does this work?

[greg2835]

This is just an example I made up to see if my understanding of datums with position tolerances is correct. Let's say I have a pin hole that is located relative to datums A, B, C with a tolerance of .002" - no problems there. Now let's say I want to locate two threaded holes relative to the position of the pin hole with a position tolerance of .002". What I am showing in my example is my attempt of saying "these threaded holes are perpendicular to A and relative to the pin hole with a tolerance of .002" AND perpendicular to A and parallel to B with a tolerance of .002"." I know I could do a simultaneous requirement for these but this was more done as an exercise for my personal understanding (still learning this stuff). Any comments are most welcome and a big thank you in advance.

Greg

 

[AndrewTT]

The composite position FCF in your example is illegal. The datums in the lower segment must follow the order shown in the top segment. If you want [A] in the bottom segment then the top segment would have to be [A] [D]. Also the tolerance in the lower segment needs to be smaller than the tolerance in the top segment for the FCF to make sense.

You will want to look into using the MMC modifier for position tolerances on threaded holes. Most GD&T authors say that measuring position on threaded holes at RFS is very difficult. "It (the MMC modifier, used with the position tolerance of a threaded hole) does, however, allow the inspector to use a fixed size thread gage for inspection" (GD&T, Applications, Analysis, & Measurement - James Meadows - page 286).

Also since the two holes are threaded you should consider if you need to project the tolerance zones (circle P modifier).

Lastly, you don't need the 2X for the basic .50 dimension. There is an implied basic zero between the distances of the two threaded holes from datum D, since they are drawn in line with each other.

 

[greg2835]

Thanks Andrew, all of your points are new information for me so I appreciate you bringing them up.

I have attached a rev2 of the dwg with your suggestions and a new FCF for the threaded holes (I also updated the tolerance values). I removed the composite form and simply have the position tolerance to datums A, D, B. Just to restate it here - my goal is to call out the position of the threaded holes relative to the pin (without simultaneous requirement, just for understanding purposes). What I am *hoping* this new FCF is saying is "these threaded holes are perpendicular to A and relative to the pin hole and parallel to D with a tolerance of .001" at MMC."

Also, quick question: would the MMC tolerance for the threaded hole be based on MMC of the minor diameter? I assume it's not based on the pitch as that would be hard (impossible?) to measure.

Thank you,

Greg

 

[AndrewTT]

Every thing in your current drawing is legal per the ASME Y14.5 standard.

Your current position FCF for the threaded holes could be read to state that the tolerance zone for the each of the two tapped holes is a cylinder of diameter .001 (when the feature is made at its MMC size, with bonus tolerance available). The tolerance zones are as tall as the produced part and located at true position, as described by the basic dimensions. The tolerance zones are perpendicular to datum feature A and located to datum feature D (clocked by datum feature B). The axis of each pitch diameter must reside inside its tolerance zone.

If you want the position tolerance to apply to the minor diameter instead of the pitch diameter then you will need to add a note under the position FCF that says "MINOR DIA". By default the pitch dia. is being controlled by your current FCF. As it stands the current FCF could be gaged by a fixed size threaded gage screw. If you add the MINOR DIA note to the FCF then the threaded holes could be gaged by a fixed size pin (smooth pin, no threads).

Usually with threaded holes the position tolerance zone gets projected beyond the part to account for the thickness/height of the mating part. You can account for the mating part in other ways but using a projected tolerance zone is the most clear description of design intent to the person reading the print, imo.

 

[SwinnyGG]

MMC of a threaded hole is based on the pitch diameter unless otherwise specified.

Using the pitch diameter allows the use of actual gauging, and MMC condition relative to the pitch diameter also accounts for the built in tolerance gap between the fastener PD and hole PD.

 

[greg2835]

Thank you gentlemen, that was all that I needed.

I did intend to include the projected tolerance zone but somehow I lost it. I will include it in this attachment just for completeness.

Kind regards,

Greg