pin hole pattern as datum offset from CL
pin hole pattern as datum offset from CL
(OP)
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.
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.





RE: pin hole pattern as datum offset from CL
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: pin hole pattern as datum offset from CL
RE: pin hole pattern as datum offset from CL
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.
RE: pin hole pattern as datum offset from CL
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: pin hole pattern as datum offset from CL
RE: pin hole pattern as datum offset from CL
Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: pin hole pattern as datum offset from CL
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.
RE: pin hole pattern as datum offset from CL
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.
RE: pin hole pattern as datum offset from CL
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.