensional reference objects for the inspection of coordinate
measuring machines and machine tools in the form of ball and hole
plates allow the errors of such machines to be measured with less
effort (only one reference object is needed), thereby faster (typically
in one day), and more cheaply (a reference object costs about $10000)
as compared with other methods that require typically five days,
require a set of costly reference measuring devices (typically $100000
of worth in total), and are difficult to handle: usually, two different
laser interferometers are needed, plus two electronic level meters,
plus one straight edge, plus one granite square [123].
In spite of these obvious advantages, a break-through in the spread
of such reference objects has not yet been reached because so far,
commercially available ball plates were not optimised neither economically
nor from the technical point of view. Better acceptance would be
more likely if the reference objects were optimised insofar as they
are individually tailored to the dimensions of the machines to
be inspected,
have arrays of probing elements with a small raster spacing (between
20 mm and 50 mm),
are of light weight so that they can be handled by one person (less
than 20 kg),
allow materials with low thermal expansion to be used,
are not more expensive than the existing ball and hole plates which
do not have all the properties above (about $10000 in 1996).
A main factor of the costs for a ball or hole plate of state-of-the-art
plate design is the extensive fine machining required for the bulk
material used. For example, the ball seats are milled with narrow
tolerances to allow to thermally shrink the balls into their matching
seats (holes). Plates from rolled steel additionally require annealing
for dimensional stability and surface treatments for corrosion protection.
Reducing weight by applying bores in the plate material turned
out to weaken the structure in certain directions (e.g. low stiffness
in the diagonals reduced the accuracy with which squareness is represented).
The milling of pockets leaving a thin wall in the plate's center
plane and furnishing ribs for the stability against bending turned
out to be satisfactory for smaller plates but significantly increased
their price. Plates bigger than 0.8 m.times.0.8 m with holes or
pockets turned out to bend excessively (more than 0.3 mm are not
allowed to keep cosine errors negligible). No state-of-the-art design
allowed plates bigger than 0.5 m.times.0.5 m to be handled by only
one person because of their weight.
SUMMARY
It is the objective of this invention to improve the design of
hole and ball plates for the inspection of coordinate measuring
machines and machine tools concerning weight, cost and stability,
and by tailoring them in their dimensions and layout to the individual
user's needs. This is achieved by gluing the probing elements (spheres,
cylinders, or flats which establish a raster of reference points)
between two panes in the symmetry plane of the reference object;
holes in these panes allow the machine under test to probe the surfaces
of each of the elements.
The panes are preferably made of special materials like quartz-ceramics,
possessing a low thermal expansion coefficient and a high long-term
stability. Such materials are available as mass products in the
form of thin panes (e.g. about 5 mm thick). If the object were made
of one single thicker pane, costs and weight would be higher without
significant extra stability being obtained. The holes in the panes
do not require high precision and can be made at low cost.
According to the invention, the assembly is preferably done manually,
allowing individual reference objects to be manufactured to customer
specifications with respect to size and layout of the raster of
the probing elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-3 Reference object for the measurement of the errors of
geometry of coordinate measuring machines and machine tools in the
form of a sandwich structure with two panes, using alternative types
of probing elements: spheres (detail 1c), rings (detail 1a), spheres
flattened at the poles (detail 1b)
FIG. 4 Reference object for the measurement of the errors of geometry
of coordinate measuring machines and machine tools in the form of
a sandwich structure with three panes
FIG. 5 Manual assembling process for the reference object, allowing
the same tools to be used for different object sizes and for different
layouts of the arrays.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, the problems of price and weight of
ball and hole plates as reference objects for coordinate measuring
machines (CMMs) and machine tools (MTs) can be simultaneously solved,
if the reference objects are made of at least two panes arranged
in parallel (detail 2 in FIG. 1), firmly connected to each other
with sufficient space between them. In the space between the panes,
probing elements (details 1a to 1c in FIG. 1) are located. The probing
elements are advantageously used to establish a firm connection
between the panes by bonding. This box-like design yields an optimum
ratio of stiffness and rigidity as regards weight and price. Additional
spacers are provided to bond the panes together with the desired
spacing if, due to the individual layout of the array of probing
elements, there are not enough probing elements to establish a stable
connection of the panes.
The probing elements may be, e.g., cylindrical rings (detail 1a
in FIG. 1), rings with spherical inner surfaces, truncated balls
(detail 1b in FIG. 1), cylindrical plugs, complete spheres (detail
1c in FIG. 1), cones, bushings with conical inner surfaces, or pairs
of small planes inclined against each other as well as against the
reference object's symmetry plane at an angle of about 90.degree..
While the centers of the spheres and spherical rings represent points
in space, all other elements represent straight lines in space,
of which the point of intersection with the reference object's symmetry
plane must be calculated to yield the desired reference points in
space.
The described design leads to the probing elements being optimally
placed with their datum points in the symmetry plane of the reference
object. This avoids first-order errors by a slight (out-of-plane)
bending of the object.
A plate manufactured according to the invention with a third pane
in its symmetry plane allows the probing elements to be fixed to
this central pane, whereas the probing elements are not directly
connected to the two outer panes. FIG. 2 gives an example of the
layout of such a three-layer plate (cross-section). The inner pane
is fixed by spacers (detail 4 in FIG. 2) to either outer pane. The
advantages of such a design are that the positions of the probing
elements and the plate's flatness are each ensured separately and
in an optimised way.
A frame (detail 3 in FIG. 1) around the panes is attached by an
elastic bonding material (silicone glue). This frame is needed to
clamp the plate to the CMM table without introducing stress into
the panes themselves. Additionally this elastic bonding and the
frame, e.g., from aluminum protects the reference object during
transport against both shock and strain. The elastic fixing of the
frame particularly reduces thermally induced strains due to changes
of temperature in the fixtures during measurement.
No precision machining is required except for the probing elements.
The probing elements may be low-cost mass products. They are made
of hard (HB=60), corrosion-resistant material with a surface finish
to Ra<0.2 .mu.m. The form of the probing elements need not be
better than 5 .mu.m because of the identical probing point patterns
during the calibration and during the use of the plate. The holes
(detail 1e in FIG. 1) in the panes need not be made to narrow tolerances,
neither with respect to their diameters nor with respect to their
positions. 0.3 mm is a satisfying tolerance for either diameter
and position.
The entire reference object is economically assembled by manual
labour. This allows the object to be tailored to the size of each
individual client's machine at no extra cost. Tailoring is favourable
to cover a machine's working space with as little object positions
as possible during the tests, reducing the machine's down time for
the tests to a minimum. The described design with its large tolerances
allows the panes to be pre-fabricated, including the holes, by the
supplier of the pane material, so that the final assembler need
not perform chip removal operations like milling. FIG. 3 shows the
steps (denoted by A to G in FIG. 3) of a possible manual assembly
process where the probing elements are positioned and glued to the
first pane: In step A, the probing elements (detail 1) are covered
with resin at one end and inserted, with the resin-covered ends
downwards, into the holes (detail 6) of a template (detail 5). After
the first column has been fixed, a square (detail 7) helps to align
the template orthogonal to the first column (step B), while the
template is locked in one of the already fixed probing elements
of the first column. In this way, arbitrary rows of probing elements
can be successively glued (step C), followed by arbitrary columns
(steps D, E, F, and G). A plurality of templates used simultaneously
accelerates the assembly process. After fixing of all probing elements
to the first pane, the second pane is bonded on top, matching the
holes to the probing elements.
The design according to the invention also allows highly efficient
batch production: Holes in the panes may be punched with sufficient
accuracy; it is possible to punch a stack of several plates simultaneously.
The gluing can be done using a ready-made two-dimensional mask to
position the probing elements.
For the panes, materials with negligible thermal expansion coefficients
are favourable. Such materials are, e.g., used for the windows in
doors of furnaces. Panes of this material are available from stock
and are thus cheaper than massive blocks of the same material. The
use of material with a low thermal expansion coefficient increases
the accuracy with which a machine can be inspected and reduces costs
at the same time as compared with materials like aluminum and steel
which have a high thermal expansion coefficient and thus require
a costly and time consuming correction of thermal expansion.
The design according to the invention is applicable with advantage
both to one-dimensional (hole or ball beams) and two-dimensional
reference objects (hole or ball plates). |