Primary Datum on a Curved Surface? GD&T

[ahaslam]

Can some one verify this? An engineer called out a true position of a set of holes using a curved surface as the primary datum. When constructing a reference point for your true position you MUST use a plane (as defined by GD&T to be flat or level surface) as your primary surface. I tried explaining this to him and he says he has never heard of that (with attitude) and that I should be able to make the part the way the print is. He wants proof that this is true but I can't seem to find with in any reliable source. A website link or something would be nice.

[mkd]

His face imprint on the back of your hand should be proof enough.

 

 

if the holes and the curved surface axes are on the same plane he could have just as well called out the centerline as primary.

[RaiderX]

ask him how to check it

[ahaslam]

I said that. He said A CMM. I said you need a level plane to establish a primary. He said something like "well sounds like you may need to get a better CMM." This guy really urks me!

 

I told him I would find some proof.

[mkd]

i'm thinking of a cylinder with some holes in the end.

 

imagine two planes, 90° apart, intersecting in the centerline of the cylinder. basically these are your flat primary datums.

 

with this scheme you could have the end of the cylinder as your secondary.

 

some way to clock the cylinder would be your tertiary datum.

[mkd]

if he had the end of the cyclider as primary, then perpendicularity could throw off inspection of the holes, when the design doesn't need squareness.

[ahaslam]

i'm thinking of a cylinder with some holes in the end.

 

imagine two planes, 90° apart, intersecting in the centerline of the cylinder. basically these are your flat primary datums.

 

with this scheme you could have the end of the cylinder as your secondary.

 

some way to clock the cylinder would be your tertiary datum.

 

 

I'm confused by that statement.

 

This is assuming I am working with a cylinder that has two flat edges. Mine does not because of the curvature one side of the hole is defined using splines. Splines as you know do not have a center point like an arc and there for you can not define two center points to construct a line on to use to defining its projection.

You can't define a true position using a datum as the hole you are defining. (if I was working with a cylinder you could use the outside to define the inside but that would be concentricity, not true position.)

You can have the two at 90 deg but you will need one as the primary that sits perpendicular to the cylinder to fully define the location, unless you were using another cylindrical object to define on of the said planes.

[Del.]

I think it can be now.

 

 

https://books.google.com/books?id=Ms7lVOepeNsC&pg=PA196&lpg=PA196&dq=can+a+primary+datum+be+a+CURVED+surface&source=bl&ots=5TyY1hS6u2&sig=VgLB6eOpP2HD-Fbv6ncDWAE7f_s&hl=en&sa=X&ved=0CDAQ6AEwAmoVChMIpv2u2v3lyAIVzDgmCh1Ngwfi#v=onepage&q=can%20a%20primary%20datum%20be%20a%20CURVED%20surface&f=false

[ahaslam]

[mkd]

I'm confused by that statement.

 

This is assuming I am working with a cylinder that has two flat edges. Mine does not because of the curvature one side of the hole is defined using splines. Splines as you know do not have a center point like an arc and there for you can not define two center points to construct a line on to use to defining its projection.

You can't define a true position using a datum as the hole you are defining. (if I was working with a cylinder you could use the outside to define the inside but that would be concentricity, not true position.)

You can have the two at 90 deg but you will need one as the primary that sits perpendicular to the cylinder to fully define the location, unless you were using another cylindrical object to define on of the said planes.

i'll come at it from a slightly diff direction.

 

your flat primary datum will be fully defined , (EDIT): perfectly perpendicular, from the centerline of curved surface (thinking shaft OD or spline pitch diameter )

the location of this datum , up and down shaft will be the secondary datum (perhaps the end of the shaft or some other feature.)

 

edit: might be talking out my a$$ cause i don't quite understand your print at this point

[taperlength]

I hate to point this out, but theoretically, he is correct.

 

http://files.engineering.com/download.aspx?folder=4c156c25-25e6-4f1c-933b-f419e7d4192d&file=Contoured_features_as_datum.pdf

 

A cmm with a scanning probe can take thousands of points on a curved surface and fit a datum from a CAD file to it.

 

However, from a _practical_ standpoint he is wrong and should have some conventional datums to align the CMM for inspecting the holes.

 

What you could do, if you have the cad model is, create a plane which intersects his datum surface in 3 points, record those points and probe them to create a theoretical flat plane on the cmm.

 

You would have to create a Start Alignment first, from other reference surfaces to find the location of those 3 points.

 

Hope this helps.

[mkd]

I hate to point this out, but theoretically, he is correct.

 

he who?

[ahaslam]

it should be noted that they say to interpret drawing per ASME Y 14.1 and this was not legal till ASME Y 14.5 but... No need to be that picky, right?

[jlw™]

Best $14 I ever spent:

 

https://www.tec-ease.com/store/shopexd.asp?id=123

[Watcher]

Best $14 I ever spent:

 

https://www.tec-ease.com/store/shopexd.asp?id=123

 These guys are the best people in the world concerning GD&T. I attended a training with them in Houston, around 2010 - Think about guys that live what they do.

[crazy^millman]

Best $14 I ever spent:

 

https://www.tec-ease.com/store/shopexd.asp?id=123

 

I have also attended one of the classes and they do a great job.

[The Cathedral]

The major caveat of that new spec is that it must be a mathematically defined surface, ie geometric, not organic. If it's someone's flowing pipe dream of a mold, it does not fall within those specifications and therefore cannot be used.

[Rstewart]

Best $14 I ever spent:

 

https://www.tec-ease.com/store/shopexd.asp?id=123

 

Technically it was $20.95 after shipping (to here at least).

 

I just ordered it as a reference, as my GD&T is pretty weak   

[Machineguy]

I attended a class by a instructor that is on the committee and wrote the book on y ansi 14.5. What a eye opener.

[crazy^millman]

I was going to respond to this thread before, but t was going to be a lengthy response and just didn't have time. With MBD and Datum Targets you can define any free form shape. The trick is the datum Targets and if the process is not defining Datum Targets then it becomes more difficult, but can be done. This is the one area where Verisurf shines head and shoulders above anyone on the market. The software was created and developed for free form shapes. CMM Software cannot handle the free form shapes because of the way the logic was written. It was all approached from Prismatic Shapes being the driving force behind the alignments. Verisurf was started out of the need to work with and inspect free form shapes. Since the algorithms and such were written with that kind of logic it can handle it differently and more robustly than most of the heavy hitters in the CMM Market can. Their logic is flawed, but because they are the biggest it must be the right way. You get enough of your people on the committee and guess what that way becomes the accepted way. Sorry all flawed and when you understand GD&T in its purest form what has been called out is correct and when the software allows you the freedom to support it then you get it. Verisurf gets it and would eat the most complex free form shape for breakfast and ask for 5 more meals on top of it.

 

Plate GD & T process tells you no it is not possible, but I see it as the same thing as saying well if you can't machine it on a bridge port then I guess I can machine it on a 5  or more Axis machine. Sorry not even the same and I can do things on 5 or more Axis machine never even dreamed of just 10 years ago. That progress and that direction is easily support by the correct tools to do it.

 

3 Points define a plane, 2 Points define a Line and 1 Point defines the lock. Using that basic of all methods you create the 6 degree of freedom to lock a part in. I have machined plenty of parts with not flat surfaces or edges and had them checked with no issue. Like touched on here you capture enough points to constrain the part and fit the feature you are locking to then you can align to and check just about anything. I always trained people doing castings to use Verisurf to inspect he casting to the finished part. Best fit the collected points to see if the finished part will clean up before you ever put the 1st tool to the part. When I was there we taught and they still teach people who to work with free from parts like mentioned here.

 

Yes it can be and yes it is possible and sorry, but the lack of CMM and Quality experience is showing here. Get a better software for your CMM and get some training of what is out there. You would be surprised how much the technology has changed in just the last 5 years.

 

That is not a complete thought about this topic, but hopefully it opens up some more communication and thinking about it.

[guitar]

The engineer is correct. I have seen this done many times in aerospace apps. 

[Müřlıń®]

Yes they changed the laws so injuneers can use the CMM as a crutch to shortcut the amount of data on the print....but...

 

This is a quote from del's link.  

 

 

 

So if it has to relate to a 3 point plane already then that 3 point plane should be the datum...

Using a curve for a datum that takes 100's of points in order to be able to define the data is a total waste of time even though it can be done and its legal.

 

Biach slap lazy  injuneer....

[jlw™]

Technically it was $20.95 after shipping (to here at least).

 

I just ordered it as a reference, as my GD&T is pretty weak 

 

 

Sorry about your luck brother, bought mine on Amazon and got free shipping.

 

Here's another little gem I use on a daily basis:

 

http://www.amazon.com/Engineers-Sheets-Formulae-Reference-Equivalent/dp/0958057125

[ahaslam]

Thanks 5th. Correct me if I am wrong but without defining target points, even with the right software we can't have a complete inspection. I am going to pass your response onto our quality guy. We do have a CMM with programmability but have never been trained on its use (kind of a touchy subject in the shop) but it sounds like we will need to figure it out for this part.

[crazy^millman]

Thanks 5th. Correct me if I am wrong but without defining target points, even with the right software we can't have a complete inspection. I am going to pass your response onto our quality guy. We do have a CMM with programmability but have never been trained on its use (kind of a touchy subject in the shop) but it sounds like we will need to figure it out for this part.

 

Yes with the new methods you can use a free form surface to define a datum. Like I said don't I agree with that process, but I understand it and with the right software it can be supported.