New to GD&T

[AbidingDude]

Hello, so I recently got my first job in a machine shop. I'm not new to the work foce by any means - just new to machining.

A brief bit of background: I worked for about ten years in the semiconductor industry (manufacturing). The factory closed. A while later I happen to run into an old coworker at Barnes & Noble. We catch up. He says he thinks I would be a good fit for his place (the machine shop). A month later I have an interview and get hired.

The boss hired me on as an inspector because I did a lot of inspecting at the chip-maker (and have a good eye for detail) and wants to make the place more high-tech by working more computers into the place.

It's a fairly small shop. They just got their first CMM a few years ago, and the one guy who (sort-of) knew how to use it got fired. We basically have one IT guy there. He and I are the only ones that know anything about computers.

So my math and computer skills are good, just no machining experience.

My confusion is with GD&T. I had one engineering graphics course in college. It just covered the different views and tolerancing - no datums or datum reference frames.

I feel pretty lost when I'm looking at a datum reference frame and trying to figure out what it is I'm supposed to measure. One of the biggest things I'm struggling with is true position. A lot of times I'll see, for example, say a rectangular plate-like object with a flat bottom. The bottom plane will be designated "Datum A". There will be a thru-hole perpendicular to the datum plane. There will be a true-position callout for the hole and it references Datum A. It's a plane and the hole goes all the way through. What positioning is there?? Position implies a distance from something. A distance away from that plane for example.

The GD&T symbolics strike me as far from intuitive. I'm really not liking it so far, but of course I realize I'm not used to it. I'd really like some help with this.

 

[Belanger]

But realize that parts have thickess, so when measuring position, you must also verify that the hole is perpendicular to the face that is identified as the datum feature.

Imagine if that typical flat plate had a small error where the top face and bottom face were not perfectly parallel. Can you see how the hole might be "positioned" better to one of the faces?

Certainly a basic GD&T course would be of help, too...

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

 

[AbidingDude]

Thank you for responding. I can definitely see how perpendicularity would be an important aspect, but if that's the case, why not use the perpendicularity symbol?? GD&T has one. Use it!
Likewise, if parallelism is an important factor, use the parallel symbol on a callout for the faces!

One of the many GD&T articles I read mentioned that true position was a "powerful" symbol because (I'm paraphrasing) it could describe different features with just that symbol. That struck me as a crap argument. Ambiguity is not a positive thing with prints. Multi-purpose is great for gadgets but not for instructions.

(Please don't take any of my cynicism as directed towards you in any way. I'm grateful to have the discussion. I'm just frustrated.)

I was actually thinking of asking the boss to get us a GD&T course. Several other guys in the shop are confused on some other things as well.

Again, thanks!

 

[CheckerHater]

Get the course.

If your boss is not in the hurry to start one, start educating yourself. Most people on this forum will recommend buying copy of Y14.5 standard - you will need it, but sometimes some moments are better explained in reputable textbooks. There is no need to buy them all, but check what you can find in your local library.

When it comes to position / perpendicularity controversy, you have to realize that when the shape and size of tolerance zone is the same, either can do the the job, but position symbol is already there. So, there is no ambiguity - just keeping your drawing clear.

Good luck!

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[JNieman]

Back when I was first learning the subject at all, this book was a great help:

http://www.amazon.com/Geometric-Dimensioning-Tolerancing-David-Madsen/dp/1566379776

I notice the author has a newer version:

http://www.amazon.com/Geometric-Dim...IP8_1_2?s=books&ie=UTF8&qid=1439985098&sr=1-2

Get a book. Read it. Reference it often as you go. That's my suggestion.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[ctopher]

Definitely go to training. It will make more sense with someone explaining it to you in person.

Chris, CSWA
SolidWorks '15
SolidWorks Legion

 

[AbidingDude]

Thank you all for the suggestions. Looks like I'll be pushing for a class. I may pick up some books too.

 

[capnhook]

You'd better do both, AbidingDude. You are a novice.
The class will have a book I'm sure.


Proud Member of the Reality-Based Community..

[green]To the Toolmaker, your nice little cartoon drawing of your glass looks cool, but your solid model sucks. Do you want me to fix it, or are you going to take all week to get it back to me so I can get some work done?[/green]

 

[JNieman]

Agreed. The book I used was the one I was required to get for the class I took. I happened to be smart enough (or not desperate enough for cash at the time) to not sell it back to the library, figuring that it'd be a handy reference in my profession. It's proved true. I've had no fewer than three coworkers buy a copy of the exact book I have, just so they could have the reference too. It has great pictures.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[powerhound]

To provide another answer to your question regarding perpendicularity vs. position: Position locates the feature, perpendicularity does not. They do not do the exact same job. All perpendicularity can do is ensure that the considered feature is perpendicular to its datum reference, it can't tell you where the feature is to be located. Position does both of those things. It controls location AND orientation.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[AbidingDude]

I do get how perpendicularity has no bearing on position. For me, the un-intuitive thing is that position encompasses perpendicularity.

Given that I'm working as an inspector, I want explicitness as to what exactly I'm supposed to check. Location is important? Show me the position symbol. Perpendicularity is important? Show me the perpendicularity symbol.

I still have a lot to learn I realize.

 

[Belanger]

Do you understand how parallelism on a surface can also control flatness of that surface? If so, then it's a similar analogy to position's relationship with perpendicularity.

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

 

[3DDave]

To simplify a bit -

Perpendicularity on a hole is:
the center point of each tiny segment of the hole ending up within a small cylinder.

Position on a hole is:
the center point of each tiny segment of the hole ending up within a small cylinder.

For the simple case the main difference is that for position the location of the small cylinder is fixed. Since that location is fixed for both ends of the small cylinder, it also fixes the orientation of the small cylinder. The definitions are identical and only the meanings of the references change.

 

[JNieman]

As an inspector, you don't really get to establish your own criteria for language. Your job is to understand what standards are being worked to, and see that the product conforms to that - not that it conforms to your personal standards.

If you are inspecting parts according to prints that declare ASME Y14.5 in some manner - you need to understand that position CAN control perpendicularity. If you understand the symbols then you would see that it /is/ implicit.

I think the best step is to get a well-recommended book, read through it lightly, take a class, and keep a book for future reference. Having access to a copy of the actual ASME Y14.5 standard you are working to would also be rather critical, too.

*edit* Removed statement made due to me mis-reading.
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[AbidingDude]

Do you understand how parallelism on a surface can also control flatness of that surface? If so, then it's a similar analogy to position's relationship with perpendicularity.

That actually does help. Thank you.

As an inspector, you don't really get to establish your own criteria for language. Your job is to understand what standards are being worked to, and see that the product conforms to that - not that it conforms to your personal standards.

Understood. I was not trying to imply that GD&T standards should bend to my whimsy. My apologies if I did that. I only meant to express my consternation.

Thank you all for your help.