Sde12
Mechanical
- May 3, 2020
- 1
Hi Guys,
I am working as a Designer with basic knowledge of CMM. While browsing through the threads on CMM i came across some thread
Link
The thread is about Datum feature should not use MMB and only RMB can be used on datum feature if part is inspected using CMM.
I gave some thought on having datum at RMB will always ensure zero datum shift or not. In case of using checking fixture it will ensure, but in the case of CMM inspection with part inspected is located (with some clamps) at CMM inspection table. I have attached here some images in doc file. Fig A shows the drawing (it may not be complete but enough to explain)(sorry if my handwriting and drw is difficult to read). In Fig B it shows actual manufactured part with hole centers apart 41 mm vertically (Dont bother about horizontal spacing, its not a matter of concern here). Fig C shows part is inspected with checking fixture with B at RMB. You can easily see that there is no scope for movement of part with datum B DRF at theoretical center of hole B pattern ( at 20 mm), so there is no datum shift. IN my Opinion, CMM always locate center of pattern of actual part to determine DRF. However if you put no fixtures and inspect part on cmm table with simple clamps (See fig D), there is shift of 0.5 mm of the center of pattern holes, (see dim 20.5 mm ie center of 41 mm) i.e. datum B DRF point. so there is datum shift of +/-0.25mm vertically in this case. So with CMM we introduce datum shift even with datum feature B at RMB. I hope you have got my point. So guys, am I right here? If not what is right? Can someone please help me with this?
Thanks
I am working as a Designer with basic knowledge of CMM. While browsing through the threads on CMM i came across some thread
Link
The thread is about Datum feature should not use MMB and only RMB can be used on datum feature if part is inspected using CMM.
I gave some thought on having datum at RMB will always ensure zero datum shift or not. In case of using checking fixture it will ensure, but in the case of CMM inspection with part inspected is located (with some clamps) at CMM inspection table. I have attached here some images in doc file. Fig A shows the drawing (it may not be complete but enough to explain)(sorry if my handwriting and drw is difficult to read). In Fig B it shows actual manufactured part with hole centers apart 41 mm vertically (Dont bother about horizontal spacing, its not a matter of concern here). Fig C shows part is inspected with checking fixture with B at RMB. You can easily see that there is no scope for movement of part with datum B DRF at theoretical center of hole B pattern ( at 20 mm), so there is no datum shift. IN my Opinion, CMM always locate center of pattern of actual part to determine DRF. However if you put no fixtures and inspect part on cmm table with simple clamps (See fig D), there is shift of 0.5 mm of the center of pattern holes, (see dim 20.5 mm ie center of 41 mm) i.e. datum B DRF point. so there is datum shift of +/-0.25mm vertically in this case. So with CMM we introduce datum shift even with datum feature B at RMB. I hope you have got my point. So guys, am I right here? If not what is right? Can someone please help me with this?
Thanks
