true position tolerance on a BC

[jerry1423]

I have a question on a "true position" tolerance for tapped holes that mate to a flange based on ASME/ANSI B16.5.
The BC on the flange is 8.5" dia and there is 8X 3/4" bolts on it.

Does anybody have any tips on determining the true position tolerance that I can put on the tapped holes, used on the part that I am designing, that mates to the flange?

The reason that I am asking is because I have seen some drawings where I feel the TP tolerance is way too tight, but I really don't know what to base the TP tolerance on - as I do not know the clearance hole size on the flange or any other variables that I may need to consider.

 

[drawoh]

jerry1423,

There are notes on how to calculate positional tolerances in appendices in the back of the various versions of ASME Y14.5.

Your positional tolerance is a function of bolt/screw configuration, and your hole clearance. If you have a pitch circle with tapped holes, you have another with clearance holes.

The fasteners in each of your tapped holes potentially occupy space in your clearance hole. Effecively, this is the tapped hole major diameter, plus the positional tolerance. You may have to apply a projected tolerance zone, especially if your clearance hole flange is thick.

At MMC, your clearance holes must be outside the space potentially occupied by fasteners. Your MMC clearance hole diameter equals the tapped hole major diameter, plus the tapped hole positional tolerance, and it is located exactly at nominal. You need to increase the hole size to allow for your clearance hole positional tolerance.

I strongly recommend positional tolerances for pitch circles. If you apply ± tolerances to pitch circle angles, you must account for the pitch circle diameter. This gets very accurate, very fast.

JHG

 

[KENAT]

Drawoh, as I read it the problem seems to be not knowing the tolerances or even explicit sizes for the holes on the existing flange in ASME B16.5.

I'm often in similar situations, not for flanges, but for things like electrical components, off the shelf motors... that have nominal sizes and location but incomplete, if any, tolerances.

One approach I'll sometimes take is just to spec the tightest tolerances I can on the part I'm designing without making the price crazy high. I'll also inspect any real parts I have access to, so at least I have some feeling on what the real sizes might be.

Bear in mind that the flanges are probably designed/toleranced for nut & bolt connections - so a floating fastener situation. As such I'm not surprised that threaded hold patterns designed to align to them look tight.

As drawoh says, the calculations are in the back of 14.5, but it sounds like you don't have the necessary input data to run those calcs.

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