DRAWING TO NOMINAL

[cappy]

Hello,

 

I just read two major posts on modeling 3D to nominal and drawing for 2d lathe work to nominal. I have used MasterCAM for about 2 years now and have stuggled with too long. I am required to hold tolerances on the print +/-.001 but the geometry is not in the middle of the tolerance band. I am seeking advice from some of you older fellow cammers and wondering how do I approach my upper management with this issue. APT was used here with great success because the numbers were inputed, but now MasterCAM is used. I figure if the engineering department is drawing or modeling the parts anyway, why not do it to the mean of the tolerance. Any advise on this would greatly be appreciated. I am done redrawing parts.

 

Thanks,

Greg.

[Guest]

greg,

 

the single biggest driving factor of modeling to nominal is the desire to manufacture and inspect to "electronic data".

 

If your company is pushing to ONLY use the electronic data to create NC and/or CMM programs, then everything NEEDS to be modeled at nominal. This allows the tolerance during machining process to be met and held across the entire part.

 

Integrity and reliability of the "electronic data" in this environment meeds to be maintained.

 

then it comes down to time, you can do your job faster if you do not have to recreate portions of what already exists. If it were created properly in the first place, many many hours could easily be saved and scrap parts as well over the course of a year.

 

The change in an engineering dept at one place took time and there was whining and crying, "we've done it this way all along" they eventually got used to it and over a couple of months they were all modeling to nominal dimension.

[cappy]

John,

 

Thanks for your reply. I have spent countless hours on getting the geometry "Right" Re-drawing doesn't make sense to me anymore. We have a lot of old school employees that don't even understand CAD/Modeling CAM or programming for that matter. I need educate some upper management about this problem not only to allow me to be more efficent but the company as a wohle.

 

Due to increasing orders I have contracted back an older gentleman to run APT. CAM is not worth the time right now if I am required to check every tight dimension.

[Guest CNC Apps Guy 1]

Draw to nominal ALWAYS!

 

JM2C

[Bruce Caulley]

We had this discussion before and back then James you agreed that all models should be modelled to mid-limit and the 2d drawings only to reference critical features. Has something made you change your mind?

 

Personally I would prefer it as I just described. If a feature is say 12mm +.2/-0.0 then the feature should be modelled at 12.1mm. If this tolerance is below what is stated on the title block, or geometric tolerancing like MMC comes into play, then it needs to be dimensioned on the 2d.

 

There is nothing more painful than having a group of pockets/holes/features that are all modelled nominal, but have different tolerance bands. If they are modelled to mid they can be chained in the one operation (assuming same depth) and all controlled together (again assuming they are equally "critical" features.

 

Bruce

[robk]

Models are nice and they make our lives much easier, but bluepring is God... It is my responsibility to make the program to the print and not the model.

[Guest]

quote:

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but bluepring is God.

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Not in MANY companies today Rob.

 

The model is god and it should be be IMO.

 

The model then can be used to DRIVE all of the necessary information.

 

A model should drive a print, not a print driving a model

 

JM2C

[Dave Ball]

We are going through this painful transition now. Currently we have tens of thousands of models that are modeled both high and low in the same model. Outside dia. usually on the low and Inside dia. usually on the high which is obviously the worst case scenario. Since my arrival ALL new models are built to the mid. The designers whined at first but now all is wonderful...except all those old models, I think I'm going to get a co-op to begin fixing all of them.

[heavychevy2155]

You could just use the amount to leave in the finish toolpath. Just leave .1mm as in Bruce's example. I know it would also leave it in the Z axis as well, but most of the time that tolerance is not as tight.

[heavychevy2155]

But that is just a workaround.

[chris m]

quote:

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You could just use the amount to leave in the finish toolpath

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Not if you have different tolerance on different steps on your part.

 

We've gone round and round on this issue here for years and I have more or less thrown in the towel on this and just recreate the geometry.

 

I don't understand why Solidworks and Inventor and the like don't have a "mean-ize" feature where the designer can model it wherever he wants and the software can massage the model on its own.

[IntegrexMan]

I have been fighting the "Model to the Mean" issue at my place for 4 years. The blueprint is God here. I have spent countless hours in the past re-drawing parts to the mean so I can machine them properly. What a waste of time!!! Now when I am assigned a new project, I simply tell them I want the models to the mean...I don't care what the prints say. If they want me to make their timelines and make good parts, then I ussually get what I want, even if PD has to make "Manufacturing Models". Current project has a total of 125 individual parts to program. I told PD that it will take me 3 months to redraw the models and then I can start programming or I can start proramming now and have parts for them starting next week. Needless to say, I now have "Models to the Mean". It is much faster for them to change models in Pro/E then it is for me to recreate geometry in MasterCam.