Qualifying errors

[joe.sha]

I've been getting errors up to .002" when calibrating a Mistral CMM. Had a B&S engineer in for the last three days trying to find the fault. He replaced the drive belts. Drive belt bearings were greased. Bearing lifts were checked. All the reader heads were removed, cleaned & replaced. Scales were cleaned. Z axis counterbalance was reset. All motors were retuned and tested.
It gave good calibration results for two days so yesterday he left. This morning the gremlins were back. It seems to happen more so in the morning when the machine is cold and if you run the calibration program a couple of times in a row the reults come good and usually stay good for the day and the next morning back to errors. I'm going to rewrite the Qualifying program to see if that helps. Anybody experience this before??

[T Miller]

I had something like that happen. I had cold air blowing from the air vents onto one side of the machine and untill I run the machine awile it would do the same thing. I had a small loop program that I run in the morning that warmed the thing up then I did calibrations. Once the air thing was figured out and the flow was diverted away everything was fine.

[joe.sha]

The CMM is in an enclosure, I'll get the operators to note the temperature when qualifying and see if that's the problem

[craiger_ny]

That is a good point Tom, I wonder if the temperature of the air going from the compressor into the CMM fluctuates.

Craig

[John Kingston]

How stable is the temperature in that enclosure? Even without air blowing on it, large variation swings in temperature will cause the different materials to expand and contract at different rates.

Craig,
I've never thought of the air line air temp. Interesting. I wonder how it affects the machine.

[craiger_ny]

Craig,
I've never thought of the air line air temp. Interesting. I wonder how it affects the machine.

I have no idea I'm just stabbing in the dark at that one.

Craig

[Hilton Roberts]

Do you turn your machine off at night? Do you have a holding tank for your air supply?

Depending on the draw of your machine, a holding tank allows the machine to operate with a stable volume of air that is replenished when the pressure drops before a certain level. If the machine is off and the air bearings are not flowing air, the holding tank reaches the cutoff pressure and the compressor shuts off. The tank is then not constantly filling and emptying. The result can be the tank ( depending on where it is ) has a volume of air that either warms up or cools down. When you switch your machine on in the morning, that is the air you will be using until the system balances out. That could cause the problems you are seeing.

We leave our machines on 24/7 and only turn the PCDMIS computer off at the end of the week. The controller stays on except during plant shutdowns over holidays.

HTH

[Jan d.]

Oh, there is no doubt that compressed air temp has an effect. Where the air bearings hover over the machine, a natural cold spot occurs (because of the way the air flows out from under the bearing). That may cool the rail. Therefore it is always a good idea to park the machine with all the axes close to an extreme before leaving the machine overnight. In those positions, the effect of cold spots under air bearings is the lowest. If you leave the carriage sitting in the middle, it may cool the middle of the rail and cause large angular errors to occur. These will disappear as you start to use the machine. A warm up cycle like T Miller suggests is therefore almost always a good idea.

We always tried to leave a long loop of airline somewhere in the measuring room so the air gets time to heat up a little. The longer the better.


Jan.

[joe.sha]

The machine is left on all night but we don't have holding tanks. I'll get the operator to leave the CMM with all axes close to extreme as suggested. I do think the problem is more specific to the CMM and enclosure. I have a global, a microxcel and 5 zeiss that are behaving.

[joe.sha]

Just an update, Friday 13th and the machine is still playing up so I wrote a warm up program as Tom suggested and this seemed to work

[Virgil UK]

Had to get rid of my Mistral, wasn't up to the job.
Not hitting the correct measuring co-ordinates, B&S did agree with this and replaced it with a scirocco.