huangyhg

pattern as a datum & timing?
i have a plate that has two patterns of holes. i need to maintain a relationship between the two patterns. the first pattern of holes is locationed using 3 datum planes (a, b, c) and then that pattern becomes a datum (d) to locate the second pattern. my question is this, does the timing/clocking of the first pattern get used when locating the second pattern (is it assumed)? i have attached a simple example. i'm trying to figure out if the ?.257 holes are timed in accordance with the first pattern, because it uses datum "d" i've read some old posts that mentioned that one hole must be used a a tertiary datum.
there seems to be some confusion on our end with several people having different ideas of how datum "d" is used?
any info would be greatly appreciated.
no tertiary datum is needed for the second pattern since d establishes 2 planes at once. if pattern d were to be clocked in one way or the other, the second pattern would clock with it. as far as i can tell, your example looks fine.
assuming you work to asme y14.5m-1994 then take a look at section 4.5.8 & figure 4-22, they explain how a pattern of features provides 2 datum planes.
kenat,
just an observation...if the holes are clearance for bolts or pins then i would recommend positioning them at mmc for maximum use of your tolerance.
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runz,
using the 4-hole pattern as a datum feature is a bit more complicated than it might seem. kenat has correctly pointed out that y14.5 deals with using a pattern as a datum feature, but it only deals with the case in which the pattern is referenced at mmc. the fixture component for the datum d pattern would have 4 pins fixed at virtual condition size. this would allow the possibility of datum shift, which would be undesirable since you want tight control of the relative clocking of the patterns.
unfortunately a pattern datum feature referenced at rfs is a gray area in the standard. exactly how a coordinate system would be established on the 4 imperfect holes on an actual part is not clearly defined. most fixturing and cmm methods would use some sort of best-fitting, averaging, or arbitrary choice of particular holes for location and clocking. the uncertainty involved in establishing the coordinate system on the datum d pattern would likely be comparable to the position tolerance for the .257 holes.
it might be more workable to maintain the clocking of the .257 pattern to the .375 pattern by referencing both patterns to the same datum features. there are a variety of ways to encode gd&t to do this. the optimal way would depend on:
-how accurate the shape and spacing needs to be for the .257 pattern
-how accurate the orientation (clocking) of the .257 pattern relative to the .375 pattern needs to be
-how accurate the location of the .257 pattern relative to the .375 pattern needs to be
-how accurate the orientation and location of the .257 pattern relative to datum features b and c needs to be
a brief explanation of the functional relationship between the two patterns would probably shed some light on this.

evan janeshewski
axymetrix quality engineering inc.

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