Uploading Model files and associated drawing GD&T 1

[ChristieBiels]

Is there a best practices for how much GD&T is required on drawings that are accompanied with CAD and are being machined in overseas. Specifically, the used of Datums and tolerance blocks. How detailed do the drawings really need to be?

Background: I have detailed small-ish parts with drawings and models that are going to be machined overseas in quantities of 50. I assume that the machinists will just use the model to machine the parts on a CNC. There are a few key features that need to defined on each part for fit (I have put bilateral tolerances on these). The tolerance block is currently calling out .X +/- -.3 and .XX +/- 0.13 mm.

I'm new to the company and in the previous drawings of these parts, dimensions were being defined to three decimal places, which I assume is driving up cost and not achievable. Thus, I removed them. And there were datums on surfaces that could not be measured or over controlled surfaces. Thus, I removed them. Now, my boss is questioning my interpretation of the drawings because he wants everything to fit. And so do I.

CB

 

[TheTick]

Do you know what needs to be controlled for the part to function? That's how much GD&T you need.

 

[ewh]

You need to know what tolerances are actually required to make a good part before you go changing them.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Nereth1]

Short and to-the-point answer, they need as much detail as is required to make sure the part functions. If you have a good relationship and understanding with a supplier and want to bet on that, that might be less than is explicitly required to define the part (e.g. if you know they will use a CNC lathe programmed to nominal size, you know it consistently achieves +/-0.05, you know you only need +/-0.3 for part function). If you have no relationship or the part is existential to your company, then it's every possible detail you can think of, such that even a malicious supplier couldn't mess you up (without breaking the instructions on the drawing).

Ranty, marginally off topic answer:

Title block tolerances with values driven on number of decimal places frustrates me. Because of them, you are in a situation where you are moving the average size (e.g. 10.05 -> 10.1) in order to change the range (tolerance) on a size.

I'm a pretty disagreeable guy so in your situation, being frustrated, I would make a line in the general drawing notes that overrides the title block tolerances, then say that specific (directly toleranced or GD&T) tolerances on the drawing override both.

Then I would tolerance all of the tolerances not caught by my new drawing-notes-tolerance, to what they actually need to be. Which in a lot of cases may be much wider than title block would have allowed.

This would take me 3x longer and maybe cost everyone more in my time than it saves in part cost... but it would be right. I'd work some overtime and feel good about it. Then eventually I'd find a chance to convince someone to change the title block tolerances to something more useable on a day to day basis that doesn't drive up cost when you write in the actual size that you want the feature to be. Then I'd size/position things for what I want them to be and tolerance them for what I can allow them to be forever more.

Or, if I was making one-off-parts where engineering costs >> part costs, that thus don't justify this level of detail, I'd probably be having phonecalls and emails with a preferred supplier confirming reasonable achievable general tolerances (good to know these for future anyway), and then adding those general tolerances in the notes even if they're a bit narrower than you might be able to allow if you were very investigative about part function.

 

[drawoh]

ChristieBiels,

I just posted a thread, thread1103-480169.

This is what happens when you don't do long term planning with your documentation. Complete GD&T clearly communicates what you want from your vendors, and it actually is not a whole lot of work once you get the hang of it. GD&T is a basic design engineering skill.

Someday, your CAD vendor comes up with an unacceptable business model, or you will get taken over by a big corporation that mandates new CAD[ ]software, or someone will come up with cool new technology that you want to adopt. The worst case scenario is that you will have to go through your library of PDF and STP files, and re-model everything.
[ol a]
[li]You need to do it.[/li]
[li]This is good training on the new CAD.[/li]
[/ol]

This all can be done quickly and efficiently if your drawings are complete. I wonder how much B-2 bomber documentation was done on a drafting board?



--
JHG

 

[3DDave]

Thinking more of what probably happened to the Stealth bomber is COTS. In exchange for providing only an Interface Control Drawing and a performance spec the supplier cut the price over a re-procurement package including all the manufacturing process information. COTS, the grift that keeps on grifting.