GD&T - True Position Callouts for Sections of Studs

M

mr_bridger

#1
Hi.

I have been reading a lot on here, but finally found something I can't find an answer to. I hope someone could help me out please.

I'm trying to work out the correct way to dimension a pair of studs. I've attached an image of what we need to achieve. Hopefully it makes what I'm about to explain easier.

The left hand stud in the picture needs to be controlled from the intersection with the surface [A] up to 5mm from that surface with a tolerance of 0.5mm. Now in the past I've seen it done with a tolerance like:-

[TP][0.5][(P) 5.0][A]

IE a projected tolerance, but I believe this is only for checking "off" the part? Like past the studs end? Would it be prudent to just have a standard callout of [TP][0.5][A] and then a not saying "First 5mm from [A]" ?

This I'm unsure of.

The second one, (it is actually the same stud, needs 2 tolerances) it to be checked ONLY at 15mm from [A] to a tolerance of 1mm. I'm thinking it should be like:-

[TP][1.0][A] and a note "Measured at 15mm from [A]" or, put a boxed 15mm dimension line on the stud and TP pointing to the stud at that line?

Is this right? Are there other symbols or terminology that I am missing?

Hopefully I've explained it right, and any help is appreciated.

Thanks.

Ross.
 

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D

David DeLong

#3
Re: GD&T - True position callouts for sections of studs.

We have one stud that requires a positioning tolerance to be measured in heights of 5 and 15 mm. relative to primary datum A.

Do not use a projected tolerane zone. This is applicable to blind threaded holes and not in your situation.

Also, positioning tolerance is not applicable since you do not have a pattern of features or do not have any other datums than primary datum A. The onlyi relationship from datum A is perpendicularity. Your feature control frame should show a perpendicular requirement from datum A with a diametrical tolerance zone (checks the tilt in any direction) of value of 0.5 mm. The location should be shown with a chain line (like a center line but thicker) at a basic dimension of 5 mm from datum surface A. The basic dimension (rectangular box around it) and is a theoretical dimension. A leader line should be shown from the feature control frame to its location reflected by the chain line.

The second feature control frame would be a perpendicularity with a diametrical tolerance zone of 1 mm relative to datum A with an leader line showing that the location that is 15 mm above datum A. The 15 mm also should be shown in a basic dimension (box around it).

With perpendiularity, the 0.5 or 1 mm tolerance is the maximum tilt in any direction. If one reflected position with the center shown from datums A, B and C, the diametrical toelrance zone of 0.5 is really a radial tolerance of 0.25 maximum off location.

Again, positional is not appropriate if there is not pattern of holes or studs or the location (center) is not shown with datums B and C.

I hope this helps rather than confuses.
 
M

mr_bridger

#4
Re: GD&T - True position callouts for sections of studs.

Hi, Thanks for the help so far. I'm pretty sure I should have explained better though! lol. I did mention there were a pair of studs, but not about the pitch.

Here is a new picture, explaining in a little more detail and wondered if what is in blue would work.

I'm only interested in the pitch, not position in the other axis.

Prefer to use GD&T rather than +/- dimensions as this works better on CMM reports.
Thanks
Ross.
 

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D

David DeLong

#5
Re: GD&T - True position callouts for sections of studs.

Hi, Thanks for the help so far. I'm pretty sure I should have explained better though! lol. I did mention there were a pair of studs, but not about the pitch.

Here is a new picture, explaining in a little more detail and wondered if what is in blue would work.

I'm only interested in the pitch, not position in the other axis.

Prefer to use GD&T rather than +/- dimensions as this works better on CMM reports.
Thanks
Ross.
This is a bit better explanation.

The area in red should be shown as a hatched area with the upper line at 5 mm as a chain line (per ASME Y14.5M-94 and also 2009). You don't even need the hatched area as long as the 5 mm dimension is shown as a leader line or one could use a phantom line. .

The positional tolerance at both the bottom and top should be shown with a diametrical tolerance zone since the feature of size is round or cylindrical. The top feature control frame used MMC which is great and conducive to a checking fixture. You also have MMC in the bottom feature control frame but that is tough to make a checking fixture. I would probably suggest taking the MMC out of the more restrictive positional tolerance taken at 5 mm above datum A.

You don't have to any note below the feature control frame as long as the leader line is directed to the hatched area or the leader/phantom line on the cylindrical stud located 5 mm above datum A.
 
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