CMM Fixturing Measurement Techniques

Does CMM correct a bad fixturing of a workpiece?

Does CMM correct the impact that a bad fixturing has on GD&T's, lengths, diameters and angles ?

I think, the part fixturing and part alignment into the fixture is the main factor in CMM incorrect measurements. If we define the part fixturing and alignment incorrectly as a concept the CMM will be used incorrectly. 

If the CMM result of a trueposition of a feature to datum -A- is out of the tolerance can the CMM correct the true position if it the datum A is measured as a cylinder and the part is 3 degrees skewed from being parallel to the CMM axis. I never have assumed that the CMM corrects the errors of fixturing, and the part can be measured without a precise fixturing/alignment.

First question I will ask is what’s our benefit if the CMM does that?

We do not need a precise fixturing-alignment of the part into a fixture and consequently we save some time/cost on this.

Practically, how it is going to work when we measure parts on CMM and we see a red CMM result?

If we take for granted what we are being told that CMM corrects the errors of fixturing-alignment of the part, we need to know the answer of some very simple questions.

First, I would like to know how much error of fixturing in angles or .xxx” (in numbers) the CMM corrects? 

Secondly, the CMM corrects that for all kind of dimensions and all GD&T’s or only for some of them? 
As we know, there are 2-d and 3-d dimensions, simple features and sphere features, and the roundness is not the same as the perpendicularity, or the parallelism as the true position. The CMM may correct the perpendicularity, or parallelism when there are errors of fixturing but may not correct the fixturing errors for the the roundness or the true position of cylindrical features.

On the other end, if there is not an “error correcting” number, for example up to 3° from being parallel to Z axis, or .005” in Z direction, or a list of GD&T’s how can we accept assumption that CMM corrects the errors of fixturing?

When we use a CMM program how we are going to understand the borders that the CMM corrects or does not correct the errors of fixturing-alignment of the part and to react, to fix it in order to get e good measurement? 

We want to know that number with the simple purpose to apply that knowledge and having accuracy and precision in measurements.


What CMM does if the work piece is aligned wrongly  -0.2” in Z direction and 3° skewed from the Z axis?

Calypso knows the old position of the workpiece from the base alignment when is defined the workpiece alignment, when the coordinate system is established by probing physically the part, if you used the standard method to define the workpiece alignment, the 3 known steps of the part alignment, level-rotation-origin.

First time Calypso will scan the first feature of the base alignment according to the old data position. For example, if the first feature is a bore with the axis positioned perpendicular to Z axis, Calypso on the first time will touch only half of the bore and you can see how the probe will scan the air for the other half. After Calypso gets the new points Calypso will scan the same feature again, but this time correctly because the new features position now is recognized and being corrected. The old position is overwritten. So, Calypso corrects this kind of fixturing errors.

Practically, there will always be an error of fixturing, it does not matter what kind of base alignment you are using, automatic, or manual alignment or start alignment, Calypso will always correct the old alignment to a new alignment.

Calypso corrects the workpiece to a new position, but Calypso cannot correct that 3° angle of a bad fixturing. The angle will still be 3° to the CMM axis and it will have an impact, for example, on the roundness of a bore which could be as bad as .002” or more when the actual Roundness is .0003. 

Without a good fixturing it is not possible of forming datum planes or axes from their high point contact, right angle, distance, circular form, orientation, location, center axis of the feature. No good fixturing may form incorrectly datum frames. 

No good fixturing will also cause the probe measurements will not be taken enough perpendicular or parallel to the probe body which will produce errors and give results that are less repeatable and accurate.

Use always 3-jaws-clamping devices, vises, magnet and tall pins to set the part when it is being programmed on CMM. A good fixturing allows the datums to be repeatable.

From my experience a bad fixturing has a huge impact on GD&T's, a medium impact on lengths and diameters.