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acnme (Aerospace)
6 Feb 11 11:43
I have a customer ask us how do we established datum target area of .250 dia, I take 1 point in this area s.b enough to define datum target area. The customer did not like my answer? so I need your advice on how to simulate datum target on the cmm machine.Thank
Helpful Member!  MikeHalloran (Mechanical)
6 Feb 11 12:12
No doubt your custimer thinks, as do I, that the workpiece should be resting on or clamped by a fixture button of .250 diameter at the datum target area while it's being inspected.

I.e., if you're inspecting a datum, or simulating a datum whatever that means, you're doing it wrong.

 

Mike Halloran
Pembroke Pines, FL, USA

acnme (Aerospace)
6 Feb 11 13:07
if I have a primary datum -A- with 3 datum targets A1,A2,A3 with .250 diameter area shown on Blueprint, how do I set up datum -A- plane on the Coordinate measuring machine? I need your help. thank for the last email.
dingy2 (Mechanical)
6 Feb 11 13:21
Well you would be the first CMM Operator to actually set up on a .250 diameter area rather than using the spherical stylus of the probe.

The only way one could ever perform this task on a CMM is to have three (3) .250 diameter wafers that are of a set dimension in thickness. Place them on the locations, take your reading on the wafers and accommodate the wafer thickness.

I realize why the customer would ask this. A checking fixture would use the .250 diameter zone and the CMM does not which could result in differences. The checking fixture results would supersede the CMM since the checking fixture set up complies with the drawing.

Dave D.
www.qmsi.ca

MikeHalloran (Mechanical)
6 Feb 11 13:24
The blueprint is telling you to support the part with a fixture while it's in the CMM, and it's telling you what the fixture should look like, i.e., tooling balls or pads at the datum target locations.



 

Mike Halloran
Pembroke Pines, FL, USA

acnme (Aerospace)
6 Feb 11 13:54
In many years working as cmm programmer, I alway use spherical ball probe to pickup datum targets A1,A2,A3 to establish datum -A- plane.
when the customer ask the question, I was surprised.You are right about setting up on fixture is the right way to inspect the dimensions because it complies with the Drawing requirement.But it is costly because taking much more time to inspect with hight gage and fixture than on cmm; or every cmm program must have locating fixture with the right size of tooling points pin for datum structure.What is your normal practice on cmm machine?
MikeHalloran (Mechanical)
6 Feb 11 17:16
Existence of datum targets suggests that the part is flexible, and measurements will therefore be affected by how it is supported.  I would therefore, were I the part designer, expect you to put some tooling balls in a piece of tooling plate, or put some planer jacks on the bed, or whatever it takes, to actually support the part as specified.

My personal preference is to avoid CMMs because they don't really save any time on most parts.

 

Mike Halloran
Pembroke Pines, FL, USA

Belanger (Automotive)
6 Feb 11 17:30
Think about a regular flat surface datum.  When the part sits on the gage table, it may hit as few as three points, and those three points could be anywhere on the surface.    So if using a CMM in this instance, you can't just use a spherical probe to hit any three points; you must probe the three highest points.  (To do this, you'd actually probe the gage table or surface plate.

Well, the scenario you describe with three circular datum target areas is the same, except that the range of the surface being sampled is reduced to selected areas only.  So ideally, you shouldn't just hit any point within that area.  You are still supposed to find the three highest points (which would work out to be one high point in each of your .250 areas).  The tricky part is finding that highest point.  That's why I might agree with the last point made by Mike H.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

acnme (Aerospace)
6 Feb 11 18:00
thank for your reply, John. Any advice on picking a highest point of datum target area of .250 dia on cmm.?
cmm does save time when the production parts need to be inspected some critical dim; you just make 1 program and repeat for let say 100 parts.
Belanger (Automotive)
6 Feb 11 18:50
Hmmm...   Are the three areas level with each other?  Or are they on three stepped surfaces?

It may entail creating a fixture with three flat-topped pins of .250 dia each.  Probe the tops of the gage pins to get the computer to create a plane.  Then set the part on those three pins and continue measuring the rest of the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

DeanD3W (Mechanical)
6 Feb 11 18:58
I don't think the circular wafers suggested are a good plan...  They'll simulate the tangent plane for each circular area but each will have a differing orientation, so you would still be left to find the high point on each wafer which would be incorrect anyway because the high point of the area's tangent plan is not what is specified.

I think a planar datum feature placed on a surface plate may touch at only one point if the surface is convex or three if it is not.  I agree with John-Paul's description of the objective...  You're after the high points of the selected/specified areas only, where those high points are relative to the plane that is established by them.

Are you able to do the following?:
1) Probe about 10 points in each of the three small areas (the quantity depends upon how imperfect the feature is and how tight the tolerances on the part are).
2) Use all 30 points to establish a best-fit plane.
3) Choose the point from each of the three areas that has the largest residual (deviation) in the appropriate direction from that best-it plane.
4) Use those 3 high points, one from each of the three areas, to create the plane you will use as the datum plane.
5) Verify that no other of the 30 points are higher than the datum plane (to validate that you really do have the three high points wrt the datum plane your establishing).
6) Ask you customer if this extra work is worth the expense, and/or ask whether it would be cheaper to build a fixture that provides the three circular pads.  Since the 3 circular areas must be located wrt the datum reference frame, that fixture is not necessarily a simple one, so I think a good, rigorous CMM approach might still be best.

I hope this helps.

Dean Watts
D3W Engineering LLC
MikeHalloran (Mechanical)
6 Feb 11 19:22
Grinding three pin ends coplanar establishes a datum plane that automatically finds the 'high points' of the part.

The discussion about probing the part or the pins in order to establish a plane in the CMM's coordinate space nicely highlights why CMMs are such a big waste of time.

 

Mike Halloran
Pembroke Pines, FL, USA

dingy2 (Mechanical)
6 Feb 11 19:33
I have not come across anyone running a CMM who has tried to simulate a round flat datum target area of .250". All use a spherical stylus on the probe and pick up the 3 points as shown.

I would love to see how someone would find the high point within the .250 diameter datum target area.

I would suggest turning this back to the customer and ask them how they would do it. The checking fixture would have the datum target area while on the CMM, one picks up the 3 datum target spots using a touch probe.   
 

Dave D.
www.qmsi.ca

DeanD3W (Mechanical)
6 Feb 11 19:45
Mike,
It's not so simple.  Once you have those three pins with their nicely coplanar ends (let's say to constrain rotation about X, rotation about Y, and translation in Z), how do you ensure that the part is located correctly in X and Y?  You can include additional datum features/targets as part of the fixture, but my experience is that if you try to include more than one datum feature in a fixture, then friction, clamping forces and clamping sequence can have a large affect upon repeatability.

I think an improperly used CMM may be a waste of time, but a carefully and properly used machine driven CMM, by an experienced and well trained programmer can be a great time and money saver.

I'm new to this forum.  Has the need to iterate while establishing a datum reference frame been discussed?  I admit that this is a necessary part of using a CMM.  Probing a part directly or using some of the new point cloud processing software can be very effective if all the proper considerations are taken into account though.

So, as with anything, whether hard tooling or point sampling, it works well if you do everything right, but not so well if you don't  smile.  It's not a CMM shortcoming that leads to poor quality data, it's a programmer knowledge and training problem.  This tends to give CMMs a bad rep.  I think the reputation of CMM data would be better if there were a training a certification program for dimensional metrologists.  There's talk of this by a CMSC group, but they want to stick to portable, large scale, CMM operation only.  GDTP certification is part of the picture but how to apply GD&T in metrology in a way that properly addresses GD&T specs is a necessary and, I believe, currently somewhat neglected subject.

Dean Watts
D3W Engineering LLC
acnme (Aerospace)
6 Feb 11 22:26
thank for all the info; I would love to attend a online training for dimensional metrologist; anybody know about this kind of training.
axym (Industrial)
7 Feb 11 10:42
Dean,

Welcome to the forum.  Based on your posts, it appears that you have a lot of good information to contribute.

Dave,

Back in my CMM days, I did occasionally program the machine to find the high points of datum target areas (using a regular spherical stylus).  I used a technique similar to what Dean described - taking a few hits on each area, finding the high points, and iterating.  This was on really rough sand castings, where the local surface variation was so large that taking only one hit on each area resulted in significant errors.

Mike,

I agree with you that if the part is flexible, then a physical fixture works best because it will restrain the part correctly.  Fixtureless CMM inspection of parts with datum target specifications often has errors because of this restraint issue.  But for rigid parts, The CMM's ability to eliminate the need for a physical fixture can be a major time and cost saver.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

HGMorgan (Aerospace)
7 Feb 11 15:37
The issue is not with CMM, one would have to account for the datum target zones however the product was being measured. That type of product alignment can only be handled quickly & easily one way: fixture.
As an example; a fixture would have a base plate with 3 -A- datum index buttons, a long angle with 2 -B- datum index buttons, and a short angle with 1 -C- datum index button. The buttons would be flat and .250 diameter. We make fixtures like this in a couple of hours with-in .002 or less. If product tolerance required it, the fixture could be made closer, costing more time & money.
The fixture is bolted to CMM table and aligned to the buttons.
The product is indexed to the fixture buttons and measured. That program is saved & used for repeat mode. A bunch of parts can now be checked faster and more accurately than any other method.

You really don't see this type of Datum Alignment in aerospace very much anymore, it was more used in the old casting days.

Fixed base CMM's are still the most accurate measuring instruments out there. Portable CMM Arm's, lasers, & photogrammetry can do a good job, and can go to the product, but they still don't have the same level of accuracy.

I have to admit, I don't care for the CMM being a big waste of time statement. That is incorrect, and doesn't help the OP any.
We use our CMM's daliy to save time & money, and, a lot of our customers would not use us if we didn't have one.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

dingy2 (Mechanical)
8 Feb 11 8:22
Evan:

I, too, used to layout castings so many years ago. We did make sure that the locating area was free from sand imperfections, etc.

We are talking about a .250 (6 mm) diameter datum target here. Would you place it on a scan mode? (area is too small?) How many contacts would you take? Would you try to create a plane? Is it possible that the high point could change (orientation) once the datum surface was mathematically set up?

The end result is that a checking fixture with a .250 diameter contact will be different than the results from a CMM.

  

Dave D.
www.qmsi.ca

axym (Industrial)
9 Feb 11 10:31
Dave,

I didn't use scan mode, it was discrete points.  As far as I remember, I used to take one hit in the center of the datum target area and an 8-point circle around the outer periphery.  The thinking was that this would get close to where the high points were most likely to be.

If the datum target areas were coplanar, then I would create a plane using all the points and find the high points of that plane.  If the datum target areas were not coplanar (i.e. some sort of step datum feature), then it was more of a pain.  The CMM software didn't have a canned routine for it, so I wrote a custom routine to find the high points.  There were typically secondary and tertiary datum targets involved as well, so the high points would change after the calculation.  Usually the alignment had to be iterated two or three several times before it converged sufficiently.

Fortunately, the success of the alignment can be tested.  The datum target areas can be re-probed after the alignment, and each area should have one point that reads nominal and all other points  reading below nominal.

In hindsight, this does seem like a lot of work, doesn't it?  Now that I think about it, the targets were half-inch diameter and not quarter-inch.  I'm not sure that I would have done all this on quarter-inch targets, because the potential error would not have been as big.

The reason I was going to all this trouble on sand castings was that the CMM program was pre-screening the castings before machining.  The castings were often so warped and distorted that they couldn't be machined properly - there would be insufficient machining stock in some areas, too much in others, etc.  The CMM program was used to screen out the bad castings so that time wouldn't be wasted fixturing and machining them.  So the program probed the part on datum target areas that corresponded exactly to the machining fixture locators, then inspected the critical surfaces.  We could have built an inspection fixture, but this was a very small company and there weren't a lot of in-house resources for things like that.  There were also several different castings that needed this treatment, so several different fixtures would have been needed.  I was able to use the generic CMM fixturing kit to hold all of the different castings.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

dingy2 (Mechanical)
9 Feb 11 10:44
Evan:

Thanks for the info. I wouldn't do it for a .250 " diameter but larger ones would merit your approach.  

Dave D.
www.qmsi.ca

DeanD3W (Mechanical)
9 Feb 11 13:31
Dave,
To follow the drawing specifications, it could be said that when a datum target area is specified, then you must find the high point of each area, no matter how small the area is.  Of course you can then bring up the fact that there is uncertainty in all measurement and the single point per area just introduces more uncertainty (which may still be acceptable, if associated tolerances aren't too tight), so that may be the practical way to follow the datum target area specifications.

If it was not the desire to have the inspection cost include the work of finding the high point in each area then datum target points should have been specified rather than areas.  Whether to specify, and inspect in accordance with, datum target areas depends upon how imperfect the datum feature surface is and how tight the tolerances are that include reference to that datum feature.  Design should make that determination and inspection should follow what is specified (in a perfect world anyway).

I hope the datum targets were specified based upon functional contact with the mating assembly...  If the mating feature has three small circular areas then to base the datum from the high point of each circular area is doing what simulators normally do, which is to model an ideal/perfect mating feature.  If the mating features for the specified datum targets are spherical bumps that are located accurately enough then maybe datum target points model the mating relationship.  I generally don't think it's a good idea to specify targets just to improve measurement repeatability...  The determination of whether to specify targets, and what type of targets to specify should be based upon how the mating assembly contacts the datum feature.  I'm taking this position because the point of the specifications is to capture design intent, which means producing data that relates to function, which can only be done if the datum feature/target specs model the way the mating assembly contacts the datum feature.

Please pardon my wordiness (was that my "outside wordiness" smile).

Dean Watts
www.d3w-engineering.com  
dingy2 (Mechanical)
9 Feb 11 14:39
Dean:

Most Designers have trouble with datum selection. Which should be the primary, secondary and tertiary. Should one use a hole or a surface for a secondary?  When it comes to datum targets, it usually is requested by Quality or Manufacturing Engineering and, in my experience, not by the Design group. As a matter of fact, some Designers aren't readily agreeable to placing them on the drawing and really want process instructions to include datum targets rather than the part drawing.

Quality personnel need datum targets to set up on the datums to measure the part consistently. If the part does not have datum targets, someone usually hand bombs the datum targets in pencil on the marked up drawing just as they number all dimensions.

If the part really touched 3 points of a diameter of.250 on the mating surface rather than the full surface, then those 3 areas should be shown on the drawing using a chain line boundary and then reflected as the datum - not the datum target. One would then create a plane using the three (3) datum areas shown in the chain line boundaries.

Just some food for thought.  

Dave D.
www.qmsi.ca

DeanD3W (Mechanical)
9 Feb 11 19:53
Sorry Dave, but that food doesn't taste good at all.

1) Design should agree that the datum targets, if applied, adequately model the function of the part, or else the inspector should be after the points that would contact a proper datum feature simulator.  Arbitrarily choosing their own datum target points will still produce data but, if the datum feature's form has significant imperfections, that data will not be of adequate quality.

2) Partial datum features (designated by chain lines) would not be used as you describe, datum targets would be used.

Best Regards,
Dean Watts
www.d3w-engineering.com
 
axym (Industrial)
9 Feb 11 20:45
Dave,

I can understand why some designers don't want datum targets put on the drawing.  It's probably because they want their drawing to communicate the intended requirement!  If quality or manufacturing wants to introduce error by setting up on non-functional datum targets, that's their business.  

I agree with Dean that datum target areas are the proper tool to use  for the three .250 diameter areas.  I don't see the point of annotating chain line boundaries in this case - the datum target areas convey exactly the right intent.
 

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

dingy2 (Mechanical)
10 Feb 11 10:31
Dean:

I enjoy discussions like this since the standard is vague in some areas and other areas, in my opinion, not really practical. From these online discussions, I have created web pages covering the controversial topics so bear with me and don't be offended.

I am going to use your description of the mating part having three (3) flat round features that contact on our part. I don't think this example is practical but I will use it anyway. The tops of the three (3) flat round surfaces on the mating part would be its primary datum.

The first question would be - is the full surface on our part the primary datum? If so, the geometric counterpart must contact on the full surface. You stated in your hypothetical example that the mating part has three (3) .250 flat round features that contact on the part rather than a full surface contact.

I could not find anything in the ASME Y14.5-2009 with this example but I did find a situation in ASME Y14.43-2003 (gauges and tolerances). There is a drawing where on the workpiece datum A is 1/2 the full surface as shown by a chain line boundary. To me this means that the mating part only contacted on the surface bounded by the chain line, not the full surface. Using this same concept, should have 3 areas on our surface shown in chain line boundaries (not the full surface) designated as datum? We could still use the datum targets on those 3 areas but the datum is not the full surface but only the areas where the mating part contacted.

Flat round contacts are usually shown on castings since surface is rather rough. Sometimes the flat round datum targets are the same size as the machine set up points for broaching or milling a surface. I have also seem flat round datum targets on a plastic automobile dash with 24 points shown on 24 tabs. The part was rather flexible and one needed it to be stable for measurement.

I have found datum targets are process rather than design generated. We need them in Quality to set up on the product consistently to achieve a good repeatability and reproducibility. If the drawing does not have the datum targets shown, how should a CMM Operator set up the part? Scan the surface looking for high points :(. That just doesn't give a stable set up.

We use a 3 point set up on a rigid part, hopefully, about 120 degrees apart towards the perimeter but not on the edge of the part. It is best to have the datum target shown on the drawing to make it official or valid but I would never suggest to a CMM Operator to send out a marked up drawing without some sort of set up criteria.     

Dave D.
www.qmsi.ca

axym (Industrial)
10 Feb 11 13:40
Dave,

Have a look at Fig. 4-48 in ASME Y14.5-2009, with the three datum target areas A1, A2, and A3.  If the mating part had three 6mm flat round features that contact the part, then this datum target specification would make perfect sense.  I still don't see what your objection is, and why you would use the chain line boundary method to try to depict the same thing.

Part of the problem here might be that we're not all using the terms in the same way.  I hate to be the terminology police, but we have to be very careful here.

A datum feature is a surface (or group of surfaces) on the part.  In 4-48, the datum feature is the group of 2 roughly rectangular surfaces, that are nominally coplanar.

A datum feature simulator is a perfect inverse feature that contacts the datum feature.  The default datum feature simulator covers the entire area of the datum feature.  In 4-48, this would be two perfectly coplanar plates that cover the entire datum feature.

Datum target specifications specify different (non-default) geometry for the datum feature simulators.  In 4-48, the datum target areas specify that the datum feature simulators are to be three circular areas 6mm in diameter.  See the three flat-tipped pins in the "means this" diagram.

A datum is a theoretically exact entity defined in the datum feature simulators.  In 4-48, Datum A is a plane that passes through the three circular areas.

So datum target specifications do not change what the datum feature is.  They change the geometry of the datum feature simulators, which means that they change the geometry of the fixture that the part will be held in to establish the datum reference frame.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

dingy2 (Mechanical)
10 Feb 11 15:55
Evan:

Fig. 4-48 makes perfect sense but please note that the datum surface is the 2 coplanar top surfaces of the part - datum A.

Here is where things seem to go wrong but maybe I misinterpreted Dean's thoughts. He stated "If the mating feature has three small circular areas then to base the datum from the high point of each circular area".

I think that he is talking about the mating part having 3 small circular areas that actually contact onto our part on the datum surface. The full surface is now not the datum but just the areas of contact from the mating part. Am I wrong to conclude that the mating part does not fully contact on the datum surface but only in the three (3) cylindrical areas noted??

As I stated, fig. 4-48 makes perfect sense using datum targets to set up on a plane and I have actually performed this many times in the past. Both Processing and Quality really need datum targets on a drawing but generally have to request them.

   

Dave D.
www.qmsi.ca

DeanD3W (Mechanical)
10 Feb 11 17:53
Dave,
You were correct to take my meaning to be that the only contact with the datum feature in question would be within the three circular areas.

Datum targets should only be specified if they model the functional contact with the mating assembly or possibly if you're dealing with an as-cast or as-forged part.  If someone in inspection is choosing "their own" datum target points then their data will likely not model function as well as it should, since those points may not be the high points that would contact a set of datum feature simulators.

For the case we're discussing here, unless uncertainty is deemed insignificant, or the additional work too expensive (almost the same case said two different ways) the inspector should be looking for the set of three high points, one from each of the the three circular datum target areas.  They need to do this since those would be the points that make contact with a datum target simulator.  To build a fixture with 3 cylindrical pins is one approach, but to properly locate that set of 3 pins relative to the part is not so easy...  Including 2 "B" targets and 1 "C" target as part of the fixture was mentioned in an earlier post.  That may be OK, but there aren't many parts in the world that really function as a simple rectangular block with three mutually perpendicular planar datum features.  Most/many parts will end up with a cylindrical datum feature if a proper, function based, approach is used to select datum features...  This makes building that "simple fixture" not so simple.  Maybe a lot of parts are handled like a simple rectangular block, but that doesn't mean that they should be.

Processing and quality do not need datum targets.  They may want datum targets, since that makes their job a bit easier, but if the designer says "no, there are no datum target points, lines, or areas on the datum features because the mating assembly does not contact the datum feature in that way...  The mating features have the full footprint of the datum features or more" then the downstream processes need to build or model a set of full datum feature simulators.  That perceived/invented need you describe is part of the reason poor quality data is gathered every day and part of why cmms get an unjustified bad rep...  Just sampling a few points and thinking/using a "plane/line/point" approach within the software does not produce a proper datum reference frame.  All the inspectors using CMMs should be 1) Sampling an adequate quantity of points on each datum feature to do an adequate job of finding the points that would contact a physical datum feature simulator, and 2) iterating when establishing a datum reference frame, so they can repeat the process with the very same points each time (an important thing whether datum targets are specified or not), and 3) examining additional points on the surface to verify that they've adequately modeled what a physical datum feature simulator set would produce (meaning no "higher" points are found that would have contacted a physical set of simulators)(not necessary if datum target points were specified).  Of course the amount of rigor depends upon whether one is building toy trucks or Trident submarines, that's why the word "adequate" is used a couple of times above...  It depends upon how imperfect the parts are relative to how tight the specified tolerances are.

Have you ever done a data correlation between a good, careful knowledgeable metrology lab and a typical supplier's metrology lab?  If so, then you know that there will be very significant differences in the data.  Those differences almost always include a less than good enough process for establishing the datum reference frame(s).

Dean
www.d3w-engineering.com
dingy2 (Mechanical)
11 Feb 11 8:56
Dean:

Thanks for the clarification although I certainly do not agree with you on this one - not even close.

Dave D.
www.qmsi.ca

DeanD3W (Mechanical)
11 Feb 11 11:00
OK Dave, We don't agree for sure.

The reason GD&T was developed was because it was found that the pre-GD&T method of directly toleranced dimensions was not enabling adequate data to predict whether a part will function or not.  The "shop floor assignment of datum target points" that you describe as method will absolutely tend to make data of lower quality with regard to predicting whether the part will function or not.  Why bother with GD&T if you're going to implement practices that significantly degrade its benefit?

What if those shop-floor-selected datum target points happen to fall into low points on the surface that, relative to the level of precision of the tolerance specified, cause a significant error in the establishment of the datum reference frame?

Dean
www.d3w-engineering.com

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