Good drafting practices, GD&T and culture shock

Mechanical Engineering general discussion Drafting & GD&T
#1

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QUESTION
I’m a lead engineer for a medical device manufacture that has a long history in the automotive and consumer products industry. I have a good background and understanding of GD&T and have been through many courses. With that being said, the company I’m currently with does not practice GD&T and does not have good drafting standards & many untrained designers (Untrained in good drafting practices). I’m making an attempt to create standards based on ANSI Y14.5.1 and also instill good drafting practices in the designers. The questions and issues I have are these:

  1. I’m trying to make a good argument for complete manufacturing drawings because I’m constantly getting resistance to this because we supply 3d cad data and the parts are typically produced from this data. Other than incoming inspection, if the parts fits, the drawing rarely gets looked at. What are other arguments?

  2. Does anyone have a good approach to driving GD&T and good drafting practices into a culture?

I was going to add more but if I could get some resolution to these issues, we’ve taken a huge step. Any input is appreciated.

REPLIES

ewh

  1. The times are definitely changing. Now that more and more manufacturing centers can read CAD data, fewer drawings are actually needed. ASME Y14.41-2003 DIGITAL PRODUCT DEFINITION DATA PRACTICES is the governing standard for those that choose to rely more on the model and less on a drawing. While most CAD packages are not yet fully complient with the standard, they are moving that way. Any information that used to be on a drawing can be in the model file. Granted, it seems most companies are fighting to keep their drawings for reasons mentioned above. However, with the right software, drawings will be unnecessary.
    Sorry I can’t help your argument on this point.

  2. The most effective way to instill this discipline that I am familiar with is to have a knowledgeable, dedicated checker who is given the authority to reject poorly drawn drawings. Checking against applicable standards is his only function, and his word is law. While there will be much grumbling that the changes he requires may be unnecessary, in the long run the drafters (and designers) will thank him, as creating a proper drawing becomes second nature to them.

#2

Unless you work for a government type of company, I’ve never seen that happen. Often bad drawings are modified as required and not rejected. Likely the ‘kiss of death’ to the rejector, unless you’re with the government.

Dik

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#3

On my last job, the policy was to show critical drawing dimensions and leave everything else off. Unless otherwise specified, the fabricator was to work to their standard process tolerance. The fabricator was to submit a First Article Inspection report on all the tolerances actually indicated. The company was knowledgable about and actually rather obsessed with GD&T.

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#4

I wish. At one past engineering job, I went looking for first article inspection reports on ‘old’ parts, found none, and got appointed as First Article Inspector for everything, forever.
Doing the inspections personally actually gave me a better appreciation for good dimensioning practice, and for the details of the process.
I used a spreadsheet template to start the reports, but I was frustrated that a single simple template could not handle our stackups well.
Eventually, I had to add odd things like the depth of a thread controlled in ‘turns’, with a tolerance range of 1/8 turn total. That’s probably better than a depth dimension, at least on an automatic screw machine, where the cams are designed that way anyway.
Gradually, I figured out that the ‘old’ assemblies had been totally redesigned, by an assertive idiot who couldn’t do math, and the ‘new’ assemblies were inferior in every way.

BTW, I am not a fan of ‘critical’ dimensions, because on those assemblies, everything was critical to some function or interaction; there was no slack in the design anywhere.

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#5

I am fighting this battle. Slowly making progress.
We need 100% inspection. No sampling.
The inspectors require fully dimensioned drawings because the talent we can find is not ready to use CMM tools.
So attempts to use CAD control have failed. Switching back to 100% dimensioning on drawings has also been flawed.
Needless to say our designers and inspectors need time and coaching to return to consistent drawing practices.