Unilateral tolerances-why?

[sendithard]

We just onboarded a new customer. All their prints have unequally disposed surface profiles. And their CAD models are all made at MMC condition. That causes some issues with surface hits on the CMM with tight tolerances. So I'm in the middle of altering their models to what we make the parts at. We are dealing with some goofy .05/.025mm profiles on position and profile, which is basically beyond the capability in my opinion of our CMM. I'm trying to get a Gage RR done at the moment on these ridiculous internal callouts,. Uncertainty is at a minimum of 2 tenths or 5 microns.

GDT is for communication. Can someone explain to me why you would ever desire to basic a print at the MMC with one sided tolerance? What is wrong with the basic at what the part should be made at and a bi-lateral tolerance zone?

The customer just provided a print at Profile .025 U 0 and the tolerance zone is dashes in the direction the part can grow which is opposite the callout. They don't understand the callout nor do our engineers. Run an external profile of .025 U 0 by two random engineers and then run an internal profile of .025 U 0 by them later in the day and see if you get the right answers. No chance. I cannot stand unilateral tolerances, and would enjoy hearing why anyone would use them?

Thanks.

 

[3DDave]

It's a preference, just not your preference.

ASME Y14.5 is for communication, just not to the CMM operator alone. This may make certain calculations for stress and interference easier or trivial. It means the basic profile is exactly equal to one boundary of the tolerance zone.

There are schemes where the nominal value doesn't lie inside the tolerance zone.

There should be no "dashes" used in conjunction with the "U" symbol and, if graphically depicted, it should be a phantom font.

There aren't "internal" and "external" profile tolerances; it's even a bit dicey to call anything an internal feature as every feature is on the outside of the material of the part, but the committee needs to please the crowd rather than explain the sort of rules the computer graphics people use for determining where material is and isn't based on surfaces. The ASME calling them different adds to the confusion you noted.

Perhaps make up a guidebook in advance of accepting new customers that tell them what you will and will not accept on drawings.

 

[mfgenggear]

OP
what type of part, and geometry.
how are the parts machined. or manufactured.
if the parts are quoted with those tolerances
then it has to meet those requirements

 

[mfgenggear]

OP
Also if the CMM does not have the precision required. Then it requires a layout on the granite surface plate.

 

[drawoh]

We just onboarded a new customer. All their prints have unequally disposed surface profiles. And their CAD models are all made at MMC condition. That causes some issues with surface hits on the CMM with tight tolerances. So I'm in the middle of altering their models to what we make the parts at. We are dealing with some goofy .05/.025mm profiles on position and profile, which is basically beyond the capability in my opinion of our CMM. I'm trying to get a Gage RR done at the moment on these ridiculous internal callouts,. Uncertainty is at a minimum of 2 tenths or 5 microns.

Thanks.

Terminology...

	Modelled size	Max	Min
Nominal	⌀40	+0.3	+0.1
Min	⌀40.1	+0.2	0
Median	⌀40.2	+0.1	-0.1
Max	⌀40.1	0	-0.2

I would prefer to do nominal modelling and then apply the tolerances. If people are doing to do 3D printing, or any form of moulding, median modelling is the way to go, as you note. The vendor wants to work off the model.

Chat with your customer. As mechanical designer, I have had these conversations throughout my career, and the desired processes have changed along with the technology. Make it clear to them that a preferred modelling technique will reduce fabrication costs.

 

[3DDave]

(Last edited: Tuesday at 11:22 PM)

We just onboarded a new customer. All their prints have unequally disposed surface profiles. And their CAD models are all made at MMC condition. That causes some issues with surface hits on the CMM with tight tolerances. So I'm in the middle of altering their models to what we make the parts at. We are dealing with some goofy .05/.025mm profiles on position and profile, which is basically beyond the capability in my opinion of our CMM. I'm trying to get a Gage RR done at the moment on these ridiculous internal callouts,. Uncertainty is at a minimum of 2 tenths or 5 microns.

GDT is for communication. Can someone explain to me why you would ever desire to basic a print at the MMC with one sided tolerance? What is wrong with the basic at what the part should be made at and a bi-lateral tolerance zone?

The customer just provided a print at Profile .025 U 0 and the tolerance zone is dashes in the direction the part can grow which is opposite the callout. They don't understand the callout nor do our engineers. Run an external profile of .025 U 0 by two random engineers and then run an internal profile of .025 U 0 by them later in the day and see if you get the right answers. No chance. I cannot stand unilateral tolerances, and would enjoy hearing why anyone would use them?

Thanks.

"their CAD models are all made at MMC condition. That causes some issues with surface hits on the CMM with tight tolerances"

That's a problem between you and your CMM software maker.

"So I'm in the middle of altering their models to what we make the parts at."

They cannot predict problems with your processes.

"We are dealing with some goofy .05/.025mm profiles on position and profile, which is basically beyond the capability in my opinion of our CMM. I'm trying to get a Gage RR done at the moment on these ridiculous internal callouts,. Uncertainty is at a minimum of 2 tenths or 5 microns."

That's another item to put into the handbook for reasons to refuse models and contracts that you give to potential customers before onboarding them.

"why you would ever desire to basic a print at the MMC with one sided tolerance?"

You don't mention any process, so for those that involve removal of material one can program the CNC system to have a very, very small negative offset (into the material) from nominal and thus maximize the chances that any out-of-tolerance conditions can be resolved by removing material by minimizing the risk of accidentally removing too much. If any feature fails then the offset for that feature can be corrected.

Needless to say, I did not see the edited version of the comment. So that means I will entirely quote every single comment you make from now on in my replies. I see that drawoh is already doing that. Consider this the first item that is going into my handbook.

 

[ctopher]

I'm trying to understand why are you inspecting your customer models? Doesn't your customer give you an outline of what they need, then you supply the product? As long as you tolerance the parts to meet their dimensional envelope?

 

[jassco]

Hi, sendithard:

Do your customer prints have a note to indicate you can use its 3D model? If not, then the models mean nothing to you.

Best regards,

Alex

 

[dtimberlake]

"GDT is for communication. Can someone explain to me why you would ever desire to basic a print at the MMC with one sided tolerance? What is wrong with the basic at what the part should be made at and a bi-lateral tolerance zone?"

Can't really speak for "why".
Sometimes I have to dig pretty deep to even be sure of the "what."

But +0/-.0005" is how the OD of Metric ball bearing with bores around 80-120 mm are toleranced in catalogs.
I have no clue what the manufacturing drawings show.

 

[Mech1595]

GDT is for communication. Can someone explain to me why you would ever desire to basic a print at the MMC with one sided tolerance? What is wrong with the basic at what the part should be made at and a bi-lateral tolerance zone?

For functional fixtures/gauges, I'll model critical features at MMC/LMC and specify the limits as needed on the drawing. This simplifies the design process on my end, and is no issue for the fab shop.
These are also for one-off/limited qty components used internally, so I'm not really concerned about a CMM operator setting up a program [edit: for long-term production].