DRF of casting and machined part 3

[SeasonLee]

Please ref to the drawing attached for details, the upper half is a casting sketch and the lower part is a machined sketch, both of them use the spherical surface (R.633 should be SR.633) as primary datum feature, and both of them choose different features as secondary and tertiary datum respectively.

My question is: The primary datum feature is only a small portion of the spherical surface, I think it will not make a repeatable and reproducible DRF, but I want to know what you think and I am willing to listen to any of your suggestions on the DRF.

Thanks of all comments and recommendations.

SeasonLee

 

[MechNorth]

I will ignore all evident errors on the drawing (datum identification symbols, placement thereof, etc.) and stick to the question about Datum Feature-A.

In the top figure;
The SR segment is small; if the other features were coaxial with the surface, then I wouldn't be concerned about this however there is not enough surface area on datum feature A to constrain the 5 dof that it should eliminate as a primary datum. The issue is that the offset arm (the axis of which seems to be intended as datum C) will generate a moment, inducing a twist motion, overriding some of the dof's that A is supposed to constrain. This could be overcome by referencing A-C as primary.

There are a lot of other issues with the drawings.



Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[SeasonLee]

Thanks for your comments, Jim.

The axis of datum feature C is perpendicular with datum feature A, they are not coaxial features, I can catch how to establish A-C as primary. Yes, there are a lot of drawing issues, any comments are welcome.
Thanks

SeasonLee

 

[PaulJackson]

I am with you on the errors and instability comments Jim... but as a sphere (however limited the segment) [A] can only constrain translations so it is confined to 3 DOF. then in the casting is limited to 2 rotations and [C] is improperly attached to an axis or centerline.

I the machining [A] is repeated as a spherical segment stopping translations, definition as a feature is obsecure (if not undefined),and [C] is a surface relegated only to stopping as many rotations that it may once is more intelligably defined. [C] does not stop the one translation that it is most capable of doing since [A] has already done that job!

It would be interesting to see how this assembles and works ( how dof's are constrained in the assembly), how the casting is constrained or balanced for measurement, and how the casting is registered for machining. That tale will reveal what the relevant functional datum features may be.

Paul

 

[MechNorth]

Wow, talk about a brain fart! Tks Paul. I read and saw SR, but in my mind I jumped to a taper ... why, dunno. So, let's try again.

For the casting drawing:
First, usually cast datums will have a different letter callout vs the machined drawing. By industry convention, Z/Y/X are the primary/secondary/tertiary datums for castings, forgings and molded articles. This helps to distinguish the datums from machined datums.
Next, There is no control on datum feature-A; because it's not a FOS, you'll need a surface profile, and make SR.633 a basic dimension. Datum feature-B is not adequate in size to effectively constrain rotations as a secondary datum; datum feature-C would be more appropriate (and successful) as a secondary datum. Datum Feature-C needs to be identified; putting a datum callout on the centerline is illegal (ISO and ASME), so you need to establish the datum FEATURE used to establish the datum axis. Secondary datum feature (B as it is now, C as proposed) needs to be controlled back to primary datum-A). The tertiary datum feature (C as it is now, B as proposed) needs to be controlled back to the primary and secondary datums.
What feature is the .629 diameter? The "bottom" ring of the conical taper boundary of the SR? Why is the relationship so important on the casting?

For the machined drawing:
You need to properly call out datums features-A, -B, -C, control the datum features and relate them back to higher precedent datums.
Profile of a surface callout cannot be attached to the basic R.433 (which needs to be SR).
Datum feature-B is redundant because datum-A eliminates the 3 translational dof's (per PJ). The machined diameter of the pin may make a useful secondary datum feature. The feature labeled datum feature-C is a good choice for tertiary datum; consider it for use on the cast drawing as well.

That should be a good start.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[SeasonLee]

Thanks for your valuable comments, Paul and Jim.

Sorry for I don’t have further information for the moment regarding to its function and assembly, will try to get it. Anyway, thanks again and have a wonderful Thanksgiving holiday.

SeasonLee

 

[KENAT]

Seasonlee, do you have a copy of Y14.8 if working to ASME stds?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SeasonLee]

Yes, Kenat. I have a copy on both of them
ASME Y14.8M-1996
ASME Y14.8-2009

SeasonLee