GDT Dimensioning method for 2 datum holes that break thru a wall

[chimps12]

Hello,

I was wondering if the dimensioning method I show in the attached drawing is acceptable. Originally, this part had two holes, but it was suggested that the holes break through due to issues with the holes being so close to the wall of the part. There are 2 mating pins that pass through the ".063" holes to align the part and is why these need to be datums. I am questioning this because once these holes break through they technically are no longer holes and become a notch. Alternatively, if I can no longer treat these features as holes do I dimension these as if they are slots? In this scenario do I remove the diametrial true position and control the midplane of the slot and or just use profile?

 

[powerhound]

I would use profile.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[pmarc]

This is very interesting case. Theoretically, since the holes have become half cylinders, they are not really features of size any more and for that reason position tolerance should not be used.

Looks like good application for profile relative to A applied to both notches, but I guess the open point will still be what features should be designated as datum features B. If the mating features are two pins, what if you draw two phantom-line circles, one in each center of the dia. .0633 and use datum target symbols B1 and B2 with additional notation that they represent cylinders? (just like in semiond's post from 4 Jun 18 16:57 in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=439628).

The diameter of the datum target cylinders would be set at certain basic value (say dia. XX) determined by the extreme condition (size, orientation and spacing) of the mating pins. Then in any geometric callout referencing B, the letter B would have to be followed by [dia. XX] notation, see para. 4.11.6.3 in Y14.5-2009. Unless the diameter of the cylinders can be equal to the inner boundary of the notches, then the letter B can be followed just by (M) modifier.

Just a thought...

 

[CheckerHater]

I would draw two thin line circles and stop right there.
Consider them irregular features of size.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[AndrewTT]

What about drawing the holes as actual holes (not slots) and add a note stating that they are allowed to break through the wall?

 

[semiond]

AndrewTT, i suppose the OP doesn't want to do something like that because that might lead to a situation where the holes break through just a little bit leaving ugly "cracks" on the external surface, or very thin sharp and deformed layer of material in case they get too close without actually breaking through.

 

[mkcski]

chimps12: You might want to consider Y14.5-2009 figure 4-29 on page 71.

Certified Sr. GD&T Professional

 

[chez311]

Is there an opportunity to either rotate the location of the holes/pins or move them slightly closer together so that holes instead of slots can be used? Keep in mind that if this is a machined part the slots may lead to additional expense, though with holding a tight tolerance on a small 0.0633" dia hole (i assume we're working in inches here - otherwise thats truly tiny!) it might be a wash. You would have to talk to your production guys or vendor.

As far as controlling the slots, profile would be the simplest from a drafting perspective however potentially not from a manufacturing perspective as it will be difficult to achieve an analog to your current allowable position vs. size/form tolerance and will instead hold everything to a tight profile tolerance. The only thing I can think of is set one as a datum, perhaps with datum targets, then hold the other to that datum with multiple single segment or composite tolerance - HOWEVER that seems somewhat complicated. Doable, but complicated for what you're trying to achieve.

Edit: I should clarify I know +/-.0005" = .001" total size tolerance isn't THAT tight but its certainly not wide open.

I would draw two thin line circles and stop right there.
Consider them irregular features of size.

CH could you provide an example of what you're talking about?

 

[pylfrm]

chimps12,

Due to rounding error(s), the 36.7° dimension is not exactly compatible with the .746 and 1.000 dimensions. One of the three should be removed.

You mentioned alignment. Can you explain the functional goal in more detail? What does the tolerance scheme of the mating part look like?


pylfrm

 

[chimps12]

Hello Everyone,

Thank you for the quick and multiple responses. To follow up with Semiond's point. We are having issues with break through already with the part and are experiencing the issues you describe which is why there was a proposal to make this a slot. Originally the part had a slot in the upper left corner and a hole in the bottom right they are experiencing issues with break through on the hole on the lower right corner. The hole was used as the main alignment feature and the slot was used for secondary alignment. Typically, here we use a hole and slot when mating to two pins so we can precisely control alignment, but no to over constrain the assembly.

Chez311 I cannot change the location of the holes due to having to change the mating part. This part mates with multiple versions of the mating block and to make a change to both would be an enormous effort.

Pylfrm I have included some screen shots of how the this part mates. I left a lot of the features off the original drawing I had attached for clarity. The part is a plastic connector with spring contacts and mates on top of a PCB (which is not shown) The connector and PCB need to be precisely aligned with one another to ensure each spring contact from the connector makes contact with a pad on the PCB. Both PCB and this connector are aligned using the same guide pins on the housing where they mate.

So far from what I can see it looks like using profile may make the most sense.

 

[chimps12]

Also, here is an ISO view of the mating parts. I added the PCB for further clarity.

 

[semiond]

Chimps12,
Consider calculating the virtual condition of the pins on the mating part, and defining the slots with profile of a surface, unilaterally (u modifier) inside the material, while the basic dimensions defining the profile are what you got as the VC for the pins.

 

[DeanD3W]

chimps12,
The partial cylinder at the end of the slots is a "partial feature of size". No standard tells us how complete a feature of size needs to be (and no standard should attempt to tell us this in my opinion). For example, a key slot in a shaft should not change the definition of what type of feature the shaft is, and it should not change how any tolerance applied to the shaft will behave. How much interruption of a cylindrical feature is too much to still consider it to be a feature of size? That's up to the user to decide, I believe. So, let's just assume that you've decided that 180 degrees of arc subtended is sufficient. Maybe a repeatability study of measurement data will prove this to be a case where profile should have been applied instead..?

The two pins and so also the two slots in the mating parts, are equally important, so one should not be the secondary datum feature and the other the tertiary. A pattern of two datum features is easy enough to handle and it captures function better. The use of coordinate axes to represent the datum reference frame can clarify the origin location and the desired orientation of the datum referenced frame, so I've used them in the attached mark-ups. I am in favor of always making it clear what the coordinate axes are based on, so even though there is only one DRF on the part, I like to show the datum feature letters in square brackets with the X, Y, and Z axis labels. This is required only when there is more than one DRF, but I think it's always good to make things as clear as possible.

As pmarc brought up, the secondary datum feature could be referenced at MMB for both of the drawings. I'll leave that for later.

One other thing to change about the mating assembly drawing (the part with the two dowel pins pressed in) is that I don't think that datum feature A would allow an origin location that would be best, and it doesn't appear to be functional either. In the mark-up I propose to use the mating face as datum feature A.

Would an approach as shown in the attached mark-ups of the two drawings make sense? In a couple of places in the mark-ups we need to rely on the fact that Y14.5 does not prohibit what is shown. One is applying size and position to partial cylindrical features and the other is adding the datum feature letters to the axis labels when it's not required.

If you decide against size and position applied to the partial cylinders, then using profile of a surface instead would be fine.

Since a pattern of two features is used as the secondary datum feature, as note such as "DATUM FEATURE B IS A PATTERN OF TWO FEATURES. THE ORIGIN OF THE DRF [A,B] IS CENTERED BETWEEN THE TWO FEATURES" would be worth considering.

Best Regards,
Dean
www.validate-3d.com

 

[semiond]

To add to the above advice by DeanD3W: if you choose to apply position and size on the partial cylinders, they can be easily verified with go/no go gages simulating the mating part at Virtual Condition/ Resultant Condition. So i would say both profile and position are good options.

 

[pmarc]

To add to the above advice by DeanD3W: if you choose to apply position and size on the partial cylinders, they can be easily verified with go/no go gages simulating the mating part at Virtual Condition/ Resultant Condition. So i would say both profile and position are good options.

Only if the partial cylinders contain opposed points (i.e. they are more than 180 degrees). Otherwise, at least in theory, the cylinders can be very close to each other and the gage will not be able to catch this.

BTW: Good to see you back, Dean.

 

[semiond]

pmarc, you are right. I didn't think of that. A solution to that would be to make the slots more narrow than the partial holes diameter.

*Edit: making the slots width smaller than the holes diameter will also help to constrain the part in assembly and prevent translations in the diagonal direction along the slots.