Ballbars

Is this what you are looking for?

Richard

Didn't we go through all this about a month ago ..you were going to build your own to micron accuracy...what's happened to that idea .

Ive still no idea what this ballbar is for.

perhaps :)

all thew best.markj

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 week... I can buy such things off the shelf in the USA, from for instance:

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

Dave

similar, but I just want a plain fixed ballbar, that appears to be a renishaw probe in the shape of a ball bar.

Dave

How peculiar. all of the ones on that page are within half a thou of a round number x 50 mm...

.Mark Rand RTFM

The pattern 150MM, 200MM in the product description confused you too much. :-) 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.

Nick

Strange for a precision device to describe it in a way that may be inaccurate by .0127mm though :-(

Mark Rand RTFM

That would be the next step. *After* learning to write units properly.

Nick

The actual length to within a few mm is not (in this case) important, only that we know what it is....

Dave

Good point. So you could just araldite two ball bearings to any old rod so long as you measure it accurately afterwards?

Yes sort of. I think that we will have to go that route this time...

Dave

In article , Nick Mueller writes

...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).

Optimist!

David

What many (most) americans don't realise is that their national standard unit of measurement, held, I believe, at the Smithsonian, is the metre!!!!!!

Some of us can do simple mathematics in order to do the conversion. The political class seems least able to understand numbers and accounts but I have a feeling that isn't just a US problem.

Wes

What is a ball bar if it isn't a renishaw type device? Normally these are used to evaluate the positional accuracy of VMC's, HMC's, and lathes.

Wes

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 can evaluate the measurement volume by using multiple poses of the bar a several angles etc. The renishaw device is used to evaluate the tooling, as they move, and the measurement is taken from the probe as it is forced to change length by the machines movements. 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 is required.

Dave