Using width and length of a recess as dimensioning datums. 2

[Tunalover]

Folks-
I often work with designs where a rectangular panel is mounted to a hole pattern within a rectangular recess. The rectangular panel is usually carefully sized to closely fit inside the recess when the holes come in at worst-case tolerance conditions. If I were to use the length and width of the recess as secondary and tertiary datums, would I take all dimensions from the CENTERLINES of those two features? If this is legal, how would this be handled in inspection? Is this an inspection nightmare? It seems to me that one could gain bonus tolerances by invoking the positional tolerances when the length and width datums are at MMC.


Tunalover

 

[MikeHalloran]

At my current shop, we dimension to centerlines all the time. It works out okay, because our homemade tubes in exotic alloys are not always perfectly round anyway, and because our standard tolerance zone is plus or minus the width of a dull Sharpie point.

In most other kinds of shops, it would be an issue, because it's hard to hook the jaw of a caliper over a centerline.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[ewh]

If it is a basic dimension, it must be connected to a datum, either through other basic dimensions, or directly. Otherwise, you are adding additional tolerances.

 

[ctopher]

Dim the length and width of the rect. Add datums (primary & secondary) to one side of each dim inline with the dims. Basic dim the holes. Inspection will position the holes within center of the part (if that is what you want). Can you attach a pic of the part you are dimensioning? It would make it much clearer to us. thanks

Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1

 

[ajack1]

I was taught never to use a centre line as a datum, if possible use the most critical feature.

If you take a very simple example of a square plate with a hole in one corner, the hole is tightly tied up from two edges that say mate to other parts but the other two edges are in free air.

If you dimension from the hole two sizes have a tight tolerance (hole to edge) but the overall dimensions do not need a tight tolerance.

Now do the same from the centre line, both the overall tolerance of the outside and the tolerance from the centre to the hole need to be incredibly tightly toleranced to ensure that you get what you want, just try this you will see what I mean.

Many hours are wasted trying to achieve something that does not need it.

This of course only works on simple 2D, 3D forms create a whole new set of problems.

 

[Tunalover]

All-
I had a conversation this afternoon with Lowell Foster in Minneapolis. Mr. Foster is one of the pioneers of Y14.5. He stated that it is best to originate the dimensions from the datums associated with the width and length dimensions. That is, the primary is the perpendicular datum, the secondary is the length datum (centerplane bisecting the length), and the tertiary the width datum (centerplane bisecting the width).

Guys, this is a VERY COMMON design scenario:

Imagine a rectangular, flange-mounted housing (the lengthwise and widthwise cross-sections look like the cross-section of a hat). The flanges have mounting holes in them for attaching the enclosure to a flat surface (using holes or studs). If the the internal length or internal width dimensions are used as secondary and tertiary datums (simulating the fit with what is inside the enclosure!) then the dimensions are taken from the centerplanes (avoiding the term "centerline" here). Functionally speaking, this is a VERY NATURAL way of dimensioning the holes because, by virtue of the way they are dimensioned, the holes are naturally centered about the midplanes of the housing! Furthermore, bonus tolerances can be granted when these features of size come in at MMC! That is, the pattern of holes can be allowed more float when the internal length and internal width are the greatest!







Tunalover

 

[ctopher]

good one tunalover

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP0.1 / PDMWorks 05
ctopher's home site

 

[ewh]

When in doubt, go to the source! It's hard to argue with Mr Foster!

 

[ringman]

My observation is that the description of 'primary is perpendicular datum' should be the 'primary datum feature would be one of the surfaces of the panel'. The choice of secondary datum feature should be the slot dimesnsion having the greater length and therefore providing a more stable or definitive portion of the feature. The tertiary would then be the other, or the smaller of the two features. Not sure if Mr F. Stated thusly or not.

 

[Tunalover]

ringman-
You're right that USUALLY one would want to use the length (the greatest dimension of the two) as the secondary.


Tunalover