basic dimension versus tolerance dimension 3

[lokho]

Hello,
I am really confused by GD&T. Sometimes I am told to use basic dimension to dimension an aligning hole, sometimes I am told to use tolerance limit +.005, -.000 for mating part 1, and +.000, -.005 for mating part 2.
My question is when to use basic dimension and when to use limit tolerancing?

Thank you

 

[KENAT]

MechNorth,

That sounds kind of like how I'd expect, or at least like to think, it should happen.

Here they're loathed to change suppliers (don't get me started) so if the suppliers resist it the suppliers get their way. Competative tendoring to some degree would probably help but that is also a rarity here, again don't get me started.

I think the idea of explaining it to suppliers & maybe getting training or what ever was nixed before I started here.

 

[MechNorth]

Kenat, ... another anecdote.

A couple of colleagues and I kept running into the "purchasing" walls ... "we don't do it that way", "that's now how we do it", and "they're an honest supplier and they come through for us when we need it", etc. While I was fighting the GD&T battle, a colleague saw the crap being spoon-fed to us about the cost of some new parts (PVC tubing with rounded edges and two grooves inside). My colleague's buddy owned a machine shop, so we chatted...got our manager's ok to get a quote directly without purchasing's involvement...got the quote; it was 75-80% lower. We didn't get to use the new supplier (politics again) but we did get the price down to the same range at least. Ticked off Purchasing, but we got the point across and some changes were made.

I've learned through many concussions that beating your head against the wall doesn't work ... if an engineer wants to make changes it's very important to remember that it's easier to get forgiveness than permission. If the suppliers need training, suggest to your management team that it become a requirement. You don't have to pay for it, the supplier does; it's a cost of doing business.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[KENAT]

I'd love to just go ahead and do it but it would catch up with me, and I like my pay check! Perhaps I just have too much of a yellow streak!

Also GD&T is just one aspect of the larger battle of drawing standards and some parts of purchasing have been allies in certain aspects so we don't want to upset them at this stage.

That said in a couple of areas I've pushed the 'it's easier to ask forgiveness than permission' line so maybe I'm just too strategic for my own good

 

[MechNorth]

Nah, Kenat, not yeller, just practical. I was kept around for a long time because I made waves and I took my lumps (& was thanked behind closed doors). We each have battles that we choose to fight, and those that we're thrown into. More often than not, I was in the second camp. That, plus I was too darned stupid to just say NO!

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca