I think that in this case, the datum feature simulator for C at MMB should be made according to the Virtual Condition that takes into account the .0060 position at MMC and not the zero perpendicularity at MMC. Because, the slot's (datum feature C) center plane can be produced offset from the datum B axis within an at least .0060 wide tolerance zone. Am I wrong?
A co-worker requested to replace the slot with a hole and align that hole at the centerline of 2 holes. Is his request better than using a slot and at the centerline of 2 holes?
Hello GD&T Guy! First of all, I want to thank you for these series of videos you've been doing so far, I am great enthusiast of GD&T and having such a great content to rely on really helps on my everyday taks as a designer. On this particular video, there's something I'd like to ask about; I see you dimensioned the hole pattern after a center coordinate system, let's say, derived from the Datum system, mainly from Datum B. What are the advantages of doing that rather than dimensioning the pattern from the datum B itself? I've seen both approaches and I'd like to dive a bit more about which one is better or if it is a matter of context and application of the part. I hope you can give some more insight about it Thanks again and I look forward to reading from you!
Hi @vanzz3863! If I understand your question, I think you absolutely could dimension all of the screw holes directly from Datum B, in this case the centermark on the .2521 hole in GDTG00402. This is opposed to the way that I dimensioned most of the holes off of centerlines that I drew across the part. I would just note that I also applied basic dimensions from the datum features to the centerlines. Basic dimensions do not "stack" tolerances. They provide the nominal location of a feature within a datum reference frame or coordinate system. We can go forwards and backwards - it's all the same. So what I've done is show the basic location of all of these features, including in this case the outside edges of the part. It does not change the fact that the 0,0,0 of the part is on that .2521 hole. So why do it this way? In one or more of these videos, I mention one of the 'fictions' of this design. Let's say the CAD for this part was sent to me by some 24 year-old engineer - or a 50 year-old laser physicist - asking me to make the drawing. They are probably not thinking about it as being 0,0,0 on that hole. They designed it as roughly symmetrical about some centerplanes, which is how they are likely thinking about the design. With that in mind, I elected to dimension the part this way so they can see very clearly the relationship between the centerplanes of the part, the screw holes, and the locating features. It doesn't change anything about the inspection or how the part will locate. One final thing to mention here. I think it would be a mistake to apply +/- dimensions from datum B to the edges of the part. If you did that, the edges of the part would not be controlled in orientation because nowhere in those dimensions is datum C mentioned. See how in GDTG00402, I provide basic dimensions to the centerlines of the outside features, dimension the outside features as features of size, and then I give them position tolerance within the datum reference frame. This gives those features the orientation control to give us a 'squared up' part, located on the datum features. Get it?
Basic, but the explanation of datums is probably the best I've seen. Simply explained and less GDT jargon. Wish I had this when I started years ago.
I think that in this case, the datum feature simulator for C at MMB should be made according to the Virtual Condition that takes into account the .0060 position at MMC and not the zero perpendicularity at MMC. Because, the slot's (datum feature C) center plane can be produced offset from the datum B axis within an at least .0060 wide tolerance zone. Am I wrong?
A co-worker requested to replace the slot with a hole and align that hole at the centerline of 2 holes. Is his request better than using a slot and at the centerline of 2 holes?
Hello GD&T Guy! First of all, I want to thank you for these series of videos you've been doing so far, I am great enthusiast of GD&T and having such a great content to rely on really helps on my everyday taks as a designer.
On this particular video, there's something I'd like to ask about; I see you dimensioned the hole pattern after a center coordinate system, let's say, derived from the Datum system, mainly from Datum B. What are the advantages of doing that rather than dimensioning the pattern from the datum B itself? I've seen both approaches and I'd like to dive a bit more about which one is better or if it is a matter of context and application of the part. I hope you can give some more insight about it
Thanks again and I look forward to reading from you!
Hi @vanzz3863! If I understand your question, I think you absolutely could dimension all of the screw holes directly from Datum B, in this case the centermark on the .2521 hole in GDTG00402. This is opposed to the way that I dimensioned most of the holes off of centerlines that I drew across the part. I would just note that I also applied basic dimensions from the datum features to the centerlines. Basic dimensions do not "stack" tolerances. They provide the nominal location of a feature within a datum reference frame or coordinate system. We can go forwards and backwards - it's all the same. So what I've done is show the basic location of all of these features, including in this case the outside edges of the part. It does not change the fact that the 0,0,0 of the part is on that .2521 hole.
So why do it this way? In one or more of these videos, I mention one of the 'fictions' of this design. Let's say the CAD for this part was sent to me by some 24 year-old engineer - or a 50 year-old laser physicist - asking me to make the drawing. They are probably not thinking about it as being 0,0,0 on that hole. They designed it as roughly symmetrical about some centerplanes, which is how they are likely thinking about the design. With that in mind, I elected to dimension the part this way so they can see very clearly the relationship between the centerplanes of the part, the screw holes, and the locating features. It doesn't change anything about the inspection or how the part will locate.
One final thing to mention here. I think it would be a mistake to apply +/- dimensions from datum B to the edges of the part. If you did that, the edges of the part would not be controlled in orientation because nowhere in those dimensions is datum C mentioned. See how in GDTG00402, I provide basic dimensions to the centerlines of the outside features, dimension the outside features as features of size, and then I give them position tolerance within the datum reference frame. This gives those features the orientation control to give us a 'squared up' part, located on the datum features. Get it?