Explaining BASIC tolerencing

[charlie2go2ubet]

Please help me.

I need a specific, clear cut source explaining how you tolerence two tapped holes and mating cover plate so they will always clear. Forget about Geometric tolerencing, or the screw tolerences?.... I am trying to explain why the clearance hole can be no smaller than 2x the tolerance range per part. I am failing to connect...I can similify it no further and I am STILL being told:

Plate hole distance= +/-.010"
Tapped hole distance= +/-.010"

Clearance hole= +.004" ?????

It is not like any of us are dumb...people just can not envision the way I am explaining it and I need

 

[KENAT]

ASME Y14.5 appendix B gives formula for this type of calculation, not sure it gives the explanation in the way you need it though. In its simplest form for a fixed fastener case as you describe it.

H = F + 2T

H = minimum diameter of clearance hole (MMC limit)
F = Max dia of fastener (MMC limit)
T = postional tolerance diameter

For your example above with +-.010 inch spacing and let's say a 1/4 inch thread then the formula in ASME Y14.5 gives the clearance hole as .306. However this is converting the +-.010 to equivalent positional tolerance which it sounds like you don't want to do.

When you say BASIC do you mean as in simple or as in the way basic is used in GD&T, from what you put in your post I assume the former.

A picture paints a thousand words, if you have access to basic CAD or even reasonable hand drawing I suggest you draw an example, you may need to exagerate the values for it to be clear but it should help.

For instance, simplify it to a one dimensional problem, ie points on a line.

Draw the two threaded holes at max spacing on the max thread diameter (typically this is also the 'nominal' screw size). Now draw the two clearance holes in the mating part at minimum spacing and minimum size. These should be centered on the midpoint of the line between the two threaded holes.

Do this for a variant of their tolerancing and also for yours.

For the parts to fit at worst case the circles representing 'threaded holes' will need to be completely inside the circles representing 'clearance holes'.

You could post this in the tolerancing forum:

http://www.eng-tips.com/threadminder.cfm?pid=1103

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

If you have the time & money you could even create parts deliberately machined at max tolerances with their and your tolerance schemes, then ask them to assemble them.

KENAT, probably the least qualified checker you'll ever meet...