Actual Part Center of a Pattern
Actual Part Center of a Pattern
(OP)
I am told this subject is a "can of worms", however I want to know... I think...
A number of our parts have a pattern of holes called out as a datum. The patterns are normally a functional interface which meet datum selection requirments. Our company uses CMM and has a philosophy of cost savings by eliminating hard gages.
I have as an example of an exaggerated part to hopefully show the question at hand, which is identifying the pattern center of the actual part.
After much head banging and mind torture, I plead ignorance and would appreciate input. For part setup on a CMM are pins for holes still required? or is it best fit? or what? And of course where is the pattern center of varying produced hole sizes and their produced locations.
Please see attached
A number of our parts have a pattern of holes called out as a datum. The patterns are normally a functional interface which meet datum selection requirments. Our company uses CMM and has a philosophy of cost savings by eliminating hard gages.
I have as an example of an exaggerated part to hopefully show the question at hand, which is identifying the pattern center of the actual part.
After much head banging and mind torture, I plead ignorance and would appreciate input. For part setup on a CMM are pins for holes still required? or is it best fit? or what? And of course where is the pattern center of varying produced hole sizes and their produced locations.
Please see attached





RE: Actual Part Center of a Pattern
Did you mean to specify perpendicularity instead of position?
If you call up a positional tolerance of zero at MMC, an accurate pattern of Ø4mm holes would be your datum. Your Ø1mm tolerance is more challenging. Perpendicularity is not meaningful to me, at least from a fixturing point of view.
RE: Actual Part Center of a Pattern
Ok, lets use your senario drawOh. My main point though is to define the pattern center of a produced part so that it can be positioned on the CMM.
The actual part holes were overlayed on the perfect model so I know they fit. Two holes are high and outward of center, while the lower two are inside and above the center plane (line). The holes vary in size as produced. Now where is the center of the pattern in order to postion on a machine to start measuring?
Are you inferring that hole pins are used for setup on a CMM?
RE: Actual Part Center of a Pattern
But if this is how it really is, then the datum could be thought of as the center of the four-hole pattern but taken as a best-fit through the holes while still maintaining proper position/orientation. This is called "regardless of material boundary" (formerly RFS) so if a hole or two wander off as in your actual sketch, the datum is still supposed to be based on the centers as designed. To understand this, think of a fixture of four expandable pins that are located exactly 20 x 40. They expand as much as they can until they hit the edges of the holes; but they will not self-center into each hole. The entire part may skew a little, but the datum can still be thought of as a perfect rectangle.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
I did not mention a major factor which is that we are working to the 1994 standard. I have heard material conditions are signifcantly changed in the 2009? std.
I do want the profile anchored to the holes.
Considering the 1994 std, is your reply the same?
RE: Actual Part Center of a Pattern
But the symbology on the drawing and the interpretations are the same for both standards.
So again, the hard part for the CMM is to probe each of the 4 holes but then mathematically construct a perfect 20 x 40 rectangle that "plugs into" the actual pattern.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
Assuming there is MMB specified on datum B within profile FCF. How CMM would work then?
RE: Actual Part Center of a Pattern
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
Some CMM software can establish an optimized datum reference frame using a "soft gaging" analysis like what JP described in his last post. Most CMM software, however, will fit the pattern of actual axes to the pattern of true axes using a least squares "best fit" algorithm. This algorithm minimizes the sum of the squared deviations. The advantages of the least squares algorithm are that it is repeatable, stable, and computationally simple. The disadvantage is that it does not get the same result as the physical gage or soft gage!
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Actual Part Center of a Pattern
I am attaching upgraded sketch to focus on my question. I changed from perpendicularity symbol to position and removed the profile FCF. I am only concerned with inspection the 4 holes called out as datum B. How is the pattern center defined on the actual part to place on the CMM. I believe you have satisfied my questions with the expandable pins and Evan's software /" best fit". Just a little more clarification please.
Belanger,
It is my understanding that datum material modification modifiers are not legal with in a profile FCF,
and obviously illegal in the FCF tolerance block. Do you not agree?
If I am getting this correctly, as shown the CMM setup would need to utilize adjustable pins or best fit to software generated zones. I presume the best fit is accomplished by physically moving the part around on the CMM to fit in the zones. Is there a template or marking of the zones on the CMM table or is this done by display readings from the CMM, or is it that the part is placed on the CMM table, probed and then the software is "zero'd" in on the part position?
With the upgraded sketch I hope to get an answer about the verification of the 4 datum holes, pattern center. I would think it is pretty much as described on the preceding posts. As shown the callout is at MMC to datum A. If actual virtual gage pins were used in conjunction with the CMM, would the company be looking at cost of pins for each part configuration, size, tolerance, and positional tolerance for every hole pattern? At this point I am thinking that this would be the case.
RE: Actual Part Center of a Pattern
To really get what you want, you still probably want a "position" symbol instead of perpendicularity on the four holes. (The rest of that feature control frame remains the same.)
I'll let others who know more about CMMs answer the particulars there, but I think you're on the right track.
For the last question, yes, each unique hole size/position would require physical pins made to those unique conditions (if it is decided to go that route of physical simulation).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
To clarify a couple things; the fcf controlling the pattern of datum features must be modified at MMC, per '94, but does not have to be modified at MMC for '09. The datum established by the pattern of FOS, when referenced in an fcf, SHOULD (good practice) be referenced at MMC also (with the exception of the datum references in a profile control).
Once you secure the workpiece on the CMM bed, you don't move it ... it would change your results. The CMM needs to sequentially probe each of the pattern's datum features of size, then internally processes the data to get the least squares fit. If you move the workpiece, any inspection done to that point is voided and you will have to re-establish your datums again. Costly and induces errors.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Actual Part Center of a Pattern
I think you have helped me understand the basics here.
The GDT reference chart for ASME Y14.5M 1994 by "Effective Training Inc. that I am looking at for profile reads;
a column for "datums used"; yes ( profile of a surface); no (profile of a surface -coplanar)
column "MMC or LMC modifier"; NO
column "Bonus Tol"; NO
I take it regarding what you have written that all 3 columns are for the FCF tolerance block?
I am not sure how it would work with at MMC if you can move the part around lets say on a hard gage with virtual pins, then the tolerance zone inner / outer "bands" move too? or are increased in size to include allowance for datum shift?
RE: Actual Part Center of a Pattern
It looks to me as I "appended" the new pdf which shows the original with the new on the bottom, at least thats what I see.
Thank you very much for your reply it helps alot, same to all !
RE: Actual Part Center of a Pattern
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Actual Part Center of a Pattern
There are several cans of worms here, so be warned ;^)
In a profile FCF, the considered feature cannot have a material condition modifier but the datum features can.
Most CMM software can inspect the position tolerance on the 4-hole pattern relatively easily. Usually, a temporary coordinate system is first created using 2 of the holes. Then the software tweaks the translation and rotation of the coordinate system until the best (i.e. smallest) actual value of position is achieved. So you generally don't need physical pins for this.
Things get more difficult when you have to inspect another FCF that references the 4-hole pattern as a datum feature at MMC. The datum reference frame (coordinate system) is defined in the set of 4 perfect virtual condition pins. If there is any clearance between the 4 actual holes and these pins, the part can be shifted around to get the best actual value. This shifting is where things get difficult for the CMM software. So you can either build a physical fixture with 4 pins, create the CMM coordinate system on the fixture, and physically more the part around to get the best actual value for the FCF. Or you can get "soft gaging" software that essentially does the same thing by digitally shifting the measured point cloud around relative to digitally defined virtual condition pins.
As you suspect, the virtual condition geometry is generally different for every position tolerance so making physical fixtures can get very expensive.
Now just a few thoughts on the pattern center. First of all, don't concern yourself with the pattern center! A position tolerance defines zones for the individual features in the pattern and does not directly control the pattern center. Second, the pattern center can only be defined on a pattern of perfect holes and not on a pattern of real holes. Figure 4-22 on p. 69 of Y14.5M1-1994 implies that the actual part has a "hole pattern axis" but that is wrong. It only works because the actual part in that diagram has 4 holes that form a perfect square. It doesn't work on real parts like the one in your diagram. CMM software can calculate a "pattern center" using least squares algorithm, but it's not the same as the center you would get if the proper 4-pin fixture was used. So the center of the real 4-hole pattern is the center of the perfect 4-pin fixture that the holes fit over. Whew.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Actual Part Center of a Pattern
Not sure if that would be of much help.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Actual Part Center of a Pattern
Evan, it may be "whew" for you to write; however, it does help make this topic clearer to me. Thank you
RE: Actual Part Center of a Pattern
thd1103_237732, thd1103_235809.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Actual Part Center of a Pattern
Some of the GD&T referene charts I've seen have an asterisk next to that column saying that profile tolerances can't use MMC/LMC, and then a footnote clarifies that it's OK for the datum references. I'm sure a chart by Effective Training would have some sort of allowance for modifiers on the datum references in profile. If not, I can provide a version that does :)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
I have wondered what one would use as the pattern center concerning a pattern of irregular holes. Thought maybe to pick the most extreme holes vertically and horizontally and use that center. We have decided here to only use 4 holes with a separate hole callout for the remainder. Never crossed my mind to use "arbitrary" point.
thanks for the thread references...
Belanger,
No asterick and I did scan the standard's examples for any referencing a datum with material condition. Doesnt show any. I do recognize that the examples are not neccessarily complete and certainly not exhaustive.
Uncle... enough... enough... (:
Want to ask question on new thread regarding feature of size defined by basic dimensions...
RE: Actual Part Center of a Pattern
The new 2009 standard avoids this problem by explicitly saying that MMB/LMB are OK in a profile feature control frame (paragraph 8.5).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Actual Part Center of a Pattern
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Actual Part Center of a Pattern
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Actual Part Center of a Pattern
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com