GD&T and manufacturing process
GD&T and manufacturing process
(OP)
Hi all
I m working on a typical problem regarding identification of the relationship between the GD&T and different manufacturing processes.
I would like to know that if there is any formulas or guidelines available for assigning different feature tolerances like circularity,cylindricity,parallism for different machining processes like planning ,milling etc?
I m working on a typical problem regarding identification of the relationship between the GD&T and different manufacturing processes.
I would like to know that if there is any formulas or guidelines available for assigning different feature tolerances like circularity,cylindricity,parallism for different machining processes like planning ,milling etc?





RE: GD&T and manufacturing process
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Apr 30, 2008)
RE: GD&T and manufacturing process
You need to chat with your fabricator. There is a chart in the Machinery's Handbook showing the ISO tolerance classes that can be achieved with a given manufacturing process. Search around the limits and fits pages.
You are not supposed to provide manufacturing instructions on your fabrication drawings, but I have rarely proceduced fabrication drawing where it was not obvious what fabrication process would be used. When you know this, you should know what tolerances can be reliably specified on your drawings.
JHG
RE: GD&T and manufacturing process
However, you don't want to end up with tolerances that can't be met except by expensive processes unless you really have to. Sometimes a change in design can allow looser tolerances and this is good practice, to know when to do this you need some idea of the process capabilities.
If you have your own shop, or one/few external shop/s that does/do most of your work you may be best off talking to them.
The chart in Machineries is probably based on ISO2768. This is a DOG of a standard that should not be invoked in its entirety by any self respecting Engineer/Designer (see recent posts on the subject by myself and others). However using it as guidance on what are typically achievable tolerances may have merit. Extracts of this standard are available on the net if you do a google search. I may have even linked them in my earlier posts on this standard.
Also, while I'm not sure it has exactly what you need, Ctopher posted a few months ago a pretty good link to a machine shops website with information on what causes them problems/make it easier for them. Take some of it with a pinch of salt as it's biased heavily to making the machinists job easy without much consideration for function but it has some good stuff.
Finally I seem to recall Drawoh having a website with a little such info, or was it dingy, one of them.
KENAT, probably the least qualified checker you'll ever meet...
RE: GD&T and manufacturing process
actually you all have make my problem easy,but while surfing about ISO2768 I found another standard i.e.ISO 2768-2:1989. it is something about "General tolerances -- Part 2: Geometrical tolerances for features without individual tolerance indications." but on some of the sites it is written that it is highly recomended that u should not use this standard! can anyone expalin about this standard and about the notification?
RE: GD&T and manufacturing process
As to why this standard is a @#%%$^@#$% @#$%$@%!@# @#$%$# piece of #$%@#$5234; I'd have hoped that looking at my and others earlier posts on the subject would have answered that. Take a look at these links then feel free to draw your own conclusion.
thread1103-216401: GD&T (GPS) per ISO 1101, 8015, & 2768
thread1103-196260: Tolerance analysis ISO2768
thread1103-197786: tolerance analysis to ISO 8015 & 2768 part 1 (again!)
thread182-208001: tolerance of investment castings ISO2768
thread182-197787: Tolerance calculations to ISO 8015 & 2768-1
ht
In summary the last paragraph of part one says:
Most designers/engineers I know would argue that a major point of the drawing is to define what tolerances need to be met to maintain function, and that hence invoking this standard in its entirety may not be that smart.
Out of interest what drawing standards do you work to? Most of the people that really have issue with 2768 are I believe ASME or ex BS users. Perhaps to an ISO die hard it's more palatable.
KENAT, probably the least qualified checker you'll ever meet...
RE: GD&T and manufacturing process
To expand on what Kenat stated, 2768-1 (when invoked) is a general tolerance for untoleranced dimensions. If you like to rely on a general "title block" tolerance to define an entire drawing, this standard may cause problems. (See Kenat's quote). Since the standard only applies to untoleranced dimensions, you should add a specific tolerance to any dimension that you don't want the standard invoked on.
Joe
SW Office 2006 SP5.1
P4 3.0Ghz 1GB
ATI FireGL X1
RE: GD&T and manufacturing process
KENAT, probably the least qualified checker you'll ever meet...
RE: GD&T and manufacturing process
Hope this helps.
RE: GD&T and manufacturing process
thanks for the links that u have given.
I have another doubt regarding GD&T,while refering to the ANSI Y14.5 standard I came to know about the formulas like the fixed fastner and the floating fastner that are used to allocate the postional tolerances.
Is there any other such formulas avilable for allocation of different geometric feature tolerances ?
RE: GD&T and manufacturing process
I can't think of anything else off the top of my head.
KENAT, probably the least qualified checker you'll ever meet...
RE: GD&T and manufacturing process
RE: GD&T and manufacturing process
If you are looking for appropriate tolerances for specific interfaces, then Machinery's Handbook, ISO 286, and other standards which specify fit classes are a great start.
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca
RE: GD&T and manufacturing process
Although I'm tempted to say that Germany where the standard originated probably re-tooled significantly in the 50s so the figures may not be quite based on 30s tooling - though it's not beyond belief.
The same goes for the charts you see showing cost increase with tighter tolerance.
I'm forever having people tell me they spoke to the machine shop and they say they can routinely hit +-.002 or tighter with no problem/cost increase.
KENAT, probably the least qualified checker you'll ever meet...
RE: GD&T and manufacturing process
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: GD&T and manufacturing process
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca
RE: GD&T and manufacturing process
I've worked places where sure the newer machines could do it day in day out but some of the older machines struggled a bit. Putting tighter tolerances meant that when there was a lot of thru-put they couldn't use the older machines so things got delayed.
While what constitutes a tight tolerance may have come down a lot, I think the principle of not specifying tighter than you need holds.
Otherwise I'll just change our block tol to +-.001 and never do another tol calc
KENAT, probably the least qualified checker you'll ever meet...