basic dimension versus tolerance dimension 3

[lokho]

Hello,
I am really confused by GD&T. Sometimes I am told to use basic dimension to dimension an aligning hole, sometimes I am told to use tolerance limit +.005, -.000 for mating part 1, and +.000, -.005 for mating part 2.
My question is when to use basic dimension and when to use limit tolerancing?

Thank you

 

[ewh]

Use basic dimensions to locate the holes, limit tolerancing to size the holes.

 

[looslib]

Use basic dimensioning to locate features, use tolerances on the size of features.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli
Sr IS Technologist
L-3 Communications

 

[Heckler]

If you're confused with GD&T then may I suggest that you get some training before attempting to use it. At the very minimum do you have the specification that govens GD&T? GD&T is a language and can create costly mistakes if not used correctly.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[swertel]

The other confusion is that people forget about the 'D'. There is a science to dimensioning and not just adding a basic dimension with a positional locating tolerance. Sometimes, a dimensional tolerance limit is all that is needed to satisfy the function of the part, other times more control is needed.

Other than taking a GD&T class, performing a tolerance stack-up analysis on your assembly will probably answer your question.

--Scott

For some pleasure reading, try FAQ731-376

 

[lokho]

Thank you all for your input.
I took a basic CAD course in my senior year of college, and unfortunately it didn't focus on GD&T.
I do need some training, and your advice about the best website to learn about GD&T will be greatly appreciated.
Thank you
Lokho

 

[shaun123]

Lokho, try

http://www.tec-ease.com/index.htm

some very useful information.

Shaun

 

[lokho]

Thanks Shaun,
That is some really useful website.

 

[swertel]

I understand the convenience of online training, but for something like GD&T, only a classroom experience works, in my recent experience anyway.

Look up the ASME website,

http://www.asme.org,

to find when they may be having the next training seminar in your area.

--Scott

For some pleasure reading, try FAQ731-376

 

[Heckler]

Lokho,

Here is a website for good solid GD&T Training:

http://www.geotol.com/abouttci.htm

Another plug for Al Neumann

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[MechNorth]

Tec-Ease provides in-house and seminar training where there is interest. Excellent trainers (Don, Brett and Frank) and mentoring style. They worked with me as I studied for my GDTP-S exam. I found their materials and methodologies much easier to follow and more informative/applicable than the Neumann method that so many trainers follow verbatim.
I have also used iigdt (

http://www.iigdt.com)

for training, and their material is also pretty decent.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[KENAT]

Drafting Zone (draftingzone.com) is another website, I believe from Genium.

They also give training.

Gary Whitemire who is the ceo or something is on the ASME commitee for 14.5 if memory serves.

 

[tank627]

use basic if you want to confuse the manufacturing personel

 

[ringman]

lokho,

As a general rule, basic dimensions on hole patterns work well when interchangeability is a consideration. Otherwise they may not be preferred. Example that comes to mind, rivet patterns.

 

[KENAT]

Hole pattern positions using positional tolerance is one of the simplest applications of GD&T.

If your manufacturing personel can't handle it you have a problem.

Use positional when a pattern of holes in one part has to match a pattern of wholes in another part or for interchangeability as above so you don't have to match drill.

 

[aardvarkdw]

Basic dimensions are a theoreticly perfect dimension. If you make something basic, it ignores the standard tolerence block and therefore must have a FCF that defines the tolerence. Like most everyone else said, features of size use limits or standard tolerences, locating dimensions use basic as long as you have FCF that defines it.

 

[tank627]

you'd be surprised, kenat

 

[KENAT]

No I wouldn't be. I've encountered the problem, we outsource all our machining and have revised a number of parts using GD&T to actually reduce the tolerances will maintaining interchangeability etc. We frequently end up getting quoted higher prices to the new drawings.

In my opinion this is a problem.

My solution would be to find vendors that know what they're doing but it's not that simple apparantly.

 

[ewh]

Sad, but true. You can redimension a drawing using GD&T and actually loosen all of the tolerances, but the quotes will still come back higher.

 

[MechNorth]

Been there, done that, got the quotes to prove it.

A good supplier is easier to train than to replace. If you have in-house GD&T training, or external seminars that you subscribe to, compel your suppliers to get on board.

I had a meeting a few years back with some 40+ suppliers who would be impacted by a GD&T implementation. It was an "Introduction To GD&T" session with discussions and introductory training. I explained that the business was going GD&T, and all suppliers would eventually be required to work in GD&T without impact to us. One supplier asked what would happen if they didn't comply; another supplier said "Which parts do you make? I can take care of them." That was the end of the resistance.

I had one supplier quote a part for $500 that had cost $80 before; the tolerances had been opened wide and surface finishes roughed to allow sawing with no grinding or milling afterwards. The in-house buyer thought that "proved" that GD&T was going to cost us money. I had the supplier and the buyer in a conference room after that and educated them both; the buyer became a "reluctant believer" and the supplier lost the business to someone else who understood the drawings and dropped the price below the original $80. Word got around to other suppliers and suddently I had much more interest in attending training, and requests for GD&T drawing corrections started to come in. The message will get out there.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca