pin hole pattern as datum offset from CL

[bxbzq]

Hello,

I have a simplified part drawing drafted like attached. There are two pin hole pattern 2X Φ16 used to position the part relative to mating part. The 2 pin holes as a pattern is offset from the cylinder centerline. If I use the two hole pattern as tertiary datum C, how to call out the clearance holes and cylinder diameters relative to the datum A, B and C?
Alternatively, if I use the cylinder Φ560 as tertiary datum D, it will be quite easy to call out the clearance holes and cylinder diameters. This is a straightforward approach but functionally I think the two pin holes should be tertiary datum.

 

[powerhound]

I'm having trouble seeing what datum feature B is. What is it? All I see is a line

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[bxbzq]

The cylinder is actually bolted together from upper and lower part. So datum B is the mating surfaces of upper and lower part. I just want to simplify the image so many features are omitted.

 

[dtmbiz]

Datum B

All datums should use the datum symbol
(triangle to datum block)

Remember the surfaces are used to establish
a datum plane (in this case) and they don’t appear to
be accessible once the 2 halves are bolted together?
BTW … Don’t see features that would “bolt the halves together

On that consideration alone (accessibility); Datum B as shown
would not be valid.

Much more that could be commented on, however starting with
datum B is most important

It does appear that DRF |A|B|C| would do the trick, however
I really couldnt speak to the functionality of the features
or the mating interfaces from what is shown.

 

[powerhound]

Yes, I agree with dtmbiz. Your datum B callout is not valid. Is the placement of the seam of the two halves really critical? You could call all the features out with respect to the same single axial datum or flat surface primary/axis secondary and then let simultaneous requirements do the rest. Then add a note saying that none of the holes can violate the seam and give a minimum distance. Would that work?

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[bxbzq]

During assembly, the lower halve assembles with a mating part axially first (datum plane A acts as mating surface and the 2 pin holes do the positioning), then comes a shaft, then comes the upper halve to close the cylinder. But in the product definition we combine the 2 halves together because they together function as a cylinder. Attached draft shows the lower halve only. I don't show many other features because I'm now only interested in how to call out the M24 bolt holes (I was wrong in saying clearance hole in the first post) and diameters relative to A, B and C.

 

[powerhound]

You still have to have a datum that is accessible. Datum feature B is accessible in this drawing but not in the original one you posted. This also violates fundamental rule 1.4(o). You have the same features specified on two different levels of drawings. The idea is that the features specified on the detail drawing are toleranced such that they will function when assembled.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

bxbzq,
I will only share some thoughts on your second picture...
From your description of how bottom half is assembled with its mating part, I do not think you need two flat top surfaces as secondary datum features. All you need is datum feature A (like you already have it) and the two-hole pattern as secondary datum feature B. This will define DRF containing 3 mutually perpendicular datum planes and constraining all 6 degrees of freedom. Pattern of 17 M24 holes should be located with the use of positional tolerance from that datums. The same datums should be used when orienting/locating two planar top surfaces - but this time profile of surface should be a way to go. Of course for both - the pattern of M24 holes and the two surfaces - true position / true profile should be defined by basic dimensions originating at the point being a middle of the distance between datum feature holes B and lying at common axis passing through both holes B.

 

[bxbzq]

It would be a long description if I try to put everything clear. To make one thing clear though, there is no drawing solely for lower or upper halve part. We only have one drawing for the cylinder and this drawing is not assembly drawing. In this drawing, both upper and lower halves are defined. Common features, like cylinder diameters, are defined in the views showing both parts together. Most of the features are same on both parts. For some features it is still good to show them in a cylinder view even though they may be located differently on upper and lower halves. The second draft was only to argue the datum surface B is accessible.
pmarc, I agree with you. Datum B is not needed to call out those M24 holes. I was actually looking for a way to call out the M24 holes as easy as polar coordinate dimensioning, but looks I have to go rectangular coordinate dimensioning if using the two pin holes pattern as secondary datum.

 

[pmarc]

bxbzq,
You can still take advantage of polar coordinates. The center of pitch circle is simply vertically offset from the line connecting centers of two datum feature holes. Dimensions for the offset and the pitch circle diameter must be basic and that would be all.