V-grooves as datum

[Alexane]

Hello,
I need some help with a part that consists of 2 V grooves which I want to determine as datums.

Both 36 diameter pins should be the primary datum. The dowel holes must be perpendicular to the plane that connects the pin's center lines.
I am not sure on how to even start.
Any assist will be much appriciated.

Thank you,
Alex.

 

[3DDave]

Use a note that describes how you want this to work. Are the two cylinders to be as exactly parallel and as exactly spaced as possible? Or, are the cylinders able to settle independently and not be parallel or at a fixed distance?

Is this an ISO drawing or ASME Y14.5 (and which year)?

 

[Alexane]

Hi,
This is an ASME Y14.5-2009 drawing.
The two cylinders should be parallel and exactly spaced.

 

[Burunduk]

The two gage cylinders can be used to establish a datum without any critical issues. There might be some difficulties creating reliable constraints as it seems from the large angles of the V grooves and diameter of cylinders but the design can probably be adjusted to fix it and there is nothing principally flawed with the idea. The part can be balanced on the two cylinders and they will constrain together 5 degrees of freedom.

Usually, when it is intended to use gage cylinders making tangent contact along them to establish a datum, datum target lines are the standardized application. However, in your case designating 4 datum target lines will not work according to your intent because it would actually imply 4 cylinders, not two. Also, in your case, the two cylinders are intended to be at fixed spacing from each other but the line contacts should not be at fixed spacing - their distance from each other may change from part to part accomodating the variation of the V angles. Datum target lines would force fixed spacing between the line contacts.

So the appropriate solution is to designate the cylinders themselves as datum targets, for example, A1 and A2. They can be shown in two adjacent views with the datum target designations appearing attached to the circular outline in one view and the straight outline in the other view. The Y14.5 standard defines datum targets only as lines, points, or areas, but an extension of a principle is possible to any shape. For example, in the digital product definition standard, ASME Y14.41 there is an example of planes tangent to a cylindrical surface used as datum targets. It is shown in thread1103-463798 in a post by pmarc. There is no reason why cylinders tangent to planar surfaces like in your case can't be designated as datum targets.

 

[SolidWorks Guy]

What if you made one of those pins datum A and control the other pin with a tight tolerance feature control frame. Then make that new pin datum B. Now you have established datum A and datum B. Then control those features you need to control as primary datum A - B. You can do that.

 

[Alexane]

Thank you very much for your detailed responses.
Whether I designate both pins as datum A or A for one and B for the second, how do I demand that both pins must be tangent to the V-groove surfaces?

 

[Burunduk]

Alexane, I don't think this is an issue and I don't see any other reasonable interpretation being possible. Do you?

 

[greenimi]

Make first pin datum feature A and control it with FCF: position Ø xxx| A-B|
Make second pin datum feature B and control it with FCF: position Ø xxx| A-B|

For the parallellsim between each other you might need to use composite callout.
Also, I am thinking that the form of the V-shape grooves could be profilled datumless.

 

[Burunduk]

I think those are just gage pins (part of an inspection fixture). This is probably not an assembly level drawing where pins need to be controlled with feature control frames. Gage-makers tolerances apply on the pins.

Alexane, am I wrong?

 

[Burunduk]

Alexane, one more thing for you to consider:
If those V-grooves are the actual interface features for this part in its assembly then the supports that contact them are probably not at fixed spacing. They can probably be adjusted to mate with each V-groove on two faces and thus create a stable assembly. Then if you choose the datum targets method I suggested you should apply the movable datum targets concept as defined in ASME Y14.5-2009.

 

[TheTick]

I think it's prudent to deconstruct this.

Simple question: what datums can be legally derived from a pair of non-parallel surfaces?