GD&T and manufacturability.

[ScottS]

Our company is in the beginning stages of utilizing GD&T in its designs. I work on the manufacturing side of things. My job is to interpret the drawings and create fixturing to these prints. I've heard more than a few times from those in Corporate Engineering the concept “I don’t care how you build it, just build it to print.” Is this really the way the manufacturing of the part(s) is viewed by those who advocate for the use of GD&T? All my training has been done in-house, so I’m hearing just from those within these walls.

My way of thinking is that GD&T is a valuable tool, but you still need to be able to know what the capabilities are of your manufacturing floor. For example, I’ve received welding prints where one of the 3 functioning datums is an imaginary plane going down the center of the part. Great in theory, but how do I locate off this? My job is to design fixturing for these types of parts, and I’m forced to design these fixtures with the parts at LMC condition, then have the guys on the floor center these parts using feeler gauges. This is more time consuming and more expensive.

I’d really like to hear how those outside of my company who are strong believers in GD&T view this topic. Is the manner in how parts made really of a secondary concern when designing with GD&T?

[Belanger]

Believe it or not, what you've heard from the Corporate Eng folks is correct, and it's (usually) best for both you and them and the customer. Think about it: If the print says "drill 3/4 inch hole," you must "drill" that hole to the correct size. But if the prints says "Ø.750," then you can make the hole any way you want, as long as it meets the size requirement of 3/4 inch (within any tolerance, which I left out of this example).

Now if for some reason the manufacturing method is really important to getting something done correctly, then it's OK to spell that out on the print. But it most cases, the print simply tells you what the finished part looks like and they don't care how you get there.
In my example, why should we assume that the hole is machined? What if it's a plastic molding process that makes the part? What if I go out back and toss the part in the air and shoot a bullet through the part? It doesn't matter -- as long as the hole meets the given specs.

This isn't me just saying so, or the Corporate Eng people. It's one of the "Fundamental Dimensioning Rules" laid out in the official drawing standard, which is ASME Y14.5-2009. It's paragraph 1.4(e).

[Kelly_Bramble]

First, let’s define the knowledge sets that are required in the product development process to produce an optimal end item or assembly. Note that each of the following knowledge/skill sets are sometimes specialized tasks within organizations however a well-rounded design engineer will be capable in all.

Also, these knowledge sets are overlapping with the other knowledge sets or skills. This list is not comprehensive.

• Communication: Team work, interpersonal skills and sharing of ideas is critical in the design engineering process. Communication across professional and expert lines is critical.
• Technology Utilization: CAD, FEA, Windows OS, Linux, and other associated productivity technologies and Tools.
• Drafting – We need to document our end item requirements. Drafting is an overlapping skill set with dimensioning and tolerancing.
• Dimensioning and Tolerancing: This is where GD&T comes in. Be aware that one can well versed in GD&T dimensioning and tolerancing however lacking in Design for Manufacturing and Assembly and other knowledge sets. Also high GD&T abilities do not mean the individual fully understands Design for Function either.
• Engineering and Analysis: Understanding stress, strain, heat transfer, engineering materials, galvanic capability (Corrosion control), quality and functional testing requirements is required to produce reliable end items.
• Manufacturing: Design for Manufacturing and Assembly DFM and DFA. One of the first questions design engineering should be asking is “How do we build this thing?” How many are we going to build? Where are we going to build? Materials? Is there a price point or target? And so on… These questions are mission critical as the geometries, tolerance structures and manufacturing process capacities of perspective manufacturing processes are different. Designing a machining friendly part that will be manufactured as a casting or extrusion will likely cause extra iterations and significant challenges in manufacturing.

“I don’t care how you build it, just build it to print.”

This is an interesting statement to me.. Design engineering should have considered the manufacturability of the part or assembly during the design process. Design engineers whom collaborate with manufacturing folks during design tend to get products out the back door to customers faster, there will be less NRE, quality is improved, fewer change notices, fewer meetings with manufacturing and so on. Everybody should care how the end item is built.. Keep in mind that functionality (it has to work) is just as important.

My way of thinking is that GD&T is a valuable tool, but you still need to be able to know what the capabilities are of your manufacturing floor.

I agree..

where one of the 3 functioning datums is an imaginary plane going down the center of the part.

Are we talking about attaching a datum to a center line? ASME Y14.5-2009 is very explicit in that we only identify datums as real and touchable features. See my statements above on DFM and DFA.

then have the guys on the floor center these parts using feeler gauges. This is more time consuming and more expensive.

Again, design engineering should be working concurrently with manufacturing during the design process to mitigate these challenges.

I’d really like to hear how those outside of my company who are strong believers in GD&T view this topic.

GD&T is an overlapping skill set to drafting, design , DFM, DFA, etc.. Dimensioning and tolerancing to GD&T industry standards is important however is not the only knowledge set required for robust and cost effective end items.

[ScottS]

Kelly and Belanger, thanks for your input!

Kelly, you brought up something that I felt was counterintuitive and something I didn't understand from the feedback I was getting from our guys in Corporate. there's been a big push not only for GD&T here at my company, but also for DFM. I couldn't understand how an engineer could tell us on the manufacturing side "I don't care how you build it..." and still state we're designing with manufacturing in mind. I'm relieved to see that, in fact, isn't the way it is supposed to be. I've downloaded ASME Y14.5-2009 and plan on getting much more familiar with it.

[Kelly_Bramble]

Adding to my above post, within ASME Y14.5-2009, Section 1.4 "Fundamental Rules" sub-paragraph e states the following:

"The drawing should define a part without specifying manufacturing methods."

This statement is sometimes construed to mean that manufacturing methods is NOT a consideration. In general manufacturing details are not A,B,C,D specified however the target manufacturing process is often specified on the engineering drawing. Again, manufacturing process geometry , mechanical tolerance, finishes and so on are manufacturing process dependent. Design engineering should have incorporated DFM in their design and communicated manufacturing intent to all concerned parties.