straightness on unit basis & m modifier

[Madhu454]

1) need to know the applicabality of M modifier when straightness is applied on unit basis? is it legal? please refer to fig 6-4 of 1994 std. can we add m modifier to straightness callout.?

2) Also I need to clarify one more thing from u. please ref to fig 6-3 of 1994 std. i have understood the drawing like this, please correct me if i am wrong. the bonus tolerance is depends on the actual loca size not the actual mating envelope. if i assume a cross section which measures 16, the straightness allowed is 0.04, if i assume one more section next to it say which measures 15.89 the tolerance allowed is 0.15. if my interpretation is correct, then over the length of the shaft, the tol zone size also varies from section to section. then the tolerance zone will not be cylindrical.

i have understood, this requirement in fig 6-3 can easily checked with a guage designed for acceptable boundary. and size limits needs to be checked separatly. but seeing at the explanation of fig " As each actual local size depart from MMC , an increase in the local diameterof the tolerance cylinder is allowed which is equal to the amount of such depature". i interpreted the dwg as i said above.

can any one help me to understand this? in the explanation they use the term Actual Local Size.

 

[axym]

Madhu,

1) The standard does not show an example of Straightness at MMC applied on a unit basis, but I would say that this application would be legal. There is nothing that indicates otherwise. In fact, I can see this application as being very functional and easy to gage on things like rod stock material.

2) You're entering very murky waters here! But your understanding of Fig 6-3 sounds correct. The size of the Straighness tolerance zone depends on the local size at each cross section. In general, the tolerance zone will not be cylindrical. The figure in the standard hides this, by showing a simplistic as-produced feature that has constant cross-sectional size. On real as-produced features that have changes in cross section - taper, barreling, waisting, etc. the tolerance zone is a nasty surface of revolution.

As a result, Straightness at MMC is very difficult to check by finding the Derived Median Line and comparing it to a locally-expanded tolerance zone. But it is relatively simple to evaluate with a gage that compares the surface to an acceptance boundary.

Another complicating factor is the ambiguous definition of Actual Local Size. Certain statements in Y14.5 indicate that an Actual Local Size is a 2-point distance (i.e. many Actual Local Sizes per cross section). Other statements indicate that an Actual Local Size is a property of the circular element (i.e. one Actual Local Size per cross section). There is still endless debate over this.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca