How do you determine gage hole size using RFS. 3

[tooldeziner]

I am checking the drawings for some mylar inspection gages that are intended to verify profile and hole locations on flat parts by laying the part on top of the mylar. The engineer specified the holes to be .1285 -.1385 dia and true pos'n w/in .020 RFS. I know that the .020 positional tolerance is added to the hole size, but I'm not sure which hole size to use: smallest, largest, median? I have looked through the ASME standard and can't find anything that addresses this. Any help would be greatly appreciated.

Peace Through Superior Firepower!

 

[KENAT]

Actually, I think the fact it's RFS messes up your hopes of of using this inspection process, but I may be wrong.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

tooldeziner,

If your positional tolerance is RFS, you must inspect the diameter and position separately. Regardless of Feature Size means exactly that.

You can implement a quick and dirty GO gauge using pins. The pins must be located exactly at nominal position. Their diameter must be the minimum size minus the positional tolerance. In your case, the diameter would be .1085". There is lots of stuff not being inspected, but this might catch your primary functionality.

I do not know what your holes are required to do. There is no guarantee that a 4-40UNC screw will pass through your clearance hole into a tapped hole.

JHG

 

[dingy2]

One cannot use GO gauges when the positional tolerances are in RFS - regardless of feature size unless one is willing to make a gauge for every increment in size. The position should be confirmed using a CMM type measuring equipment.

If I had a drawing reflecting RFS, I would go back to the Designer and request MMC. Now you can develop a gauge at its virtual condition size.

Dave D.

http://www.qmsi.ca

 

[drawoh]

One cannot use GO gauges...

That is why I called it quick and dirty.

MMC also is a better description of a lot of design intent. Presumably, we are trying to clear something. Ø.020" is a sloppy positional tolerance.

JHG

 

[restedup]

I think Kenat made a good point. To follow up with that, I would ask, what version of ASME is applicable on your drawing?

 

[tooldeziner]

IMHO, I liked it better when the designer had to specify the M, L OR S. Now an oversight on the engineer's part, accidentally omitting the modifier, may give a result that was not desired or intended. But, that's for another post...

 

[pmarc]

On one hand I agree with Dave. Fixed in size pins should not be used for verifying tolerance of position on RFS basis. Dia .1085 pin basically located from datum reference frame would work fine if TOP was applied on MMC basis.

On the other hand, I would not automatically disqualify such TOP with RFS callout on a drawing and go back to a Designer for changing it to MMC without asking him what is the function of analyzed component. There are some cases when TOP with RFS is better than with MMC (centering, symmetry, alignment). Of course the inspection is then almost always more expensive and time consuming but if the function requires such position callout a designer should go for it.

 

[drawoh]

pmarc,

You make an interesting point. I just assumed that the holes were for clearing screws. If I provide holes for cable ties or for cable pass-throughs, I would apply sloppy positional tolerances. Functionally, I would not care where the holes are.

The gauging I proposed above, looks for a worst case in which the part interfaces with a rigidly located external feature. The external feature may not be rigidly located, or the intent may be for it to be located by the hole.

JHG