Wel, its exactly like it says, it a bar of fixed length with to
accurate spheres on the ends.
Because the spheres are accurately round you can measure them at
several points and
from that get a 'virtual' point center. They are used to calibrate
things like CMMs, where
you know that the length of the bar is a certain distance, and so you
the measurement volume by using multiple poses of the bar a several
The renishaw device is used to evaluate the tooling, as they move, and
is taken from the probe as it is forced to change length by the
If the (CNC Mill for instance) describes a circle then the length wont
change, and so you
can determine th emachines accuracy from the renishaw results.
In this case I need to calibrate an optical metrology system, so a
fixed, known length bar
Yeap, the super accurate one is still in progress, thats a research
project / long timescale item (its still not certain that it is
possible to make such an item to the required tolerance), but I need a
different (somewhere around 200mm - 300mm) long one to calibrate a
pair of sensors into a single measurement volume, for a demo, for next
I can buy such things off the shelf in the USA, from for instance:
but Id prefer to source in the UK if I can, not to mention the
vagaries of shipping (customs) from the US.
A ball bar is a bar and a pair of balls, where the center to center
distance is accurately known. the balls are a surface a known distance
from a point (in the center) and as such you can use it to calibrate
metrology equipment, usually CMMs, but in this case its an optical
fringe projection system that I work on, theres a bit more info at
The pattern 150MM, 200MM in the product description confused you too
We all know that people of the US of A do have problems adopting to the
metric systems. That already starts with the correct usage of units. mm,
not MM (that would be MegaMega).
I guess they'll catch up in some decades. <G>
...except in the oil & gas business, where MM (or mm) is one million (M
(or m) = 1000). Don't ask me why, it's completely barking mad, but it's
been like that since forever and shows no sign of changing. Drives me
mad (I work in that area, and trying to persuade US colleagues to use SI
units is a hard job).
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