Center plane determination with CMM

[ringster]

Problem: Rectangular part with a pattern of holes related to center planes of the part.

If a rectangular part were to have datum centerplanes in the vertical and horizontal planes, and these were specified in a FCF at RFS, and the part was to be verified by CMM, how would it be accomplished.

Also if it were at MMC using CMM, how?

 

[wallyd999]

IT LOOKS LIKE YOU HAVE A SQUARE PLATE IN WHICH YOU WANT A STANDARD 4 HOLE PATTERN.

YOU NEED TO ESTABLISH ANOTHER VIEW THAT IS SURFACE -A-. WHICH IS USUALLY THE LOCATE FACE. THEN ALL YOU SHOULD HAVE TO ASK FOR IS TRUE POSITION TO -A-B-C- AS YOU HAVE PARTIALLY SHOWN.

WHEN YOU COMPLETE THE DRAWING WITH DIMENSIONS IT SHOULD BE COMPLETE WITH THE ADDITIONAL VIEW AND THE TRUE POSITION TO -A-B-C THEN THE CMM SHOULD NOT HAVE ANY ISSUES WITH THE DRAWING.

 

[DesignBiz]

Interesting question. Here is my shot at something I know little about but would like to know more about.

Wouldn’t it be reasonable to believe for RFS, that after the part is positioned according to the referenced DRF, a probe would touch off of both ends of the part to establish the vertical center plane and the same for the
width surfaces for the horizontal planes. using those planes to measure to each hole's center?

I would suspect that the CMM software can calculate the ‘pass’ locations for the holes based on tolerance zones derived from their FCFs.

For MMC, the part would be positioned again according to the DRF but with movement allowed for “datum shift” relative to any DRF ‘feature of size’ datum. The part could possibly be physically adjusted in the setup depending on the calculate amount of “datum shift” for that part.

It has been my understanding that CMM software has the ability to calculate tolerance zones based on inputs per part FCF's. For RFS the part is immobilized and measurements are taken. For MMC there must be some ability to move the part physically within the DRF or possibly the software makes adjustments for "datum shift".


DesignBiz

 

[ringster]

DesignBiz,

I also thought it to be an interesting question and was hoping for more responses. Perhaps there are not that many on this forum with CMM backgrounds.

 

[HGMorgan]

It depends on the CMM software capabilities as far as "auto" set-up. However, the following process could also be input by hand on most all CMM's.
On our CMM, I would set it up as follows:
1. Create -A- plane (3 or more points on part feature -A-).
2. Take points B1 & B2 on -Y side of part @ X C/L holes.
3. Take points B3 & B4 on +Y side of part @ X C/L holes.
4. Obtain mid points from B1 & B3, B2 & B4.
5. Use those 2 ea 'B' mid points to create -B-.
6. Take points C1 & C2 @ -B- on +X & -X sides of part.
7. Obtain mid point of C1 & C2 to create -C-.
(On our CMM the check points & mid points can be driven by the software, and, a repeat program created.)

Now you have A|B|C and can go measure part features.

The B1 thru B4 points could be recalled and output from the A|B|C for use in MMC. Might want to take more points on 2 ea -B- and 2 ea -C- if MMC condition is desired. Would need to see the actual call out.

 

[ringster]

I am beginning to believe that my theory of centerplanes not being a practical means of dimensioning is substantiated. Any one have an opinion in support or opposition?

 

[Tunalover]

ringster-
Width datums are used widely and they are centerplane datums. Why even define width datums in 14.5 if they have no practical value? This question needs to be referred to a metrologist with good GDT knowledge.


Tunalover

 

[axym]

Hi, I'm a metrologist with good GD&T knowledge. I would have responded to this earlier, but I've been dealing with serious PC problems all week.


Regarding the practicality of centerplanes, I think that they have their place. If the datum feature is held in a vise-like fixturing device or mates with a slot, then the centerplane datum represents function.

Finding centerplanes using a CMM isn't as easy as it looks. CMM software has good functionality for finding tangent planes, midpoints, "symmetry" points, etc. but the centerplane presents a different problem. To find the proper centerplane, the software must find the "minimum circumscribed parallel planes" which is difficult. If the centerplane is a secondary datum, the parallel planes must be oriented to a primary datum and it's even more difficult. Duplicating the DRF that would result if a physical vise-like fixture was used is very difficult, and generally can't be done with a couple of points on each surface.

If the datum features have the (M) modifier, then most CMM software isn't able to calculate the datum shift properly. Special software with soft-gaging functionality is needed.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca