Is Parrallel Datums required?
Is Parrallel Datums required?
(OP)
Greetings
I am not an"expert" at GD&T however I know what I know and it is based on very good training and many years of machining expereince and then design experience. I was confronted by an "old" = "expereinced" checker this week making suggestions for improvments to an initial rough draft of a model I was detailing. This model is of a cast stainless part cast into an aluminum cylinder housing. I espablished Datum A in the bore axis, I established B on the surface mating to the crank case, and I established C on the surface that the support post is mounting to. (horizontal engine cylinder) B is perp to axis A, and C is parralell to A and perp to C. On parrallel surfaces to B and C I have different machined holes and grooves for various reasons and I have location callouts for the tapped holes according to what type of limits I found nessacary to accomplish the task at hand. I have a fourth surface perp to C and angled to B so I called it Datum D. I turned my rough draft into this checker and he came back with a suggestion that I should establish the parrallel surfaces to B and C as E and F, his reasoning was if I didnt I would assume unwanted tolerance stacks and cause trouble in the asembly. I asked him if he was talking about projected tolerance zones and he said yes. I said that I can still do a projected tolerance zone with out making parralel datum planes. I would like your feed back, In the application, my expereince tells me I am right. He is asking me to go to the ANSI book and show where I am right, I dont have the time, so I told him to show me where he was right. He backed down and said he didnt know if he could specifically find any definition to what he was speaking of. I believe he is surrenduring to my view. I welcome any comments.
Norb.
I am not an"expert" at GD&T however I know what I know and it is based on very good training and many years of machining expereince and then design experience. I was confronted by an "old" = "expereinced" checker this week making suggestions for improvments to an initial rough draft of a model I was detailing. This model is of a cast stainless part cast into an aluminum cylinder housing. I espablished Datum A in the bore axis, I established B on the surface mating to the crank case, and I established C on the surface that the support post is mounting to. (horizontal engine cylinder) B is perp to axis A, and C is parralell to A and perp to C. On parrallel surfaces to B and C I have different machined holes and grooves for various reasons and I have location callouts for the tapped holes according to what type of limits I found nessacary to accomplish the task at hand. I have a fourth surface perp to C and angled to B so I called it Datum D. I turned my rough draft into this checker and he came back with a suggestion that I should establish the parrallel surfaces to B and C as E and F, his reasoning was if I didnt I would assume unwanted tolerance stacks and cause trouble in the asembly. I asked him if he was talking about projected tolerance zones and he said yes. I said that I can still do a projected tolerance zone with out making parralel datum planes. I would like your feed back, In the application, my expereince tells me I am right. He is asking me to go to the ANSI book and show where I am right, I dont have the time, so I told him to show me where he was right. He backed down and said he didnt know if he could specifically find any definition to what he was speaking of. I believe he is surrenduring to my view. I welcome any comments.
Norb.





RE: Is Parrallel Datums required?
Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-05)
RE: Is Parrallel Datums required?
You need to go back and correct one of the sentences that states C is perpendicular to C. This cannot be. You need to use spell check on the word 'parallel' too.
With three datums, you should be able to locate just about whatever features you desire on the part. Including projected tolerances.
The sequence in naming the features is NOT important. The sequence in the FCF IS important.
A sketch would help.
RE: Is Parrallel Datums required?
A picture would be benificial.
RE: Is Parrallel Datums required?
All tolerance should relate to these datum’s in the order of importance. You may have positional to C B A and profile to A B C.
Cheers
RE: Is Parrallel Datums required?
Ring man your correct C is perp to B.
RE: Is Parrallel Datums required?
ProEDesigner00, I'm curious why you set up a new datum-D on the angled face. The only practical reasons that I can foresee would be (1) that whatever features are being related to that new datum-D are totally independent of the original datum structure, or (2) the setup of the new datum structure in inspection grossly overwhelms the cost of the setup and the error introduced (with respect to the original datum structure) is not critical. If that isn't the case, the features should be related back to the original datum, and scrap the one on the angled face.
Stick to your guns with the checker.
Jim Sykes, P.Eng, GDTP-S
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